Wednesday, February 27, 2013

March 2013 Outlook

This is the March 2013 outlook. This outlook will not include a graphical outlook. Rather, a region-by-region summary will be included in this article.

Dates are shown at the bottom of this image as (day/time), so 4/12 would be March 4th at 12z (6 AM CT)
We're going to start with something called the Arctic Oscillation, or AO. The Arctic Oscillation involves the strength of a massive low pressure system in the Arctic that we call the polar vortex. Think of how you used to swirl your straw around in your drink in restaurants when you were a kid. If you swirled it fast, you could form a tornado-like vortex. If you only swirled it a little, the attraction to the center of the cup and the vortex is weak. This can be applied to the polar vortex. When the polar vortex (swirling your straw around) is weak, cold Arctic air is released down to lower latitudes, and can penetrate into the US. This is known as the negative phase of the AO. In the positive phase, a stronger than normal polar vortex (swirling your straw around fast) is observed. This locks up the cold air in the Arctic and provides for a warmer pattern in the United States. If we look above, we see the long range forecast of the Arctic Oscillation from the American ensemble forecasting system. The black line with intermittent circles is what is called the control run of this ensemble system (an ensemble member that does not have its initial parameters changed, unlike other ensemble members), and the green lines illustrate the mean forecast. The blue lines are the maximum and minimum of all the ensemble members' forecasts for the Arctic Oscillation. Analyzing the forecast above, we see that the AO is negative at this time and has been negative recently. This promotes stronger chances of cold air due to the weak polar vortex losing its grip on cold Arctic air. Looking ahead to the future, we see there is a strong consensus for this negative AO to continue into the middle of March. However, at the end of this forecast, the ensemble mean's green lines spike towards neutral territory, and this would indicate a strengthening polar vortex and thus less chances for cold air. As far as I can see, the first half of March has fair potential to be chilly in parts of the US. I will address this further at the end.

Now shown above is the forecast for the North Atlantic Oscillation, or NAO. The same rules about colors and types of lines in this forecast apply. The NAO has a positive and negative phase (in the following description, the positive and negative phase word colors above will correlate with word colors in the next few sentences). When there is high (low) pressure over Greenland, the NAO is said to be negative (positive). As a result, low (high) pressure forms over the Eastern US, and the subtropical jet stream, which helps Nor'easters form, is strengthened (weakened). Thus, when the NAO is negative, one can anticipate better chances for cold and storms in the East US, while the positive NAO brings about warmer weather. Looking at the forecast above, once again from the American ensembles, we see that we are currently in a weak negative phase that will be intensifying in coming days. Looking towards the longer-term, we see a tendency to go back to that medium-weak negative phase, and this could continue through the first half of March. Like the Arctic Oscillation, we see a spike towards more neutral and positive phase territory at the end of this forecast, which could mean warmer times to come. I want to point out that future forecasts of the NAO could be more negative than what we're seeing above. Based on my extrapolation of forecasts over the North Hemisphere, I feel that the negative NAO could be enhanced more than what is being depicted, but only time will tell if that comes to fruition.

For our last forecasting indice, we have the Pacific North American index, or PNA. The PNA has a positive and negative phase. In the positive phase, high pressure forms in the West US. In accordance with Newton's Third Law of Motion, an action will have an equal and opposite reaction, meaning high pressure in the West US will provoke low pressure in the Central and East US. The positive PNA is the favored stage for storms and cold in these two regions. In a similar fashion, when the PNA is negative, low pressure dominates the West US. Going back to the Third Law of Motion, high pressure then tends to form in the East US. This type of pattern gives the North Plains the big winter storms, a feature we have seen much of the winter (the PNA was negative during much of the winter, so the Plains storm train can be attributed to that). The forecast of the PNA starts out with us in a solid modest positive phase. This looks to continue into the first days of March before we go into neutral territory to round out the first week of March. Beyond this, we drop well into negative territory with the PNA, and you know what that means: storms diverted away from the East US, warmth dominates. And look at those blue maximum-minimum forecast lines- for a couple days at the end of the second week in March, the forecast is off the charts, while the maximum forecast barely breaks neutral territory. That is not a good sign for those in the East and Central US wanting cold weather for March, but an excellent sign for the warm-weather lovers in those areas.


This is a long range forecast from the most recent American ensembles, and this time they are forecasting 500 millibar height anomalies. Reds and oranges tell of high pressure, while blues indicate low pressure. I outlined the three indices we talked about above- the AO, NAO and PNA. We see very strong, blocking high pressure from the Bering Sea into the Arctic, something the American ensembles have been hinting at for quite a while now. There is no low pressure in that area, meaning the polar vortex has been exiled from its domain. On a related note, large low pressure in northeast Asia shows where the polar vortex may have been shifted to. The fact that the polar vortex isn't even allowed in its domain means cold air is much more likely to flow south and into lower latitudes, the extent of which is to be determined. We see a solid negative NAO over Greenland, shown by high pressure in that region. Now, I said how I believed the NAO may be more negative than what is currently being forecast, and the reason for this is because of that strong -AO. There is certainly plausible evidence that the negative Arctic Oscillation's high pressure could influence and strengthen the high pressure over Greenland. I will be watching for this in the next week or two to see how it plays out. As far as the negative PNA goes, it would be strengthened by high pressure in the Bering Sea into the Gulf of Alaska. As a result, you can see a small shade of orange in the South. As small as it is being shown to be, I can assure you the high pressure would be much stronger if this forecast comes true.

After reconciling these atmospheric factors and adding in a few more complicated indices, this is my March 2013 forecast by region.

Pacific Northwest: Starts more or less warmer than normal, gradually turns stormier and colder.
Southwest: Continued instances of stormy weather, possibly increasing after mid-month.
South Plains: Turbulent temperatures in first half of March, gradually turning warmer.
Southeast: Chilly start gives way to warm end.
Northeast and Mid-Atlantic: Bouts of cold and chances for snow give way to warmer end to March.
Ohio Valley: Chances for cold and snow lessen as month progresses; turning warmer.
Midwest: Chilly start to month ends up moderating as month goes on. Storm track shifts north.
North Plains: Cold start, stormy end. Wintry region of the month.

Andrew

Tuesday, February 26, 2013

Sudden Stratospheric Warming Could Mean Robust Spring

The chances of a late-winter sudden stratospheric warming could mean a robust severe weather season this spring.

Shown above is an animation of temperature anomalies at the 10 millibar region in the atmosphere. To give you an idea of how high up that is, we live at the 1000 millibar mark, commonly known as surface level. 500 millibars is where storm systems are most commonly seen, and 300 millibars is where airplanes fly. So 10 millibars is quite a high position in the atmosphere. If you look at the animation, you will see it start with sudden cooling after a large sudden stratospheric warming in January. From early February to now, we have seen a very chilly Arctic, meaning the chances of anomalously cold air reaching the lower latitudes and the polar vortex becoming more unstable are lower than normal. If we look towards the Himalayan mountain range, however, we can see some oranges (and more recently, reds) bubbling up in the mountain range. This does signify that the stratosphere is warming in that area, and sudden stratospheric warmings have been known to occur off of mountain ranges, particularly the Himalayan range.

From here, I can reasonably assume that the warmth in the Himalayan mountain range will continue to increase in the upper stratospheric 10 millibar level. In the next 1-3 weeks, we should then see movement of this body of warmer than normal temperatures propagate north and likely into the Arctic. This is almost an identical set up to the sudden stratospheric warming that was experienced in January- warming bubbles up in the Himalayan mountain range for a few weeks, then propagates north into the Arctic where it blossoms into a sudden stratospheric warming event.

So what does this have to do with spring? Well, the sudden stratospheric warming phenomenon involves warm air being forced up into the stratosphere. As a result of warm air going into the stratosphere, anomalously cold air must then be pushed out of the stratosphere due to air displacement. This cold air then moves down the stratosphere and troposphere until it hits the surface. This moving down takes a good 2-4 weeks, so if the sudden stratospheric warming event did come to fruition, we would still have to wait 3-6 weeks before we see its effects. This leads us into spring, the season of severe weather. If the atmospheric pattern is able to allow this cold air to flow into the US and not other parts of the world, we would most likely see additional chances for wintry weather, including snow and cold. The increased temperature gradient in a cold front would most certainly excite severe weather potentials, and this in turn could lead to more damaging wind and general severe weather events.

This all depends on if the Himalayan warmth even propagates into the Arctic, which there's not a 100% confidence in right now. However, based on past experiences with Himalayan stratospheric warming, I think what I typed above has at least a decent chance of coming true.

Andrew

Sunday, February 24, 2013

Himalayas Provoking Upper Stratospheric Warming

The upper stratosphere is showing signs of additional warming over the Himalayas and Southeast Asia. Posted above is the animation over the past 30 days of the 10 millibar layer of the stratosphere, considered the upper stratosphere.

Looking down towards the Himalaya Mountain range, we see there has been a trend of increasing warm temperature anomalies. The mountain range is one of the ideal spots for stratospheric warmings, as air is easily provoked to be forced up and into upper levels of the atmosphere. It was in the past several weeks that we saw a sudden stratospheric warming originate from the very same mountain range. While we are at the end of February and it would take a month to see this warming propagate down to the surface, the stratosphere does not effectively 'shut down' for the winter until well into the spring and into summer. Until then, stratospheric warmings are still on the table.

The EP Flux forecast at the bottom of this multi-panel image will also be on the rise in the next several days. This increase means the stratosphere is becoming more supportive of warming in the stratosphere. Personally, I don't believe we will see another sudden stratospheric warming in the next 10-14 days. However, that increasing warmth in the Himalayas will be something to watch- if it is able to strengthen and detach from that mountain range into the Arctic, we could then see a solid sudden stratospheric warming event. Climatologically, that chance is decreasing by the day, but in weather, anything is possible.

Andrew

Serious Threat to Florida Citrus Crop in Coming Days

There could very well be a serious threat of damage inflicted on the Florida citrus business and other warm-season economies in the Deep South in coming days.

The forecast above depicts 850 millibar temperatures in light color contours, where below freezing temperatures are north of the thick blue freezing line, and above freezing temperatures to the south of that thick blue line. As you can see, the thick blue line is edging very close to the southern tip of Florida, leaving Jacksonville, Tampa and Orlando in below-freezing temperatures. Light precipitation over the latter two cities could indicate snow - yes, SNOW - would be falling in Florida. This solution has been shown a few times, so the idea of such cold temperatures hitting the state is not entirely out of the realm of possibility. The orange/citrus economy that is based in Florida could be severely damaged if such a solution came true- if snow fell, only time could tell what the impact on the citrus economy would be.

Andrew

February 24-27 High-Impact Blizzard

500 millibar analysis reveals that our storm system is currently in the Rockies, and is at the pre-mature stage of a positive tilt. The positive tilt is shown by how the vorticity values (in red outlines) seem to be trying to push to the southwest. In the next 24-48 hours, the system will attain a negative tilt, where the vorticity values will slide east and appear to be pushing to the southeast. In similar fashion to the most recent snow event, this storm will dump intense snowfall on the Plains and occlude (weaken) as it progresses east.

The time for analyzing forecast models has come and gone, as we begin to see swaths of the country stamped with various winter weather headlines. In purple, we see winter weather advisories, which are issued for generally lighter snow events. Pinks show winter storm warnings, and these are the areas that are most likely to see these heavy snowfall amounts in the next couple of days. In an orange-red color we see blizzard warnings, which extend from North Texas and western Oklahoma into southwest Kansas and portions of Colorado to the east of the Front Range. This is where the high-impact blizzard portion of the storm title comes from. These areas will see the worst of this storm in terms of travel impacts and ability to travel.

This is the short range NAM model, an American-based weather model that does fare well in these types of late-winter storm situations. Its forecast for this event holds as much as 31 inches of snow in western Oklahoma in that small red dot. That's nearly three feet of solid snow! The apricot color signifies 24-30 inches, and the light blues reflect 20-24 inches of snow. This is essentially a repeat of what we saw with the most recent snow event: It dumps all of its snow early, occludes and leaves the leftovers for the Midwest and Great Lakes. However, this time, the 'leftovers' may not be so meager. Multiple forecasting sources have informed me that cities like Chicago, IL , Gary, IN and even the region of southern Michigan could be in the running for more accumulating snow than what fell earlier last week.
I did put in city forecasts for this event in addition to the coloring scale. Light Blue is a general 1-3 inches, dark blue is 4-8 inches, and light pink is upwards of 8 inches. I used a blend of the NAM model and forecasts from the National Weather Service, as well as my own opinion on the matter. As far as commutes go, I am anticipating an incredibly messy one for Wichita and Amarillo, where blizzard conditions are expected, especially in the latter city. Chicago and Des Moines will really be 'game-time decisions', as upper air thermal profiles are still a little murky and will ultimately determine how much liquid and snow falls over these cities.

Andrew

Friday, February 22, 2013

February 24-27 Potential Significant Winter Storm

Current satellite water vapor imagery indicates that the storm system in question is currently over the waters just offshore the Pacific Northwest. Enhanced water vapor values across the Pacific Northwest would indicate the presence of precipitation, and radar analysis confirms that precipitation is ongoing in the oceanside states. Despite its appearance of close proximity to the United States, 500 millibar analysis reveals the storm is still too far away for weather balloons (that are launched in the US Mainland) to get into the storm and put data into the models that would help significantly improve their forecasts for this storm.

For this post, we will be using the European model and ensemble forecast out of consistency for this storm. American model remains out of touch with current trends, which will be reflected in this post. Shown above is the Hour 72 (times will be expressed in hours for this post. If you want to know how many days out that is, divide the number of hours by 24) from the European ensembles, forecasting 500 millibar heights. The system is clearly indicated by a deep depression in the Rockies and Plains. Such a strong depression does indicate a strong storm. Looking closer at the storm, we see it is almost pointing straight down with those light blues. This indicates a neutral tilt storm. This means that the storm's 500mb appearance is not pointed towards the southwest (positive tilt) or the southeast (negative tilt), it is simply pointing south. For future reference in this post, a negative tilt means the storm is at its strongest point.

Let's go back to the winter storm we just had across the Plains and Midwest. If you recall, the concern in the days leading up to the forecast was that it would reach a negative tilt too quickly, and would then occlude (or weaken) as it progressed northeast towards the Midwest. This did end up happening, and the Plains got the brunt of the storm. This was thanks in part to how early the storm attained a negative tilt. By the time it was exiting the Rockies, it already had developed a negative tilt. However, looking at the forecast above, we see it is holding on a neutral tilt- a good sign for those east of the Plains as far as snow potential goes. Another thing relating to this recent storm is where it exited the Rockies. I did mention in a recent post how it would be exiting the mountains in southeast Colorado. This is further north than what is being shown above. The European ensemble's forecasted south placement of this storm system relative to the previous storm is more good news for folk in the Midwest and Great Lakes that may be affected by this system.

We now progress ahead to Hour 96 of the European ensembles forecast, again looking at the 500 millibar level. We find that the storm has now exited the Rockies and has attained a negative tilt, shown by how the storm seems to be pushing into the southeast. This pushing motion/negative tilt does a few things. For one, it strengthens the storm. The presence of a negative tilt does mean the storm has reached maturity, as I mentioned earlier in this post. Second, the system begins to dig down towards the southeast. In a similar situation to the most recent snowstorm, there will be high pressure to the east of this storm system, and that feature is exhibited well by large arcs of greens and yellows in the East Coast and Northeast. This high pressure, combined with the strongest areas of vorticity pushing southeast (the technical definition of a negative tilt) results in the cold front catching up to the warm front, and this results in the final stage of occlusion. Occlusion of a storm means that the negative tilt swings north so the storm seems like it's pointing to the east or even northeast. When this happens, significant weakening of the system occurs and precipitation weakens in response. This is exactly what happened in the Midwest and Great Lakes with the most recent storm system. Luckily for those folk, the European ensembles believe that this will happen when the system is beginning to cross into Canada in the central Great Lakes just east of Michigan (the mainland Michigan, not the U.P.).

Now it's time to look at what the atmosphere will be supporting as far as storm tracks. Shown above are two different atmospheric indices, forecasted by two different government weather agencies. The ESRL/PSD agency has their forecasts on the left hand side of this multi-panel image, while the NCEP agency is on the right hand side. The two indices being forecasted are the Pacific North American index (PNA) and the North Atlantic Oscillation (NAO). For ease of viewing, I superimposed arrows pointing towards the date of this storm's timeframe. The two agencies generally agree on a slightly negative to neutral PNA. A negative PNA means there are negative height anomalies in the West US (low pressure), thus positive height anomalies must form in the East US (high pressure), as dictated by Newton's Third Law of Motion. The negative PNA discourages the big coastal storms known as Nor'easters, and are typically detrimental to the formation of any big winter storms in the central and East US, save the North Plains. However, as I stated with the European ensembles, there should be very little influence from the PNA at this time, which leaves more influence up to the NAO. Both agencies have the NAO negative during this timeframe. In the negative NAO, high pressure builds over Greenland. In response, the jet stream drops south into either the Central or Northeast US, bringing cold to the region. The negative NAO then enhances the subtropical jet stream, which is basically another jet stream along the Gulf Coast and South US. This is the jet stream that drives the Nor'easters up the coast. As the subtropical jet stream (STJ) is enhanced, high pressure in the East is suppressed, meaning that if there were a solid negative PNA, it would have to fight the negative NAO. Luckily, there will not be a strong negative PNA this time around.

Here's my current thinking on the system. I'm going to buy into the European model and ensemble's consistent solutions and insert a large swath where significant snow may fall. It should be noted that the significant snow area may have to be pulled south a bit if some indications of mine become stronger. I also put in a potential icy area. Temperatures are shaky right now in the icy zone, and those areas could see a potentially major icing event. Don't begin preparing yet- besides the system not being in our upper air network yet, ice storms are a much shorter-range forecast than snow, as ice is much harder to forecast. Nonetheless, keep up to date on the latest forecasts if you are in the icy zone.

Confidence checks (out of 100%)

Confidence that a significant winter storm will hit the Plains and/or Midwest: 85% confidence
Confidence that over a foot of snow will hit somewhere in the Midwest: 40% confidence
Confidence that a major icing event will occur somewhere with this storm: 90% confidence
Confidence that a snow day will be called for someone in this storm: 90% confidence

Comments are welcome, please don't ask for your location's snow or snow day chance. You will not get an answer, because we are too far away from the event.

Andrew

Wednesday, February 20, 2013

February 24-27 Potential Winter Storm

**Notice: Due to continued confidence in my February 21-23 forecast, there will not be a new post.**


There appears to be potential for yet another winter storm across the nation from February 24th to 27th.

Shown above is a side-by-side comparison of the European ensemble set for Hour 144, or a week away from today. The left side shows 850 millibar temperatures and mean sea level pressure, while the right shows 500 millibar heights and mean sea level pressure. The European ensembles are predicting a pretty strong storm system to hit just south of Chicago, IL at this timeframe, in a solution that would most certainly bring accumulating snow to portions of the Plains and Midwest. A quick background on the European ensembles: They originate from the European model, which is a very accurate model, widely regarded as the best forecasting model in the world at the moment. This means its ensemble set, which is composed of 52 individual members, is also very prestigious. Think about the accuracy of the European model, and multiply it by 52. That's the European ensemble set for you. Additionally, the fact that it is showing such a strong storm this far out means there are individual ensemble members that are much stronger with this system, as these images are a mean of all the members combined. That said, there are also individual ensemble members that are weaker than this mean. Just a little something to keep in mind.

The 500 millibar forecast on the right signals the presence of elongated high pressure across the Eastern Seaboard into the Canadian Maritimes. There is a general lack of pressure anomalies in the Rockies, which concerns me. If you want to map out a storm's track, you want to have at least some signal from the Rockies, as its placement upstream of the Eastern US means it does have a profound influence on what the storm will do.

Shown above are two different atmospheric indices, forecasted by two different government weather agencies. The left half of the 4 panel image contains forecasts from the ESRL/PSD forecasting branch, while the right half shows forecasts from the NCEP (essentially the American model and ensembles). The top half shows forecasts for the Pacific North American index (PNA), which I will explain later, and the bottom half shows the North Atlantic Oscillation (NAO), which I will also show later on. I superimposed arrows pointing towards the date of this storm's timeframe. The two agencies generally agree on a slightly negative to neutral PNA. A negative PNA means there are negative height anomalies in the West US (low pressure), thus positive height anomalies must form in the East US (high pressure), as dictated by Newton's Third Law of Motion. The negative PNA discourages the big coastal storms known as Nor'easters, and are typically detrimental to the formation of any big winter storms in the central and East US, save the North Plains. However, as I stated with the European ensembles, there should be very little influence from the PNA at this time, which leaves more influence up to the NAO. Both agencies have the NAO negative during this timeframe. In the negative NAO, high pressure builds over Greenland. In response, the jet stream drops south into either the Central or Northeast US, bringing cold to the region. The negative NAO then enhances the subtropical jet stream, which is basically another jet stream along the Gulf Coast and South US. This is the jet stream that drives the Nor'easters up the coast. As the subtropical jet stream (STJ) is enhanced, high pressure in the East is suppressed, meaning that if there were a solid negative PNA, it would have to fight the negative NAO. Luckily, there will not be a negative NAO this time around.

I'm not saying this storm will happen. But if it did happen, I don't think we would see the storm go into the Midwest. I have a better time believing that the atmosphere will favor a more suppressed storm, possibly something that could affect the East Coast. Again, this is all with a lot of uncertainty, this is just what I'm looking at right now.

Please no questions about snow for your area; it's worthless to ask this far out.

Andrew

Tuesday, February 19, 2013

February 21-23 Significant Winter Storm (Updated 2/19)

Edits as of February 20, 2013...

-No new information to add, still holding to my forecast graphic. Chicago's totals continue to be the most controversial, will hold my amounts there as models are in shaky agreement.

Post title removed the 'High Impact' just because it was getting too long to write and the high impact snows (well above one foot) are looking less synoptic (large scale) and more mesoscale (localized).

Latest 500 millibar analysis shows that our storm system is beginning to work its way onshore of the US. Recent model runs have had the privilege of getting weather balloons into the system, so the model data is more reliable as it ingests data from the storm. These weather balloons could not get into the storm earlier, as the system was over the Pacific. Nonetheless, now that the storm is beginning to shift onshore, weather model data is becoming more and more reliable with each step the system makes to move fully onto land.


I decided to add in choice cities to my forecast graphic today, so those of you in the crosshairs will be able to get a better look at what snow amounts you may receive. Wichita, KS into Omaha, NE are definitely looking like the big winners with this storm, where amounts will likely flirt with (and even surpass) one foot of snow. Localized areas in central Kansas to the west of Wichita could see amounts trying to reach above 18 inches, but the spectrum of that potential remains to be determined. The Quad Cities are on the border of the 6-10 inch line. I decided to go conservative in that area, as the Quad Cities eastward are going to be the tricky spots as far as how snow accumulation will work out. The Quad Cities into Chicago will be ground zero for hair-pulling, as forecasters will be dealing with how to forecast snowfall amounts after storm has begun to weaken.

One city I want to highlight in particular is Chicago. I have been surfing multiple weather forums and have seen local forecasters with amounts past a half foot. I even read one testimonial about a government official (not the big Washington politics guys, we're talking government meteorologists) saying this city would receive over a half foot of snow. However, other nationally-forecasting models show lighter amounts. Due to known bias' with these local models, I have decided to go for the conservative 3-6 inches, with 6 inches being the high end if the local models end up right. Then again, if they do, both the Windy City and Quad Cities will have to be bumped up.

Andrew

Monday, February 18, 2013

February 21-23 High-Impact Significant Winter Storm (Updated 2/18)

500 millibar analysis reveals the storm system in question remains a while away from the United States, with enhanced wind readings being found in the Gulf of Alaska. This is the storm system in question, and is the storm system that is at the center of this big mess of confusion amongst the models.


There continues to be a large spread amongst the forecasting models as far as which solution is the most likely to verify. Shown above is the North American Ensemble Forecasting System, or NAEFS forecast for Hour 96, which converts to 4 days away from today. We see our system is in Kentucky at this time. If you've been reading my recent posts, you will understand that there has been a north trend with this storm that some models have had. This trend then resulted in the storm going to Wisconsin and other areas in the Northern Plains/Upper Midwest. It has been within the last 48 hours that we have seen a sudden revolt, so to speak, among the ensemble systems. This includes the NAEFS, the Canadian ensemble system and the American ensemble system. All three are considered big ensemble forecasting systems and have all been in fascinating agreement with the system going south rather than north.

So why is there so much confusion? Well, shown above is the NAEFS 500 millibar height anomaly forecast for 3 days away. Colder colors correspond to low pressure systems, while warm colors are supportive of high pressure systems. The model disagreement appears to stem from how strong high pressure in the Central and East US will be. The high pressure originates from persistent low pressure in the West US. As a result of below normal heights in the West, there must be above normal heights in the East US, as Newton's Third Law of Motion would indicate. These lower heights in the West and higher heights in the East are called the negative Pacific North American index, or negative PNA. In the negative PNA, storm systems that form in the Southwest as a result of persistent low pressure in that area are forced north into the Northern Plains, due to high pressure formation in the Southeast US. The models are catching onto this idea, but why are the ensembles going south? One reason is the presence of high pressure to the east of Greenland, called an east-based negative North Atlantic Oscillation. You can probably guess why the East based negative NAO is called east-based, but the negative NAO implies high pressure is over Greenland. This high pressure then acts to strengthen the subtropical jet stream, which is essentially another jet stream that sits along the Gulf Coast and Southern US rather than the North US (which is the big time jet stream). The subtropical jet stream, which is also called the STJ, is strengthened in a negative NAO. This negative NAO also acts to suppress the Southeast Ridge of high pressure, which would then act to verify the ensembles prediction of the storm sticking to the south.

Now that we've identified the indices at play here, let's look over that 500mb anomaly map and identify why the models are not handling this well. In that image, we can see high pressure centered over Canada and stretching into the US. This stretch into the US is helped by the negative PNA. By Hour 72 (when this image is valid), the high pressure system has started to extend north, leaving a gap of high pressure in the Southeast that is weaker than the high pressure in the Upper Midwest. Some models, like the European model, want to take this system north and have it ride the stronger high pressure wave into the Upper Midwest. The ensembles seem to believe it will go towards the weaker area of high pressure and this take a more southern route. The logical solution would be to take this system to the south, towards the weaker area of high pressure, as the ensembles are showing. I can understand why the models want to take the system more into the Upper Midwest, and it's due to where the system ejects from the Rockies. The farther north it comes out of the mountains, the farther north the track is going to be. Likewise, the further south the system ejects from the Rockies, the further south the storm track will be, like the ensembles are forecasting. In my personal opinion, if the system will be moving out of the Rockies from Southeast Colorado and the jet stream will be flowing towards the northeast (with winds over 100 knots), I could see the Northern track work out.


I combined both possible storm tracks in the image, with the yellow 'L's illustrating the north track and purple 'L's showing the southern storm track. Amounts were put at a wide spread of 2-6 inches for those in the light blue. Most should reach the middle of that spread, while some closer to the dark blue will flirt with the half foot mark. The dark blue contains an even wider spread of 6 to 12 inches. The wide-ness of the forecast spread is due to how low confidence still remains. The system is not yet onshore; we still have another day or two before the system makes landfall on the US. The pink shows amounts surpassing one foot. Southeast NE and north KS will bear the brunt of the system if either track succeeds.

Andrew

Sunday, February 17, 2013

February 21-23 High-Impact Significant Winter Storm

Have removed the 'Potential' from the storm title, as the event is within the timeframe of reasonable certainty.

Satellite imagery shows the storm system of interest is in the Gulf of Alaska and nearly onshore in Alaska at this time. Analysis of the infrared field determines that the center of low pressure is on the coast of Alaska, with a very ragged comma shape still present in association with this storm. Surface analysis indicates the storm is in the 990 millibar range, so it is a fairly decent storm at this point in time.




Model uncertainty still remains a priority to deal with at this time, as it does appear there are two possible solutions evolving. Shown above are 500 millibar height anomalies for Hour 120, or 5 days away. Cold colors suggest the presence of low pressure, while warm colors favor areas of high pressure. The bottom image shows 850 millibar temperatures and mean sea level pressure for Hour 132, meaning it is 5 days and 12 hours away. This model is the North American Ensemble Forecasting System (NAEFS), a set of ensembles which I understand is derived from the Canadian and American ensemble forecasting systems. The NAEFS has done very well in the past two years for predicting temperature anomalies during the winter, this will be my first time seeing how it fares with storm systems. The NAEFS is predicting the storm system to ride east-northeast through the Plains and into the Midwest. This is not what recent model trends have been showing- the latest forecast models have taken it further north.

The issue with the models is how strong that high pressure system will be in both Canada and the East US. If we look at the top image of the NAEFS, we can see that the high pressure system is strongest from Canada into Greenland. Models and ensembles differ as to where the center of this high pressure system will be. The NAEFS proposes the high pressure system to be centered in north Canada, which then allows the storm system to progress further east and into the Midwest. The European model, as you will see below, takes a much different track.


This is the most recent forecast from the typically-reliable European model. It is called the ECMWF model in the weather world, but for practical purposes we will refer to it as the European model. This forecast also has two images: the top image once again shows 500 millibar height anomalies, while the bottom image depicts 850 millibar temperatures and mean sea level pressure values. The 500 millibar chart is valid for Day 5 (Hour 120), while the bottom image of mean sea level pressure is for Hour 144, or 6 days away. If we do a comparison between the NAEFS and this European model, it's pretty obvious that the European model is stronger with the high pressure in the East than the NAEFS. We see more oranges stretching down south from Canada and into the Eastern US than the ensemble system. The reason for this is a much weaker connection between the high pressure in Greenland and high pressure in Canada. Almost like a bridge, the European model nearly breaks off the connection between these two masses of high pressure, which would allow the Canadian high pressure system to then push further south and allow the European solution to prevail. If the European model solution did prevail, one would expect lower snow totals with any snow further to the north. Quite an odd solution in my opinion, and we will evaluate it further later on in this post.

Something that needs to be brought up is that strong high pressure system in the Pacific. That big high pressure system, combined with the low pressure in the West US creates what's called a negative Pacific North American index, or a negative PNA. The low height anomalies in the West provoke above normal height anomalies (high pressure) to arise in the East US, which typically can push storms into the North Plains in a similar fashion to the European model. The NAEFS ensemble system also has this high pressure system in the Pacific, but is more widespread than the European model. The more widespread it is, the stronger the high pressure is. As a result of a stronger high pressure, the 500mb heights must become lower and this would push the storm track further to the south. However, we now go back to the previously-mentioned high pressure over Greenland. When high pressure is established to the east of Greenland, it is called and East-based Negative NAO; 'East based' for being east of Greenland, and the negative NAO (North Atlantic Oscillation) implies high pressure over Greenland. The negative NAO provokes the subtropical jet stream to become more active and pushes the overall storm track south. It also suppresses the high pressure in the Southeast, which is why I'm not confident in the north-bound European model. Both images above show the east-based negative NAO, but both have different solutions. It all comes down to which factor has the stronger influence. Because the negative PNA is upstream of the Midwest, it will have a stronger effect than the negative NAO, which is far up in Greenland. However, by the time the storm system is able to reach the Plains, the negative NAO will hold some influence and ought to be able to keep the storm system south, definitely further south than the European model's solution.



I can understand why there have been trends north. Shown above are 500 millibar heights (not anomalies, just the regular values) valid for Hour 96 (4 days away). We can see our low pressure system just beginning to emerge from the Rockies. The issue from here is where it comes out of the Rockies. The NAEFS forecast in this image has the low pressure system in far southeast Colorado. When you get storms coming from that area, there is a decent possibility that high pressure could push it north, and I believe this is where the European model is getting the idea of a northward trend from. Additionally, the storm will be reaching what's called a negative tilt by the time it is exiting Colorado. A negative tilt implies that a storm system's highest vorticity values are pointing towards the southeast, and this is where we find the big difference between the southern solution (NAEFS forecast) and the northern solution (European model).

I put together the NAEFS and ECMWF (European) models together for the same timeframe. They are both forecasting 500 millibar values for 4 days away. The black line illustrates the tilt of our storm system, while the red line illustrates the tilt of the high pressure system in the Central US that is preceding this storm. The issue between the northern and southern solutions is that the European model wants to bring the system to a negative tilt faster than the NAEFS ensembles do. This is shown by the ECMWF image's black line tilted to the southeast, while the NAEFS is more neutral-tilted, meaning the highest vorticity centers are not tilted in any particular direction. In response to the negative tilt by the European model, the storm system digs to the Southeast more. This situation then responds similar to if you were to push some form of a wedge (shovel, for example) into the ground- the shovel (storm system) tries to dig down further south, but the ground (high pressure system) is provoked to move north in response, and this is how the European model gets its northern solution.

Considering there are three different ensemble systems rooting for a southern solution and just one model and one ensemble system rooting for a northern solution (the most recent American model jumped south a bit), I'm going to have to go with the southern solution. I am a bit hesitant, because the European model and its ensemble set are very prestigious and reliable forecasting systems, so it's a very tough forecast to make when it's essentially Europe vs. the world.

Despite this, I am willing to stick with my prediction that was made yesterday for this system. To me, there are three stages of the models. The first is the Long Range track, where the models get a consensus roughly 10 days away from the event. The second stage is what I like to call the '60 Hours of Chaos', where the storm gets within 8 to 5 days away (Hours 180-120 on the models) and the models go in every which way. The third and final stage is the final track, where the models converge on a solution within 5 days of the storm. This final track can sometimes be nearly the same as the first Long Range track. We already observed the first stage, and are now clearly in the second stage. The system will get on land in the next few days, so model solutions should clear up as the time comes closer. I should mention that the Chicagoland area into southern Wisconsin and north Indiana and southwest Michigan may see that 'Accumulation Possible' dropped if dry air manages to start that region with virga (precipitation is being produced, but evaporates in a dry layer of air before it hits the ground) as some sources are suggesting. There will be a lot of dry air in place just a day or two before the event, and this could very well hinder the already-lowered totals. I will most likely have my first accumulation map out either tomorrow or Tuesday; confidence remains low.

Andrew

Saturday, February 16, 2013

February 21-23 High-Impact Potential Significant Winter Storm (Posted 2/16)

Storm title is kept the same despite crucial model differences. I do this because, despite said model differences, it is quite obvious that someone will get a thumping of snow out of this storm system. The question right now is: Who?

Satellite imagery indicates that our storm system is moving into the Gulf of Alaska at this hour, with elevated levels of infrared imagery suggesting the center of this storm system is located around the Aleutian Islands. Pressure observation charts suggest the storm system has a minimum pressure of 988 millibars, a pretty solid storm system. The comma shape of the infrared imagery depicts the storm as a well-functioning system that will certainly be able to make some noise on land, both on the wintry side and the severe weather side.

This continues to be an unresolved issue between the European model and several other reputed models. Shown above is the Day 5 forecast for 500 millibar height anomalies. Cold colors indicate low pressure systems, while warm colors display high pressure areas. We see our storm system in the Southern Plains at this time, with high pressure in the East US and additional high pressure near Greenland. This high pressure system in the East US is what is causing such intense model differences. Both the European suite and American model suite agree that the storm will be ejecting from the Rockies in the position shown above in the apricot color. That's where the similarities stop. The European model believes high pressure in the East will force the storm system to move north-northeast, cutting out many regions of the Midwest from snow that was previously shown by this model.

These are the American ensembles for the same timeframe and same parameters as the European image above. We still see our storm system in the Southern Plains, although there is no longer an apricot shade. This is to be expected, as the ensemble members begin to diverge at this stage and therefore the mean solution is not as strong as the single European model. In comparison, the high pressure in the East US is weaker and centered further north in Canada than the European model has it. This is a crucial difference, as the weakness of the high pressure does not force such a northward movement anymore. It does move northeast, but nearly as drastically as the European model. Also of interest is stronger high pressure in the Pacific than in the European model. This stronger high pressure would force the storm further south.

I'm having trouble understanding why the European model takes the storm directly into the high pressure system. It is apparent that we will see some northward movement in response to persistent low pressure anomalies in the West and high pressure anomalies in the Northeast Pacific. These two factors make up the negative Pacific North American index (PNA). The negative stage of the PNA responds to the low pressure anomalies in the West by provoking high pressure in the Southeast US, and this does correlate with the issue the models are having with the high pressure in the East US. If it was just the negative PNA, we would most definitely see the solution being shown by the European model. However, we now go back to the previously-mentioned high pressure over Greenland. When high pressure is established to the east of Greenland, it is called and East-based Negative NAO; 'East based' for being east of Greenland, and the negative NAO (North Atlantic Oscillation) implies high pressure over Greenland. The negative NAO provokes the subtropical jet stream to become more active and pushes the overall storm track south. It also suppresses the high pressure in the Southeast, which is why I'm not confident in the north-bound European model. Both images above show the east-based negative NAO, but both have different solutions. It all comes down to which factor has the stronger influence. Because the negative PNA is upstream of the Midwest, it will have a stronger effect than the negative NAO, which is far up in Greenland. However, by the time the storm system is able to reach the Plains, the negative NAO will hold some influence and ought to be able to keep the storm system south, definitely further south than the European model's solution.

It's times like these when you have to stop looking at the models and think about how the atmosphere works. In times like these, I like to use my aforementioned river analogy. If this storm was a leaf that had fallen off a tree and into a river, would it want to move towards the rock in the river or towards the open river channel? If you think it wants to move towards the open channel, you've just explained the big paragraph I typed above. And in all honesty, the models are worthless. Until the storm gets on land, we might as well be trying to throw a dart, in the dark, blindfolded, spun around several times, with the correct solution being the dartboard bullseye three millimeters wide and a mile away from you. But if you're going to look for hints, always look at the ensembles. They stand the best shot of being right from a time this far away, which is another part of the reason why I support the cross-country solution.

The 12z run of the American model shows very heavy snow in the Plains. I am very confident that Nebraska and Iowa will certainly cash in on heavy snowfall. Even South Dakota could receive some extreme snowfall. Amounts could very well exceed 18 inches based on snow-to-water ratios above the usual 10:1 (10 inches of snow equals 1 inch of water). We could see ratios flirting with 20:1, meaning there would be lighter, fluffier snow, but a lot more of it. Towards the Midwest is where confidence drops. The American model believes in a sharp cutoff near Chicago. A sharp cutoff will happen, but this is the first time the American model proposes a cutoff zone so far north. The American ensemble system has been more persistent in the rain/snow line remaining in central Illinois. This is the same solution previously shown by the American model, up until now.


As of now, the European model is a far north outlier, and the American ensembles have been very consistent. I preferred the American ensemble solution, because I don't buy into an extreme far north solution. To me, there are three stages of the models. The first is the Long Range track, where the models get a consensus roughly 10 days away from the event. The second stage is what I like to call the '60 Hours of Chaos', where the storm gets within 8 to 5 days away (Hours 180-120 on the models) and the models go in every which way. The third and final stage is the final track, where the models converge on a solution within 5 days of the storm. This final track can sometimes be nearly the same as the first Long Range track. We already observed the first stage, and are now clearly in the second stage. The system will get on land in the next few days, so model solutions should clear up as the time comes closer.

I will update daily here on the blog, and much more often on our facebook page at www.facebook.com/TheWeatherCentre .

Andrew

February 21 Potential Severe Weather Event

The Storm Prediction Center has outlined an area of severe weather for Day 6, or February 21st.

Prognosis... Strong storm system will be ejecting from the Rockies on the 20th-21st. As the storm system moves east, the subtropical jet stream and Pacific jet stream will merge into a single, powerful jet stream that will be centered over the outlined region. Current thinking is that the abundance of moisture and strength (albeit weakening) of the storm system will provide a base for severe weather on the 21st. Tornadoes and damaging winds would certainly be possible, but model uncertainty forbids me from making any definitive conclusions on threats for this day.

I will have more updates in coming days.

Andrew

Friday, February 15, 2013

February 21-23 High Impact Potential Significant Winter Storm

Have downgraded system from Blizzard to Significant winter storm on account of recent model trends. Will maintain rest of storm title.

Infrared satellite imagery of the Pacific Ocean shows the storm of interest near the Aleutian Islands. We can see some convective activity associated with the front end of the storm system, characterized by a nearly vertical line of elevated infrared values. The storm itself remains far out to sea, away from any nearby weather stations in the Mainland that would otherwise help figure out model forecasts. We will need to wait until Tuesday or Wednesday for this system to crash ashore and the National Weather Service's weather balloons to get inside the storm and help out model forecasts.

Previous model agreement has been wiped away as we enter the dreaded timeframe where the models go every possible solution before they convene on one solution. This process typically takes a few days, but can last more than 5 days. I expect we will see model agreement come Wednesday evening when the system gets onshore. Model disagreement appears to be stemming from the forecasted strength in high pressure that will be over Canada and into the East US when the storm digs into the Southwest. When this storm system hits the West Coast and shifts south, it is typical to see high pressure arise in the East. The big question is, how strong will this high pressure be? A stronger high pressure would result in a more northerly track, while a weaker ridge would indicate a more cross-country, west-to-east storm track. This cross country idea is what earlier model solutions were showing. This dilemma is shown on the 500 millibar forecast from the American ensemble prediction system above. Our storm system is shown in the West US, with high pressure in the East. In this situation, the base of the high pressure system is in Canada, so the storm takes a more cross-country track. The actual American model has the base of the high pressure system in the Great Lakes, which is how we get that north track, as shown below:



My thinking is that this storm is going to hit areas that the models were in good agreement yesterday. The reason? Well, let's go in order here. First, we'll start in the West US. We have low pressure in the area, and that can instigate high pressure response in the Southeast US, which would go hand in hand with the American model solution above. However, looking at Greenland in the upper part of both images, we see high pressure to the east of Greenland. This is what's known as an east-based negative NAO. You can probably figure out why it's named east-based, but the negative NAO infers high pressure is in place in Greenland. This high pressure can then force the subtropical jet stream (located along the South US) to act up, and this brings the storm track further south. Also, it can oppress formation of the Southeast Ridge that the West Coast low pressure is trying to do. These two indices that are battling for the Southeast may just cancel out and we could see that cross-country solution as I suggested. The third (and probably most important) reason is that storms don't head right into high pressure systems. Don't get me wrong here, they can, but in a river, does water try to go to the place where it will get stuck? No, it goes towards the most open part of the river to keep on moving. This means that the low pressure system would theoretically not want to go north, right into the Canadian high pressure system. Rather, it would want to travel west-to-east (cross-country track), where it would encounter less high pressure anomalies. Adding onto that, if we see the base of this high pressure will be in Canada, the weaker high pressure anomalies in the Southeast would not be able to properly force the storm system north, like recent model solutions have been showing.

It's times like these when you have to stop looking at the models and think about how the atmosphere works. In times like these, I like to use my aforementioned river analogy. If this storm was a leaf that had fallen off a tree and into a river, would it want to move towards the rock in the river or towards the open river channel? If you think it wants to move towards the open channel, you've just explained the big paragraph I typed above. And in all honesty, the models are worthless. Until the storm gets on land, we might as well be trying to throw a dart, in the dark, blindfolded, spun around several times, with the correct solution being the dartboard bullseye three millimeters wide and a mile away from you. But if you're going to look for hints, always look at the ensembles. They stand the best shot of being right from a time this far away, which is another part of the reason why I support the cross-country solution.

I maintain the high impact part of the storm title because I do believe that there will be an intense snowstorm somewhere with the storm. The system in question will tap into pure Gulf of Mexico moisture that will allow heavy snow to fall across a potentially wide swath of land. Right now, a North Platte, NE to Cedar Rapids, IA swath is looking good for the most snow, with similar amounts east until Gary, IN.

There will be more developments as we progress through the weekend and into next week. My thoughts will undoubtedly change, but that's what weather does: change. Please don't ask for your location's snow, I will not answer if you ask.

Andrew

Thursday, February 14, 2013

February 21-23 High Impact Potential Blizzard

I have upgraded this storm title from 'Potential Significant Winter Storm' to 'High Impact Potential Blizzard' on account of recent model forecasts.

Current satellite analysis from the Japan Meteorological Agency (JMA) reveals that our storm system continues to kick up quite a ruckus in the North Pacific. A solid dry slot and enhanced cloud curving around said dry slot/low pressure center affirms that our storm system is quite a strong one. Further analysis on other charts indicates that the storm system is down to 968 millibars, with hurricane force winds. Usually, these types of systems are reserved for the Bering Sea, but this is the one we're watching to hit potentially a very large swath of the nation with intense accumulating snow.

In the interest of model forecasts and public awareness, I have upgraded the storm title to reflect my current outlook on the situation. Shown above is the latest ECMWF (henceforth referred to as the European model) model forecast for Hour 192, or 8 days from today (henceforth called Day 8). We see a very strong area of low pressure on the Illinois/Indiana border at this time, with very cold temperatures in the storm's wake. Analysis of pressure values around the Plains and Midwest reveals this storm to have a central minimum pressure of 995 millibars. That's a pretty strong storm system. Tight pressure contours to the west and north of the system suggest a blizzard potential with this system, hence why the title has been changed. The European Ensemble system, commonly regarded as the best ensemble prediction center known to man, agrees with the potential for a winter storm in this timeframe:

Somewhat low confidence is reflected by the broad-ness of the system by the ensembles. This is very common to see in the long range; the surprising thing is that the ensembles are even showing such a potential this far out. Their support behind this very interesting storm system adds confidence to my prediction for this storm system. Several other models and ensemble sets are in agreement but will not be shown to save space. Among the supporters are the American model, the American ensembles, and the Canadian weather prediction system has been known to previously support this idea.

A big concern is that this storm will be going extremely slowly. If the storm goes extraordinarily slow, we could see this storm system drop significant amounts of snow for over 24 hours. If current forecasts verify, some places may see over 36 hours of continuous snowfall. Accumulations would only grow if the storm continues to become slower in future model forecasts. This is another reason why I added both the High-Impact and Blizzard titles to this storm system.

For those of you wanting to see the European's snowfall forecast, here it is through 10 days out:



Here is an excerpt from yesterday's post on this storm. I am reposting it because it still contains my best thoughts as to what the storm could do.

I could go on and on about other models and ensembles supporting such a solution, but we need to take a step back and analyze the atmospheric pattern that will be in place when this storm potentially occurs.

This is the American ensemble forecast, yet again for 9 days away from today. The colors show 500 millibar pressure anomalies, with blues signifying below normal heights (low pressure) and reds depicting above normal heights (high pressure). First and foremost, this system will be originating from the Pacific. If we look at the Pacific ocean, we find a very tight jet stream, shown as the 500mb pressure contours grouped very close together. This tightened jet stream adds increased energy to disturbances that flow through this jet stream. The pieces of energy may start off in the Bering Sea, where persistent low pressure is holding strong in this forecast. As they swing out from the Bering Sea, strong high pressure in the Gulf of Alaska tightens the Pacific Jet stream further. In response to the extreme above normal height anomalies in the Gulf of Alaska, low pressure would be tempted to develop in the Southwest. If we recall Newton's Third Law of Motion, every action produces and equal and opposite reaction. The presence of extreme high pressure in the Pacific will not only enhance low pressure in the Bering Sea, it will also do so in the Southwest. A concern from those two anomalies of pressure is that a Rex Block may set up. A Rex Block occurs when high pressure is stacked north of low pressure. We can see this trying to happen in the West US, with that Pacific high pressure apparently trying to progress into the Pacific Northwest region. Persistent low pressure in the Southwest would finish out the Rex Block. A Rex Block induces a more zonally-oriented (west to east) wind flow that is more favorable for warm temperatures and not big coastal storms. Fortunately, these concerns are alleviated when looking at other ensemble forecasts near this timeframe.


What is apparent is that a negative Pacific North American index (PNA) will be present during this timeframe. A negative PNA acts to produce persistent low pressure in the West US, and this then instigates high pressure formation in the Southeast US. This high pressure formation is a barricade to Nor'easters and any winter weather for the East US. We do see a pretty solid negative PNA in the forecast by February 21st as shown in the image above. If this does happen, we can expect high pressure formation in the Southeast US in response.

However, all hope is not lost for those in the Midwest. Rather, it is enhanced. This is the forecast from several global models and ensembles for the North Atlantic Oscillation (NAO). In the negative NAO, high pressure forms over Greenland and stays there. In response, the jet stream will dip south into portions of the US and produce an environment favorable for coastal storms (aka Nor'easters). We see a fair consensus of a negative NAO during the timeframe of this potential storm. The strength of this negative NAO is TBD, but the general idea of a negative NAO is in place.
The North Atlantic Oscillation then gets more specific with the West-based and East-based phases. The West-based negative NAO means that high pressure over Greenland is centered to the west of that land mass, hence west-basednegative NAO. It is this specific phase that is best for Nor'easters. The East-based negative NAO means that high pressure is strongest to the east of Greenland, and this means storms and cold are preferred in the Midwest over the Northeast. Looking over more model forecasts, the negative NAO will be nearly evenly distributed among both regions, but the East-based is definitely a favored phase by the time this storm's timeframe rolls around.

The negative PNA will set off the storm originally going through the Southern US, but high pressure in the Southeast will try to force it north into the Northern Plains, a common storm track this winter. However, the east-based negative NAO will suppress that high pressure to the point that the storm system is able to stay on a more eastward track that then goes through the Midwest and Lower Great Lakes, delivering snow to those areas. For those wondering if the Southeast ridge could disappear and the storm goes up the coast, chances are nil. The negative PNA, having a stronger influence on the US weather as it is upstream from the nation, will make the Southeast ridge prevail to an extent that matches my description in the first few sentences of this paragraph.

My current prognosis remains the same as yesterday. It will have to be altered to spread further north (just to cover some more of the Plains, that's all), but I want to see another round of model forecasts before I make a serious change.


Andrew

Wednesday, February 13, 2013

February 21-23 Potential Significant Winter Storm

The potential for a storm system between the 21st and 23rd continues to unravel as the time draws closer.

Pictured above is the ECMWF model (henceforth called the European model), forecasted for Day 9 (9 days away). The image shows mean sea level pressure values for this system, and the system is defined in the Midwest with a central minimum pressure of just above 1001 millibars. This is not a particularly strong system, but is still worth watching closely, as this model is projecting widespread amounts over 6 inches for many big cities in the Great Lakes and Midwest.


Now pictured above is the GFS Model (henceforth called the American model), also valid for Day 9. This time, however, the American model is projecting mean sea level pressures to be as low as 993 millibars, indicating a very strong storm system. Storm placement is relatively similar, with both models indicating the storm will be somewhere in the central half of Illinois. There is definitely that big strength difference, and this strength difference will eventually determine just where the storm goes and how much snow is produced. The American model prints out nearly a foot of snow in the Upper Midwest in response to this storm system, higher than the European model.


Now we see the American model's ensembles for the same timeframe. Now shown are 850 millibar temperature values and mean sea level pressure contour lines. The ensembles take the system to a very similar placement as the European model; not surprising given that most forecast models tend to follow the European model. Notice how the system is weak and spread out in this forecast. This denotes uncertainty among the models- typically, uncertainty in ensembles is denoted by a very broad low pressure system, and that is what is being observed here. Personally, the addition of this ensemble set to the pack of forecasting systems supporting such a snowstorm enhances confidence that this event will actually happen.

Now shown above are the European model's own ensemble set, forecasting the Day 9 forecast of 850 millibar temperatures and mean sea level pressure contours. The ensembles are slightly further south when matched up with the American model, American ensembles and European model. This is to be expected; all models and ensembles, no matter their forecasting accuracy will have problems in the long range. Nonetheless, its presence in the relative same area as the other models is indeed supportive of this event coming to fruition.

I could go on and on about other models and ensembles supporting such a solution, but we need to take a step back and analyze the atmospheric pattern that will be in place when this storm potentially occurs.

This is the American ensemble forecast, yet again for 9 days away from today. The colors show 500 millibar pressure anomalies, with blues signifying below normal heights (low pressure) and reds depicting above normal heights (high pressure). First and foremost, this system will be originating from the Pacific. If we look at the Pacific ocean, we find a very tight jet stream, shown as the 500mb pressure contours grouped very close together. This tightened jet stream adds increased energy to disturbances that flow through this jet stream. The pieces of energy may start off in the Bering Sea, where persistent low pressure is holding strong in this forecast. As they swing out from the Bering Sea, strong high pressure in the Gulf of Alaska tightens the Pacific Jet stream further. In response to the extreme above normal height anomalies in the Gulf of Alaska, low pressure would be tempted to develop in the Southwest. If we recall Newton's Third Law of Motion, every action produces and equal and opposite reaction. The presence of extreme high pressure in the Pacific will not only enhance low pressure in the Bering Sea, it will also do so in the Southwest. A concern from those two anomalies of pressure is that a Rex Block may set up. A Rex Block occurs when high pressure is stacked north of low pressure. We can see this trying to happen in the West US, with that Pacific high pressure apparently trying to progress into the Pacific Northwest region. Persistent low pressure in the Southwest would finish out the Rex Block. A Rex Block induces a more zonally-oriented (west to east) wind flow that is more favorable for warm temperatures and not big coastal storms. Fortunately, these concerns are alleviated when looking at other ensemble forecasts near this timeframe.


What is apparent is that a negative Pacific North American index (PNA) will be present during this timeframe. A negative PNA acts to produce persistent low pressure in the West US, and this then instigates high pressure formation in the Southeast US. This high pressure formation is a barricade to Nor'easters and any winter weather for the East US. We do see a pretty solid negative PNA in the forecast by February 21st as shown in the image above. If this does happen, we can expect high pressure formation in the Southeast US in response.

However, all hope is not lost for those in the Midwest. Rather, it is enhanced. This is the forecast from several global models and ensembles for the North Atlantic Oscillation (NAO). In the negative NAO, high pressure forms over Greenland and stays there. In response, the jet stream will dip south into portions of the US and produce an environment favorable for coastal storms (aka Nor'easters). We see a fair consensus of a negative NAO during the timeframe of this potential storm. The strength of this negative NAO is TBD, but the general idea of a negative NAO is in place.
The North Atlantic Oscillation then gets more specific with the West-based and East-based phases. The West-based negative NAO means that high pressure over Greenland is centered to the west of that land mass, hence west-based negative NAO. It is this specific phase that is best for Nor'easters. The East-based negative NAO means that high pressure is strongest to the east of Greenland, and this means storms and cold are preferred in the Midwest over the Northeast. Looking over more model forecasts, the negative NAO will be nearly evenly distributed among both regions, but the East-based is definitely a favored phase by the time this storm's timeframe rolls around.

The negative PNA will set off the storm originally going through the Southern US, but high pressure in the Southeast will try to force it north into the Northern Plains, a common storm track this winter. However, the east-based negative NAO will suppress that high pressure to the point that the storm system is able to stay on a more eastward track that then goes through the Midwest and Lower Great Lakes, delivering snow to those areas. For those wondering if the Southeast ridge could disappear and the storm goes up the coast, chances are nil. The negative PNA, having a stronger influence on the US weather as it is upstream from the nation, will make the Southeast ridge prevail to an extent that matches my description in the first few sentences of this paragraph.

Here's my current prognosis:

Andrew