Wednesday, September 17, 2014

New LRC Winter Pattern Projected To Be Snowy

Long range climate models are in agreement that the month of October may feature above-normal precipitation in the North and East US, which could play into a very snowy pattern later on in the winter LRC cycles.

What we'll do first is analyze four precipitation forecasts, each one slightly different from the other based on their initialization and model foundations, from the CFS model. Shown above is the first member of the CFS suite, showing precipitation accumulated over the next 45 days. The end of this forecast period puts us at the very end of October, on Halloween night.

In this forecast, we see a swath of substantial precipitation values extending from the Plains into the Upper Midwest, giving the Great Lakes some of those wet conditions as well. A very dry forecast is noted in the southern Midwest, Ohio Valley, and into the Gulf Coast. We then see somewhat-dry conditions into the Mid-Atlantic, with wet conditions again present in the Northeast.

The next 45-day precipitation forecast from the CFS ensemble members shows a pretty similar story across the board as the first forecast image. In this new projection, we see what appears to be a swath of wetter-than-normal conditions extending from the Southern Plains and into the Midwest, once again hitting the Upper Midwest and Great Lakes the hardest. In a twist, we now see the Eastern Seaboard receiving similarly-heavy precipitation totals, quite a difference from the first ensemble member. Overall, once again, it appears a wet end to September and most of October would be expected for the Upper Midwest.

This third ensemble member from the CFS once again retains a similar projection as the first two members for the next 45 days, but differs in the East. For the Central US, heavy precipitation values are displaced across most of the Midwest and west-central Great Lakes, extending back into the Plains. In this forecast, the Mid-Atlantic is dry, in a similarity to the first ensemble member. The Northeast then appears very wet, all in all looking like a combination of the dry scenario from the first member and wet scenario from the second member.

This fourth member I specifically saved for last, since it appears to be most radical with its forecast in the Midwest (that should be taken as a caveat as well). In this final forecast member, we see very heavy precipitation values over the month of October and late September in the entire Midwest and most of the Great Lakes, stretching down south into the Plains, and even hitting the Ohio Valley in the process. The Eastern Seaboard is forecasted to be predominantly dry, save for the New England region. This could mean that the wet-East projection from the second member we analyzed may be a false forecast, but that's not something we'll investigate right now.

So, why should we care what these four members say about precipitation in the next month and a half?

Because we could be staring right at our winter precipitation pattern.

Something I've discussed on here more than a few times is the concept of the Lezak Recurring Cycle, or LRC. The LRC was developed by meteorologist Gary Lezak, and discusses the idea that weather patterns which develop in October leave a 'footprint' of sorts that is repeated in a regular interval, between 40-60 days through the winter and following spring. In other words, the weather patterns that develop in October repeat themselves for the better chunk of the next year.

So, if we take the four members above to be true (which is NOT something I'd advise to do; I'm showing them in collaboration with additional forecasts below), the Upper Midwest, Midwest, Great Lakes, Plains, and other parts of the East could be in for a very snowy winter.

In the image above, we see an average of precipitation anomalies over the month of October, compiled from eight global forecasting models. In this graphic, we see a familiar trend. The Plains are projected to receive above normal precipitation, leading directly into the Midwest and Great Lakes, as well as the New England area. Does this look familiar? Because it's nearly identical to some of the projections we were viewing earlier in this post.

Based on the average of these eight global model projections, as well as the four CFS ensemble members we dissected earlier, it's safe to say the current trend favors a very wet October for the Midwest (particularly the Upper Midwest), portions of the Great Lakes, Plains, and Northeast. If these projections end up verifying (again, there are many long range caveats associated with this), then the aforementioned areas may want to prepare for a pretty snowy winter, so long as the LRC cooperates.

Since we don't have as many projections for temperature as we do for precipitation, I won't discuss temperature projections for the LRC in-depth right now. However, based on the average of the eight climate models, October could be a warm month, which might then transfer over into a warm winter. I personally don't agree with the diagnosis for now, but the possibility will be re-examined later on this fall.


Sunday, September 14, 2014

The Story Behind The Polar Vortex

Ever since last winter, the polar vortex has taken over as the go-to thought as soon as the prospect of cold weather is brought up. Let's take this time today to go over what the polar vortex actually is, and disprove some inaccuracies I've seen swirling around the world.

1. The Polar Vortex Exists Year-Round
The phrase 'polar vortex' is used to describe a strong low pressure system (vortex) that is semipermanently placed over the Arctic region (polar). In the summer, the vortex significantly weakens, and is replaced by high pressure. During the fall months, however, the vortex gains strength yet again and low pressure dominates the Arctic as the polar vortex gears up for winter.

2. The Polar Vortex Extends into the Stratosphere, Not Just the Troposphere

The image above shows the 10-day forecast from the GFS model in the stratosphere, at the 30 millibar level. That's pretty high up the sky when you consider us humans are at the 1000 millibar level. In this forecast, we see relatively weak low pressure establishing itself over the Arctic Circle, with weak high pressure surrounding it. That low pressure body is the "birth" of the polar vortex for this cold season, as it begins to come back to life and strengthen for the winter.

Some people assume the polar vortex is no different than your average storm system that brings snow to your neighborhood. The vortex is actually quite different. In order for it to be the polar vortex, it must exist at both the tropospheric and stratospheric levels. That's how big and controlling it is in the atmosphere.

3. There is only ONE Polar Vortex

This is something I've been hearing ever since January 2014, when this polar vortex business really took off. The belief is that there is more than one polar vortex in the atmosphere. This might stem from the aforementioned idea that some believe the polar vortex is no different than your average storm system, but regardless of its origins, the belief is false.

Take a look at the image above. In this image, we see a view of 500mb height anomalies across the Northern Hemisphere on January 7th, 2014. Warm colors correspond to high pressure, which brings warmth and generally quiet conditions. Blues and purples indicate the presence of low pressure, which permits stormy and chilly weather. Just taking a glance over this map, we see one strong upper level low that seems to be the strongest one in the entire hemisphere. That upper level low, seen scraping the northern US and inundating Canada, is the tropospheric version of the polar vortex. As you can see, there is only one low as strong as the one in North America, hence there is only one polar vortex. Those other low pressure systems are just disturbances compared to the polar vortex.

To give you a better idea of how there's only one polar vortex, let's take a look at the stratosphere on January 7th, 2014. In this image, we see what appears to be two strong areas of negative height anomalies, being pinched by two bodies of ridging/high pressure on either side of the globe. If you thought that this image shows two polar vortexes, you are incorrect!
What's actually happening is the two bodies of high pressure are trying to tear apart the polar vortex, something that happens rather often each year. The difference between other years and this past year is that the ridging forced the main part of the polar vortex into North America; that other area of negative height anomalies in Asia is just a weaker part of the single polar vortex that was torn off of the main body. Some of you more experienced weather enthusiasts may know this type of set-up as Wave-2 stratospheric activity, as two bodies of high pressure push into the Arctic.

To close out this post, here's a stellar graphic made by the folks at the National Weather Service office in New York in early January, when the hype over the polar vortex was in full swing.

To summarize:

• The Polar Vortex exists year-round, varying in strength. It does not (and will not) "come back" for the winter, as it's already present and cannot "come back".

• The Polar Vortex exists across both the troposphere (where we live) and the stratosphere (where Felix Baumgartner jumped from his capsule in October 2012); it does not exist at the surface.

• There is only one polar vortex; our world would be much different if more than one polar vortex existed.

Additional points of summary are included in the NWS graphic above.


SOI Continues To Plummet As El Nino Emerges

The Southern Oscillation Index, a key indicator for the emergence of an El Nino or La Nina, continues to free-fall as our El Nino looks to finally come to the surface.

The image above shows a 30-day average of the SOI values from January 2012 to the present day. The Southern Oscillation Index is calculated by examining the pressure differences between Tahiti and Darwin, Australia. When values drop below -8, conditions are considered optimal for El Nino formation. Values above +8 are considered optimal for La Nina conditions.

 As the chart shows, we had been seeing predominantly-positive anomalies this past spring and early summer, which is likely a reason why the the historic Kelvin Wave never brought about the Super El Nino so many (myself included) had discussed. However, beginning right around the start of July, we saw SOI values tank, and they've remained below zero for the entire period since. Values have gone both above and below the El Nino 'benchmark' of -8 since that time, but have remained below zero the entire time.

Due to potential copyright infringement concerns, I will refrain from posting a forecast of the SOI I came across yesterday. What it shows, however, is continued negative SOI values for the next few weeks, and possibly beyond.

The animation above shows a depth-by-longitude animation of equatorial temperature anomalies, in Celsius, since mid-July to present day. In this animation, we see the Kelvin Wave-induced warm waters spreading east and pushing to the surface as they do so, now just beginning to hit the surface.

The SOI being negative indicates that trade winds along the Equator and areas in the neighborhood of the Central Pacific have shifted to a state that will allow this warmth to stay and ferment on the surface, likely into an El Nino. Back in April, with the historic Kelvin Wave warmth, trade winds wee unfavorable, and prohibited the warmth from sustaining itself on the surface.

In coming days and weeks, we'll likely see this winter's El Nino bring itself to the surface, likely at a weak, potentially moderate strength. I'll have more on the projected El Nino strength in the near future.


Saturday, September 13, 2014

September 27-October 1 Potential Storm System

The September 27th to October 1st period is being monitored for a potential storm system.

Tropical Tidbits
The image above shows 500mb height anomalies over the West Pacific area on the afternoon of September 21. In this image, blues indicate the presence of below-normal height anomalies, which can then lead to cooler than normal temperatures, as well as stormy conditions. Warm colors are associated with positive height anomalies, which generally bring warm and quiet conditions.

As we see in the image above, a trough is moving through Japan on the 21st, according to the latest GFS Ensembles forecast. We see negative height anomalies across the entire country of Japan, but the real point of interest is the depression in the contour lines. This depression indicates a trough moving through the area, and that's what may provide us with the chance for a storm system in the September 27th - October 1st period.

Forecasted teleconnections can give us an idea of where this potential storm may end up.

On the top-left, we see the forecast for the Pacific-North American (PNA) index. We see that at the end of the forecast period, on September 27th, the PNA is projected to be descending from a strong positive state. This tells us that ridging in place across the Western US may be dissipating by the time this potential storm rolls around. Typically a positive PNA means the Midwest and Ohio Valley are most favored for storm systems, but it could be more for the Plains if the PNA is descending to neutral or negative territory.

Additional updates will be provided in the near future.


Long Range Regional Outlook (Ohio Valley): September 20-October 12

This is the latest Long Range Regional Outlook forecast for the Ohio Valley area, valid for September 20th through October 12th.

CMC Ensembles
I have elected to use a combination of the CMC and ECMWF ensembles for this outlook, after finding the GFS ensembles to be in disagreement with the two aforementioned systems, which do hold a consensus.

The image above shows the CMC ensemble mean 500mb height contours on the date of September 22nd. In this forecast, depressions in the contours indicate troughing (cold and unsettled weather) while arcing motions in the contours depict ridging, which results in warm and relatively quiet weather. In the graphic shown above, we see a broad depression in the contours across the Plains and into the Midwest, though we then see slight ridging emerging over the Ohio Valley. This would tell me that the forecast period of September 20th to October 12th likely opens with some seasonable to above-normal temperatures, before colder weather moves in.

ECMWF Ensembles
Now shown above is a two-panel forecast from the ECMWF ensemble system. We see forecasted 500mb height contours and anomalies (with legend on the right) in the left-hand panel, with the ensemble 'spread' on the right image. An ensemble 'spread' indicates the degree of uncertainty among individual ensemble members on a particular area and its forecast. For example, we see deep purples on the right panel over the Great Lakes, which the left panel says will experience troughing (due to the depressed contours). This tells us that the ensembles are uncertain as to how likely this particular factor is to actually occur, and results in a wider spread of ensembles.

In the image above, valid for September 21st, we see a similar layout as that of the CMC ensembles. We see suppressed ridging over the Western US, created by some stormy weather in the Gulf of Alaska (see green shadings of below-normal height anomalies). This ridge in the West results in cooler weather for the Plains and Midwest, as the CMC ensembles depicted, but that then leads to some slight ridging over the Ohio Valley. This jives well altogether with the CMC ensembles.

Tropical Tidbits
In the image above, valid September 17th, we see a swath of negative height anomalies overtaking Japan as a rather strong upper level low scrapes the nation to the north. Seems pretty mundane, sure, but the consequences here at home are far more than mundane. As has been discussed consistently for the past couple of years, the weather in East Asia can have a significant impact on weather here in the United States. Utilizing the East Asian correlation of ridging over Japan equals ridging in the US 6-10 days later, and the same situation with negative height anomalies, we can foresee long range weather patterns weeks out at a time. This mechanism is referred to as the Typhoon Rule, and states that weather patterns found at the 500mb level can replicate themselves over North America 6-10 days later after they appear over Japan.

If we use this rule for the Ohio Valley, we might expect to see some cold weather, though I wouldn't call it a 'cold blast'. In this forecast, the upper level low looks displaced north enough that the core of the cold would likely stay in Canada. This fits in with the CMC and ECMWF ensemble projections of a weak trough moving east from the Plains and Midwest (as we discussed earlier), and is a reason why I disagreed with the GFS ensembles.

* The image used above is a forecast from the GFS ensembles, but the part I disagree with is for its forecast in the Northeast Pacific, not over Japan. Still, this part of the forecast must be monitored closely for the discrepancies described above.

Tropical Tidbits
Once again using this Typhoon Rule, we see the forecast on September 21st calling for continued deep troughing over Japan, now pushed deep into the country. Using the guidelines set forth for this rule, we might expect some chilly weather in the Ohio Valley around the September 27th - October 1st period.

After going over the factors listed above, the temperature and precipitation outlooks for the Ohio Valley over the September 20th - October 12th period are as follows:

Temperature Outlook:

The Weather Centre
Temperature Outlook
Temperatures for the Ohio Valley are expected to remain around average to slightly below-average, due to a warm start to the forecast period and a few weak cool shots.

Precipitation Outlook:

The Weather Centre
Precipitation Outlook
The precipitation outlook calls for predominantly below-normal precipitation over the forecast period, due to somewhat dry signals over the Japan region for the next week or two, among other factors.

The next Long Range Regional Outlook will be published Saturday, September 20th.