Wednesday, October 30, 2013

Pattern Change Coming for Early November

Ensemble guidance is indicating that the weather pattern over the Pacific waters and over North America will be changing in coming days.

The GFS Ensembles above show the development of intense troughing in the western US, along with high pressure in the East SU US as a byproduct. This comes as the overall pattern in the Pacific also appears to be in line for changes, including the troughing pattern being replaced by a more persistent ridging pattern. Longer range model guidance indicates that this ridging in the pacific will strengthen, but it is possible we see the West US troughing evolve in the waters west of the coast of California. This would be good news for those in the Central and East US who want the cooler fall weather, as offshore troughing could incite ridging in a portion of the Rockies, which could then lead to some cooler weather in the Central and East US. The Central US would be more likely to see this cold weather, as it is possible some slight ridging would appear in the East US. This trend is partially shown in the GFS ensemble temperature departure forecast for 9 days out:



Andrew

Saturday, October 26, 2013

Halloween Potential Major Storm System

There is the potential for a major storm system in the Plains and Great Lakes during the Halloween timeframe.

There are currently two different model projections, so we'll start with the ECMWF model. The sea level pressure forecast for the morning of Halloween shows deep pressures across northern Minnesota and Wisconsin, with a tight pressure gradient across the eastern Midwest and Great Lakes. This tight gradient is suggestive of a high wind field in those regions, and the ECMWF confirms this with 5000' wind speeds peaking just over 50 knots in Michigan, Wisconsin, Illinois and Missouri on Halloween. This translates to some awfully gusty winds for the aforementioned states, and windy conditions across the north central states in general. As far as precipitation, the ECMWF hints at non-accumulating snowfall across the upper Plains, and overall precipitation isn't horribly more impressive. This would be more of a big wind event rather than a big winter storm.

The GFS model keeps this storm system further south, with the lowest pressure centered over southern Wisconsin and northwestern Illinois/northeast Iowa. The GFS actually follows the ECMWF with low to practically non-existent snowfall over the upper Plains, but differs from the model with a rather high amount of precipitation falling across the southern Plains and parts of the Midwest.

I'm thinking that a compromise between the two models may be appropriate, with wind being a pretty good portion of the storm, but some wet conditions also arising in the Midwest and southern Plains. With the Pacific North American (PNA) index rising to a weak-moderate positive during this storm, but the North Atlantic Oscillation (NAO) in a strong positive phase, expect the storm to go through the upper Midwest.

Andrew

Thursday, October 24, 2013

Final 2013-2014 Winter Forecast

"Slow start to winter should deliver harsh January, February for Central US..."

Hello everyone, this is the Final 2013-2014 Winter Forecast from The Weather Centre. This post will finalize my projections for this winter, with 3 month-averaged temperature, precipitation and snowfall graphics. Month by month descriptions will be written below each graphic. If you do not wish to see the discussion, you may scroll down to the graphics to see the forecast itself.

There are a lot of factors to look at, and we're going to start out with the El Nino-Southern Oscillation.

The El Nino Southern Oscillation (or ENSO) phenomenon involves anomalous sea surface temperatures across these four regions on the chart above. When sea surface temperatures are below normal in this graph, it is said that a La Nina is in place. In similar fashion, above normal sea surface temperatures (SST's) in the Nino 3.4 region correspond to the term 'El Nino'. Both the La Nina and El Nino have different effects on the United States- an El Nino brings about a snowy and cold East Coast, while allowing warm and dry weather to flourish in the Plains and Midwest. A La Nina permits cool and snowy weather to hit the Ohio Valley and Midwest, while warming up portions of the Southern US. Looking at the chart above, from TropicalTidbits, we see a neutral ENSO, meaning the water temperatures are not cold enough to be declared a La Nina, and are not warm enough to be called an El Nino. Because the ENSO is in a neutral state, we are once again going to see other atmospheric patterns affect our winter this year, like last year, because the ENSO is not strong enough to have a pertinent influence on our weather.

An animation of subsurface water temperatures under the ENSO regions shows disagreement over what phase the oscillation is currently in. This confusion confirms the idea that we are in a neutral ENSO state, and a lack of any agreement over temperature anomalies underwater suggests the rest of the fall (and most likely throughout the winter) will also see this neutral ENSO condition.

We will now move on to something called the Quasi-Biennial Oscillation, or QBO for short.

This phase space, drawn up by Mike Ventrice, illustrates the strength and phase of the Quasi-Biennial Oscillation throughout the past several months. As the chart shows, the QBO has been slowly traversing the middle phases of this diagram, to where we are now entering Phase 6 at modest strength. Extrapolation and a glimpse at the current QBO status through other diagrams tells me the QBO will be able to move through Phase 6 in the early part of winter, before transitioning to Phase 7 in time for the January-February part of the winter.

Each QBO phase results in different atmospheric effects during the winter months. The diagram above (also made by Mike Ventrice) shows 500 millibar height anomalies for the November-March period for each QBO phase. If we take a look at Phase 6 on the right-hand side of this image, we see that ridging across the Gulf of Alaska and troughing in the waters just west of the Bering Sea is favored. This sort of pattern can lead to lower pressures in the Rockies, which can then lead to ridging in the East US. However, the ridging in the Gulf of Alaska appears to be close enough to the West Coast so that the East US ridging is suppressed. It is not completely suppressed, however, as deep negative height anomalies are shown across Greenland. This Pacific pattern and strong +NAO indicator (the troughing over Greenland) tells me we will see a pattern that supports a warm-ish pattern in the East US, with some stormy weather across the eastern Rockies/Plains. Bear in mind that the strongest warmth ought to be kept down in the Southeast and maybe portions of the Mid-Atlantic, but I'm not seeing a full-on torch for December.

When the QBO hits Phase 7, you can see that the situation dramatically changes. Height anomalies in Phase 7 do a complete reversal and give way to a quite favorable winter pattern. Extreme ridging is spotted across the Arctic Circle, and this gives an indication that the chance for those stratospheric warming events are enhanced for January and February, when the QBO does enter Phase 7. At the same time, ridging is also observed across the western coast of North America, which is a key component to a colder pattern for the Central and East US. While I don't have temperature charts for each phase of the QBO, I would expect solid below normal temperature anomalies for the Central and East US during Phase 7. If we go solely by the QBO, I would be calling for a warm early half of the US and -- dare I say it -- harsh second half of winter.

Another factor we're going to watch for this winter is how the solar cycle plays out. We are entering a time period in the next decade or so that will see the sun go into a hibernation, with sunspot levels plummeting to levels we have not seen since the Maunder Minimum. As we enter this upcoming winter, the sun looks like it is starting that downhill trend. The chart above shows the forecasted sunspot cycle, but as the observed sunspot numbers tell, that forecast is not going to verify. The latest sunspot values for September really dropped off, per the most recent dot on the chart above. Upon some research I did a few months back, there is a positive correlation between spiking or plummeting sunspot values, and temperatures a couple months later. This was best shown by the intense sunspot spike we saw prior to the winter of 2011-2012. It's no secret that the winter ended up warmer than normal. In the same context, we saw a spike in sunspot numbers just three or four months into this year, which led to a large heat wave in June into July. Now, September sunspot numbers have dropped off a cliff. Using the research I did on this, we should see a cooler trend in time for December and January. I did express my concern over the warmth potential in December with the QBO, and those concerns are still valid, but we could see this warmth tempered if the sunspot correlation ends up verifying this winter.

An additional benefactor of the low solar activity is the stratosphere. When the solar cycle is at low levels, ozone is able to build up in the upper layers of the atmosphere, as ozone-destroying energy (i.e. certain solar rays and radiation in general) is tempered by the anomalously quiet sun. This helps the warming process in the stratosphere, which then not only weakens the almighty polar vortex, but theoretically raises the chances of stratospheric warming events as well. With the sun quickly dropping off, I have a feeling this ozone build-up will come into play, but it may take the QBO switching phases in Phase 7 before the stratosphere starts experiencing some turbulence in terms of the state of the polar vortex. This leads into the next (and my favorite) topic, the stratospheric implications for this winter.

This is a 240 hour forecast of what is referred to as the lowest part of the stratosphere, the 100 millibar level. 240 hours (or 10 days) from now, we see that the forecast calls for three very distinct things.

I. Intense warming over the Bering Sea.
This has massive implications for this winter, folks. I cannot stress enough what this would mean for the winter season if this part of the stratosphere becomes entrenched in the weather pattern. It is known that warming in the stratosphere leads to lower pressure and cold weather, which is why winter weather people love sudden stratospheric warming events. At the same time, colder areas of the stratosphere indicate a preference for high pressure and warmth, and we will touch on this particular aspect in just a paragraph or two. But for now, focus your attention on the Bering Sea. That intense warming, should it hold its ground in the weather pattern from now through winter, would help stormy prospects in the Bering Sea. With the use of Joe Renken's Bering Sea Rule, which illustrates a correlation between strong storm systems in the Bering Sea and a cooler/stormier weather pattern in the Central/East US roughly 2.5-3 weeks later, the stratosphere may be able to give a helping hand to the chances for some stormy weather this winter, as long as the warmth in the Bering Sea can translate to stormy weather in that body of water. A caveat for this scenario involves that the stratospheric temperature pattern changes and we see the warm Bering Sea temperatures dissipate, which is a real possibility. However, with this warmth in the Bering Sea staying put in the next 10 days (and possibly beyond), it looks to me like this pattern isn't really going to change in the relatively-long range, and that's good news for winter weather folk.

II. Warmth in the Central/East US
The warmth over the Central and East US will also be here to stay for the next 10 (or more) days, and this should contnue the cooler weather pattern we've been having. I'll discuss how this cold weather will affect this winter in the LRC portion of the winter forecast, but for now we'll stick to the stratosphere. If the temperature pattern in the lower stratosphere remains the same throughout the rest of the fall and into winter (which I'm cautiously optimistic it will), expect the brunt of the cold to really hit the Plains, Midwest and parts of the Rockies and Northeast. On a side note, the lack of warmth in the Southeast US may be helping to notify us of what could be ridging in the Southeast this winter, which would favor a storm track through the Midwest and Ohio Valley, but that's speculation at this point.

III. Cool temperatures over Greenland
Again, cool temperatures in the stratosphere can help prospects for ridging, and this is no different. The forecast for the lower stratosphere suggests we will see a gradual cooling trend over the Greenland area, which, in due time, should lead to ridging over the Arctic land mass. Should this cool-down continue, expect the chances of a negative NAO to rise for this winter.

Finally, bearing in mind the cold weather stratospheric temperature trend that is able to produce ridging, take a look at the 10 millibar (upper stratosphere) forecast below.

That intense cold air mass over the Arctic Circle will be intensifying from now until the 10 day forecast, which is shown above. If this stays into winter, expect the chances of not only a negative NAO, but a negative AO to skyrocket. It remains to be seen if this cold weather will translate to the surface, but for right now, I am encouraged by this development. Further helping my opinion on the stratosphere is the new stratospheric warming event that has just started in Asia, mainly due to persistent positive mountain torque values across Asia. The 10mb animation of stratospheric temperature anomalies is shown below:

While I am skeptical of this warming event having extreme effects on the winter, history points to this coming winter featuring a stratosphere that, at the very least, should not end up anomalously cold. If this stratospheric warming event is to have a big effect on this winter, it should keep the stratosphere in a relatively warm state throughout the winter, possibly in spite of the positive QBO. Take a look below:

These three charts show temperature anomalies from the years of 2005 (top left), 2006 (top right), and the current year of 2013 (bottom). Take a look at the bottom chart. If you look closely, you can see a sharp cut-off of the slight below normal temperature anomalies, and an emergence of slightly above normal temperature anomalies in the stratosphere. I took a look back in history to find years that also featured warming events in the stratosphere in the fall, and the years of 2005 and 2006 were included in this. Glancing over 2005, I circled the warming event in black. Following this warming, the stratosphere did cool for a while, but when December rolled around, the polar vortex never really got going, mainly due to the multiple stratospheric warming events that occurred in the end of the year. Could this be a side effect of that fall stratospheric warming? I'm not in a position to say yes or no, but the possibility to consider is yes, this warming event did set the stage for December to feature a relatively warm stratosphere. It gets better than this- taking a look at 2006 on the top right, the stratosphere was bombarded with at least two big stratospheric warming events throughout January and February. Again, I cannot say if the fall stratospheric warming event influenced these late winter massive warming events out of uncertainty, but I would not rule out the idea that it did. In 2006, this same scenario played out again, with a fall stratospheric warming event leading to a rather warm stratosphere for the end of the year. A look at early 2007 shows that the stratosphere was not nearly as excited with warming events as it was in early 2006, but the January-February 2007 period still featured as many as 4 separate warming events, two of which were decent warming events in terms of how much of the stratosphere was affected. It should be noted that the winter of 2005-2006 had a solid negative QBO, which enabled the polar vortex and stratosphere to naturally be open to stratospheric warming events, but the 2006-2007 QBO is very similar to the QBO we are currently experiencing, and this historical connection bodes well for the idea that while the +QBO should hamper efforts by the stratosphere to enact sudden stratospheric warming (SSW) events (especially in the first half of winter), the stratosphere is likely to be hit with at least one stratospheric warming event, per the factors I have covered thus far.

It's now time to analyze the next piece in the winter forecast puzzle- the SAI.

Next, let's take a gander at a pretty special tool that can be used for forecasting the winter Arctic Oscillation, titled the Snow Advance Index (SAI). The SAI involves monitoring the snow cover anomaly over Siberia during the month of October. In short, if the snow cover anomaly maintains an above normal stance for the month of October, the Arctic Oscillation should be watched for a negative state this winter. On the opposing end, below normal snow cover in Eurasia tends to lead to a positive Arctic Oscillation in the succeeding winter. Per the graph above, which shows northern hemisphere snow cover anomalies from November 2012 to the present day, we see a spike in snow cover anomaly in late September, which has led into strong above normal snow cover anomalies in October... that is, until October 19th. If you look closely, you may be able to see that the snow cover anomalies tanked just around that date, and this is shown by a quick drop on the graphical depiction on top, as well as a drop-off from blue to a tinge of red on the bottom depiction of snow cover anomalies. Medium range snowfall forecasts insist that the snow cover will be built back up in due time, but I am a little concerned that high pressure wavering around Siberia may hurt the snow cover. I expect the snow cover to build back up in the face of this drop-off, so do not be too concerned right now. Overall, the SAI does currently favor a negative Arctic Oscillation for this winter.

I have taken into account other things, such as the Pacific Decadal Oscillation, and the Atlantic Multidecadal Oscillation. They will not be covered in this forecast, as the PDO, AMO status has not changed since my Official winter forecast.

The last item left to discuss is the Lezak Recurring Cycle. The Lezak Recurring Cycle, or LRC, is a tool developed by meteorologist Gary Lezak that, in essence, can enable forecasters to predict the overall weather pattern months in advance. The gist of the LRC involves a cycling weather pattern that develops in October and November of each year; no pattern is the same from year to year. Around mid November, the LRC begins to repeat, meaning we start to see a similar weather pattern in mid November that we saw in early October. This means that the cycling pattern has begun, and it will continue to cycle on a 40-60 day interval for the next ~10 months before it dissipates over the following summer.

The first item to address is the October 4-6 winter storm, which delivered close to 5 feet of snow in a few isolated spots across western South Dakota. This storm system trekked northeast after strengthening over the Plains and dumping all of its snow. In the wake of this storm, a cold front meandered south through the southern Plains, Midwest and the Eastern US. Just on a very preliminary look at the winter storm, and the environment surrounding it, I would not be surprised to see this storm stay in the Plains and Midwest through the next couple of LRC cycles through this winter. To show why, take a look at the 500mb map for October 5th:
You can see the storm system in the Plains as a closed low, signified by the closed contour lines. There's a reason it went northeast rather than east- there was a ridge of high pressure in the Southeast. This ridge means it really can't just dive south or go due east unless the ridge moved, and in this case, it didn't. That said, considering the LRC should maintain a similar atmospheric pattern through its winter and spring cycles in the upcoming year, the storm system should go northeast again when it re-cycles in mid-November. I don't see this storm system being one that will become a big snow producer for the East, given the position of the ridge in the Southeast, but I would also be surprised to see extreme snowfall amounts like the ones seen in early October from this storm. Depending on how the jet stream changes from now until the next cycle in mid-November, the storm track may change slightly, but the overall pattern should remain similar enough to keep the storm track at least relatively on the same track as we saw in early October. With another storm system about to move into the Plains again, this region is looking at multiple snow and cold chances this winter.

The next piece of the LRC puzzle is the new weather pattern that has developed as of mid-October, and continuing now and into the next week or two. As the 500mb height anomaly chart on the left shows, this new pattern involves strong ridging over western North America into the Gulf of Alaska. This ridging provokes deep troughing in the Central and East US, and this is intensified by the favorable negative North Atlantic Oscillation (NAO) over Greenland. There is little doubt in my mind that we see this pattern cycle again for late November/early December through mid/late December, and again into early February, possibly through a majority of that month. The point is, this cold snap should last a good ~15-20 days when it actually ends, with modified cooler air possibly lasting beyond that ~15-20 day point. This means the LRC, if all goes as is expected, will allow the Central US to experience some frigid weather this winter as a direct result of the LRC. If you live near the Great Lakes, especially Michigan, Huron and Superior, get ready for what could be a big lake effect snow event when this part of the LRC repeats again.

Taking into account all of that information, and then some, here is the Final 2013-2014 Winter Forecast.

Temperature
Confidence: High
December
I'm anticipating a warm start to winter, with that idea coming from Phase 6 of the positive QBO as well as the LRC, which should see the warmth observed in early October repeating in mid-November into early December. This warmth should be held up throughout the East US, and into the Great Lakes and Midwest. Expect the northern Plains to escape the worst of the warmth.

January
As the QBO transitions to Phase 7, and the LRC goes into its next cycle, the temperature trend ought to be on a decisive colder path. The Plains region into the Midwest and Great Lakes should see the brunt of the cold in January, with slight ridging in the Southeast offering the opportunity for warmer than normal temperatures in that area. Depending on if the negative Pacific Decadal Oscillation is still present by the first month of 2014, this ridging in the Southeast could be enhanced, and the overall storm track would be affected. Prospects of a negative Arctic Oscillation for this winter, as well as the heightened chance of stratospheric warming events in the latter half of the season should see a colder trend in much of the nation.

February
More of the same from January is a good summary of what the nation can expect in terms of temperatures. The stratosphere should be in a decent amount of turmoil for February, as long as the current stratospheric temperature trends and changing QBO phase cooperate. Once again, eyes will be on the Plains, Midwest and parts of the Northeast and Great Lakes for the brunt of the cold for the last month of traditional winter.

Precipitation
Confidence: Medium
*Due to low confidence, this discussion will be grouped into the three month December-January-February period.

December-January-February
Signs are pointing to ridging in the Southeast this winter, and that should enable a rather dry set-up to be provoked in that region of the nation. The ridging in the Southeast, aided by what should be a stormy Bering Sea, ought to outline a swath of above normal precipitation that could stretch from the Oklahoma panhandle to western New York. This would all depend on the eventual storm track for this winter, which has yet to be determined due to a few select atmospheric factors that I will need more time to monitor before making decisions that pertain to the storm track. Confidence goes down for both coastal regions, as it is (decently) possible we see a stormy East Coast and possible we see a dry West Coast. Again, I will need more time to decide what the forecast will be for those two regions, but if I had to choose, I would expect a rather wet trend for the East Coast and maybe a drier trend along the West Coast (but that is in question due to the potential storminess of the Bering Sea and how it could affect high pressure prospects in the Pacific Northwest).

Snowfall
Confidence: Medium to Medium-High
*Due to low confidence in conjunction with the precipitation forecast, and because the snowfall pattern will follow the storm track (which is somewhat TBD), the snowfall description will be grouped into a three month December-January-February description.

December-January-February
The lake effect snow machines will be in overdrive this winter, as the Lezak Recurring Cycle and end-half of winter pattern allows intense lake effect snow events to develop. Lakes Michigan, Superior and Huron will be affected the greatest, so long as the current outlooks for the core of the cold to stay in that region locks in place. Lakes Ontario and Erie should not see as intense of lake effect snow outbreaks, but the coastlines of those lakes will still experience some hefty lake effect snow events. As far as synoptic snowfall projections, current trends highlight the Plains, Midwest and western Ohio Valley for the best chances of above normal snowfall. The Southeast Ridge, stratospheric temperature trends and LRC ought to see this happen, but I am not confident enough to say it will happen, purely because of the 'little things', in terms of mesoscale teleconnection patterns that have yet to be determined for this winter. The East Coast will have the opportunity to see some hefty snow events this winter, and if you really want to, you could envision an above-normal snowfall outline across the coastal regions of the East Coast in addition to the Plains/Midwest. I may have to adjust the southern Ohio Valley's snowfall projection in the event my idea on the storm track changes, and the same may be said for the northern Rockies. Any adjustments will most likely be made in mid-November, when I will re-examine the forecast. Do not expect a whole other winter forecast to be issued at that point; if any changes are made, it will just be a single, moderately-long post.

Here is the overall graphic for the winter of 2013-2014, subject to slight changes.



Well, there you have it, folks. This concludes The Weather Centre's Final 2013-2014 Winter Forecast, and thus concludes the release of winter forecasts for this year. As I see fit, adjustments will be made and the changes will be published, but the Final winter forecast usually heralds the end of the publishing of major winter forecasts.

If you have any questions, don't hesitate to comment below, comment on our Facebook page, or tweet us.

Andrew

This winter forecast took into account data produced by: Joe Renken, Eric Webb, Steven DiMartino, Michael Ventrice, and Josh Herman.

Tuesday, October 22, 2013

Stratospheric Warming Event Commences

A temperature animation of the upper stratosphere shows that East Asia is now experiencing a stratospheric warming event, something that could be a sign of things to come this winter.

As the animation shows, the last few days have seen a big jump in stratospheric temperatures in Eurasia as a result of a persistent rise in mountain torque values. Without going too in-depth, high values of mountain torque in certain regions of the world (in this case, East Asia) can lead to sudden stratospheric warming (SSW) events. This is the first such warming event this winter, and one can only question if this is a sign of things to come.

Observed mountain torque values (Note the elevated East Asian mountain torque values in red on the right side of the graph).
This warming event could help to weaken the polar vortex in the short and medium term, despite projections for the polar vortex to strengthen in the next several days. Implications on the winter could range from nothing to this warming event being a big blow to the polar vortex this winter. My money's on this warming event having little effect on the winter's polar vortex, especially because we are still in a positive Quasi-Biennial Oscillation (QBO) phase, which favors a stronger polar vortex. However, a stratospheric warming event in January or February would have a very different impact on the winter, and this is something I will address in my Final 2013-2014 Winter Forecast, which comes out this Thursday at 5:00 PM Central Time.

Andrew

Monday, October 21, 2013

Final 2013-2014 Winter Forecast Release Date

The Final 2013-2014 Winter Forecast will be released on Thursday, October 24. The release time will be 5:00 PM Central Time.

Andrew

Saturday, October 19, 2013

Why Early November Will Be Colder Than Forecasted, and Other Long Range Hints

It is likely that the recent forecasts of a dissipating cold pattern in the 14-16 days range will end up incorrect, as the west Pacific continues to produce typhoons, one of which will end up in east Asia to confirm a continuing cold weather pattern through at least the first week of November.

In just 84 hours, on October 23, typhoon Francisco will begin to approach East Asia at just about the same time a closed low traverses eastern mainland Asia. It has been demonstrated that the weather pattern in East Asia can be (and often is) reciprocated in the East United States 6-10 days after the weather pattern in East Asia is observed. In the case of recurving typhoons, it is expected that a deep trough appears in the eastern half of the United States in the 6-10 days succeeding the typhoon's curving motion around East Asia.

Two days later, ensemble forecasts have the typhoon and closed low combining into a swath of negative height anomalies covering a decent portion of East Asia. This should lead to an enhanced period of colder weather in the Central and Eastern United States as a result. Ensemble forecasts then try to build ridging in East Asia after the troughing, but with that same ensemble set not catching on with the probable troughing pattern across the East US due to the typhoon scenario in East Asia, I am wary of trusting that solution.


The GFS model does show another big storm system hitting East Asia around the 140 hour forecast period, which could then prolong the cold pattern further, as far as ~November 4th, as the image above shows, but there's another piece of information that may have significant implications in the middle or near-end of November.

See that massive system around the Aleutian Islands? That storm system will be meandering north around the time period of this forecast image, which is roughly October 25th, and likely into the Bering Sea. This brings into effect something a knowledgable AccuWeather Forum moderator named the 'Bering Sea Rule'. The Bering Sea Rule involves a strong storm entering the Bering Sea, and a strong storm system affecting the East US 2.5 to 3 weeks later. With the large storm system projected to enter the Bering Sea in late October, it is possible/likely we see a storm system/cooler pattern forming in the Central and/or East US in time for the middle of November. You can find the full description of the Bering Sea Rule at this link.

What does seem apparent is the continuation of this cold weather pattern that as started in the last few days into early November, despite model projections of the cold weather pattern ending to kick off November. Beyond that, some colder weather may be provoked in the middle and early-ending periods of November.

Andrew

Thursday, October 17, 2013

Intense Cold Snap Here to Stay

A cold snap which has just begun in the Plains and Midwest is here to stay, and will expand east in the process.

The atmospheric pattern has changed in the last couple of days, with ridging starting to build over the Gulf of Alaska and western coast of North America. This has started to lead to troughing/a colder weather pattern in the Central US, and this will intensify in the next several days, with the 10 day forecast above showing this pattern continue.

GFS ensemble temperature forecasts show this cold trend continuing and intensifying in the next several days, before the cold air mass slowly drifts east and weakens with time. I'm monitoring the GFS ensembles for moving the cold air out too quickly, as a recurving typhoon in the short and long range should keep at least a cool air mass across portions of the nation in the long-long range (i.e. 20-25 days).

This actually does have implications for the coming winter, and I will go in-depth with it when the Final 2013-2014 Winter Forecast is released next week, possibly on Thursday (one week from today).

Andrew

Saturday, October 12, 2013

2013-2014 Winter Update: Midwest and Plains Deemed Frontrunners for Harsh Winter

This post is an update on the current outlook for the winter of 2013-2014, and will address why preliminary indications show that the Plains and Midwest may be frontrunners for a harsh winter this year.

Before we get into the discussion, I want to lay out the Lezak Recurring Cycle, which will be used in today's post. The Lezak Recurring Cycle, or LRC, is a tool developed by meteorologist Gary Lezak that, in essence, can enable forecasters to predict the overall weather pattern months in advance. The gist of the LRC involves a cycling weather pattern that develops in October and November of each year; no pattern is the same from year to year. Around mid November, the LRC begins to repeat, meaning we start to see a similar weather pattern in mid November that we saw in early October. This means that the cycling pattern has begun, and it will continue to cycle on a 40-60 day interval for the next ~10 months before it dissipates over the following summer. Now that we are in October, it is time to start looking for atmospheric phenomena that may factor into the LRC this year. Using the LRC, and long range model guidance, some early returns on who may get hit hard this winter are surfacing.

The first item to address is the October 4-6 winter storm, which delivered close to 5 feet of snow in a few isolated spots across western South Dakota. This storm system trekked northeast after strengthening over the Plains and dumping all of its snow. In the wake of this storm, a cold front meandered south through the southern Plains, Midwest and the Eastern US. Just on a very preliminary look at the winter storm, and the environment surrounding it, I would not be surprised to see this storm stay in the Plains and Midwest through the next couple of LRC cycles through this winter. To show why, take a look at the 500mb map for October 5th:

You can see the storm system in the Plains as a closed low, signified by the closed contour lines. There's a reason it went northeast rather than east- there was a ridge of high pressure in the Southeast. This ridge means it really can't just dive south or go due east unless the ridge moved, and in this case, it didn't. That said, considering the LRC should maintain a similar atmospheric pattern through its winter and spring cycles in the upcoming year, the storm system should go northeast again when it re-cycles in mid-November. I don't see this storm system being one that will become a big snow producer for the East, given the position of the ridge in the Southeast, but I would also be surprised to see extreme snowfall amounts like the ones seen in early October from this storm. Depending on how the jet stream changes from now until the next cycle in mid-November, the storm track may change slightly, but the overall pattern should remain similar enough to keep the storm track at least relatively on the same track as we saw in early October. With another storm system about to move into the Plains again, this region is looking at multiple snow and cold chances this winter, even though we're only ~25% into the LRC.

Let's take a look at what the models are predicting for the next week or two.


This image shows the European ensemble forecast (left) and the American ensemble forecast (right) for October 19, predicting 500mb height anomalies across the Northern Hemisphere. The grand idea here is that the weather pattern will be experiencing a drastic change in the medium-range. Both ensemble projections agree that a strong ridge will set up across Alaska and western Canada, and it may be strong enough to incite a cross-polar flow, which would enable cold air to be transported from Siberia to North America. Regardless of if this cross-polar flow does set up, significant negative height anomalies will form in the Central and East US as a result of this intense ridging in the western part of North America and ridging in Greenland (a +PNA signal and -NAO signal, respectively). A ridging pattern will be maintained to varying strengths in the Southeast US, which should keep the strongest cold weather in the Plains/Midwest and Great Lakes, hence why these regions are outlined for frontrunners for a harsh winter; the cold will be stronger when the LRC brings this cold pattern back in a couple of cycles for January. These negative height anomalies in the Central and East US will be enhanced by a recurving typhoon in the west Pacific named Wipha:

The American ensembles take this pattern further, maintaining it into the 2-week range:

The American ensembles continue to maintain ridging across western North America, aided by deepening negative height anomalies spreading south from the Bering Sea. Thanks to the continued ridging in western North America, persistent negative height anomalies are maintained in the Central and Eastern US. I'm a little skeptical of the negative height anomalies being shifted so far east, opposed to being in the Plains and Midwest, as ridging in the Southeast should still be present in this timeframe and the negative NAO pattern (ridging in Greenland; favorable for a chilly Northeast US) will have either weakened or dissipated by this time per the ensemble projections. On the other hand, a typhoon crashing into East Asia in the next 3 to 4 days (as shown below) should lead to a continued troughing Central/East US pattern beyond the next 10 days, enhancing the argument by the GFS ensembles.

Typhoon seen moving ashore in East Asia in 96 hours.
The weekly CFS model maintains a positive PNA pattern until the beginning of November, and if this verifies, the Midwest and Plains would continue to be frontrunners for a harsh winter this winter.

CFS PNA forecast through mid-November.
Andrew

Thursday, October 10, 2013

Cold Pattern Evolving in Near Future May Repeat this Winter


A cold pattern that will be showing up in the near future could have a prolonged impact for this winter.

The image above shows 500 millibar height anomalies for October 19th, with the European ensemble projection on the left and the American ensemble projection on the right. Both ensemble sets are in agreement that a ridge of high pressure will develop over western Canada in cooperation with a developing trough over the waters in and around the Aleutian Islands. In response to the ridging in western North America, which we call a positive PNA (Pacific-North American index), a cold pattern will set up over the US, particularly for the Great Lakes, Midwest, Ohio Valley, and Northeast.

Both ensemble sets have the western North America ridge connecting with a weaker high pressure pattern over Greenland, which is described as a negative North Atlantic Oscillation (NAO). The negative NAO and positive PNA patterns are two major atmospheric patterns that help usher in cold air into the United States. While the ensemble systems have slight variations over the general Northern Hemisphere pattern, the concept that a colder weather regime will evolve over the Central and Eastern US is agreed upon by both ensemble systems. The temperature anomaly graphic below, which shows the American ensemble's temperature anomaly forecast for one day per panel, agrees with this colder regime:


But we aren't finished yet. The American ensemble insists that this cold weather pattern sticks around ... for quite a while.

This is the American ensemble 500 millibar height anomaly for October 25th. Does the pattern look familiar? It should, because it is nearly identical to the pattern being shown by both global ensemble sets at the top of the post for October 19th. A natural caveat for the long range forecast models is their notorious lack of accuracy. Taking that into account, it's possible we don't see this cold pattern stick around this long. At the same time, we have to examine all the possibilities, and one of them is that this pattern becomes more prolonged in nature. Looking at the temperature anomaly graph just above this 500mb anomaly chart, the ensembles are willing to maintain a cold air mass over the Central and Eastern US in the bottom row of panels. It is this bottom row of panels where the forecast period ends, and we cannot see projected conditions beyond October 26th (the far bottom right panel). The last 11 - that's right, ELEVEN - American ensemble runs consistently agree on this cold air mass being maintained in much of the country. Will it actually happen? It's too far out to tell, but model consistency does help the point of this colder pattern possibly becoming more prolonged in nature.

But the fun is not over yet. It is possible we could see this cold pattern return a few times throughout the winter through the Lezak Recurring Cycle.

The Lezak Recurring Cycle, or LRC, is a tool developed by meteorologist Gary Lezak that, in essence, can enable forecasters to predict the overall weather pattern months in advance. The gist of the LRC involves a cycling weather pattern that develops in October and November of each year; no pattern is the same from year to year. Around mid November, the LRC begins to repeat, meaning we start to see a similar weather pattern in mid November that we saw in early October. This means that the cycling pattern has begun, and it will continue to cycle on a 40-60 day interval for the next ~10 months before it dissipates over the following summer. Now that we are in October, it is time to start looking for atmospheric phenomena that may factor into the LRC this year. We already saw the big winter storm in the Plains, and time will tell if that becomes a part of this winter's LRC, and now we are looking at this possibly-prolonged cold weather pattern to see if it will factor into the LRC. Considering it should occur in mid to late October, chances would be good that we see this in this year's LRC. However, in meteorology, nothing is ever 100% guaranteed. Again, time will tell, but it's possible this cold fall pattern returns in November and January as a long(ish)-term frigid weather pattern.

Andrew

Sunday, October 6, 2013

Long Range Lookout: Pattern Change in the Offing

It does appear we will see a shift in the atmospheric pattern in the next couple of weeks, as the hyperactive typhoon regime in the Pacific tapers off in response to a changing Madden-Julian Oscillation.

This graphic, provided by the NCEP, shows a composite of tropicalcyclones that have formed in the month of October by each phase of the MJO. The graphic indicates that typhoons tend to reach maximum occurrence in phases 5-7, and taper off as the MJO rounds out its later phases. We have been in Phases 5 and 6 for nearly a full month now, which explains why the last few weeks have had remarkable typhoon formation in the Pacific. Among those that formed in the last ~30 days were:

Typhoon Usagi (September 16- September 24)

Typhoon Pabuk (September 19- September 27)

Typhoon Wutip (September 25- October 1)

Tropical Storm Sepat (September 29 - October 2)

Typhoon Fitow (September 29 - Present Day)


Typhoon Danas (October 3 - Present Day)



These typhoons have come from the MJO being restricted to Phases 5 and 6 in the last month. However, the latest European forecast for the MJO shows gradual progression into Phases 7 and 8, phases which are shown by the NCEP graphic at the top of this page to include a tapering off of the hyperactive typhoon regime in the Pacific.

All of these typhoons have to go somewhere, and they go north into the Bering Sea and general northern Pacific. With these extratropical storms in the northern Pacific, they then propagate down into the West US and western Canada, where they dig deep into the Western US to provoke ridging of high pressure in the East US. As long as the typhoons continue to form in the western Pacific, there is little that should alter this pattern for at least the next several days. The MJO forecast above indicates the formation of typhoons will lessen after these next several days, but until then, the prospect for warmer weather in the East for as long as the next week appears very possible. The main point to get out of this typhoon discussion is that the warm weather will wind down as the typhoons wind down, and that should happen in the ~7-10 day period.

We're going to stay in the west Pacific and move our discussion into the East Asia region. The last couple of days have seen a massive ridge building up across East Asia that is just now beginning to weaken and pull east into the Pacific. Those who have followed this blog for a long time understand that there is a 6-10 day correlation between height anomalies in East Asia and height anomalies in the East US. Hence, this massive ridge in East Asia will translate to ridging in the East US in just over a week.

The ECMWF model (left) and GFS model (right) nail this correlation in showing a massive ridge across eastern Canada and the East US in the 8-10 day forecast period. This ridging in the East does something that the ridging in Asia couldn't do- it propagates north and into the upper latitudes. Both global models project that this strong high pressure in the East will exert tremendous influence on the pattern in and near the Arctic Circle in what could become an upper latitude blocking pattern. Long range GFS ensembles project a steadfast negative NAO developing due to ridging across Greenland, and a cool pattern then takes place over the Plains before gradually shifting east. It does appear the cool Plains/Midwest idea should end up verifying, and as long as the East US ridging stays on track to flow north into Greenland, the negative NAO should work out as well. Projected rising of the Pacific-North American index would certainly help cool Plains prospects in the long range in addition to the -NAO:


Summarization
The Eastern US will encounter warm weather for the next week to 10 days before the threat of any big heat moves out and seasonal-ish temperatures move in. The Plains will remain seasonal to slightly below normal as another storm system is projected to swipe the Central Plains and hit the Northern states. An overall cool down is anticipated beyond the 10 day forecast point. The West US will be inundated with cool weather for the next week and a half before rising temperatures make their return.

Andrew

Thursday, October 3, 2013

Trifecta of Weather Extremes Collides Friday

A trifecta of weather extremes will bombard the United States beginning tomorrow, and continuing on through the weekend.

Part I: Blizzard Buries Plains

Model guidance has finally come into agreement that a blizzard will develop out of an anomalously strong storm system in the Plains, delivering amounts exceeding the two foot mark. I outlined a 1-6 inch forecast across portions of the Dakotas and Minnesota, with such a large difference in amounts due to how sharp the cutoff may be with this snow event. South Dakota will get slammed the hardest, with 12'' to 24'' amounts possible. Latest model guidance suggests isolated pockets of 30'' totals are possible, but to anticipate any model exaggeration bias, I held down totals to the 12-24'' mark, with higher amounts in the realm of possibility. I'm not sold on anything higher than the 24'' mark due to this possible snowy model bias.

Those in South Dakota (and portions of North Dakota that may get in on these high totals) should prepare for widespread power outages, in large part due to trees that may not have lost all of their leaves yet. Snowfall will build up on trees and leaves, which can easily lead to partial or total destruction of many large tree branches, which can then cause power lines to collapse. This is expected to be a significant weather event, and anyone in line for these significant snows should be aware of the potential of this major storm system.

Part II: Severe Weather Swamps Midwest

On the warm side of this major storm system, a severe weather outbreak is expected. Severe thunderstorms are anticipated to fire from Oklahoma to Wisconsin, but supercellular, tornado-producing thunderstorms find the largest potential of formation in Iowa and southern Minnesota. High resolution model data paints a very intense severe weather event, though such forecasts cannot be taken at face value. That said, I do expect a substantial severe weather event tomorrow, on October 4th.

Those in Iowa and southern Minnesota should prepare for the possibility of damaging and life-threatening thunderstorms tomorrow. Any storms that manage to become supercellular will have the ability to produce tornadoes in this environment. With a strong storm system producing what ought to be a tight temperature gradient across the Plains and Midwest, storms should not have too rough of a time forming.


Part III: Tropical Storm Karen Likely to Landfall on Gulf Coast

Tropical Storm Karen formed earlier today in the Gulf of Mexico, and is expected to eventually make landfall on the Gulf Coast sometime Saturday. Tropical storm conditions are anticipated across a narrow band of land where Karen hits, and the rainfall amounts could reach the 10'' mark. At this time, it does not look like TS Karen is a significant threat to life and property, but with any tropical cyclone, high winds, heavy rainfall, and the threat of tornadoes does mean anyone in the potential landfall zone should keep a close eye on Karen. Model guidance is not perfect, and the cyclone's track could change.

Andrew