Thanksgiving Potentially Significant Storm System

Model guidance is now hinting at this Thanksgiving storm system to impact the Central and East US.

PSU

Beginning with the ECMWF model, with this forecast going out to 10 days, we see a deep trough pushing into the Southern Plains, neutrally-tilted, as the isobars pushing due south show. Note how pressure tendencies have rotated to the southeast corner of the vort max, likely telling us that this trough will start its maturing phase into a negatively-tilted trough in the next few days, if this model forecast were to go out further. A better description of tilted troughs is below from theweatherprediction.com .

The Weather Prediction

Given the freezing line / 540 thickness line in the top-right image being pulled all the way to the Southern Plains on that ECMWF graphic, as well as the expected negative tilt to this trough, the going ECMWF forecast would likey deliver severe weather to the Southeast, as well as some snow to those in the Great Lakes and southern Midwest, with exact areas to be determined.

Tropical Tidbits

Moving ahead to the GFS-Parallel forecast for the same November 24th timeframe as that ECMWF graphic, we see a very similar forecast. Once again, a strong trough is located in the Southern Plains, as shown by the deep negative height anomalies centered over Texas. If you guessed this trough was neutrally-tilted, you are correct! The height contours seem to be 'pushing' due south, which means it's neither positive or negatively-tilted. Unfortunately, I don't have access to the jet stream forecast for this timeframe from the GFS-Parallel model, which means we cannot tell if this trough is preparing to tilt negatively or not.

We don't have much to work with right now, since the storm's still about 10 days away, so here's a few graphics of precipitation forecasts from the GFS and GFS-Parallel model with this storm.

Tropical Tidbits

Tropical Tidbits

Andrew 

November 25-29 Potentially Significant Winter Storm

We are beginning to nail down how this potentially significant winter storm may evolve across the country in the days surrounding Thanksgiving.
For my humor and your reading pleasure, I've divided this post into multiple sections, each concerning a different timeframe of this storm.

I. The Instigator 

Let's begin with a refresher on how this potential has come about.

OPC

On the morning of November 8th, we saw the remnants of Typhoon Nuri reach peak strength via Ocean Prediction Center observation. As the chart shows here, the massive storm bottomed out at 924 millibars. This means the storm, located at about 170E and 55N, didn't break the record for strongest extratropical cyclone on record, but it certainly came close.
Why do we care about this storm? The Bering Sea Rule states that a strong storm that hits the Bering Sea can produce a consequential storm in the United States about 17-21 days later. This is the same for high pressure in the Bering Sea. You probably get what I'm getting at here with this observed strong storm in the Bering Sea, so let's keep reading...

NWS

The graphic above, produced by the National Weather Service in Alaska, shows the observed mean sea level pressure of the storm at that 924mb reading, while the NWS office measured it at 930mb on the chart on the left, where strongest extratropical cyclones in the North Pacific are depicted. For multiple reasons, including the possibility that past storms may have been stronger than that 924mb reading, this storm was not declared the strongest on record in the North Pacific. Despite this, it's quite clear this storm was a historically-strong one, relative to storms in the last 60 or so years.

NOAA

Purely for comparison purposes, the team at KOPN Weather identified a strong storm in the Bering Sea on April 7th, 2011, bottoming out at 936mb, that was about 10 degrees W of where this very strong storm was observed yesterday in the Bering Sea. If you recall what happened about 2-3 weeks after the date of April 7th, we saw a certain tornado outbreak strike the South US, devastating thousands across the country, and causing millions on millions of dollars of damage. If we look at where the resultant storm ended up in mid-late April, utilizing the Bering Sea Rule, we find the storm situated in the Ohio Valley.

WPC

Now, keeping in mind that this sort of correlation is a tough one to use at best, not to mention all the caveats associated with long range forecasting, we could theoretically juxtapose the remnants of Typhoon Nuri and this storm in the Bering Sea on April 7th to get an idea of where the consequential storm in the US may end up. Recalling that the storm in 2011 in the Bering Sea was at about 180 degrees longitude, and almost the exact same latitude as the one observed in the last day or two, we find the remnants of Nuri placed about 10 degrees west of that 2011 storm. If we take the location of that storm system in late April (pictured above) and move it west, like the remnants of Nuri were west of that 2011 Bering Sea storm, we end up with a map like this:

WPC

Continuing this correlation, just to see what would happen, we notice that the remnants of Typhoon Nuri are moving eastward (a bit northeast in the process) in the Bering Sea right now, slowly at that. If this storm somehow does end up in that potential location outlined above, and if enough cold air is available (this will be discussed later), a significant winter weather event may strike the Central Plains, Midwest, Great Lakes, and Ohio Valley. Similarly, if the correlation works out, a severe weather event may strike the South US. Confidence remains low, but the potential for this correlation to verify is on the rise.

II. The Japan Connection

Now that we know where this potential is coming from, let's start to use it to our advantage, in the form of the Typhoon Rule (click here for explanation on the rule).

Tropical Tidbits

The image above shows the ECMWF model's forecast of 500mb geopotential height anomalies over the West Pacific. Here, cool colors denote stormy and cold weather, while warm colors depict mild and generally quiet weather. If we take a look at this forecast graphic, valid on November 17th, we find a rather strong trough/storm system pushing eastward into Japan. We can see this trough by the depression of height contours, and associated blue shadings. As this trough pushes through Japan, it looks to close off, a phrase used to describe when those contour lines literally close off and make a circle, indicating a closed low. Until then, we see this negatively-tilted trough hitting Japan around the 17th and 18th. Extrapolating that out using the Typhoon Rule, we find the potential for a storm hitting the US around November 23rd to 28th, oddly enough right around that timeframe that we saw with the intense Bering Sea low.

Tropical Tidbits

Moving on to the next graphic, we find the GFS-Parallel model forecast for November 17th, again forecasting 500mb height anomalies. The GFS-Parallel is not the same as the regular GFS; this Parallel model is the new, enhanced version of the current GFS model, which is set to be 'retired' in the next few months, where the new GFS will take its place. Among new corrections are bias fixes and increased accuracy, etc. The difference from the ECMWF model to this GFS-Parallel forecast is the Parallel model closes off this trough a bit quicker than the ECMWF, as that circular contour line over Japan shows. At this point, it's not so much a question of if this storm will strike Asia, so much as it is what strength will it be and when will it close off. Regardless, the ECMWF and GFS-Parallel both support this storm threat.

Tropical Tidbits

To add in a bit of diversity, let's check out the ECMWF ensemble mean 500mb height anomaly forecast for November 18th. In this image, we see the average of all 52 - yes, as in fifty two separate ensemble members - forecasts favoring a non-closed trough over Japan. This is a bit surprising, as the guidance we went over above has this trough closing off over Japan or just after it leaves the "mainland" of the island nation. Despite this disagreement, which will no doubt be worked through as the time between now and November 17th/18th approaches, the ensembles agree with the other two models on this being a substantial storm crossing Japan, with a consequential storm in the US around November 23-28.

III. The Set-Up

We're now in the timeframe where we can get a view of model projected set-ups for the storm environment (with typical low-confidence, of course). Let's go through the projected set-ups.

Tropical Tidbits

The image above may seem confusing, but it's not that difficult to interpret once you get a feel for it. This image shows the GFS ensemble forecasted 500mb height anomalies, averaged out across the ~20 ensemble members, valid on November 26th. Let's first begin with the positive height anomalies in the West US into British Columbia. We see what are interpreted to be slightly above normal height anomalies, but if this forecast verifies, you'll see these anomalies increase to more extreme values as confidence among all ensemble members increases. For now, confidence is low, so the anomalies aren't as pronounced.
Those positive height anomalies in the West are enabling a positive Pacific-North American (+PNA) pattern to set up. In a positive PNA pattern, ridging in the West forces the jet stream south. If resultant ridging forms in the East, the jet stream then bends north, to enable frigidly cold air to strike the Central/East US, also driving the storm track through that area. This pattern across North America is a classic +PNA pattern, and should be treated as though the Great Lakes/Plains may see the brunt of this storm... initially.
My concern rests with that big upper level low stationed just west of Greenland. If that low becomes too strong and pushes too far south (which is a plausible result), the storm track may be suppressed, and the big winners could end up being the Ohio Valley and interior Northeast. This is something to watch closely in coming days. For now, due to how the Pacific appears to be controlling the pattern, I would favor a Midwest/Great Lakes impact, but let's keep analyzing.

Tropical Tidbits

This graphic is the same type of forecast chart as the one we just discussed, but now comes from the Canadian ensembles, and is valid for November 25th. Despite this slight time difference, note that trough in the Central US, which could be our storm (though the timeframe's a bit fast for my liking). Once again, we see a positive PNA pattern, somewhat suppressed by lackluster ridging in the West US, but compensated by intensified ridging along the East. This would bend the storm track in favor of the Midwest and Ohio Valley for any big snows, and the Gulf Coast could then see some severe weather. Again, this will all sort itself out in due time, and this is merely something to watch for now.

IV. The Storm

It's all been leading up to this, folks. While I don't trust individual model guidance to show what the storm will be like (since it changes from forecast to forecast; no consistency), I would like to show the GFS ensembles' thoughts.

WeatherOnline

What you see above is a 'cluster' forecast from the GFS ensembles, valid on November 26th (technically the evening of the 25th in our time zones), forecasting precipitation and sea level pressure values over North America. I'll show the description of 'cluster' modelling below from the Weather Prediction Center, then I'll try to interpret it for others who may not understand at first.

Sometimes the ensemble members tend to group themselves into two or more solutions.  For example in the image above the ensembles cluster in two solutions off the Pacific NW coast  of the U.S. (a trough south of the Aleutian Islands and a trough off the Pacific Northwest coast of the U.S.).
CLUSTERING is an automated method that identifies and extracts like members and derives output from these like solutions (of which there are different methods to identify clusters).

If you didn't understand that, let's go through an example. Recall that the GFS ensembles have around 20 members, each of which produces its own, different forecast. Let's say that on November 26th, a certain number of ensembles are showing a relatively similar forecast. The 'cluster' method combines these similar forecasts, and does the same with other, like forecasts into 'clusters' of ensemble members. This allows us to narrow down how many ensembles are showing what type of solution for a certain time frame.

Going back to the GFS ensemble image above, approximately 30% of ensemble members' forecasts for this timeframe show a solution like the one above. This solution means a very strong storm would push north and east across the Midwest, like that positive PNA pattern may induce. The result? Heavy snow likely in parts of the Midwest and Great Lakes, while the South may see severe weather.
At first, you may think 30% is not that high. And you'd be right. But for a forecast 324 hours out, about a third of the ensembles showing this sort of solution isn't a bad thing to see if you're hoping for snow in the Central US.

Here's a good representation of what a scenario like the one above may result in.
I'm going on record and saying this is not a forecast! Please don't treat it as such!

Representation of one possible track for this storm.
Again, this is not a forecast, and it shouldn't be treated as such.

To summarize:

- Model guidance continues to support the idea of a strong storm system in the United States around Thanksgiving.
- The set-up for this storm looks to favor a storm track over the Central/East US.
- One model's representation might show heavy snow for the Midwest and Great Lakes.
- Low confidence, high caveats remain present.

Andrew

Thanksgiving Potential Arctic Outbreak

It is looking increasingly likely that Thanksgiving will contain a significant cold weather outbreak for much of the nation.

850mb temperature anomalies for the evening of November 27

The ECMWF ensembles and GFS ensembles have been telling of the entrance of a very cold air mass into the US, to the tune of nearly 20 degrees below normal, in Celsius (that's 36 degrees below normal in Fahrenheit). This would most likely bring about the coldest air seen in many parts of the nation thus far, with many in the Plains, Midwest, Great Lakes and Ohio Valley in line for temperatures that may flirt with single digits, like Chicago, or smash right through the single digit mark, like many areas closer to the border with Canada.

ECMWF temperature forecast for the morning of November 27

It remains to be seen if the models are correct in their estimation of a brutally cold Thanksgiving, but right now, it looks like such an outcome is within the realm of possibility.

There is also the potential for a Nor'easter in the day or two before Thanksgiving...

Andrew

Thanksgiving Potential Major Nor'easter

There is potential (keyword here is potential) for a Nor'easter to form along the East Coast in time for Thanksgiving, and if true, may drop copious amounts of snow on the inland Northeast.

The overnight 0z ECMWF model projects a rather strong piece of energy to slide along the Gulf Coast in the few days leading up to Thanksgiving, drifting almost due east as the jet stream allocates the energy in that direction. At the same time, another piece of energy will be dropping south and east from Canada, and if you know what phasing is, you know what happens next. The model then has the Canadian and Gulf Coast energies combine into a single system that forms just off the East Coast to form a strong coastal storm, as the forecast below (valid November 27) shows.

The ECMWF model has this Nor'easter bring massive amounts of snow and liquid precipitation to the Northeast, and spreads the wealth north and east to cover most of the interior Northeast. The minimum pressure is projected to be at 991 millibars, which pretty much qualifies the statement that this would be a substantial coastal storm. 

Now, the new runs of the ECMWF and GFS do not show this happening. The ECMWF takes the southern energy and pushes it out to sea as the two pieces of energy do not merge in time to form this possible Nor'easter. So why bring it up? Well, there is most likely going to be a storm system somewhere in the US in the days just before Thanksgiving. As I've told on this blog, there has been a storm forecast to hit East Asia just two days ago. It ended up verifying, and the text from a post on that discussion a handful of days ago is below.

This is a forecast from the GFS Ensembles out to the Hour 60 timeframe. While all of the colors and lines may seem a little overwhelming, we're only going to focus our attention on East Asia, which is under the deep blue swath in the top left part of the left panel. Another correlation that has been proved effective is how weather in East Asia can be reciprocated in North America 6-10 days after the weather anomaly in East Asia. This includes the presence of storm systems or high pressure in East Asia. The November 18 forecast from both the GFS Ensembles and ECMWF model indicates that a deep negative height anomaly swath will slide down over Japan. If we extrapolate November 18th six to ten days out, we come up with a potential cold weather and storm system time frame of November 24 to 28. This narrows down the timeframe previously set out by the Bering Sea correlation, and if we match up the two timeframes (November 24-28 for the East Asian connection and November 25-29 for the Bering Sea connection), we come up with a November 24-29 potential timeframe for a storm, which can be isolated into the November 25-28 period, which is shown by both connections. Based on these two connections coming up with a common timeframe for a potential US storm system, confidence is quickly rising that this event will happen. 

The evidence doesn't stop there- here's text from a post on this storm potential I made back on November 13:

Take a look at the Bering sea in this two-panel reanalysis of the weather on November 8th. If you look closely, you can see that the Bering Sea was in the midst of a decent storm system on this date, just three days ago from today. This storm system passed along those waters in the midst of a mammoth ridge of high pressure just to the south. The 500mb height anomaly chart on the left best reflects this storm system, but its presence is confirmed on the right panel, which displays mean sea level pressure contour lines and denotations, as well as cloud cover. This Bering Sea storm has a connection to this potential Thanksgiving winter storm. Based on research done by Joe Renken, weather in the Bering Sea correlates to weather here in the US approximately 2.5 to 3 weeks after the Bering Sea weather anomaly occurs. This storm system in the Bering Sea happened on November 8th, and extrapolating that out 2.5 to 3 weeks ( 17-21 days) leads us to a potential storm system impacting the US around November 25 to 29. Considering November 28th is Thanksgiving, there does appear to be at least decent potential for a winter storm around the Turkey Day timeframe.

There is something else that must be noted with the ECMWF model I discussed at the top of this post. There has been discussion that the model has a bias, in which it holds energy in the Southwest US for longer than it ends up being there. I have a feeling that if this bias ends up working out in this situation, we would see the merge occur in the Northeast, resulting in that coastal storm. That is, if the energy in the South is able to latch on to the Canadian energy- in the most recent ECMWF model run, the southern energy was unable to phase with the Canadian energy until it was too late and the potential coastal storm was already off to the Canadian Maritimes. To give you an idea of what the snow forecast might look like if this Nor'easter occurred, take a look at the overnight ECMWF forecast below (warning, don't take this as a legitimate forecast just yet):

What I'm saying is, there really is potential here that needs to be monitored.

Andrew