Christmas Day Significant Severe Weather Event

**This post is dedicated to Madeleine Hsu, age 6, a victim of the Connecticut shootings.**

The Storm Prediction Center has outlined a Moderate Risk of severe weather for Christmas Day, including a 45% chance of severe weather in the pink area on the bottom image. It is not often you see a storm system produce a moderate risk in December, so let's look at the set up.

The 500mb forecast from the NAM model shows high vorticity values digging into Arkansas in the late night hours of Christmas Day. If you look closely, you can see the highest vorticity values tilting to the southeast. This signifies a negatively tilted storm system, meaning cold air is more prevalent aloft, and thus enhancing severe weather potential.

The jet stream forecast going into the day after Christmas shows this strong storm system pushing down on the jet stream, bringing about very high winds, as high as 130 knots. Again, not a common sight to see in December. The fact that there are such high winds aloft tells me that we will definitely have enough energy to provoke strong thunderstorms on the evening of Christmas Day. I am especially concerned for Louisiana and Mississippi, as the highest deep layer shear will be located in those states as shown below:

Such a powerful storm will draw up more than enough Gulf moisture and energy to provoke a nice severe weather event, but the main question is, what type of severe weather event are we looking at- supercells or a linear formation?

The simulated reflectivity for the late morning hours of Christmas shows very strong storms over Arkansas, Louisiana and Mississippi in response to this instability and favorable tornado dynamics. I have a feeling that individual cells will arise in LA/south MS to start the event, before the situation shifts to a more linear threat, thus higher risks for damaging winds and hail.

Be VERY careful tomorrow if you are in the outlined area by the SPC. There are going to be relatives visiting the Gulf Coast who may not be familiar with the area (weather-wise), and thus may not know how to appropriately handle such a severe weather risk.

Andrew

Long Range Discussion: A Tale Of Two Patterns

**This post is dedicated to Madeleine Hsu, age 6, a victim of the Connecticut shootings.**

The next few weeks looks to be dominated by one of two patterns in the atmosphere, and both can have significant effects on who gets the most cold and snow. Let's take a look.

Set up #1 involves your average Rex Block forming on the West Coast. High pressure would be in place over the Pacific Northwest, with low pressure in place over the Southwest. However, the Rex Block would extend its high pressure more east than what a 'typical' Rex Block would do. In response to that, the northern jet stream (as it will be referred to in this post, a.k.a. the one coming from Canada) will be tightened and stretched further east, giving storms a more meridional pattern and a good basis for strengthening. Low pressure stationed in the Southwest would shed pieces of energy again and again, which would then move into the combined storm track in the South. Now, from here, things get tricky. We have high pressure stationed in the Southeast, a.k.a. a Southeast ridge, but also low pressure in much of the Northeast and Ohio Valley. I would think that storm systems would be provoked to take the path shown in green, which would favor the Midwest, Ohio Valley and some parts of the Northeast for the best snows. The lack of a good negative NAO tells me that not all of the Northeast could see this pattern as the jackpot, but nonetheless it will be interesting for all involved.

Potential set up #2 is a more moderated Rex Block pattern. The Pacific Northwest high pressure system is more confined to its aforementioned domain, thus the corresponding low pressure system is located slightly offshore of the Southwest. While this does result in a slightly less meridional pattern, the jet stream is pushed further south, good news for those along the East Coast. That Southeast Ridge is now slightly offshore of the Southeast, meaning that the Midwest and northern Ohio Valley is no longer in the sweet spot for the best snows. However, because that high pressure system is still present, storm systems will be diverted north, but will have done so just offshore the East Coast. A large depression of cold air present in New England would certainly enhance the probabilities that any storm system that follows this track to produce snowfall. It is also possible that the Southeast is too far offshore to have any effect, so the low pressure in New England covers the entire Eastern Seaboard. This would still have the desired effect of storm systems going up the Coast.

This dual-image forecast from the ECMWF (left) and the GFS (right) shows 500mb height anomalies across the globe, with blues showing low pressure areas, and reds depicting high pressure areas. The ECMWF seems to set up that Rex Block in a similar situation to Set-up #1, but the low pressure anomaly in the Northeast is too overwhelming to give any for-sure signs that the Midwest and Ohio Valley would receive decent snows from this pattern. This morning's 0z ECMWF model shows that the North Jet stream would also be amplified in a pattern similar to the Set-up #1, but the Southern Jet stream is cut off by the Rex Block. Such a scenario is certainly possible, but I would expect to see more gusto in the Southern Jet stream than is being forecasted. It appears that the GFS moderates the Rex Block, meaning it is abiding by Set Up #2.

In summary:
-There are two potential set ups for the atmosphere to take in coming days.
-Set Up 1 would favor the Midwest, Ohio Valley for the best winter weather.
-Set Up 2 would favor the Northeast for the best winter weather.
-Set Up 1 appears to be working out in the ECMWF.
-Set Up 2 appears to be working out in the GFS.

Andrew

December 25-27 Significant Winter Storm - 12/23 Evening Update

**Although not mentioned, all posts today are dedicated to Dawn Hochsprung, the principal of S.H.E.S. killed in the school's shootings.**

Above are the four major models' snowfall forecasts for the December 25-27 significant winter storm. All 4 models go out to 120 hours, except for the NAM, which can only go out to 84 hours.

The ECMWF and GFS appear to have a good handle on the snowfall. The ECMWF's snow is not showing up as significantly, because the site where this was gotten from (WeatherBell) derives the snow from precipitation forecasts, meaning they use a different algorithm to get snowfall forecasts from that model. Getting the real algorithm requires a LOT of money.

The GEM model is off in its own world, typical of one of the worse models in the forecasting industry. The NAM is heavier with the snow in the Midwest than the GFS or ECMWF models are. This is to be expected, however, considering the NAM has a colder and wetter bias than other models (a.k.a. it overestimates snow).

Personally, I'm thinking the GFS has a good solution going, mainly because the GEM is unreliable, the NAM always overestimates snowfall, and the ECMWF snowfall forecast isn't showing up correctly for some reason.

To see the full update from earlier today, click on this link.

Andrew

December 25-27 Significant Winter Storm (Updated 12/23)

The December 25-27 timeframe only continues to look better and better in terms of snowfall for the Ohio Valley and Northeast.

Above is my latest thinking about this storm system. I believe that the system will drop a good 1-3 inches in southern Illinois and much of Indiana before 3-6'' accumulations take over in central Indiana. The storm begins to strengthen at this point, and all of Ohio s under the gun for shovelable snow. Northeast Ohio looks to get in on the worst snow of the state, with 6-12 inches possible. It is not out of the question that isolated areas in northern Pennsylvania could see amounts flirting with 12 inches, but much of the state should get 3-6 inches. New York will receive the worst of the storm, with the central part of the state up for a good 12''+ accumulation. Most likely the maximum amounts will get just below 15 inches, but certainly a good storm, depending how strong the storm gets when it receives the Atlantic influence. Maine is a little tricky, with much of the state likely to near 12 inches and scattered locations surpassing the one foot mark.

Considering this is a very busy weekend, the Impact Scale will come in handy. I am projecting Low Impact conditions through the Midwest locations, save Indiana. Indiana, Ohio and Kentucky are in the middle of a 'Moderate Impact' forecast, where some travel will be disrupted (i.e. some plane cancellations, several delays). The Northeast is covered in a 'Major Impact' forecast, as the heaviest snow will fall there. Widespread travel disruptions are likely, and I anticipate a chaotic mess for the days after Christmas as dozens of flights are cancelled, and hundreds of delays are observed.

A word of advice: I heard stories about some people driving to their location rather than flying during the December 19-22 blizzard. The car is much more dangerous than the airplane, as far as potential for having an accident in a winter storm goes. The car is in direct contact with the potentially-icy ground, while the airplane (despite its delays and cancellations) is much faster and safer. But that's me, and I'm not one to decide how you get back home for New Year's.

Andrew

Christmas Day Severe Weather Threat

There is a legitimate threat for severe weather on Christmas Day, something that should not be taken lightly. The Storm Prediction Center shows a slight risk of severe weather over the Gulf Coast, but probabilities of severe weather over Louisiana and Mississippi show a 30% chance of severe weather- something seen in the spring, not the winter.

The 500mb wind forecast from the NAM model on Christmas Day shows a strong storm system in Oklahoma, with winds surpassing 90 knots. Notice how the winds are also tilting to the southeast. This tilt means the storm system is negatively tilted. In a negatively tilted system, cold air is able to flow more freely aloft, meaning higher instability and higher chances for severe weather.

The 700mb winds show a very strong value of over 60 knots present over Texas and Louisiana. The presence of very high winds in the lower level jet stream only significantly increases the threat of severe weather and tornadic thunderstorms. Considering the high gradient of dewpoints, it does appear that these storms will form along a dry line (high gradient of moisture). Drylines are commonly the focus of severe storms in the spring, so we could very well be dealing with spring-type severe weather.

Such a situation needs to be closely monitored, as, if these forecasts verify, lives and property could be threatened.

Andrew