Monday, December 24, 2012

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.


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