Monday, December 21, 2009

Severe Storms Wednesday

A very strong storm system will develop on the Leeward side of the Rocky Mountains tomorrow afternoon. This storm system will draw in very moist air from the Gulf of Mexico and collide with a strong cold front across East Texas on Wednesday afternoon. The greatest likelihood of severe weather appears to be late Wednesday Afternoon through early Thursday morning for all of East Texas with the greatest threat east of a line from Sulphur Springs to Tyler to Lufkin. All types of severe weather are possible with this system including hail up to nickel size, gusty winds to 80 mph, and isolated tornadoes.

For anyone planning on traveling around Christmas, Wednesday will not be the best of travel days. If you are planning a flight to the Midwest, this same storm system will cause blizzard conditions in some areas.

Thunderstorms will begin to develop late tomorrow as mid level instability increases tomorrow afternoon. These thunderstorms will be elevated, forming above the boundary layer. Thanks to increasing steep mid level lapse rates and shear in the mid levels of the atmosphere, a few of these storms tomorrow could produce small hail but the overall threat of severe weather is very low.

On Wednesday enough low level moisture will be across the area to allow for surfaced base thunderstorm development. With the low level winds increasing around a developing area of low pressure across North Texas, there will be enough turning of the wind with height to give much of East Texas a threat of tornadoes.

The image above is taken from the 12Z run of the NAM forecast model on BUFKIT. There are a couple of things that show the possibility of tornadic supercells developing Wednesday afternoon and night. First is the hodograph, the graph in the upper left corner. This shows the wind speed and direction as you increase in height through the atmosphere. This is showing a classic large clockwise turn, or veering, of the wind with height, along with an increase in speed. This is very favorable for the lower levels of the atmosphere to spin.

Next we see the actual skew-t diagram, on the right. You notice the solid yellow line to the right of the solid red line. This is showing the amount of convective available potential energy, or CAPE, available for thunderstorm updrafts. Now the amount of CAPE is not all that great, only around 500 J/kg. But this is more than enough when you combine the shear and forcing associated with this upcoming storm system. So we will see thunderstorms developing in the boundary layer with rotating updrafts. All this means we will have to be on the watch for tornadoes Wednesday afternoon and evening as storms begin to develop.

As we head into Thursday morning the overall trend of these storms will be to form a squall line and race east. As this happens the overall severe weather threat will become more of a strong wind event than hail or tornadoes however, we will still have a chance at seeing an isolated tornado along the squall line.

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