Wednesday, April 13, 2011

Tomorrow's Set Up Much Like 04/09/09

A storm system will be moving through the Southern Plains tomorrow with the threat of severe weather for parts of the area. Much of Eastern Oklahoma has been placed under a moderate risk of severe storms with a slight risk of severe weather from I-20 north. But as I look at the forecast data it looks like this set up for severe weather is looking much like the set up we had on April 9th 2009. For those who may not remember what happened on this date four supercell thunderstorms developed across East Texas producing hail and tornadoes continuing on into Louisiana. A couple of these tornadoes were on the ground for over 30 miles producing EF-3 damage.

So what happened that day and how does it compare to tomorrow’s event? Well, a strong area of low pressure developed at the surface dragging a dryline through East Texas during the early evening hours. There was a conditional threat of severe weather across East Texas meaning storms may not develop but if they did, watch out! Why was the threat conditional? There was a warm layer of air around 5000 feet in the atmosphere we call a cap. This cap forces rising air back to the surface putting a lid on thunderstorm development. Because of this cap temperatures were able to warm well into the 80s producing plenty of instability. At the same time wind of different speed and direction at different levels in the atmosphere was producing strong wind shear across East Texas. This wind shear allowed for any storm that could develop to rotate, producing large hail and tornadoes. As the dryline was forced through East Texas, there was enough lift to cause this cap to break in a few locations causing severe storms to explode.

A very similar set up is being forecast for tomorrow across the Southern Plains. An area of low pressure will develop dragging a dryline through the area. The next few images below show the position of the cut off low in the mid levels of the atmosphere. First the actual cut off low front April 9th 2009 then form various forecast models.

You can see that the second image and last image both place the cut off low in nearly the exact location as April 9th 2009. The outlying model is the NAM which places the cutoff low farther north. This is important because if the NAM’s solution is correct, the formation of the surface low would be farther north placing less forcing across East Texas limiting the threat of thunderstorm development.

The next set of images show the actual 850mb level low pressure analysis for April 9th 2009 followed by the NAM and GFS forecast position of the 850mb low. Again the NAM has this low much farther north which would limit the threat of severe weather here in East Texas. However the GFS is pretty much positioning the low pressure in the exact location from April 9th 2009.

Usually I like to use what the NAM is showing for my short range forecasting. This high resolution model seems to do a fairly good job in predicting the position of weather features with 48 hours. However, today the NAM is the odd model placing the low pressure much farther north than all the other computer models. All models seem to have a pretty good handle on this disturbance in its current position but then send the energy in different directions. When you look at the forecast instability across East Texas, 2000+J/kg of surface based CAPE and Lifted Index Values around -5°C, there is more than enough energy to produce strong to severe storms. You combine that with nearly 70 knots of shear and 0-1KM Storm Relative helicity values over 240, any storm that develops would rapidly become severe, rotate, and more than likely produce golfball size or greater hail and an isolated tornado. Again with such a variance between the models we will have to watch the eventual path of this developing low pressure. If it moves farther north, the chances of severe weather will be very small. If it moves farther south tomorrow could be a very interesting day. Stay tuned!

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