Monday, November 5, 2012

Severe Weather This Weekend?

Long range models are still pointing to a very active weekend weather wise across the central portions of the United States. A deep trough will swing across the Rocky Mountains Saturday and move into the Plains on Sunday. This will cause a strong area of low pressure to develop and swing a cold front through the area. Now there are still some differences on the exact timing of the event, but confidence is rising with the threat of strong to severe storms across the mid section of the nation Saturday and Sunday.

Right now it appears two areas of low pressure will develop between Friday and Sunday across the high plains ahead of the aforementioned trough. This will cause strong southerly winds to set up all across the Plains States this weekend including East Texas. The second area of low pressure that develops near the Texas Panhandle Sunday will be the one that drags a cold front through the area with the possibility of strong to severe storms Sunday afternoon.

 Right now the limiting factor across the area in preventing severe storms will be the minimal amount of buoyant energy available for thunderstorm growth. Models are paining between 500 and 1000 J/kg of Mean Layer CAPE across the area and we would usually like that number to be greater than 1000 to get strong convection or storms to develop.
 But the surface based Lifted Index, or LIs, are all negative and approach -4, not extremely unstable but are significant when overall CAPE values are low.
 By looking at the profile of the atmosphere we notice the CAPE, energy for thunderstorm growth, is what we call short and fat.
 This means even though there is not a substantial amount of CAPE for thunderstorms the updrafts that do develop have the potential to be very strong. This would increase the threat of severe weather including hail and tornadoes.

 Now the wind shear available on Sunday in more than adequate for severe weather across the Plains. The overall bulk shear up to the mid level of the atmosphere is close to 50 knots in most areas and approach 70 knots near the Red River Valley.
We normally need to see around 40 knots to see supercell development, storms that are capable of producing very large hail and tornadoes. In addition, the 0-1km shear is on the order of 30 knots across most of the area.
Levels of 20 knots or greater are usually what we look for in tornado development. Third, the winds just of the surface will be between 50 and 60 knots so any thunderstorm downdraft could easily produce winds over 60 miles per hour.

So the winds energy is more than adequate for high winds and tornadoes if storms can remain scattered.

 The good news, if there is any, is it looks as though this situation will result in a fast moving squall line which will cut down on the tornado threat. The shear vectors will run parallel to the front causing storms to rapidly form into a line. This will increase the threat of widespread wind damage but limit the overall tornado threat. Based on what the models are advertising right now it looks as though storms would begin to develop around midday Sunday. As they initially form there would be a tornado threat due to all the parameters mentioned earlier. But the storms would rapidly form into a squall line before moving into East Texas meaning our main threat would be hail and high winds.

 Again we are still a long way out from this potential event and a lot could and probably will change between now and then. The timing is 24 hours slower than that of my last update and could easily change again. The forecast will be fine tuned as we approach this weekend and all the parts of the puzzle come together. Remember we are entering a period of the year that climatologically produces severe weather. Stay tuned!

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