Monday, November 21, 2011

Today's Severe Weather Threat

A cold front is dividing the weather across East Texas today. Temperatures ahead of this front are already approaching 80 degrees while north of the front, many areas are in the lower and middle 50s. It is along and just ahead of this front where a severe storm or two may develop later this afternoon. North of the front we will see showers and storms develop as warm air is forced over the shallow cooler air at the surface. Any storms north of the front could produce large hail but with the lack of surfaced based instability, the wind and tornado threat is extremely low.

It is along and south of the front where the severe weather threat changes significantly to include the threat of tornadoes and gusty winds. The one factor missing to make this a significant severe weather event is enough lift out ahead of the front. The upper air disturbance this afternoon will be too far west to provide enough lift for wide spread convection but there will be a chance a storm or two could develop due to daytime heating.

If a storm can develop south of the front the environment the storm would encounter would be favorable not only for severe weather but tornadoes as well. MLCAPE approaches and surpasses in a few areas 1000 J/kg south of the front. This is more than enough energy to provide deep robust updrafts.

In addition to the MLCAPE being around 1000 J/kg, the 0-3 KM CAPE will approach 100 J/kg. This provides enough lift in the lowest layer of the atmosphere to stretch and tighten any surface vorticity which could allow for tornadogenesis.

In addition the effective shear in the warm sector south of the front approaches 60 knots. This shear will cause any robust updraft to rotate producing supercell thunderstorms. In addition to the effective shear being adequate for severe storms, the effective storm relative helicity is over 200 providing more than enough low level spin that when combined with the low level CAPE shown early could help with tornadogenesis.

So all the parameters are there for severe thunderstorms including tornadoes today but with the lack of real forcing, we will have to wait and see if a storm can develop. The Storm Prediction Center has areas immediately along the forecast position of the front under a 5% chance of seeing a tornado(now is only 2% based on the question will a storm develop). Again as the southerly flow hits the front and is forced upward, showers and storms will develop north of the front. There is enough elevated CAPE along with shear to cause rotating or supercell thunderstorms to form. However with these storm’s updrafts not routed in the surface layer of the atmosphere, tornadoes will not be a threat.

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