Tuesday, January 24, 2012

Severe weather possible tomorrow

The unusual wet patter for a La Nina year looks to continue as a strong storm system promises to bring heavy rain and thunderstorms to the area starting tonight. Rainfall could add up to more than 2 inches for many areas before this storm system finally exits the area late Thursday. In addition to heavy rain, there is a slight chance for severe thunderstorms tomorrow across parts of East Texas, mainly south of Interstate 20.

The way it looks right now an area of low pressure will develop tonight across extreme south Texas and begin to move northeast towards Shreveport by Thursday morning. This low pressure will pull in warm moist air from the Gulf, allowing for showers and thunderstorms to develop. Areas along and south of the low pressures track will be in what we call the “warm sector” of the storm system. It is in this area where we could see a few severe storms, especially from noon to 6PM tomorrow afternoon. Now we normally don’t expect to see severe weather during January but it is not unheard of. January 20th, 2010 was the last major severe weather event during the month of January in East Texas. Now the atmosphere will not be as volatile as we saw for that event but, the amount of energy and shear in the atmosphere is very similar to watch just occurred in the Southeast Sunday night. So for areas along and south of a Marshall to Longview to Tyler To Palestine line there is a threat of gusty winds and maybe an isolated tornado or two if an isolated thunderstorm can develop. The good news for East Texas is that as storms develop tomorrow they should quickly form into a squall line, limiting the tornado threat.

On the positive side, this system should bring heavy rain to the area. Now we cannot rule out a few local areas of flash flooding, but with 2 plus inches of rain falling over the area in a short period of time, lake levels should rise. The past few heavy rain events have occurred a little slower than what is expected with this system and over a drier ground. Soil moisture has increased with the rains of November and December so rapid run off should occur with heavier downpours causing our lake levels to rise. Stay with KLTV and KLTV.com for the latest. Any change in the track of this low pressure will cause a change in the threat area for severe weather.

Now earlier I stated there were similarities to the Sunday/Monday event with tomorrow’s storm system. But events were fairly what we usually reference to as a low instability high shear event. CAPE values in central Alabama early Monday morning were between 500 and 1000 J/kg and the forecast CAPE values here for tomorrow are around 500 J/kg, usually not what we would look for in a severe weather situation.

However the 0-3KM CAPE, or the amount of energy available closest to the surface, is forecast to be between 100 and 200 J/kg tomorrow afternoon. This is more than enough to produce healthy low level rising of air parcels, some we look for in tornado formation.
I this this low level energy that creates the stretching possibilities of any low level spin to help generate strong gusty winds along with tornadoes.
Another factor we look for in the development of tornadoes would be the low level helicity, or the amount of spin available for the thunderstorm to ingest. Values at or above 150 are usually enough to help with the development of tornadoes. Tomorrow’s forecast values are between 200 and 300.

Finally we look for the 0-1KM shear values. Anything over 20 knots is favorable for tornado development in severe weather situations. Tomorrow’s values are forecast to be between 30 and 40 knots, more than enough to help in tornadogenesis.The two major differences in tomorrow’s setup I see from Alabama’s tornado outbreak is the amount of forcing and the orientation of the shear vectors to the front. In Alabama the forcing was displaced farther to the north along with shear vectors which were more perpendicular to the front. This allows storms to separate and ingest all the available energy and spin. This lead to a couple of very strong supercells which produced at least two ling track tornadoes.
As seen in the first image, a very strong vorticity maximum will be moving into the area to provide a large amount of forcing. In addition the shear vectors will be parallel. Combining these two factors will allow for any storms that form to quickly form into a squall line. This should limit the overall tornado threat but increase the gusty wind threat.
So what we will be watching for is any isolated thunderstorm to develop ahead of the main line of storms. Any isolated storm could produce tornadoes but should be quickly ingested by the main line.The overall severe weather threat will depend on the eventual track of the developing low pressure. If this system moves farther north, all of East Texas will be under a threat of severe weather. A bit farther south and the severe weather threat will be pushed along the Gulf coast.

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