Tuesday, January 31, 2012

Severe Weather Possible Friday

The next few days will see a few disturbances rapidly moving across the area as the mild winter weather continues. The first fast moving disturbance will move through the area tomorrow near the Red River Valley providing enough lift for an isolated thunderstorm or two. With the surface temperatures being as warm as they are, there will be enough surface instability for a marginally severe storm with the main threat being small hail and gusty winds. With the surface winds remaining rather weak and a uniform shear profile tomorrow, the tornado threat at this time appears very low. Most areas look to remain dry as it only appears isolated convection but the next system heading our why for Friday looks much stronger.

Forecast models differ on the timing and position of the next system but all agree much of the area will receive rain. What is becoming interesting with this next system is the NAM forecast model, a higher resolution short range model, is starting to slow the system down while at the same time strengthening it. If what the NAM is predicting right now does come true, we could see a significant round of severe weather across parts of the area including tornadoes.

At the time of this writing the latest forecast time available for the NAM was 18Z Friday, or noon CST. At this time a strong, negatively tilted trough is forecast to be moving through West Texas. As this happens warm moist air from the Gulf will be forced north across the state of Texas adding to the amount of surface instability available for thunderstorm development. Winds at the surface will be out of the southeast around 15 to 20 miles an hour. At the same time the winds at 850mb, or about 5000 feet in the air, will be out of the south around 50 to 60 miles per hour. This will produce a large turning of wind with height in the lowest level of the atmosphere allowing for large storm relative helicity to be ingested by any thunderstorm that develops. This will help any storm that develops to be capable of producing a tornado.

The main limiting factor in severe weather development is the level of instability available for storms. Right now it is forecast to be fairly low. However, usually in strong negatively tilted troughs the amount of surface instability can be underestimated 84 hours out by the forecast model. The 700mb chart is showing very high relative humidity levels across the area indicating cloud cover.
There is an area of very dry 700mb air that will wrap around the storm system allowing for breaks in the clouds which will in turn heat the air mass adding to the low level instability.

Severe weather outbreaks are always a blend of perfect timing between all severe weather parameters. This system bears watching the next few days. The upper air pattern is fairly similar to the April 9th event in 2009 nine which produced numerous long lived tornadoes across the area. The upper air jet streak is forecast to punch just south of the area putting the Red River Valley and much of East Texas in the left exit region which gives added lift to air parcels. The entire area will also be in the area of diffluent wind flow which also adds to air parcel lifting.

So if the current trend continues, this meteorologist will be out roaming the plains looking for tornadoes Friday afternoon and evening.

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