Monday, September 28, 2009

Severe Weather Possible Late Thursday

A strong storm system is forecast to move into the middle of the country Thursday and drag a cold front through the Southern Plains. Current forecast models are coming to an agreement on the position and strength of this system. There is a possibility of severe weather along and ahead of the cold front from eastern Nebraska into Texas late Thursday through Friday morning. Right now the main threat with this line appears to be strong winds and hail but, I can’t rule out a possible tornado or two late Thursday, especially near the Oklahoma Texas state line north of Dallas.

Below is the forecast position of the 500mb Vorticity maximum late Thursday by the GFS forecast model. Along with this strong area of vorticity, this image shows diffluent heights across Ozark Plateau and Southern Plains. This will aid in the lift for the formation of strong storms late Thursday.

This storm system is still too far out to show on the short range NAM model run for Thursday evening however, it is showing up Thursday morning on this model and is a very similar position to the long range GFS forecast model. The next two images show the comparison between both models. Not much difference.

There also appears to be plenty of Gulf moisture available for this upcoming storm system. By Thursday morning both forecast models show a large area of dewpoints over 70°F along the Texas Gulf Coast with the GFS showing a larger supply of moisture.

By Thursday evening the GFS is showing plenty of deep moisture with mid 60 dewpoint temperatures being pulled into the storm system across the Southern Plains and 70+ degree dewpoints across southeast Texas.

Now the GFS is hinting at a smaller secondary surface low (shown below) developing somewhere near the Oklahoma Texas state line. This is an important feature because along and to the north of this secondary low, surface winds will have an easterly component, or what we call backing. This will increase the threat of tornadoes with any storm that can develop in this area.

We also see plenty of instability with this system. Surfaced Based lifted index in south central Oklahoma are approaching -8, more than enough for strong to severe storm development. (See image below)

There is a maximum of CAPE (Convective Available Potential Energy) near the maximum of instability in south central Oklahoma.(See image above) This will allow for stronger thunderstorm updrafts aiding in the possibility of large hail. Now, if the best instability and energy develop in the area just northeast of the developing secondary low pressure, the chances of tornadoes will increase, especially during initial thunderstorm development. The GFS is showing this threat with the 0-1km EHI (Energy Helicity Index) of 4 (Values greater than 2 have been associated with significant tornadoes) or greater near Lake Texoma. (Image below)

Other experimental severe weather indices are pointing to a possible severe weather event as well. The two images below the Craven Significant Severe weather index and the Supercell Composite Parameter. The second image is showing a bulls eye near the same area of increased instability and CAPE.

So will we see an outbreak of tornadoes? Well we won’t know for sure until we get closer to the forecast event however, the last image shows a long line of enhanced 700mb vertical motion, or lift. This indicates the storms that form will quickly form into a squall line so the best chance of isolated supercell thunderstorms will be during the early evolution of storms late Thursday afternoon. With the associated shear anticipated Thursday evening, this line of storms will more than likely take on a line echo wave pattern (LEWP). This will allow for a few isolated supercells to be embedded in the squall line increasing the threat for strong winds and an isolated tornado. I will have an update once the NAM forecast model is in the forecast timeframe.

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