Update: WEATHERAmerica Newsletter (Severe Weather Threat), 02/09

by LarryCosgrove | February 9, 2009 at 02:31 pm

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Overview

A powerful, very cold disturbance over California will emerge from the southern Rocky Mountains on Tuesday afternoon, providing what appears to be a major risk for severe thunderstorms and tornadoes to an area stretching from S OK and TX into the Ohio and Tennessee Valleys during a Tuesday evening to early Thursday time frame.
Factors Which Favor Prolific Development Of Severe Thunderstorms
One aspect of this system that stands out is its tight, intense core of vorticity associated with the 500MB vorticity. This feature is associated with a fast-moving, rapidly-deepening surface low that digs into central Texas. Once in the vicinity of Austin TX, the storm makes a rapid (or bolting) motion toward the northeast with a sharply defined triple point of convergence (cPk, cT, mT air masses). So if forecasts of dewpoints exceeding 65 deg F verify, an optimum threat for a major severe weather event will be in play until the low takes on a vortex structure over the lower Great Lakes on Thursday morning.

If we check parameters such as Lifted Index, Convective Available Potential Energy (CAPE), helicity, then it is clear that the greatest risk can be ascribed to a box bounded by Lawton OK....San Antonio TX....Conroe TX....Hope AR between 3 PM CT Tuesday and 3 AM CT Wednesday. The compaction of the positive vorticity advection may prolong/extend the supercell and bow echo coverage into the hours just after daybreak in parts of AR, SE MO, W TN, W KY and S IL. Even though instability will not be as impressive on Wednesday afternoon, the intense, pocketed vertical velocity display and PVA "stab" through the Ohio and Tennessee (accompanying a strong 250MB wind velocity maximum) should be enough to produce a few tornadic cells as well as LEWP and bow echo formations. So a scant two weeks after the worst ice storm in history, locations still reeling may be targeted by a severe weather episode!

It should also be pointed out that this feature has a very sharply (circular) defined cold pool aloft which will promote lift as well as steep lapse rates in the immediate vicinity of the storm track.
Factors Which Argue Against Synoptic-Scale Production Of Intense Convection
Because of the calendar date (February), we must use some caution in defining the threat areas for severe weather, especially those above the Interstate 20 corridor. Consider, however, the Super Tuesday Outbreak of 2008 and the St. Louis MO killer tornado of February 9, 1959, which show conclusively that "out of bounds" outbreaks of tornadoes, microbursts, and large hail can, and do, occur.

In some cases involving low-latitude digging of strong cyclones, the moisture fetch from the Gulf of Mexico is so dense that little heating occurs within the warm sector. If the frontal structure advances into a thick overcast, severe weather impacts will be greatly reduced. They key here: sunshine over Texas with scattered cumulus mediocris will give way to strong thunderstorms. Dense stratus and stratocumulus on Tuesday afternoon argue against important convection.

Another critical issue in play is timing. If the PVA and strong upper wind fields do not reach the TX and NM border by mid-afternoon, initiation may occur in a somewhat cooled, more stable environment over Texas. That would mean reduced risk to the Interstate 35 corridor and perhaps points further east.
SUMMARY
The first true severe weather outbreak of the 2009 convective season will initiate over TX and OK on Tuesday afternoon, February 10. Large hail and tornadoes are especially likely over C, E TX, extreme S OK, SW AR, and NE LA into the early hours of Wednesday, February 11. As a diurnal resurgence and strong lifting ahead of the deepening low occurs, the period from 2 PM ET Wednesday to 2 AM ET Thursday may see cases of strong winds and isolated tornadoes sweeping through much of the Ohio and Tennessee Valley.