Severe weather analysis - December 24, 2012
The potential exists for a significant severe weather outbreak over the central Gulf Coast states on Christmas Day.
As an amplifying mid- to upper-level trough over the four corners region pivots eastward during the day today (Christmas Eve), it is likely to undergo significant intensification, resulting in surface cyclogenesis across the deep south. Model prognoses suggest that the trough will experience a negative tilt (northwest to southeast orientation), which is a situation that is usually helpful for severe weather. In addition, model soundings show some rather sizable hodographs evolving throughout the day today and especially into Christmas Day, along with substantial CAPE values. Surface-based storms may evolve across portions of central and eastern Texas this evening as the aforementioned trough begins to deepen. As the day progresses, this threat is expected to be maximized by a strong low-level jet which is expected to promote a threat for damaging winds within stronger convective downdrafts, which is supported by 0z soundings across Texas and Louisiana. In addition, very large hail appears to be possible with any elevated thunderstorms that develop within this region (central to east Texas, extending to perhaps far western Louisiana). There is obviously some tornado potential in these areas, but the overall threat appears to be somewhat conditional, namely in regards to the mode of storm development (surface based vs elevated). Directional and speed shear should be more than adequate in this region to promote a marginal tornado risk.
On Tuesday/Christmas Day, the real fun is anticipated to begin as the low-level jet reaches its maximum potential. Indeed, there appears to be enough speed shear within the lower troposphere to promote the potential for hurricane force wind gusts -- perhaps up to 70 kt in some of the most favorable areas. This would occur if the primary convective mode is a squall line or Mescoscale Convective Complex (MCS). Assuming we establish more discrete storms, these are the ones that will pose the risk for tornadoes, possibly strong and long-lived. The greatest threat for tornadoes will probably occur from west-central Louisiana to southwestern Alabama during the afternoon/evening hours of Christmas Day. Areas south of these areas appear to be more at risk for damaging winds, and there exists the potential for this event to be widespread, resulting in downed trees and power lines.
In addition, with very high precipitable water values of 1.5 to 2.0 inches, torrential rainfall and associated flash flooding will pose a considerable risk even if the other threats are somehow mitigated. The Storm Prediction Center (SPC) in Norman, Oklahoma has placed areas from central Louisiana to west-central Alabama under a Moderate Risk for severe weather, which is their second highest alert.
Figure 1. Latest 2-day Convective Outlook by the Storm Prediction Center (SPC) in Norman, Oklahoma. Note the Moderate Risk area highlighted from central Louisiana to west-central Alabama.
This has the potential to be a substantial severe weather episode. Please monitor the weather, and if a warning -- particularly a tornado warning -- is issued -- take cover immediately. Heed the warnings and do not venture outside if possible.