Severe weather analysis - December 24, 2012

By: KoritheMan , 7:46 AM GMT on 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.

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

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4. WunderAlertBot
6:29 AM GMT on February 12, 2013
KoritheMan has created a new entry.
Member Since: December 31, 1969 Posts: Comments:
3. hydrus
2:51 PM GMT on December 24, 2012
Good blog Kori. I am hoping that this system does not become more intense then the mets are predicting....It would make Christmas a nightmare for a lot of folks. The GFS nailed another one.
Member Since: December 31, 1969 Posts: Comments:
2. FLPandhandleJG
8:58 AM GMT on December 24, 2012
Nice write up Kori.. I think this thing is going to get started tomorrow evening and cranking up from there.. There is quite a bit of energy building up and should release some scattered to strong cells in Texas and into LA.. This is one of the classic setups to have and specially during the winter months as well.. Its going to be interesting and probably one of those rare events that we dont want to see again.. Im just waiting on some more data and see if its going to be a Ka-Boom type system or a little boomer type system.. I just hope these ppl will heed the warnings during this holiday break.. This could be a type of system to catch some ppl off guard b/c there doing one thing while a storm could be coming for them..

Anyways i do want to watch that Christmas Tornado Movie that some ppl are raving about.. I love those SyFy movies..
Member Since: December 31, 1969 Posts: Comments:
1. louisianaboy444
8:18 AM GMT on December 24, 2012
The amount of tornadoes that this episode will spawn like you said will depend on the mode of the severe weather. 12Z model sounding data will be interesting. Usually to get discrete super-cells you want a small cap in the atmosphere around 700-800. This is usually a better set up for discrete super-cells because the convection breaks through the cap only at certain places and when it does it taps into all that CAPE above. If there is no cap then you could have to much instability and just get a widespread MCS...we will see
Member Since: December 31, 1969 Posts: Comments:

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