Convective Addiction

FCST: 01/22/12 – 01/23/12 – Substantial Severe Weather Event Plausible Across South-Central U.S.

Posted on 21. Jan, 2012 by Jesse in Forecasts

A formidable severe weather event appears to be evolving for parts of Dixie Alley late Sunday evening and into the overnight and early morning hours on Monday, with the possibility for nocturnal tornadoes and damaging winds being the primary hazards. A setup that was previously looking more mediocre than not has, per 12z/22 model runs, been looking more favorable for an appreciable severe weather event across much of the South-Central U.S. over the next 36 hours or so. The Storm Prediction Center has introduced a high-end slight risk for the region for tomorrow per their SWODY2; communities such as Little Rock, AR, Memphis, TN, Nashville, TN and Evansville, IN are in the crosshairs for potentially significant severe weather during this time frame.

SPC Day 2 Outlook from 1730z/21

A powerful trough will take on a strongly negative tilt tomorrow afternoon/evening as it gyrates towards the Ouachita and Lower Mississippi River Valley regions, with both the GFS and the NAM advertising a 90+ kt mid-level jet streak nosing into the I-30 corridor by 00z/23 (6 PM SUN).

In referencing the 500 mb mid-level profile (shown above), it should be noted that there were some slight differences between the NAM (left) and GFS (right) from today’s 12z/21 model runs, most notably that the NAM has a slightly stronger jet streak over the Red River Valley region at 00z/23 (6 PM SUN), though the two models are showing relatively similar placement of the trough itself, placing much of Dixie Alley in the favored exit region where the most favorable forcing for ascent will be located, juxtaposed with a surface boundary that will push eastward, thence serving as a forcing mechanism for igniting convection between 0z/23 (6 PM Monday) and 03z/23 (9 PM Monday).

Of note are some rather salient differences among the numerical trifecta, i.e., the 12z/21 ECMWF, 12z/21 NAM and the 12z/21 GFS.  The GFS and the NAM are both further north with the placement of the surface low, each showing a deeper low pressure system centered proximal to the I-35 corridor across IA/MN at 00z/23 (6 PM Monday), whereas the 12z ECMWF has a more elongated surface low centered across the I-44 corridor, deepening and taking aim on the St. Louis – Chicago corridor with a more southerly track, which could change the evolution of storms in reference to discrete vs. linear modes, timing, etc.  This is noteworthy because the ECMWF ignites convection across far NE TX into WC AR between 20z /22 – 00z/23, whereas the American models are trying to initiate deep convection closer to the Mississippi Delta region around 00z/23 (Monday 6 pm) ahead of the main forcing.

ECMWF showing more southerly placement of surface features - 06z/23 (1 AM MON)

As moisture surges northward, all models are showing 500-1000 J/KG SBCAPE cape peregrinating northward in advance of the surface front, which, when juxtaposed with ambient SFC-500 mb bulk shear values AOA 50kt, 0-1 km SRH > 200 m2**s2 and LI values between 5C and 8C, the stage should be set for a very respectable, widespread SVR weather event across the region.  If discrete cells can be maintained initially, a threat for tornadoes will be more substantial.  With the strong unidirectional forcing, storms should evolve into a QLCS with an enhanced threat for wind damage with embedded tornadoes.

NAM forecast sounding near Memphis, TN - 00z/23 (6 PM MON)

Interested parties in the region should maintain a close watch on the weather conditions over the next 36 hours.  This could be the first appreciable SVR weather event of the 2012 severe weather season, and nocturnal events across Dixie Alley are notorious for more capacious human impacts, namely due to their unorthodox timing.  From a chasing standpoint, the situation will be aggravated by unfavorable terrain and, perhaps, the more troublesome aspect of the best forcing not arriving until late in the day.

JLR