Convective Addiction

FCST: 02/24/11

Posted on 23. Feb, 2011 by Jesse in Uncategorized

A potential SVR weather episode is likely to unfold across parts of the Lower Mississippi River valley on Thursday, though the extent of a seemingly more appreciable TOR threat remains in question due to possible instability limitations.  A mid-level shortwave, with an associated upper-level jet core of 120 kts, will move into the S Plains by early Thursday.  An associated surface cyclone should be located somewhere over EC OK, and it’s progged to move ENE into the lower Ohio River valley by early Friday, slowly deepening along its trek.  An attendant warm frontal boundary should be located somewhere INVO the surface cylone extending EWD towards the MO bootheel, with an associated dryline extending southward across TX and a cold front moving SSE behind the surface low.

SFC-500 mb shear profiles AOA 60 kts will rapidly increase to 80+ kts after 00z, owed largely to an intensifying 850 mb SW LLJ that strengthens from 50 kts to over 70kts as it moves right up the Lower Mississippi River valley between 6 PM Thursday and midnight on Friday.  This environment will help to enhance storm motions from between 40-50kts late THUR to potentially near of above 60 kts after nightfall THUR, creating a potentially dangerous situation if the actual SVR potential is realized.  Southerly surface winds of 15-20 kts behind the warm front will help to enhance localized SRH values, though large portions of the synoptically favored warm sector in/near the region of best divergence/lift are progged to have less than optimal hodographs through 00z FRI.

850 mb LLJ shown intensfying to 60+ kts by midnight FRI

Best mid-level CAPE progged by the GFS at 6 PM THUR (00z FRI)

Most unstable (MUCAPE) CAPE as progged by the NAM-WRF at 6 PM THUR (00z FRI)

NAM-WRF forecast SKEW-T for Fayetville, AR (NW AR) at 12z THUR morning

ECMWF Skew-T: 6 PM THUR (00z FRI) for "ARKMOTEN" region

What to watch for

The evolution and track of the surface low, along with the placement of the WF and triple point, will be key to where the SVR threat evolves.  Additionally, the existence of any remnant convection and cloud cover will also play a major role as to when and where any pockets of instability materialize.  The synoptic models are not going to be able to resolve the lot of these finer mesoscale details, but it does appear that shear profiles won’t be the most problematic here.  The ECMWF has been advertising the most consistently northern track of the surface low with this system, with the GFS and the NAM slightly further south with a tad less deepening forecast.  It’s entirely possible that all of the models could fail to correctly depict where the warm front will actually end up, so that will need to be watched via morning surface observations, visible satellite imagery and the existence and placement of ongoing convection.

Interestingly, the ECMWF has been advertising an arguably decent pocket of instability THUR afternoon (300 – 1000 J/KG) across a narrow region from far NE TX across C AR and into far SW TN; however, most models want to depict higher H7 RH values and even mostly cloudy conditions across the threat area throughout most of THUR.  As the LLJ intensifies after 00z FRI (6 PM THUR), instability values make approach more seasonably appreciable categories with surface Theta-E values AOB 320k moving into the Lower Mississippi region, potentially favoring a nocturnal SVR episode ahead of the cold front.  Across the warm sector, SE of the triple point, the ECMWF advertises the best LI values >= -5 with SFC-850 mb lapse rates of 4-6 C/km between 6PM THUR and midnight on FRI.

The most appreciable TOR threat that can be discerned at this time would appear within the region circled in pink below, where seasonably acceptable instability levels would co-exist with favorable shear profiles, with potentially enhanced pockets of SRH near and INVO the triple point and/or the warm front.  If storms root in the boundary layer and utilize the enhanced shear present courtesy of the warm front, remaining discrete and riding parallel to this enhanced shear zone, then the potential for TORs does indeed exist.  Fast storm motions, poor terrain and a potentially large nocturnal timeframe for some of this event makes it a less than optimal chasing episode for this season’s first home opener.

JLR - 02/23/11

HPC's synoptic forecast for 6 PM THUR (00z FRI)