SPC MD 92

MD 0092 CONCERNING SEVERE POTENTIAL...TORNADO WATCH LIKELY FOR PORTIONS OF NORTHWEST LA...NORTHEAST TX...SOUTHERN AR...FAR SOUTHEAST OK

Mesoscale Discussion 0092
NWS Storm Prediction Center Norman OK
0233 PM CST Sat Jan 21 2017

Areas affected...Portions of northwest LA...northeast TX...southern
AR...far southeast OK

Concerning...Severe potential...Tornado Watch likely 

Valid 212033Z - 212300Z

Probability of Watch Issuance...95 percent

SUMMARY...The risk for severe thunderstorms capable of producing
tornadoes, damaging winds, and very large hail is increasing this
afternoon, and Tornado Watch issuance is likely within the next few
hours.

DISCUSSION...A warm front continues advancing northward from
portions of east-central TX through southern LA, within a
warm-advection plume preceding midlevel troughing over the
South-Central States. Embedded within broader-scale midlevel
cyclonic flow, water vapor loops indicate a smaller-scale impulse
advancing northeastward toward the lower Sabine Valley. Ascent
associated with these features is expected to support thunderstorm
development from portions of northeast TX through west-central LA --
particularly after 22Z -- with activity subsequently spreading
east-northeastward/eastward into the evening hours. Initial
boundary-layer cumulus development is already noted across portions
of east-central TX, southeast of Corsicana.

Ample low-level moisture south of the warm front, marked by surface
dewpoints in the middle 60s, will continue to build northward in
response to southerly low-level mass fluxes promoted by the
approaching upstream trough. With a well-defined elevated mixed
layer sampled by the 18Z Shreveport sounding featuring steep lapse
rates in the 800-550-mb layer, moderate instability will continue to
develop northward into the zone of developing convection. With 45-60
kt of effective shear, and an anticipated initial discrete and
semi-discrete convective mode, significantly severe hail will be
possible with supercell structures. The risk for damaging winds will
increase as convection organizes into smaller-scale clusters with
embedded bowing/rotating convective elements. While not particularly
strong, low-level flow will be increasing through the evening. The
resultant enlargement of low-level hodographs will foster an
increasing tornado risk, especially with convection evolving in
proximity to the warm frontal zone and given the steep midlevel
lapse rates.

..Cohen/Hart.. 01/21/2017

...Please see www.spc.noaa.gov for graphic product...

ATTN...WFO...LZK...SHV...TSA...

LAT...LON   32459214 31689246 31499380 32169451 34019517 34569449
            34289285 33499230 32459214 

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