Mesoscale Discussion 1897
   NWS Storm Prediction Center Norman OK
   0244 PM CST Thu Dec 31 2020

   Areas affected...Parts of the upper Texas and Louisiana coastal
   plain

   Concerning...Tornado Watch 520...521...

   Valid 312044Z - 312245Z

   The severe weather threat for Tornado Watch 520, 521 continues.

   SUMMARY...Potential for supercell structures capable of producing
   tornadoes may become primarily focused across southwestern
   Louisiana, near/south of Fort Polk, by 5-7 PM CST.

   DISCUSSION...The center of the surface cyclone continues to deepen
   across the Greater Houston Metro area, as an 80-90 kt cyclonic
   mid-level jet (around 500 mb) noses across the lower Rio Grande
   Valley, toward the upper Texas coastal plain.  However, the cyclone
   is in the process of occluding, and inflow of moist boundary-layer
   air into the upper Texas coastal plain probably will become cut-off
   within the next couple of hours.

   Although strongest south/southeasterly 850 mb flow is forecast to
   develop across the Ark-La-Tex into the Red River Valley between now
   and 23-01Z, 40+ kt speeds may still be maintained as far
   south-southeast as the southwestern Louisiana coastal plain. 
   Surface temperatures across the Louisiana coastal plain have warmed
   into the lower and mid 70s, with surface dew points in the upper
   60s/near 70F.  Despite weak lapse rates and lingering warm layers
   aloft, it appears that this is contributing to mixed-layer CAPE of
   500-1000 J/kg.

   Given this instability, in the presence of wind profiles
   characterized by strong deep-layer shear and sizable
   clockwise-curved low-level hodographs, the environment is becoming
   at least conditionally conducive to supercell storms capable of
   producing tornadoes.  

   Although strongest mid-level forcing for ascent will remain
   displaced to the northwest, strongest high-level difluence appears
   likely to overspread the region through early evening.  Aided by
   low-level warm advection, this may contribute to discrete supercell
   development ahead of the eastward advancing cold front, probably
   focused near its intersection with the surface warm front (roughly
   extending east-west across the Fort Polk vicinity).

   ..Kerr.. 12/31/2020

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

   ATTN...WFO...LCH...

   LAT...LON   30389422 30919361 31099269 30999209 30899188 30639179
               29969170 29609208 29819308 29829371 29829399 30389422