Mesoscale Discussion 1895
   NWS Storm Prediction Center Norman OK
   1121 AM CST Thu Dec 31 2020

   Areas affected...Upper Texas and southwestern Louisiana coastal
   areas

   Concerning...Tornado Watch 520...

   Valid 311721Z - 311915Z

   The severe weather threat for Tornado Watch 520 continues.

   SUMMARY...Potential for thunderstorms capable of producing tornadoes
   may still increase.  Near/north and east of the Greater Houston
   metropolitan area, this risk probably will maximize from 1-3 PM CST.
    Trends are being monitored for the possibility of an additional
   tornado watch across parts of southwestern Louisiana.

   DISCUSSION...Discrete supercells have been forming across the
   northwestern Gulf of Mexico, within and just ahead of a broken
   pre-cold frontal convective band advancing northeast of the middle
   Texas coast.  While there have been a number of substantive embedded
   low-level mesocyclones, these have generally tended to weaken while
   spreading northward across the coastal waters, at least to this
   point.

   However, a deepening lower/mid tropospheric cyclone is in the
   process of slowly accelerating/redeveloping northeastward across the
   Rio Grande River, toward the Texas Hill Country.  As this continues
   80-90 kt cyclonic mid-level flow (around 500 mb) is forecast to
   overspread the lower Rio Grande Valley through middle Texas coastal
   areas between now and 19-21Z.  In lower-levels, flow around 850 mb
   is forecast to strengthen to 40+ kt, veering to westerly in the
   post-frontal regime, and perhaps backing to an increasingly
   southeasterly component ahead of the front across upper Texas and
   Louisiana coastal areas.  

   It appears that the surface cyclone will continue to deepen and
   consolidate over the Greater Houston Metropolitan area, which may
   allow for some further boundary-layer moistening off the Gulf of
   Mexico.  Widespread ongoing precipitation will continue to temper
   destabilization, but it is possible that strengthening dynamic
   forcing for ascent, coupled with intensifying vertical shear, and
   ambient vertical vorticity may still contribute to an environment
   increasingly conducive to supercell structures capable of producing
   tornadoes inland of coastal areas.

   Potential for supercells with a risk for tornadoes may also begin to
   increase within a developing downstream warm advection regime across
   the upper Texas into southwest Louisiana coastal plain.

   ..Kerr.. 12/31/2020

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

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

   LAT...LON   29369536 30349562 30739457 30089304 29669319 29359414
               28379449 29369536