SPC AC 171231

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0731 AM CDT Wed Mar 17 2021

   Valid 171300Z - 181200Z

   ...THERE IS A HIGH RISK OF SEVERE THUNDERSTORMS THIS AFTERNOON INTO
   TONIGHT FROM NORTHEAST LA/SOUTHEAST AR ACROSS CENTRAL MS AND AL...

   ...SUMMARY...
   A significant tornado outbreak, with long-track, intense tornadoes
   is expected to begin this afternoon across parts of Louisiana and
   Arkansas, and then spread eastward and peak this evening into
   tonight across Mississippi and Alabama.

   ...Synopsis...
   A compact, intense shortwave trough over the TX Panhandle this
   morning will progress eastward over the Red River Valley today and
   the lower MS Valley tonight.  This midlevel trough will be preceded
   by a surface cyclone that will move from northeast OK across
   northern AR/southern MO to the confluence of the MS/OH Rivers near
   the end of the period.  This process will result in strengthening
   mid-upper flow over the lower MS Valley, as well as increasingly
   strong low-level southerly flow and moisture transport.  The net
   result will be a broad warm sector centered on the lower MS Valley,
   capable of supporting multiple rounds of storms/supercells today
   into tonight.

   ...Short-term, ongoing convection...
   There are several zones of ongoing convection this morning, though
   most of this convection is on the fringes of a broad warm sector. 
   The primary short-term threats will be focused with warm advection
   storms, now rooted near the surface warm front, from southeast AR
   across northern MS/adjacent southwest TN through about midday.  More
   than adequate deep-layer shear and SRH, in a slowly
   warming/moistening environment, will support a continued threat for
   all hazards with this convection through about midday.  Farther
   west, an extensive (but broken) band of convection extends from
   northeast TX into western AR in association with a bore-like wave
   propagating eastward in the warm sector.  These storms will persist
   through the morning, with primary threats for damaging winds and
   large hail.  Farther north, a loose cluster of elevated storms
   continues to spread east-northeastward from northeast OK toward
   southwest MO, in a zone of strong low-level warm advection with
   MUCAPE of 1000-2000 J/kg.  Despite the elevated nature of the
   storms, there will still be the potential for downdrafts to reach
   the surface with strong/damaging gusts through the morning.

   ...Primary tornado outbreak this afternoon into tonight...
   The ongoing convection across eastern AR/western TN/northern MS will
   tend to reinforce the surface warm front near the TN/MS border
   through early afternoon.  The northeast TX and eastern OK bands of
   storms will overspread the Ark-La-Tex and the remainder of central
   AR through mid-late morning, with the potential to produce damaging
   winds, large hail, and a couple of tornadoes.  In the wake of this
   convection, some air mass recovery is expected across AR, back to
   the surface cyclone near the AR/OK/MO border intersection by midday
   to early afternoon.  Thereafter, surface-based thunderstorm
   development is expected along the wind shift from near Shreveport to
   Fort Smith, and storms will spread eastward across AR/LA through the
   afternoon/evening.

   The destabilizing warm sector with warming temperatures and
   mid-upper 60s dewpoints, will support MLCAPE up to 2000 J/kg, and
   strengthening vertical shear will be favorable for a broken band of
   supercells capable of producing tornadoes, large hail, and damaging
   winds.  A couple of strong tornadoes will be possible this
   afternoon/evening across LA/AR.

   Farther east, the most dangerous part of the severe weather outbreak
   is expected to evolve today into tonight from central MS into
   central AL.  Here, there will be the potential for scattered
   supercell development in the open warm sector by midday, in an
   environment with MLCAPE near or above 2000 J/kg, effective bulk
   shear near 50 kt, and effective SRH near 200 m2/s2.  All hazards
   will be possible with these warm sector storms during the afternoon.
    By late afternoon and into early tonight, a low-level jet segment
   will strengthen to 50-60 kt across MS/AL and the midlevel trough
   approaches from the west, contributing to very strong low-level
   shear (0-1 km SRH of 400-500 m2/s2).  Buoyancy will be slow to
   decrease after sunset and with eastward extent based on the
   prevalence of upper 60s dewpoints, while very favorable wind
   profiles will maintain the threat for long-track, intense tornadoes
   with both warm sector supercells, as well as supercells within the
   broken band along and ahead of the surface wind shift progressing
   eastward across MS by early tonight.  West central GA appears to be
   the eastern edge of the primary severe threat area through tonight.

   ..Thompson/Gleason.. 03/17/2021

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