SPC 1300Z Day 1 Outlook

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0647 AM CST Fri Dec 18 2020

Valid 181300Z - 191200Z

...NO SEVERE THUNDERSTORM AREAS FORECAST...

...SUMMARY...
Thunderstorm development will be possible late tonight from parts of
the Texas Hill Country northeastward to the Arklatex. No severe
thunderstorms are expected.

...Synopsis/Discussion...
In mid/upper levels, a positively tilted, synoptic-scale trough is
apparent in moisture-channel imagery from eastern WY across western
CO, northern AZ, and northern Baja.  An associated/primary vorticity
lobe extends northeast-southwest across the Four Corners area.  This
trough will move eastward through the period while maintaining
positive tilt.  By 12Z, it should extend from eastern IA across
southeastern KS and west TX to north-central MX.

Continental/polar air left behind the last frontal passage will
remain the dominant boundary-layer condition over most of the
central/eastern CONUS.  The 11Z surface analysis showed a cold front
from central/southern MB through a low near MBG, to another low
between HLC-GLD, and southwestward over northeastern NM.  By the end
of the period, this front should move to Hudson Bay, eastern Lake
Superior, eastern IA, southwestern MO, north-central TX, to near
LRD.  

Ahead of that front, a plume of elevated, low-level warm advection
and moisture transport will spread north-northeastward over the
outlook area, accompanying a 35-45-kt LLJ.  Isentropic lift of
parcels with increasing mixing ratios to LFC will occur in a regime
of steepening low/middle-level lapse rates above the boundary layer.
Largest/deepest buoyancy, and greatest potential thunderstorm
coverage, should be over roughly the southwestern 1/3-1/2 of the
outlook area after 09Z -- or across portions of central to southeast
TX.  MUCAPE 500-700 J/kg may develop into icing layers suitable for
lightning generation.  Weaker buoyancy with isolated thunder
potential will extend farther north during that time frame, and
between 06-09Z throughout the area.  Sufficient over-water airmass
modification to support surface-based effective-inflow parcels still
appears to be improbable until after the end of the period on the
middle TX Coast, if at all.  See SPC day-2 outlook for more details.

..Edwards.. 12/18/2020

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