SPC 1300Z Day 1 Outlook

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0655 AM CST Fri Nov 27 2020

Valid 271300Z - 281200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM SOUTH TEXAS
TO SOUTHERN MISSISSIPPI...

...SUMMARY...
Isolated, marginally severe thunderstorms are possible from South
Texas to southern Mississippi today.

...Synopsis...
The mid/upper-tropospheric pattern over the CONUS features a
positively tilted synoptic trough, extending from far northwestern
ON across MN, much of NE, through a col over central CO, to a closed
cyclone over central AZ.  That portion of this trough located
poleward of the col will split eastward rapidly through the period,
reaching portions of New England and the coastal Mid-Atlantic by
12Z.  Meanwhile the closed cyclone -- though not completely cutoff
-- will move eastward far slower, its center reaching east-central
NM by 12Z.

At the surface, 11Z analysis showed a cold front from Lake Michigan
across central/southwestern MO, northeast/central TX, and the TX Big
Bend area.  This front is forecast to move southeastward by 00Z to
OH, middle TN, central MS, southwestern LA, the middle/upper TX
Coast, and deep south TX.  By 12Z the front should reach central/
southern GA, southern LA, the northwest Gulf just offshore from the
mid/upper TX Coast, and slightly southward through deep south TX. 
In doing so, it will overtake the western segment of a diffuse,
stationary to warm frontal zone analyzed initially across southern
GA, extreme southern portions of MS/AL, south-central LA, and east
TX.

...South TX/Rio Grande Valley to southern MS...
Scattered showers and widely scattered to scattered embedded
thunderstorms are expected through this evening -- mainly ahead of
the front but also along and behind it.  Isolated, marginally severe
hail/gusts are possible, with the gust threat being along and ahead
of the front.  The strongest convection should shift offshore
along/ahead of the front, and any prefrontal outflow pushes, late
this afternoon into this evening.

Regional radar mosaics, satellite imagery and lightning data reveal
a broad, gradual increase in convective coverage and areal extent
the past few hours from Coahuila to the Mississippi River portion of
the MS/LA border.  This activity is building within a low-level
warm-advection plume, with rich boundary-layer moisture theta-e over
south TX to southwestern LA.  Weak boundary-layer lift in the warm
sector appears to be keeping surface-based development subsevere in
character, despite the presence of 40-50-kt effective-shear vectors
and MLCAPE estimated in the 1000-2000 J/kg range.  Abundant, multi-
level cloud cover also is apparent in IR imagery.  That cloudiness
will impede diurnal heating substantially today, with peak MLCAPE
not much higher than at present.

Still, with large-scale ascent forecast to increase gradually ahead
of the slow-moving AZ/NM cyclone, and a favorable parametric 
juxtaposition of shear, moisture and buoyancy across the region,
maintaining a marginal risk appears justified for this outlook
cycle.  Upscale aggregation/clustering of convection is forecast by
some convection-allowing guidance across southeast TX near or just
ahead of the cold front; such a process may boost wind potential
locally should a sufficiently strong cold pool arise.  Otherwise,
the gust threat will be brief and with downbursts in the most
intense multicells.  Hail near severe limits is possible, but its
size and frequency will be limited by lack of stronger
1) Deep-layer lapse rates, and
2) Low-level shear for developing and maintaining low-level
mesocyclones, though transient supercell characteristics are
possible.

..Edwards/Broyles.. 11/27/2020

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