SPC 1300Z Day 1 Outlook

Day 1 Outlook Image

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0643 AM CST Sun Feb 28 2021

Valid 281300Z - 011200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM NORTHEAST
TEXAS TO MIDDLE TENNESSEE AND SOUTHERN KENTUCKY...

...SUMMARY...
Damaging thunderstorm gusts, a few tornadoes and isolated large hail
are possible from northeast Texas to Middle Tennessee and southern
Kentucky.

...Synopsis...
In mid/upper levels, a positively tilted trough is apparent in
moisture-channel imagery from central Canada across the northern/
central Plains and central Rockies to AZ and Baja.  A shortwave
trough initially  over SD/NE will eject/stretch northeastward across
the Upper Great Lakes through the period, losing amplitude, but
remaining connected to a very lengthy vorticity banner extending
southwestward around the northern/western sectors of a developing
closed cyclone over parts of NM/AZ.  The associated 500-mb low
should reach southern NM by the end of the period at 01/12Z. 
Farther equatorward, a weak but potentially influential southern-
stream perturbation over Chihuahua should eject northeastward across
west-central/north-central TX today.

At the surface, 11Z analysis depicted a surface low near the IA/MN
border with cold front southwestward over southeastern KS,
southwestern OK, and the TX South Plains.  By 00Z, the low should
reach ON, just north of Lake Huron, as the center of a well-
developed and deepening synoptic cyclone, with cold front trailing
across Lower MI, IN, the lowest segment of the Ohio Valley, the
Arklatex region, and portions of south-central/southwest TX.  By
12Z, the cold front should reach eastern NY, central VA,
northwestern GA, central MS, central LA, the mid/upper TX Coastal
Plain, and deep south TX.

...North TX to TN/KY...
Scattered thunderstorms may develop along/ahead of the cold front as
early as mid/late morning over parts of north TX and southeastern
OK, increasing in coverage and extending northeastward through the
day as the regime both backbuilds southward, shifts eastward and
expands northeastward.  An organized belt of scattered to numerous
thunderstorms is expected to evolve as a result, by mid/late
afternoon, from the Ozarks across southeastern OK to central/
northeast TX.  The foregoing warm sector will destabilize and build
buoyancy through the day from a combination of warm advection,
moisture advection/transport, and modest/erratic surface heating
under broken cloud cover.  Surface dew points in the 60s to near 70
F should be common near and south of a decaying warm front now
located over parts of TN, AR and eastern OK, and forecast to become
more diffuse with time.  This will contribute to a pre-cold-frontal
plume of MLCAPE 1500-2000 J/kg over the TX part of the outlook,
where midlevel lapse rates will be steepest, and 800-1500 J/kg
elsewhere.

Low-level and deep shear will be favorable, with effective SRH
commonly in the 150-250 J/kg range despite somewhat veered flow just
above the surface, and 60-80 kt effective-shear vectors.  The latter
will be aligned with a substantial component parallel to the main
belt of convective forcing, such that a combination of embedded
supercells and QLCS mesocirculations are possible.  Those will yield
potential for tornadoes and damaging to severe gusts.  Hail will be
more of a concern relatively early in the convective cycle and in
western parts of the outlook area, while modes still are less messy,
and amidst greater lapse rates.  The severe potential may continue
through much of the evening before weakening late tonight, except
for the next regime described below.

...South TX tonight...
Isolated to scattered thunderstorms are possible overnight -- mostly
from about 06Z onward -- across portions of the Edwards Plateau, and
adjoining parts of the Rio Grande Valley, northeastward over the 
Hill Country.  This activity may develop on either side of the cold
front, as strong moist advection via easterly flow occurs mainly in
the 850-950-mb layer.  Forecast soundings indicate this process,
underlying steep low/middle-level lapse rates, will yield weak
MUCINH, with MUCAPE ranging from around 1500-2000 J/kg over southern
parts of the outlook area to 500 J/kg in the north.  The substantial
easterly component in the inflow layer will contribute to strong
deep shear, with effective-shear magnitudes 60-80 kt possible. 
Activity developing on or just south of the front will be undercut
quickly, but may persist long enough to produce large hail.  At this
time, coverage of the severe convection is uncertain, so the
unconditional outlook will be maintained at marginal probabilities.

..Edwards/Jewell.. 02/28/2021

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