LIVE MAP (ABOVE) ... SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0744 AM CDT Tue Sep 19 2023
Valid 191300Z - 201200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS OF
THE SOUTHERN PLAINS...
...SUMMARY...
Severe thunderstorm gusts and large hail are possible today and
tonight across portions of the southern Plains.
...Synopsis...
In mid/upper levels, amplification of the pattern is forecast over
parts of the northwestern and north-central CONUS later today into
day-2, related substantially to a strong shortwave trough and
embedded 500-mb cyclone now over the AK Panhandle and adjacent
coastal waters. This trough should dig southeastward to
southwestern AB, southern BC and WA by 12Z tomorrow, while a
downstream ridge builds across parts of the upper Mississippi Valley
into ON. As this occurs, initially zonal but increasingly difluent,
westerly to northwesterly flow will prevail downstream to the
trough's southeast, over the central/southern parts of the Rockies
and Plains.
Within the increasing difluence, a shortwave trough now over parts
of western KS to the TX Panhandle will shift eastward to eastern
portions of KS/OK by 00Z. This feature may decelerate in the
difluent regime tonight over the Ozarks to Arklatex. Following
that, just upstream, a smaller perturbation is evident in moisture-
channel imagery over southeastern CO and northeastern NM. This
trough should track eastward to parts of western/central KS and
northern/central OK through 00Z, then weaken eastward in favor of
convectively driven vorticity processes, and perhaps merge with the
lead wave late in the period.
At the surface, 11Z analysis showed a cold front over south-central
FL, becoming quasistationary over the north-central Gulf, and a warm
front northwestward across southeast to north-central TX. This
boundary is expected to move slowly northward/northeastward over the
east TX/eastern OK region today, while a dryline develops across
western KS, western OK, northwest TX, to the Big Bend area. A
pseudo-warm-frontal zone of relatively backed near-surface flow may
develop over parts of OK, in the western/modifying part of outflow
air and behind morning convection, encouraged by isallobaric forcing
related to weak cyclogenesis along the dryline. There is
uncertainty on this process -- depending on mesobeta-scale
thermodynamic recovery and kinematic response behind morning
convection.
...Southern Plains...
Widely scattered to scattered showers and thunderstorms are ongoing
across portions of northwest Texas, northward and northeastward over
parts of OK, in a zone of large-scale ascent (warm advection and
DCVA preceding the leading mid/upper perturbation. This activity
should remain largely nonsevere as it shifts eastward through the
remainder of the morning, though an isolated, marginally severe
instance of hail cannot be ruled out.
Convective and severe potential should ramp up this afternoon as the
boundary layer diurnally/diabatically destabilizes south of the
morning clouds/convection (across parts of southern OK and north
TX), southwest of it atop a strongly heated/mixed boundary layer
near the dryline, and perhaps behind it over western/central OK late
this afternoon. The latter potential is more uncertain and
conditional, given time needed for both direct heating/
destabilization and warm advection in the recovering low-level air
mass immediately preceding the second mid/upper-level perturbation.
Where convection can develop over northwest TX and OK, especially
near any dryline/outflow intersection, supercells are possible amid
strong veering of winds with height in low/middle levels, yielding
well-curved and favorably enlarged hodographs. Development appears
best focused near the dryline over the southwestern OK/northwest TX
area, with sustained heating and MLCINH erosion likely, but less
low-level shear. Conversely, coverage is more uncertain in and
north of the backed-flow regime, though hodographs will be more
favorable. As such, too much mesoscale uncertainty lingers to
assign a more-specific, focused area of higher unconditional
probabilities at this time.
Forecast soundings near the trailing dryline depict small 0-1-km SRH
and shear due to uniformity of winds in that layer, however, 200-400
J/kg effective SRH and 40-45-kt effective-shear magnitudes should be
common (with larger values northward). A corridor of mid 50s to
lower 60s F surface dewpoints -- even after diurnal mixing -- should
combine with steep low/middle-level lapse rates to yield 1500-2500
J/kg MLCAPE, in a northward-narrowing moist-sector corridor. In
addition to severe downdrafts encouraged by the deep mixed layer,
isolated significant (2+ inch) hail appears possible given the
supercell-favoring shear, the shape/depth of the buoyant profile,
sounding analogs, and peak size given by 2D hail models applied to
progged soundings. Some evening/nocturnal clustering of convection
also is possible, locally boosting wind potential for a couple
hours. However, given aforementioned mesoscale uncertainties, great
spread understandably appears among convection-allowing guidance on
where and how intensely any such wind threat will organize.
..Edwards.. 09/19/2023
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