SPC 2000Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0231 PM CDT Thu Apr 20 2023
Valid 202000Z - 211200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS LATE THIS
AFTERNOON AND EVENING IN A CORRIDOR FROM SOUTHERN WISCONSIN THROUGH
THE MIDDLE MISSISSIPPI VALLEY AND SOUTHEASTERN GREAT PLAINS...
...SUMMARY...
Scattered severe thunderstorms remain possible late this afternoon
and evening in a corridor from from southern Wisconsin to south
Texas. Damaging winds and large hail appear the primary hazards in
terms of coverage, but a few tornadoes are also possible.
...20Z Update...
Large-scale mid-level troughing now east of the Rockies may amplify
a bit further, but only a slow eastward progression into the
Mississippi Valley is expected through early Friday. An associated
modestly deep cyclone appears likely to continue to gradually
occlude across the Upper Midwest and adjacent Great Lakes vicinity,
with the trailing cold front advancing into the lower Great Lakes,
lower Ohio, Mississippi and Rio Grande Valleys.
Due to the lingering presence of mid-level troughing across the
Florida Peninsula, and dry/potentially cool boundary-layer air
associated with surface ridging across much of the Southeast and
eastern Gulf of Mexico, low-level moisture return to the warm sector
of the cyclone remains rather modest. Seasonably moist
boundary-layer air remains largely confined to southern/central and
eastern Texas, where south through southwesterly flow in the
lower/mid troposphere has become modest to weak, in the wake of the
cyclone.
As a result, while conditions have become conducive to increasing
vigorous thunderstorm development posing at least some risk for
large hail, damaging wind gusts and tornadoes, in a pre-frontal
corridor from Texas through southern Wisconsin, severe weather
potential overall does not appear particularly optimized. Severe
probabilities are being maintained as outlined in the 1630Z, aside
from some change to account for the progression of the synoptic
features.
..Kerr.. 04/20/2023
.PREV DISCUSSION... /ISSUED 1126 AM CDT Thu Apr 20 2023/
...Mid-MS Valley to southern WI...
Elevated convection persists across portions of southern WI and IL,
but appears to be weakening. Subsidence in its wake has yielded
cloud breaks immediately ahead of the primary surface cyclone over
central IA, which will yield destabilization into this afternoon
across the Mid-MS Valley and IL to southern WI. The cyclone will
initially track northeast into western WI before occluding
northward, aided by an 80-90 kt 500-mb jetlet curling across IA into
central WI.
The degree of surface-based destabilization remains somewhat
uncertain with a confined warm/moist sector ahead of the
eastward-moving cold front. But guidance is largely insistent that a
narrow plume of moderate MLCAPE of 1000-1500 J/kg should develop, as
upper 50s to low 60s surface dew points in eastern IA and northeast
MO advect northeast. The synoptic pattern will support strong
effective bulk shear along with an enlarged low-level hodograph
favorable for discrete supercells consolidating into small clusters.
As such, have increased tornado probabilities and expanded the cat
2-SLGT risk northward. Otherwise, a mix of scattered large hail and
damaging wind will be possible, but the overall threat will likely
be spatially and temporally confined given the narrow warm/moist
sector, resulting in severe potential rapidly weakening in the early
evening roughly east of the IL/IN border longitude.
...Central/east TX to AR...
The cold front surging south-southeast towards the Concho Valley of
west-central TX and the Ozark Plateau is expected to help focus
scattered to widespread thunderstorms along and ahead of it this
afternoon and evening. Large buoyancy with MLCAPE in excess of 2500
J/kg, supported by steep mid-level lapse rates, will primarily
reside across central to south TX where low-level shear should
remain weak. But sufficient deep-layer shear, although somewhat
parallel to the front, should support initial supercells
consolidating into several clusters and one or more MCSs. Farther
northeast across the Ark-LA-Tex into AR, low-level shear will be
comparatively greater but buoyancy will be relatively less. Large
hail can be expected with the more discrete storms (some in excess
of 2 inch diameter, especially across central to south TX), and the
potential for damaging winds will increase with upscale growth this
evening. The tornado threat is expected to remain unfocused and
short-lived, likely dependent on favorable storm-scale/boundary
interactions.
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