SPC AC 261636
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1136 AM CDT Tue Sep 26 2023
Valid 261630Z - 271200Z
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS THE
MIDWEST...NEW MEXICO/WEST TEXAS...AND NORTHERN FLORIDA...
...SUMMARY...
Isolated severe thunderstorms are possible today across parts of the
Midwest, across parts of New Mexico into west Texas, and possibly
northern Florida.
...Midwest including Illinois/Indiana/Missouri...
An upper low (-16C at 500mb) centered over eastern Iowa this morning
will continue to drift east-southeastward toward and across northern
Illinois through tonight. A semi-moist environment, with surface
dewpoints in the upper 50s/low 60s F, will diurnally persist
near/northeast of a weak surface low and occluded front/wind shift
that will generally extend southeastward from the surface low. While
deep-layer wind profiles will generally be weak (15-20 kt effective
shear), ample CAPE with respect to low-topped storms will support
the possibility of hail with the stronger pulse-type cells. Funnels
appear quite plausible across areas including eastern Iowa, southern
Wisconsin and northern/central Illinois, and sufficient low-level
CAPE/ambient vorticity near the boundary could yield a brief tornado
risk.
Farther to the southwest, there is some potential that some strong
to severe thunderstorms could develop near the front across
central/eastern Missouri late this afternoon into tonight. MLCAPE
could reach 1000-1500 J/kg MLCAPE near/south of the front, and wind
profiles will be moderately strong, influenced by increasingly
strong cyclonically curved westerlies in the base of the Upper
Midwest shortwave trough. Thermodynamic/kinematic profiles could
conditionally support some high-based supercells aside from
multicellular clusters if/where storms develop. While near-frontal
convergence will be modest, and the likelihood and extent of
sustained deep convection may remain limited, there will be
additional late night opportunities for convective development as
warm advection/moisture transport increase, and some of these storms
could also be strong. This region will be closely reevaluated in
subsequent outlooks for a potential reintroduction of severe
probabilities.
...New Mexico/West Texas...
Within the weaker southern branch of westerlies, a weak shortwave
trough is expected to move southeastward across the Four Corners
vicinity through tonight. As it does, associated forcing for ascent
should augment convective development, particularly storms
initiating in response to heating across the mountains of
southern/central New Mexico by early afternoon. It appears
that moisture across this region will be sufficient to support
initially sizable CAPE, beneath modest westerly to northwesterly
flow, at least in upper levels. Stronger storms may become capable
of producing marginally severe hail, then a few strong to severe
gusts while propagating southeastward off the higher terrain into an
unstable and more deeply mixed boundary-layer through early this
evening.
...Florida Peninsula...
Low severe probabilities have been introduced for the northern
Florida Peninsula where storms are expected to increase/develop
east-northeastward across the region, all while deep-layer shear
modestly increases. High precipitable water content will support
heavy precipitation loading and some potential for locally strong
surface gusts.
...Coastal Oregon...
Forcing for ascent within the exit region of a strong jet streak
(including 70-90 kt around 500 mb) may contribute to an organized,
vigorous pre-frontal convective band, associated with the surface
cyclone approaching the Pacific Northwest. However, with the cyclone
forecast to occlude prior to reaching the coast near/after 27/06Z,
appreciable inland boundary-layer destabilization appears unlikely,
and stable near-surface lapse rates may tend to mitigate the
potential for downward mixing of severe surface gusts as the
convective line progresses inland.
..Guyer/Squitieri.. 09/26/2023
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