SPC AC 261230
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0730 AM CDT Tue Sep 26 2023
Valid 261300Z - 271200Z
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS LATE THIS
AFTERNOON AND EVENING ACROSS PARTS OF NORTHERN AND EASTERN ILLINOIS
THROUGH WESTERN AND SOUTHERN INDIANA AND ACROSS SOUTHERN NEW MEXICO
INTO WEST TEXAS...
...SUMMARY...
Widely scattered strong thunderstorms are possible today across
parts of the Midwest into lower Ohio Valley vicinity, and across
parts of southern New Mexico into west Texas. Some of these may be
accompanied by hail and gusty winds.
...Synopsis...
As an initially broad and deep cyclone undergoes considerable
further weakening near the northern British Columbia coast,
associated large-scale mid-level troughing may gradually lose some
amplitude while continuing to progress inland across the Pacific
Coast and Canadian/northern U.S. Rockies today through tonight. It
does appear that one vigorous embedded short wave impulse will
accelerate toward the Pacific Northwest coast, accompanied by a more
modest and compact surface cyclone late this afternoon into early
Wednesday. This short wave has been preceded inland by at least a
couple of other significant perturbations. One of these has already
slowly begun to suppress a prominent downstream blocking high,
initially centered near southern Hudson Bay, though the elongating
center of higher heights may maintain considerable strength across
southern Hudson/James Bays into the St. Lawrence Valley through this
period.
Beneath the mid-level high, a sprawling surface ridge (centered over
Quebec) will maintain a considerable influence across much of the
nation to the east of the Rockies. Seasonably high moisture content
will generally remain confined to areas along and south of a weak
surface front across southern Atlantic and eastern Gulf coastal
areas, downstream of an elongating cyclonic perturbation migrating
north-northeast of the Yucatan Peninsula, and merging into weak
larger-scale troughing over the Gulf of Mexico.
...Southern Rockies into Texas Big Bend...
Within the weaker southern branch of westerlies, and in advance of
the significant inland migrating northern branch troughing, a weak
short wave trough is forecast to slowly turn across and southeast of
the Four Corners vicinity during this period. As it does,
associated forcing for ascent may augment convective development,
particularly storms initiating in response to heating across the
mountains of southern 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 evening.
...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.
...Florida Peninsula...
Inland destabilization may support fairly widely thunderstorm
development this afternoon and evening, with high precipitable water
content supporting heavy precipitation loading and some potential
for locally strong surface gusts. However, given the initial large
degree of saturation in the thermodynamic profiles and the weakness
of the ambient mean flow, peak gusts seem likely to remain mostly
below severe limits.
...Indiana/Illinois vicinity...
To the southwest of the lingering blocking mid-level ridging, a weak
mid-level low is forecast to continue slowly digging from parts of
the Upper Midwest toward the lower Ohio Valley. It appears that
this will maintain at least broad, weak inverted surface troughing
on the southwestern periphery of the Quebec surface high, which is
forecast to become a focus for scattered thunderstorm development
today beneath modestly cool mid-level temperatures (around -14 C
near 500 mb). Beneath an evolving mid-level dry slot, mixed-layer
CAPE may exceed 1000 J/kg by peak afternoon heating, which may be
sufficient to support small to, perhaps, marginally severe hail in
stronger cells. Due to the generally modest to weak nature of the
deep-layer mean flow and shear, the potential for severe wind gusts
appears low. However, an isolated, brief tornado might not be
entirely out of the question, where surface convergence can become
locally enhanced in close proximity/beneath the mid-level low.
..Kerr/Squitieri.. 09/26/2023
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