997
ACUS01 KWNS 220556
SWODY1
SPC AC 220555

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1255 AM CDT Sat Jun 22 2024

Valid 221200Z - 231200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS LATE THIS
AFTERNOON AND EVENING ACROSS PARTS OF CENTRAL AND EASTERN
IOWA...NORTHERN MISSOURI...SOUTHERN WISCONSIN...NORTHERN
ILLINOIS...CENTRAL AND SOUTHWESTERN LOWER MICHIGAN...

...SUMMARY...
Severe thunderstorms are possible this afternoon and evening across
parts of the Midwest and Great Lakes region.  One or two clusters of
storms may evolve and pose a risk for damaging wind gusts, but a
couple of supercells posing a risk for tornadoes are possible as
well.

...Synopsis...
Models indicate that persistent mid/upper ridging will become
increasingly suppressed across and east-southeast of the Upper
Midwest later today through tonight.  As flow trends broadly
cyclonic across the Great Lakes and Ohio Valley into the Northeast
and Mid Atlantic, the slowly weakening center of this ridging is
forecast to shift from the Mid South into the southern Great Plains
by 12Z Sunday.

It appears that one notable, but relatively low-amplitude, short
wave perturbation emerging from the Great Basin will accelerate
east-northeastward, then eastward, across the Upper Midwest through
the Great Lakes during this period, as a somewhat stronger upstream
perturbation digs east-southeast of the Canadian Prairies toward the
upper Great Lakes region.  Although there is substantive spread
among the various model output concerning the evolution, the lead
impulse is forecast to support modest cyclogenesis along a surface
front, generally across or east-northeast of Iowa through the lower
Michigan vicinity by late tonight.  As this occurs, a trailing cold
front likely will advance southward through portions of the central
Great Plains, lower Missouri and Ohio Valleys.  The modifying
western flank of a preceding intrusion of cooler air may be slow to
shift northeast of the southern New England and Adirondacks
vicinity, while a reinforcing intrusion of cool air begins to
advance south of the international border into the northern U.S.
Great Plains by this evening.

Destabilization along and just south of all of these boundaries may
provide a focus for strong to severe thunderstorm development this
afternoon and evening.

...Great Lakes into lower Missouri Valley vicinity...
Severe weather potential remains somewhat unclear for this period
due to the model discrepancies concerning the surface cyclone
evolution, and the stabilizing influence of potentially considerable
lingering early period convective development across the Midwest
into Great Lakes region.  However, a belt of 30-50+ kt southwesterly
to westerly flow in the 850-500 mb layer may overspread a
destabilizing environment along and south of a left-over outflow
boundary across parts of eastern Iowa through southern
Wisconsin/northern Illinois, and the stalled frontal zone over
central lower Michigan during the day.  Seasonably high
boundary-layer moisture content (including dew points near 70F) will
become supportive of at least weak to moderate CAPE, in the presence
of moderate to strong shear, including sizable clockwise-curved
low-level hodographs.  This environment may become supportive of a
few supercell structures accompanied by a risk for tornadoes, before
increasing thunderstorm development ahead of the southeastward
advancing cold front poses primarily a risk for damaging surface
gusts into this evening.

...Northern Mid Atlantic...
Deep-layer wind fields and shear are likely to remain weak, but
moderately large CAPE (in excess of 2000 J/kg) may once again
develop within weak surface troughing, along and southwest of the
remnant surface frontal zone.  It appears that this will become
supportive of scattered strong thunderstorms with potential to
produce damaging wind gusts, as mid-level heights begin to slowly
fall late this afternoon into this evening.

...Northern Great Plains...
Stronger mid/upper forcing for ascent and mid-level cooling are
likely to remain north of the international border beyond peak
daytime heating, but an isolated strong storm or two may advect
south of the international border by this evening, accompanied by a
risk for marginally severe hail and wind.

..Kerr/Wendt.. 06/22/2024

$$