ACUS02 KWNS 180541
SWODY2
SPC AC 180539

Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1239 AM CDT Fri Jul 18 2025

Valid 191200Z - 201200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS SATURDAY INTO
SATURDAY NIGHT ACROSS PARTS OF EASTERN IOWA...SOUTHERN
WISCONSIN...NORTHERN AND CENTRAL ILLINOIS AND INDIANA...CENTRAL AND
SOUTHERN LOWER MICHIGAN...NORTHERN AND WESTERN OHIO...PARTS OF
NORTHWESTERN PENNSYLVANIA AND WESTERN NEW YORK STATE...

...SUMMARY...
Strong thunderstorms, including one or two organizing clusters, are
possible across southern portions of the Great Lakes region and
adjacent portions of the Ohio Valley Saturday into Saturday night,
with more widely scattered strong thunderstorm development across
the northern Rockies into adjacent high plains.

...Discussion...
Models indicate that at least some amplification to the mid/upper
flow across the northern tier of the U.S. is probable during this
period.  It appears that this will include digging troughing across
the Pacific Northwest through northern Rockies, downstream of
increasingly prominent ridging building across the northeastern
Pacific.  Farther east, troughing is forecast to dig across and to
the east of the Great Lakes, in advance of significant troughing
within a separate branch of westerlies, pivoting southeast of Hudson Bay.

In lower levels, the better defined remnants of an ongoing cold
frontal intrusion to the lee of the Rockies, perhaps aided by
stronger differential surface heating during the day Saturday, may
shift east of the middle Mississippi through the lower Ohio Valley.
At the same time, a reinforcing intrusion of cooler and drier air is
forecast to advance across and south of the Upper Midwest and Great
Lakes, probably in the wake of a weak eastward migrating surface low.

...Midwest through Great Lakes/Ohio Valley...
Model depictions concerning synoptic and sub-synoptic features,
including potential convective evolution remain rather varied.  In
general, it appears that low-level moistening ahead of the
reinforcing cold front will contribute to moderate destabilization
as far north as the southern Great Lakes during the day Saturday.
On the southern fringe of the stronger westerlies, 30-50 kt flow in
the 850-500 mb layer across southern Wisconsin and Lower Michigan
into the lower Great Lakes will contribute to potential for
organized convective development.  This may include a couple of
supercells and perhaps an upscale growing cluster.  Although
mid-level lapse rates may be modest, this may be accompanied by some
risk for hail, in addition to strong to severe surface gusts and
perhaps a tornado or two.

Southwestward toward the middle Mississippi into Ohio Valleys,
deep-layer mean wind fields are likely to be weaker, but shear
beneath west-northwesterly mid-level flow may still support
potential for organized thunderstorm development, in the presence of
a seasonably moist boundary-layer becoming characterized by sizable
CAPE with daytime heating.  Forcing for this activity remains
unclear, and probably will hinge on convectively generated or
augmented perturbations migrating around the northern periphery of
mid/upper ridging encompassing much of the central and southern tier
of the U.S. to the east of the Rockies.

...Lee of the northern Rockies...
Downstream of the digging mid/upper troughing across the Northwest,
moisture return to deepening lee surface troughing is forecast to
contribute to at least a narrow corridor of moderate to large
mixed-layer CAPE, in the presence of steep lower/mid-tropospheric
lapse rates.  Veering of winds with height probably will contribute
to sufficient shear for organized thunderstorm development,
including supercells, as widely scattered to scattered thunderstorms
develop off the higher terrain into the high plains late Saturday
afternoon and evening.  It is possible that severe weather
probabilities could still be increased in later outlooks for this
period.

..Kerr.. 07/18/2025

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