942
ACUS02 KWNS 111710
SWODY2
SPC AC 111709

Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1209 PM CDT Mon Aug 11 2025

Valid 121200Z - 131200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS TUESDAY INTO
TUESDAY NIGHT ACROSS PARTS OF EASTERN WISCONSIN...UPPER AND LOWER
MICHIGAN...AND PARTS OF NORTHEASTERN ILLINOIS...NORTHERN
INDIANA...NORTHWESTERN OHIO...

...SUMMARY...
Strong thunderstorms may impact parts of the upper Great Lakes
region Tuesday into Tuesday evening, accompanied by at least some
risk for severe wind and hail.

...Discussion...
The westerlies will remain confined to the northern mid-latitudes,
but models suggest that flow near/north of the Canadian/U.S. border
will trend a bit more zonal, as an initially prominent mid-level
high centered offshore of the Pacific Northwest continues to become
suppressed west-southwestward across the northeastern Pacific.
Within this regime, models continue to indicate the consolidation of
several short wave perturbations into larger-scale mid-level
troughing across Ontario into Quebec by late Tuesday night,
accompanied by significant surface cyclogenesis across far northern
Ontario into the southern Hudson/James Bay vicinity.

In lower levels, a modest initial intrusion of cooler/drier air may
overspread much of the Dakotas and portions of the Upper Midwest and
adjacent Great Lakes region by 12Z Tuesday, before slowly advancing
east-southeastward across the upper Great Lakes and eventually being
overtaken by a more substantive cool intrusion later Tuesday through
Tuesday night.  Complicating matters with respect to
destabilization, guidance continues to indicate that the lead front
will be preceded by a stalled lingering remnant frontal zone, which
may still be reinforced by convective precipitation/outflow across
parts of southern Lower Michigan and northern Illinois, into the
lower Missouri Valley and the southern Great Plains, at the outset
of the period.

...Upper Great Lakes...
Short wave developments within the evolving pattern remain unclear
due to spread within the various model output, but guidance
continues to indicate that one notable short wave impulse will
continue digging across parts of the Upper Midwest and adjacent
Great Lakes region during the day Tuesday, before pivoting across
and east-northeast of Lake Huron/Georgian Bay.  It appears that this
may be preceded by one or two convectively generated perturbations
which will become increasing sheared while merging into the
southwesterly cyclonic mid-level regime overspreading the Great
Lakes vicinity.

As this occurs, various model output (notably including calibrated
thunderstorm probability guidance from the High Resolution Ensemble
system) suggests that destabilization and forcing for ascent will
become supportive of a couple of corridors of increasing
thunderstorm development by late Tuesday afternoon.  These generally
coincide with the stalled remnant frontal zone across parts of
southeastern Lower Michigan into northeastern Illinois and the
initial cold front advancing across Upper Michigan/eastern
Wisconsin.

Along the southern boundary, which may strengthen some with
differential surface heating, mid-level thermodynamic profiles may
be relatively warm with weak lapse rates.  However, it still appears
that seasonably high boundary-layer moisture content, coupled with
daytime heating, will contribute to sizable CAPE.  Coinciding with a
belt of strengthening southwesterly lower/mid-level flow (including
30-40 kt in the 700-500 mb layer), the environment may become
conducive to organized convection, including evolving supercell
structures and upscale growing clusters with potential to produce
strong to severe wind gusts by late Tuesday afternoon into evening.

Across Upper Michigan into eastern Wisconsin, pre-frontal
destabilization remains more unclear due to the possible influence
of convective cloud cover and/or outflow.  But, with breaks in cloud
cover allowing for a narrow corridor of heating, cooling aloft and
strengthening shear along the frontal zone might contribute to an
environment conducive to strong thunderstorm development with
potential to produce marginally severe hail and localized strong
surface gusts late Tuesday afternoon.

...Southwest...
Forecast deep-layer mean wind fields are generally light, but
include a west-northwesterly upslope component to the low-level flow
beneath north-northeasterly mid-level flow.  This may aid
propagation of convection off the higher terrain of southwestern New
Mexico and southeastern Arizona, into lower elevations with a warm
and deeply mixed boundary layer characterized by modest potential
instability by late Tuesday afternoon.  As this occurs, a few strong
to severe downbursts and gusts along strengthening outflows appears
possible.

..Kerr.. 08/11/2025

$$