ACUS02 KWNS 180557
SWODY2
SPC AC 180555

Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1255 AM CDT Wed Sep 18 2024

Valid 191200Z - 201200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS IN A PART OF THE
UPPER MIDWEST...

...SUMMARY...
Scattered severe thunderstorms are possible from mid-afternoon to
mid-evening on Thursday across central/eastern Minnesota, northern
Iowa, and western Wisconsin. A few tornadoes, isolated very large
hail, and damaging winds may occur.

...Upper Midwest...
A vertically stacked cyclone near the ND/SK/MB border area on
Thursday morning will advance northeast across MB through the
period. Mid-level west-southwesterlies will remain enhanced (35-50
kt from 700-500 mb) through the afternoon to the south-southeast of
this cyclone, before the system as a whole weakens and progresses
farther away from the Upper Midwest. At the surface, a cold front
will arc south, and progress east over MN/IA. Ahead of the front, a
confined plume of mid to upper 60s surface dew points will be
maintained, but will decrease within a drier boundary layer deeper
into WI and IL. The aforementioned flow regime will yield initially
steep mid-level lapse rates from 7-8 C/km through early afternoon.
Coupled with the rich buoyancy plume, moderate to large MLCAPE of
2000-3000 J/kg appears plausible, northeast of the Mid-MO Valley.

Guidance spread does exist with the timing and related spatial
placement of initial thunderstorm development along the front. Some
models suggest scattered convection should form during the early to
mid afternoon, while others are delayed until the late afternoon.
This may be related to the strength of the elevated mixed layer and
timing of weakened MLCIN. This uncertainty within a limited
longitudinal extent of greater threat, precludes a level 3-ENH risk.
The conditional large to very large hail threat appears favorable
amid a vertically veering wind profile with height that will support
initial supercells. While some clustering is possible owing to
frontal convergence, the bulk of severe convection may remain tied
to semi-discrete supercells given the nearly perpendicular mid-level
wind profile to the front and moderately enlarged low-level
hodograph. This should support potential for a few tornadoes as
well. Clustering should become more prominent after sunset, but the
severe threat is expected to diminish soon after dusk as activity
outpaces the nocturnally shrinking buoyancy plume.

..Grams.. 09/18/2024

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