Subj : DAY1SVR: Day 1 Convective Outlook
To   : wx-storm@lists.illinois.edu
From : COD Weather Processor
Date : Mon Jul 28 2025 05:58:25

ACUS01 KWNS 280557
SWODY1
SPC AC 280555

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1255 AM CDT Mon Jul 28 2025

Valid 281200Z - 291200Z

....THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS FOR PORTIONS OF
EASTERN SOUTH DAKOTA INTO FAR WESTERN MINNESOTA...

....SUMMARY...
A derecho is likely across portions of the northern Plains, with
several gusts expected to exceed 75 mph. A few line-embedded
tornadoes and instances of severe hail may also occur. Isolated to
potentially scattered storms may product severe wind and hail over
portions of Montana.

....Synopsis...
An upper ridge will dominate the upper-pattern across the CONUS
today, with multiple mid-level perturbations poised to crest the
upper ridge through the period. The first impulse is expected to
trigger a cluster of thunderstorms that will rapidly grow upscale
into an intense MCS and produce a widespread severe wind swath with
embedded 75+ mph gusts across portions of the northern Plains. A
second mid-level impulse will traverse the northern Rockies, amid
favorable wind shear and buoyancy, to support a severe threat.

....Northern Plains...
Multicellular convection is expected to initiate and grow upscale
into an MCS across the northern High Plains at some point in the
morning to early afternoon time frame. As the MCS propagates
eastward across the Dakotas, and matures with a deepening cold pool,
it will encounter an elongated corridor of extreme buoyancy, where
mid 70s surface dewpoints beneath 9+ C/km mid-level lapse rates will
boost MLCAPE to well over 5000 J/kg. Guidance consensus shows 50+
kts of effective bulk shear coinciding this extreme instability
axis, with some hodographs depicting modest low-level
size/curvature. This buoyancy/shear profile should support an
intense MCS capable of producing a derecho, with widespread severe
winds expected, and several 75+ mph gusts likely. Given anticipated
strong low-level shear (e.g. 300+ m2/s2 effective SRH), a few
tornadoes may occur with the stronger, more persistent leading-line
system-scale mesovortices.

Some CAM guidance differs slightly on the position of the
derecho-producing MCS, including the possibility of a secondary bow
echo developing closer to the SD/NE border. The track of the parent
bow echo will be largely dependent on where the baroclinic boundary
from an MCS (ongoing along the MN/IA border) sets up with daytime
airmass modification. Therefore, some adjustments in the position of
probabilities may be needed in future outlooks. Regardless of the
aforementioned details, a higher-end severe wind event should unfold
across the northern Plains, especially over central into eastern SD.

....Montana...
Scattered thunderstorms will develop by afternoon peak heating in a
post-frontal airmass. Despite a drier low-level airmass, 8-9 C/km
troposheric lapse rates amid straight and elongated hodographs
(effective bulk/speed shear near 50 kts) will support multicells and
supercell structures capable of severe wind and hail.

...Squitieri/Dean.. 07/28/2025

$$

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