ACUS02 KWNS 021720
SWODY2
SPC AC 021718
Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1218 PM CDT Wed Sep 02 2026
Valid 031200Z - 041200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FOR PORTIONS OF
THE NORTH DAKOTA-MINNESOTA RED RIVER VALLEY INTO THE UPPER
MIDWEST...AND IN PARTS OF THE UPPER OHIO VALLEY TO MID-ATLANTIC
COAST...
...SUMMARY...
Severe storms, accompanied by predominantly severe wind gusts and
hail (including 75+ mph gusts and hailstones greater than two
inches), are possible from the North Dakota-Minnesota Red River
Valley into the upper Midwest. At least widely scattered severe
storms capable of mainly damaging wind gusts are also possible from
the upper Ohio Valley to the Mid-Atlantic coastline.
...Synopsis...
A high-latitude omega block will persist through the forecast
period, featuring upper lows along the Pacific Northwest coast and
over QC, which bracket a mid-level ridge centered along the MS
Valley. Several smaller-scale perturbations are expected to move
through the broader-scale flow regime from the northern Plains and
the upper MS Valley through the Great Lakes into the Mid-Atlantic,
aiding in thunderstorm development in those areas. At the surface,
an area of low pressure is forecast to develop through the Dakotas
into MN, while a preceding warm front lifts north through the upper
MS Valley and upper Midwest. The boundary is expected to extend
southeast through the Great Lakes into the Mid-Atlantic, where its
location may be modified from convective outflow stemming from
thunderstorms Wednesday night into Thursday morning.
...Northern Plains into the upper Great Lakes...
A plume of steep mid-level lapse rates associated with an
elevated-mixed layer (EML) will reside atop a moist boundary layer
in the vicinity of the surface low and warm front, yielding a
strongly unstable air mass by afternoon with MLCAPE increasing to
3000-4000+ J/kg. Amidst a backdrop of general height rises, the
progression of weak disturbances through the building ridge will
promote periods of forcing for ascent. The first of these may occur
Thursday morning when elevated thunderstorms appear possible from
portions of the mid-MO Valley into upper Midwest. Given the presence
of the EML and sufficient cloud-bearing shear, isolated occurrences
of large hail may accompany the strongest storms as they spread east
into the upper Great Lakes.
In the wake of the early-day storms, residual capping associated
with the building ridge coupled with the subtle forcing for ascent
results in considerable model variability in the location of
subsequent, surface-based storm development Thursday afternoon into
night. Nonetheless, the degree of instability and up to 40-50 kt of
deep-layer shear will conditionally support higher-order storm modes
including supercells and bowing structures, both of which are
depicted by the suite of various 12Z CAMs. Hail in excess of two
inches, severe wind gusts greater than 75 mph, and a couple of
tornadoes appear possible with the most intense storms.
The 15% hail and accompanying CIG1 contours have been expanded into
the ND-MN Red River Valley and in parts of WI to account for the
large hail potential. Otherwise, the forecast will remain largely
unchanged due to the spatial uncertainty in surface-based storm
development. Once finer-scale details become more clear, an upgrade
to a level 3/Enhanced Risk may become necessary, primarily for
severe winds associated with a cold-pool-driven MCS.
...Upper Ohio Valley to the Mid-Atlantic Coast...
Similar to this morning, clusters of thunderstorms capable of
isolated occurrences of damaging wind may be ongoing at 12Z Thursday
over portions of the lower Great Lakes and upper OH Valley regions.
A subset of the morning storms may intensify by afternoon over the Delmarva/southern Mid-Atlantic with additional storms forming during
the peak of the diurnal heating cycle in the vicinity of the surface
front and/or residual outflow boundaries over the upper OH Valley.
Despite weakening mid-level lapse rates with eastward extent,
daytime heating combined with dewpoints largely in the low/mid 70s
will contribute to moderate to strong instability by afternoon along
and south of the effective surface front with MLCAPE approaching
2500-3500 J/kg. The frontal zone will coincide with a belt of 40-45
kt west-northwest mid-level flow, yielding sufficient vertical shear
for supercells and organized line segments and/or bowing structures.
The primary hazard will be damaging wind gusts, though isolated
occurrences of large hail are possible with any supercells that may
evolve.
A slight northward shift in the Marginal and Slight Risk
probabilities has been made to account for stronger instability and
an associated severe risk evident in various 12z models.
..Mead.. 09/02/2026
$$
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