Source: US National Oceanic and Atmospheric Administration
SPC AC 210502
Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1202 AM CDT Mon Jul 21 2025
Valid 221200Z – 231200Z
…THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS TUESDAY INTO
TUESDAY NIGHT ACROSS PARTS OF SOUTHEASTERN NORTH
DAKOTA…NORTHEASTERN SOUTH DAKOTA…MUCH OF CENTRAL MINNESOTA…AND
ADJACENT NORTHWESTERN WISCONSIN…
…SUMMARY…
Scattered strong thunderstorm development may impact parts of the
northern Rockies and Great Plains into Upper Midwest Tuesday into
Tuesday night, posing at least some risk for severe weather.
…Discussion…
Models continue to indicate that an increasingly prominent mid-level
high will evolve across parts of the middle Mississippi/lower Ohio
Valley region during this period, within persistent ridging
encompassing much of the interior U.S. Modest, anticyclonic flow
around the northern periphery of the broader ridging is likely to be
maintained across the northern Great Basin through the Dakotas and
Upper Midwest, Great Lakes and adjacent portions of Ontario and
Quebec. It appears that mid-level ridging will also remain prominent
across much of the northeastern Pacific, though it may begin to
undergo some suppression.
Between the ridging, weak mid-level troughing is likely to linger
across the Pacific coast vicinity. As one much more significant
short wave trough and embedded cyclone in higher latitudes progress
slowly toward the Hudson Bay vicinity, it still appears that one
smaller-scale short wave perturbation emerging from the Pacific
Northwest may accelerate across and northeast of the mountains of
western Montana. However, models now generally indicate that a
slightly stronger perturbation may linger to the southwest, across
the northern intermountain region.
In lower levels, a cold front trailing the higher latitude cyclone
is forecast to advance across the international border into the
northern Great Plains late Tuesday into Tuesday night. It appears
another cold front may make further progress southward though the
southern Atlantic coast vicinity, well south of the mid-latitude
westerlies.
…Northern Rockies/Great Plains into Upper Midwest…
Beneath the warm mid-level ridging, in the wake of a prior frontal
passage, seasonably moist boundary layer air is forecast to surge
from the middle Missouri Valley and adjacent Great Plains toward the
Upper Midwest/adjacent Great Lakes, in advance of the front
approaching from the Canadian Prairies. This moisture appears
likely to once again contribute to moderate to large potential
instability within modestly deep pre-frontal surface troughing,
aided by insolation and the presence of generally steep tropospheric
lapse rates, though with warm, capping layers aloft.
Near the southern periphery of a belt of strengthening southwesterly
mid-level flow, it still appears that this environment will become
at least conditionally supportive of organized convective
development. However, south of the international border, forcing
for ascent to support convective development is likely to be mostly
tied to subtle perturbations progressing around the northern
periphery of the mid-level ridging, which remain uncertain at this
time.
There does appear a general consensus that stronger boundary-layer
heating, within the lee surface troughing, will occur roughly in a
narrow corridor from the western South Dakota/Nebraska state border
vicinity through northeastern South Dakota/adjacent west central
Minnesota by late afternoon, with a zone of strengthening
differential heating extending east-southeastward across central
Minnesota. It is possible that the boundary intersection could
become a focus for isolated late afternoon supercell development,
with warm advection along the zone of differential surface heating,
aided by a strengthening southerly low-level jet impinging on it,
contributing to an upscale growing cluster of storms Tuesday
evening. However, due to the warm and capping air aloft, and
possible weak upper support for ascent, this remains rather
uncertain.
With a more substantive short wave perturbation lingering back
across the northern intermountain region, models suggest that more
substantive destabilization is possible on moistening low-level
northeasterly to easterly flow into the higher terrain of south
central through southeastern Montana. Aided by the pronounced
veering of winds from near surface to mid-levels, it appears that
deep-layer shear may become conducive to isolated to widely
scattered supercell development late Tuesday afternoon and evening.
…South Carolina into Georgia…
Although lapse rates may be modest, high moisture content
along/ahead of the slowly southwestward advancing cold front may
support the development of modest CAPE and contribute to an
environment conducive to convection capable of producing a few
strong downbursts. It is also possible that unsaturated layers may
be sufficient to contribute to broader cold pools accompanied by
potentially damaging winds along the gust fronts.
..Kerr.. 07/21/2025
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