SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0742 AM CDT Thu Aug 25 2022
Valid 251300Z - 261200Z
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS
OF WESTERN MONTANA AND THE NORTH-CENTRAL HIGH PLAINS...
...SUMMARY...
Isolated severe hail and thunderstorm gusts are possible across
portions of western Montana and the north-central High Plains today.
...Synopsis...
In mid/upper levels, mean troughing will remain over the eastern
CONUS through the period, anchored by a large cyclone centered over
southern Baffin Island. Meanwhile, the western mean ridge will
deamplify somewhat, as progressive troughing shifts eastward and
southeastward from the Gulf of Alaska and southern AK. This will
contribute to eastward movement of a long-lived, initially cut-off
cyclone over the ID Panhandle and vicinity. By 00Z, the associated
500-mb low should reach northwestern MT, then move slowly east-
northeastward to southern AB overnight. To its southeast, a small
but prominent shortwave trough is evident in moisture-channel
imagery over western WY. This feature should move east-
northeastward to eastern WY and extreme southeastern MT by 00Z,
amidst strongly difluent mid/upper flow. By 12Z, the trough should
become more negatively tilted and reach the western Dakotas.
The 11Z surface analysis showed a cold front across northern New
England to eastern Lake Ontario, becoming wavy/quasistationary
westward to a low over west-central WI. The front became cold again
southwestward over northwestern NO, then nearly stationary over
southwestern NE, northeastern CO and south-central WY. The marginal
severe-thunderstorm threat areas discussed below should remain north
of the frontal zone through the period. The front will remain
well-displaced from the very rich/tropical moisture plume over the
Southeast, where scattered disorganized thunderstorms are expected
today amidst weak deep shear.
...Western MT...
Damaging to locally severe thunderstorm winds and isolated large
hail are possible, mainly this afternoon into early evening.
Clouds/precip now apparent in satellite and radar imagery over much
of this area should break up from west-east through the day, as
minor midlevel drying apparent in moisture-channel imagery spreads
over the area. Scattered thunderstorms are forecast to develop
predominantly over the mountains this afternoon, where diabatic
surface heating of elevated terrain and large-scale lift/cooling in
midlevels will combine to destabilize the troposphere soonest. With
favorable low-level moisture remaining in place (e.g., surface
dewpoints upper 40s to mid 50s F in lower elevations), despite
boundary-layer mixing, MLCAPE around 1000-1500 J/kg should be
common, with values near 2000 J/kg on the adjoining High Plains.
Vertical shear will be modest overall, because of a lack of stronger
low/middle-level flow. However, the wind profile should be
characterized by robust veering with height, contributing to pockets
of 30-40-kt effective-shear magnitudes. This should support a few
organized multicell clusters, and marginal/short-lived supercell
structures cannot be ruled out.
...North-central High Plains...
Widely scattered to scattered thunderstorms should develop this
afternoon over the Bighorns and northwestern parts of the Black
Hills today, moving northeastward across adjoining portions of
northeastern WY, extreme southeastern MT and western SD. Isolated
severe hail/gusts are possible.
Activity should commence as the vorticity max with the western WY
perturbation moves across the Bighorns and toward the area between
SHR-GCC this afternoon. Its preceding field of large-scale lift/
cooling in midlevels should time well to optimize deep-layer
destabilization, while overlapping afternoon diabatic surface
heating. Associated steep low/middle-level lapse rates (around 7-8
deg C/km) will overlap the western part of a moist axis extending
east-southeastward to eastern NE. Accompanying surface dewpoints in
the 50s to near 60 F will support 1000-1500 J/kg MLCAPE. Veering
low-level winds with height, beneath a channel of 30-35-kt
southwesterlies in the 650-700-mb layer, will yield curved
hodographs in the first few km above ground level, indicating some
potential for storm-scale rotation. However, that will be tempered
by weaker mid/upper-level winds near the trough itself, with the
dominant mode being multicellular and clustered. Convection should
diminish late this evening over SD as it encounters a stabilizing
boundary layer.
..Edwards/Jewell.. 08/25/2022
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