MIL-OSI USA: SPC Jul 26, 2021 1300 UTC Day 1 Convective Outlook

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Source: US National Oceanic and Atmospheric Administration

SPC AC 261244

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0744 AM CDT Mon Jul 26 2021

Valid 261300Z – 271200Z

…THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS THIS
AFTERNOON/EVENING ACROSS NORTHEAST MN/NORTHWEST WI…

…SUMMARY…
Supercells with very large hail near the size of baseballs and
damaging winds are expected this afternoon/evening from eastern
North Dakota into northern Minnesota, and the storms could grow into
a cluster or two while spreading southeastward tonight.

…Eastern ND/MN/northern WI/Upper MI this afternoon into tonight…

A subtle midlevel shortwave trough will crest the large-scale ridge
over the Rockies and move southeastward from SK this morning to the
upper Great Lakes by Tuesday morning. At the surface, a remnant lee
cyclone in southwest ND will develop slowly eastward across ND along
a baroclinic zone. The eastern extent of this front will shift
northward as a warm front today across MN/WI. In the wake of some
lingering, elevated convection this morning, strong surface heating
and deep mixing are expected near the front in ND and northern MN,
which will reduce convective inhibition and boost buoyancy. Though
low-level flow is expected to remain relatively weak, 40-50 kt
midlevel flow from the west-northwest will contribute to effective
bulk shear of 40-50 kt and long hodographs favoring supercells with
initial storm development this afternoon/evening from eastern ND
across northern MN.

Storms that form along the front this afternoon/evening will be
capable of producing very large hail (near baseball size), given the
supercell wind profiles with strong vertical shear aloft, steep
lapse rates, and MLCAPE of 2000-3000 J/kg. The warm surface
temperatures/steep low-level lapse rates will also favor strong
downdrafts and the potential for damaging winds, especially if/when
storms cluster and grow upscale this evening into early tonight
while spreading east-southeastward.

…VA/NC this afternoon/evening…
On the southern fringe of the midlevel trough over the Great
Lakes/Northeast, a weak surface cold front will drift southeastward
across the Appalachians. Surface heating/mixing and ascent along
the front, as well as a diffuse lee trough from VA into NC, will
support scattered thunderstorm development by early afternoon.
Effective bulk shear of 20-30 kt may support some transient cluster
or cell organization, and precipitation loading and downdraft
potential will support the threat for isolated downbursts with wind
damage this afternoon/evening.

…ME area today…
In the wake of morning convection, clearing will allow some
destabilization this afternoon across northern New England. Forcing
for ascent and storm coverage both appear somewhat nebulous for this
afternoon, and midlevel lapse rates will be rather poor. However,
storms that do form will have some potential to produce isolated
wind damage given steep low-level lapse rates and effective bulk
shear around 30 kt.

..Thompson/Gleason.. 07/26/2021

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