000
ACUS01 KWNS 280550
SWODY1
SPC AC 280549
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1249 AM CDT Wed Jul 28 2021
Valid 281200Z - 291200Z
...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS FROM EXTREME
EAST-CENTRAL MINNESOTA INTO SOUTHEAST WISCONSIN THIS AFTERNOON INTO
TONIGHT...
...SUMMARY...
Severe thunderstorms, capable of widespread damaging wind gusts,
very large hail and several tornadoes, are forecast across portions
of Minnesota and Wisconsin late this afternoon into tonight.
Isolated strong storms also are possible across parts of the
Mid-Atlantic and may produce locally damaging wind gusts.
...Upper Midwest...
A favorable pattern will exist over MN/WI toward Lake Michigan
today, supporting intense severe thunderstorms and possible derecho
development later this afternoon/evening. As a result, a Moderate
risk has been included from extreme east-central MN into southeast
WI with the initial Day 1 Outlook.
An upper shortwave impulse is forecast to migrate atop the Plains
upper ridge and amplify as it traverses the Upper Midwest/Great
Lakes region this afternoon into tonight. In response to midlevel
height falls, and low level cyclogenesis, 850-700 mb flow is
forecast to increase to 45-70 kt. At the surface, a reservoir of
substantial moisture is in place, with dewpoints from the mid 60s to
mid 70s F. While cloudiness and some early morning elevated
convection are possible across parts of MN/WI in a strong warm
advection regime, steepening midlevel lapse rates and at least
pockets of strong heating will result in strong to extreme
instability by afternoon. A surface low is forecast to shift
east/southeast from eastern ND into southeast MI by early Thursday
morning, sweeping a cold front southeast across the region. The cold
front is forecast to extend from northeast MN into northern SD late
this afternoon, while a warm front extends southeastward from
northeast MN into central Lower MI. Capping will limit convection
initially, however increasing large scale ascent associated with the
upper shortwave impulse as well as frontal forcing/low level
convergence should overcome inhibition.
Any initial semi-discrete convection is likely to be short-lived,
with rather fast upscale development anticipated. Nevertheless,
strong vertical shear and enlarged low-level hodographs within an
extremely unstable airmass will support supercell structures (both
initially and embedded within developing bow echo/derecho). Very
large hail (greater than 2 inches in diameter) will be possible
early in convective evolution. As upscale development occurs, a
transition toward intense, damaging wind gusts is expected, with a
corridor of 65+ kt wind gusts possible. Tornadoes also are possible
both early on near the triple point/warm front and once an organized
bow develops via mesovortex processes.
...Mid-Atlantic Vicinity...
Modest deep-layer northwesterly flow will be in place today as an
upper trough pivots eastward off the Atlantic coast. A very moist
airmass is in place with dewpoints generally from the upper 60s to
mid 70s F. Strong heating will result in strong destabilization
during the afternoon. Nebulous large-scale ascent is forecast, and
should limit coverage, with isolated thunderstorm development
expected during the afternoon/evening. While low-level winds will be
light, southerly flow beneath northwesterly winds aloft will result
in moderate effective shear magnitudes, aiding in at least briefly
organized cells/clusters. Steep low-level lapse rates and high PW
values will support isolated damaging gusts through early evening.
..Leitman/Bentley.. 07/28/2021
$$
