SPC 2000Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0258 PM CDT Sun Aug 28 2022
Valid 282000Z - 291200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
MINNESOTA INTO WESTERN WISCONSIN...AND NORTHERN ILLINOIS INTO
SOUTHEASTERN WISCONSIN...
...SUMMARY...
Severe thunderstorms should affect much of Minnesota into western
Wisconsin late this afternoon into early tonight, with large to very
large hail, damaging gusts, and a couple of tornadoes possible.
Damaging winds may also occur this afternoon from northern Illinois
into southeastern Wisconsin.
...20Z Update...
Severe probabilities have been adjusted in northern IL/southeastern
WI based on the position of a band of thunderstorms associated with
an MCV. Damaging winds should continue to be the main threat with
this activity as it spreads northeastward across this region through
the rest of the afternoon. But, a brief tornado also remains
possible with embedded circulations given modestly enhanced
low-level flow noted on recent VWPs from KMKX and KLOT.
No changes have been made to the Slight Risk across MN and western
WI. Persistent cloudiness over much of northern/central MN and
elevated convection along the ND/MN border has hampered daytime
heating to some extent. Still, northeastward advection of steep
mid-level lapse rates will continue across the Upper Midwest ahead
of a pronounced upper trough over central Canada and the northern
Plains. Current expectations are for convection to develop and
strengthen late this afternoon and evening across western MN
along/ahead of a cold front. All severe hazards still appear
possible with this activity as it spreads eastward across MN and
eventually western WI this evening into the early overnight hours.
..Gleason.. 08/28/2022
.PREV DISCUSSION... /ISSUED 1130 AM CDT Sun Aug 28 2022/
...MN and vicinity this afternoon into early tonight...
A well-defined midlevel trough will move eastward from western ND as
of mid morning to northwestern MN tonight, and will be preceded by a
weakening MCV moving from eastern ND to northwestern MN today. The
primary associated surface cyclone will progress eastward across
SK/MB, as a trailing cold front/trough likewise moves eastward
across the Dakotas to the MN border by this evening. Ahead of the
front/trough, a plume of steep midlevel lapse rates of 8-9 C/km
sampled in 12z soundings will act to cap a gradually moistening
boundary layer (mid-upper 60s dewpoints northern MN to low 70s
across southern MN by afternoon), and will allow MLCAPE to increase
to near 3000 J/kg. Stratus under the cap and lingering convection
in southeastern ND will tend to slow surface heating, but ascent
along the boundary will support weakening of convective inhibition
and thunderstorm development 21-23z across west central and
northwest MN. Storms will then spread eastward through late evening
and develop southward toward central MN, with greater uncertainty
regarding storm coverage with southward extent into IA/eastern NE
this evening.
Mixed convective modes are expected along and ahead of the surface
front/trough, with a few semi-discrete cells initially, and some
upscale growth into clusters/line segments expected by this evening.
Vertical shear will become sufficient for supercells as hodographs
lengthen and clockwise curvature increases some in the low levels,
as reflected by effective bulk shear of 35-45 kt and effective SRH
of 100-200 m2/s2. The steep midlevel lapse rates/large CAPE and
sufficiently long hodographs will favor isolated very large hail
(2-2.5 inch diameter) with the more discrete supercells initially,
and the threat will transition more to damaging winds of 60-70 mph
with upscale growth of convection. A couple of tornadoes will also
be possible with the initial supercells, and with embedded
circulations in line segments.
...Northern IL/southeastern WI this afternoon/evening...
An MCV will continue to move northeastward from southeastern IA
toward southeastern WI through this evening. Midlevel lapse rates
are poor, but surface heating in cloud breaks ahead of the MCV will
contribute to moderate buoyancy (MLCAPE of 1500-2000 J/kg), and
thunderstorm development will be possible by early-mid afternoon
near the MS River. A mix of clusters and embedded rotating storms
will be possible given modest enhancement of low-level shear ahead
of the MCV (effective SRH in excess of 100 m2/s2), with the
attendant threat of isolated damaging winds and a tornado or two.
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