SPC AC 200530
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1230 AM CDT Wed Oct 20 2021
Valid 201200Z - 211200Z
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS NORTHERN
IOWA AND SOUTHERN MINNESOTA...
Thunderstorms with isolated large hail are possible today across
northern Iowa and southern Minnesota, primarily during the morning
and early afternoon.
...Portions of the Midwest/Mid MS Valley into the Great Lakes...
A mid/upper-level low is forecast to move from near the Sandhills
region of Nebraska this morning eastward toward southern Lake
Michigan by early Thursday. A surface low is forecast to gradually
deepen as it moves from eastern NE into parts of lower MI, as an
attendant cold front moves through portions of the Midwest, mid MS
Valley, and Great Lakes.
Elevated convection will likely be ongoing at the start of the
period across northeast NE/southeast SD/northwest IA, within an
environment characterized by relatively steep midlevel lapse rates
and cool temperatures aloft, MUCAPE of 500-1000 J/kg, and effective
shear of 30-40 kt. This environment will support an isolated large
hail threat, as convection spreads eastward through the day in
conjunction with the mid/upper-level low.
Further south toward the mid MS Valley, substantial MLCINH will
likely inhibit surface-based storms along and ahead of the cold
front, though moistening near the 850 mb level may support some
slightly elevated convection. While small hail or locally gusty
winds cannot be ruled out with convection along the front, the
threat for severe hail/wind appears too limited for probabilities in
Elevated convection will likely spread into lower MI late tonight
into early Thursday morning. MUCAPE of up to 500 J/kg may support a
few stronger storms, though any severe threat appears rather limited
at this time.
...North-central Gulf Coast...
Relatively rich low-level moisture will stream northward into
portions of LA and southern MS/southwest AL during the day today,
along the western periphery of a surface ridge over the Southeast
CONUS. As this occurs, moderate buoyancy will support scattered
thunderstorms near the coast this morning, with convection expected
to spread further inland with time by afternoon. While weakly
veering wind profiles may favor some organization with the strongest
storms, poor midlevel lapse rates and generally modest low/midlevel
flow may tend to limit the severe risk in this area, though small
hail and gusty winds cannot be ruled out.
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