Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0247 PM CDT Tue Apr 21 2026
Valid 212000Z - 221200Z
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FOR PORTIONS OF
THE OHIO VALLEY AND THE CALIFORNIA CENTRAL VALLEY...
...SUMMARY...
Isolated strong thunderstorms are possible in the California Central
Valley, capable of gusty winds and perhaps a tornado or two. Other
strong storms are possible over parts of the Great Lakes region.
...20Z Update...
The primary changes to the outlook were to extend Category
1/Marginal Risk probabilities in CA farther north into the
Sacramento Valley, and expand severe probabilities in the Great
Lakes area to include far southwestern Lower MI. In CA, MRMS mosaic
radar imagery shows convection deepening farther north into the
Sacramento Valley, and where 19Z mesoanalysis shows well over 100
J/kg 0-3 km CAPE in place with a focused corridor of low-level
vertically oriented vorticity. As such, a low-topped supercell could
develop this afternoon or evening, which may be capable of producing
hail/wind and perhaps a brief tornado. Thunderstorms, with a history
of producing marginally severe hail over extreme southeastern WI,
are currently traversing far southern Lake MI, and have been slow to
diminish. A corridor of MUCAPE is developing over far southwestern
Lower MI as 8+ C/km mid-level lapse rates, coinciding with a
southwesterly WAA regime, overspreads the Great Lakes. Therefore,
Category 1/Marginal risk wind/hail probabilities were extended
northeastward into far southwestern Lower MI to account for
potential strong thunderstorms in this region.
..Squitieri.. 04/21/2026
.PREV DISCUSSION... /ISSUED 1051 AM CDT Tue Apr 21 2026/
...Central CA...
Morning water vapor loop shows a well-defined shortwave trough
approaching the coast of central CA. An associated mid/upper level
jet will nose into CA, with forcing for ascent in the left-front
quadrant of the jet aiding in the development of afternoon/evening
thunderstorms over parts of the Central Valley. Forecast soundings
show steep low and mid level lapse rates and sufficient CAPE for a
few robust updrafts. Vertical shear will be strong enough for
supercell structures, including a risk of a tornado or two, small
hail, and gusty winds.
...Northern IN/IL...
A fast-moving shortwave trough currently over IA/MN will track
quickly southeastward into WI/IL/IN this afternoon. Visible
satellite and radar imagery show a line of high-based thunderstorms
developing ahead of the trough over southern WI. As these storms
track into northern IL/IN, continued heating/weak destabilization
will increase the risk of gusty/locally damaging wind gusts and
small hail in the strongest cores this afternoon.
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