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Wednesday, October 20, 2021

SPC Oct 20, 2021 1300 UTC Day 1 Convective Outlook

SPC 1300Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0755 AM CDT Wed Oct 20 2021 Valid 201300Z - 211200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS THE UPPER MIDWEST... ...SUMMARY... Thunderstorms with isolated severe hail are possible today across northern Iowa and southern Minnesota, primarily this morning into early/mid-afternoon. ...Midwest including northern Iowa/southern Minnesota... Immediately ahead of an eastward-moving upper low, early morning convection has exhibited a modest uptick in coverage and intensity in the pre-dawn hours across far northeast Nebraska and northwest Iowa into far southwest Minnesota. For additional short-term details, see Mesoscale Discussion 1877. Steep mid-level lapse rates (12Z Omaha observed sounding) and sufficient elevated instability (upwards of 1000 J/kg MUCAPE) may support some storms capable of hail today as this activity spreads northeastward across northern Iowa/southern Minnesota. Farther south across the middle Mississippi Valley, substantial convective inhibition will likely inhibit surface-based storms along and ahead of the cold front, although moistening near 850 mb may support some slightly elevated convection. The potential for severe-caliber hail/wind is expected to remain very limited today. ...Lower Michigan and far northern Indiana/northwest Ohio... Thunderstorms are expected to develop this evening into the overnight, particularly across Lower Michigan. This convection will be rooted atop a stable boundary, with modest elevated instability (MUCAPE generally 500-750 J/kg) supportive of thunderstorms. This could include some potential for hail, but current thinking is that the risk for severe-caliber hailstones should remain low given the modest overall buoyancy. ...Middle Gulf Coast... Relatively rich low-level moisture will stream northward into portions of Louisiana and southern Mississippi/southwest Alabama 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 mid-level lapse rates and generally modest low/mid-level flow should tend to limit the severe risk in this area, although a few instances of small hail and gusty winds cannot be ruled out. ..Guyer/Leitman.. 10/20/2021 Read more LIVE: spc.noaa.gov