SPC 1200Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1233 AM CDT Tue Oct 21 2025 Valid 211200Z - 221200Z ...NO SEVERE THUNDERSTORM AREAS FORECAST... ...SUMMARY... Isolated thunderstorms are possible over the Great Lakes, South Florida, and the northwest Gulf Coast regions. Organized severe threat appears low today. ...Discussion... Strong upper trough will dig into the central Great Lakes region by late afternoon as 80+kt 500mb speed max translates across southern IL into central KY. 180m 12hr height falls will overspread much of the Great Lakes region with substantial falls expected as far south as the TN Valley. Latest model guidance suggests notable boundary-layer heating will occur across southern IL/IN into southern Lower MI by mid day, just ahead of a surging cold front. This will contribute to steep 0-3km lapse rates and weak instability as convective temperatures are breached by 19z ahead of the wind shift. Forecast soundings support this with a few hundred J/kg SBCAPE by early afternoon as 500mb temperatures cool below -24C. Isolated thunderstorms should develop within a strongly forced environment then spread downstream ahead of the progressive trough. With surface dew points expected to mix into the upper 30s there is some concern this activity could generate gusty winds; however, mean wind in the boundary layer should remain around 30kt and this may prove too weak to warrant more than sub-severe mixing/downdrafts. Isolated thunderstorms, a few possibly strong, are also expected along the trailing boundary across the lower MS Valley. While large-scale forcing will be limited at lower latitudes, instability will be considerably greater, though deep-layer shear will not be strong. Current thinking is convection should remain sub severe with the possibility for gusty winds. Elsewhere - scattered convection should once again develop across the southern FL Peninsula today, and perhaps across southern New England very late in the period as LLJ strengthens into this part of the country. ..Darrow/Moore.. 10/21/2025
