SPC 1200Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1203 AM CDT Fri Jun 27 2025 Valid 271200Z - 281200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS LATE THE AFTERNOON INTO EARLY TONIGHT ACROSS THE NORTHERN PLAINS...AND THIS AFTERNOON/EVENING ACROSS SOUTHEAST LOWER MICHIGAN... ...SUMMARY... At least widely scattered thunderstorms are expected late this afternoon/evening across the Dakotas and Nebraska, where very large hail and severe outflow winds will be possible. There will also be a window of opportunity for tornadoes this evening across the central Dakotas. Occasional wind damage will be possible this afternoon/evening across southeast Lower Michigan. ...Northern Plains late this afternoon into early tonight... A low-amplitude midlevel trough with embedded speed maxima will move eastward from the northern Rockies to the northern High Plains through tonight. At the surface, lee cyclogenesis is expected this afternoon in the vicinity of northeast WY, while low-level moisture spreads northward across the Dakotas through this evening. The moistening will occur beneath an elevated mixed layer with midlevel lapse rates of 8.5-9.5 C/km, and daytime heating will result in MLCAPE exceeding 4000 J/kg. The elevated mixed layer will also act as a cap and surface temperatures will need to reach the mid 90s in SD and the mid-upper 80s in ND to largely eliminate convective inhibition. As such, storm timing/coverage is a bit uncertain with modest forcing for ascent. Where storms form, the steep lapse rates/extreme buoyancy and effective bulk shear in excess of 40 kt will conditionally favor supercells with very large hail (potentially greater than 3 inches in diameter). There could also be a window of opportunity for tornadoes as low-level shear increases in the evening, where storms remain discrete prior to convective inhibition increasing tonight. Otherwise, any storm clusters will have the potential to produce severe outflow winds of 60-80 mph, especially in the deeper mixed environment farther south in SD. ...Southeast Lower MI this afternoon/evening... A midlevel shortwave trough and associated weak wave cyclone now over the upper MS Valley will continue eastward over the upper Great Lakes through this evening, while a trailing cold front crosses Lower MI. Daytime heating and boundary-layer dewpoints in the low 70s in the warm sector will contribute to MLCAPE above 2000 J/kg during the afternoon across Lower MI as convective inhibition weakens. Low-level ascent along the front will support a broken band of storms by mid afternoon, and the storms will spread eastward before weakening by late evening. Though midlevel lapse rates and deep-layer vertical shear will not be strong, ~35 kt midlevel flow and steep low-level lapse rates suggest the potential for occasional wind damage with multicell clusters and/or short line segments. ...Appalachians into the Southeast this afternoon/evening... Another afternoon/evening of widely scattered thunderstorms and isolated downburst potential is expected across a broad area from the Appalachians into the Southeast/FL. Local sea breeze circulations, terrain circulations and residual outflow/differential heating zones will help focus thunderstorm development. Thermodynamic profiles will be sufficient for isolated wind damage with downbursts. ..Thompson/Lyons.. 06/27/2025