854 FXUS63 KGRR 202356 AFDGRR Area Forecast Discussion National Weather Service Grand Rapids MI Issued by National Weather Service Topeka KS 756 PM EDT Mon Jul 20 2026 .KEY MESSAGES... - Storms Tonight - Upper Troughing Continues through next week && .DISCUSSION... Issued at 258 PM EDT Mon Jul 20 2026 - Storms Tonight A vigorous upper shortwave trough is phasing with troughing over eastern Canada, while a deepening surface low north of Lake Superior drives a cold front into a moist and unstable airmass across Wisconsin. The convective allowing models show a line of storms initiating in that area this afternoon then moving southeast across Lake Michigan and into western Lower Michigan this evening. They also show a general weakening trend as instability decreases this evening, although shear profiles remain impressive. The main threat appears to be downburst winds associated with bowing line segments along with isolated large hail in the strongest, most persistent updrafts. The threat should be ending Tuesday morning as the cold front pushes through. - Upper Troughing Continues through next week The heat and humidity of last week looks to be gone for a while with all the long range models showing a persistent pattern of ridging across the SW CONUS and troughing across the NE CONUS. This will keep the Great Lakes region in a northwest flow regime for the next week at least, with occasional cold front passages that could focus convection. The next one appears to be next weekend. Overall, a cooler and drier than normal weather regime for the rest of the month is expected. && .AVIATION /00Z TAFS THROUGH 00Z WEDNESDAY/... Issued at 756 PM EDT Mon Jul 20 2026 MVFR visibility with smoke may be returning this evening with boundary layer mixing settling down, and there are 4SM to 8SM vis reports to the southwest in the Chicago to South Bend area, where our air will be coming from. Smoke is likely to be less of a concern on Tuesday. Based on latest forecast trends including guidance from the WoFS, HRRR, and RRFS, the thunderstorm threat is being pushed later into the night, and remains low-confidence enough to keep TS as a prob30 for now. So far the convection in northeast Wisconsin has been fairly limited in coverage and intensity, perhaps due to the effects of two rounds of thunderstorms today in eastern Iowa and northern Illinois. Instability aloft and midlevel moisture is nonetheless expected to increase tonight and may still support scattered thunderstorm development. && .MARINE... Issued at 258 PM EDT Mon Jul 20 2026 SCA continues through Wednesday evening. We may need a Gale Warning for strong north winds Tuesday night as the pressure gradient tightens between the departing low and the advancing high. Conditions will quickly improve on Thursday as the high builds in. && .GRR WATCHES/WARNINGS/ADVISORIES... MI...Air Quality Alert until midnight EDT tonight for MIZ037>040- 043>046-050>052-056>059-064>067-071>074. Beach Hazards Statement through Wednesday evening for MIZ037-043- 050-056-064-071. MARINE...Small Craft Advisory until 11 PM EDT Wednesday for LMZ844>849. && $$ DISCUSSION...Ostuno AVIATION...CAS MARINE...Ostuno

NWS GRR Office Area Forecast Discussion