729 FXUS63 KGRR 201020 AFDGRR Area Forecast Discussion National Weather Service Grand Rapids MI 620 AM EDT Mon Jul 20 2026 .KEY MESSAGES... - Thunderstorms Tonight With Damaging Winds Possible - Dry and Cooler Mid to Late Week && .DISCUSSION... Issued at 237 AM EDT Mon Jul 20 2026 - Thunderstorms Tonight With Damaging Winds Possible Primary concern with this forecast package is for the potential of severe thunderstorms tonight. A cold front looks to trigger storms across the upper midwest this afternoon which progresses eastward Monday evening. Questions remain as to the strength of any convection, with timing favored to be late this evening into the early overnight hours. Evaluation of forecast profiles suggests the potential for an MCS on the nose of a 40 kt LLJ, with strong low-level and deep layer shear. This lends itself to a primary threat of damaging winds, with large hail in any discrete convection given steep mid-level lapse rates. Instability looks to be sufficient as well (albeit waning with eastward extent), so even with uncertainties as to convective strength, the environment warrants the Slight (Level 2/5) risk in place across most of the CWA. The threat ends Tuesday morning as the cold front exits the CWA. - Dry and Cooler Mid to Late Week Tuesday through late week will feature a relatively stable pattern with persistent upper-level troughing and surface ridging across the area. Combined with the dry airmass, dry conditons are favored at this point. Look for highs in the 70s mid-week climbing back into the low 80s by the weekend. && .AVIATION /12Z TAFS THROUGH 12Z TUESDAY/... Issued at 620 AM EDT Mon Jul 20 2026 MVFR conditions due to smoke (IFR at GRR) will continue for the next few hours before improving late morning as mixing increases. Gusty winds to around 20 knots then are expected through the evening. Showers and storms over Wisconsin then arrive after 00z tonight with MVFR and lower conditions and gusty winds possible. Given low confidence in coverage and strength of storms, have maintained PROB30s with this package. Post storms, low ceilings are then forecast to develop with MVFR conditions likely. && .MARINE... Issued at 237 AM EDT Mon Jul 20 2026 Winds and waves increase this morning ahead of a cold front with 3-6 foot waves building by this afternoon. Winds veer northwesterly Tuesday behind a cold front with waves to 5-8 feet. Winds of 20-30 knots are expected with a 40% chance of gale force gusts Tuesday evening. Hazardous winds and waves continue into Wednesday evening. Will hoist a Small Craft Advisory and Beach Hazards Statement for this multi day hazardous wind and wave event. In addition, thunderstorms, the strongest of which may have winds in excess of 34 knots, are possible this evening across the nearshore and open waters. && .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 from 11 AM EDT this morning through Wednesday evening for MIZ037-043-050-056-064-071. MARINE...Small Craft Advisory from 11 AM this morning to 11 PM EDT Wednesday for LMZ844>849. && $$ DISCUSSION...Thomas AVIATION...Thomas MARINE...Thomas

NWS GRR Office Area Forecast Discussion