241 FXUS63 KGRR 212254 AFDGRR Area Forecast Discussion National Weather Service Grand Rapids MI 654 PM EDT Fri Aug 21 2026 .KEY MESSAGES... - Scattered showers/thunderstorms today into Saturday - Cooler and dry Sunday through Tuesday, midweek rain chances && .DISCUSSION... Issued at 314 PM EDT Fri Aug 21 2026 - Scattered showers/thunderstorms today into Saturday Scattered thunderstorm cells have developed along an inland surface trough / confluence zone east of US-131 and will move east this evening. These are ahead of diminishing convection from Wisconsin with midlevel MCVs apparent on radar. The storms are in an area of MLCAPE 500-1000 J/kg. Modeled effective shear is no more than 20 knots, though it`s possible the MCVs could boost the shear values somewhat and provide greater longevity to the cells. Small hail and wind gusts to 40 overall are possible today. Additional scattered storm cells are likely tonight, with elevated bases via 850 mb moisture, then may become more surface based as they move toward Mid Michigan later Saturday. CAPE values will generally be 500-1500 J/kg and effective shear is likely to remain no more than 20-25 knots. Lapse rates both at low and mid levels should be fairly modest during much of this activity which should also help keep severe wind/hail potential low. QPF mean is about 0.10 to 0.25, though more realistically, amounts will be more hit or miss, with a few isolated totals closer to 1 inch possible. - Cooler and dry Sunday through Tuesday, midweek rain chances Northwest flow Sunday and Monday will provide slightly cooler, drier, more comfortable air as the upper-level trough turns cutoff low over the eastern/northeastern Great Lakes region. A minor return of moisture and warmer air occurs from late Tue into Wed, with scattered thunderstorms again possible with the next frontal wave. && .AVIATION /00Z TAFS THROUGH 00Z SUNDAY/... Issued at 654 PM EDT Fri Aug 21 2026 Scattered showers and isolated thunderstorms are possible over the next several hours, though coverage is too low for TAF inclusion of any thunder threat with 10-20% impacts for terminals. Chances increase later this evening into tonight with a pre-frontal trough followed by a cold front. Given high variability in short range model guidance and questions on coverage have maintained previous PROB30s for all terminals. Winds then increase from the northwest behind the cold front with gusts to 25 knots expected. The other concern is high dewpoints favor a period of low clouds and patchy fog near daybreak, particularly in any thunderstorms. Mainly MVFR is favored at this time, but IFR cannot be ruled out in thunderstorms. && .MARINE... Issued at 314 PM EDT Fri Aug 21 2026 Ongoing Beach Hazards Statement / Small Craft Advisory for near Big and Little Sable Points through Fri evening is fairly low confidence given latest data, as south winds 10-15 knots may only build up 2 to 4 foot waves with a moderate swim risk before subsiding tonight. Scattered thunderstorms are possible over the lake tonight with a low chance for gusts to 30 knots. More likely hazards from wind, waves and currents for this weekend as north-northwest winds increase behind a cold front. There remain some differences in the models regarding the strength of winds at times, but overall, waves should build to 3 to 6 feet at most beaches later Saturday morning. Then after a slight lull Saturday evening, winds likely strengthen and waves build again Saturday night into Sunday. && .GRR WATCHES/WARNINGS/ADVISORIES... MI...Beach Hazards Statement until 8 PM EDT this evening for MIZ037- 043. Beach Hazards Statement from Saturday morning through Sunday evening for MIZ037-043-050-056-064-071. MARINE...Small Craft Advisory from 8 AM Saturday to 11 PM EDT Sunday for LMZ844>849. Small Craft Advisory until 8 PM EDT this evening for LMZ848-849. && $$ DISCUSSION...CAS AVIATION...Thomas MARINE...CAS

NWS GRR Office Area Forecast Discussion