570 FXUS63 KGRR 211036 AFDGRR Area Forecast Discussion National Weather Service Grand Rapids MI 636 AM EDT Tue Apr 21 2026 .KEY MESSAGES... - Breezy and milder, afternoon chance of showers or thunderstorm - Chance of thunderstorms Friday - Another rainy weather system possible early next week && .DISCUSSION... Issued at 333 AM EDT Tue Apr 21 2026 - Breezy and milder, afternoon chance of showers or thunderstorm A rapid return of warmer 925-850 mb air is occurring with 30-40 knot WSW winds early this morning. Gusts 20 to 30 mph should develop mid- late morning with daytime heating and a deepening mixed layer. Each of the 00Z models within the HREF and the experimental MPAS models have trended earlier and farther north with the development of showers, so have introduced slight chance of measurable precipitation to US-10 early afternoon. The shower chances move southward and increase toward I-96 and I-94 by later afternoon to early evening. These showers appear to develop via increasing moisture brought by 800-700 mb WSW winds underneath cooler 700-500 mb temperatures in a WNW flow pattern. A subtle surface trough and 500 mb shortwave trough will also be present. HREF mean MUCAPE builds toward 100-250 J/kg south of Holland, Grand Rapids, and Lansing, with convection potentially reaching -20 C, making some thunderstorm cells possible. Any better- developed convection could initiate downdrafts into a pretty well mixed boundary layer, with HRRR suggesting DCAPE at least around 500 J/kg. With background low-level flow around 25 knots, isolated downdrafts and potential cold pool development could bring gusts over 40 mph near I-94 during the evening. - Chance of thunderstorms Friday Warm temperatures Wednesday to Friday are favored while Michigan is under the narrow ridge portion of a developing omega block. Ensembles have been fairly consistent with instability building into Friday with a cold front or frontal triple point moving over Michigan, connected to the anchored Northern Plains low (the left foot of the omega block). The amount of instability is still somewhat in question, though should be on the modest side as cooler upper level air should remain west closer to the upper level low. Effective shear may be fairly weak also, making the severe weather threat marginal at most. Spotty rain amounts under thunderstorms may be around 1 inch, but overall the median QPF for the area is about 0.25 to 0.5 inches. - Another rainy weather system possible early next week (Issued at 245 PM EDT Mon Apr 20 2026) We do not see much cooler air move in behind the front for next weekend. Again this is because we really do not see the upper low make it to our area. It stays to our west, and we will be able to clear out with just slightly cooler temperatures. Instead of the 70s on Friday, highs will be in the 60s over the weekend. We will see additional short wave energy rotate in on under the upper low, and then head NE toward the area by next Monday. This wave will have a chance at drawing additional moisture from the Gulf and into the system. This system being seven days out has time to adjust. && .AVIATION /12Z TAFS THROUGH 12Z WEDNESDAY/... Issued at 636 AM EDT Tue Apr 21 2026 Scattered to broken VFR cloud layers are expected much of today. LLWS is favored until 13 Z, then trending to gusty surface winds with daytime surface heating. Probability of rain showers appears a little higher this afternoon and evening, but with relatively low confidence on time of arrival within a 3 hour PROB30 window and low confidence on the intensity of rainfall affecting visibility. There is a slight chance of a thunderstorm among the showers, for now not 30 percent, but relatively higher in more southwestern areas. If convective cells are even somewhat robust as they move through aerodromes, they may produce local downdraft-induced LLWS and higher surface gusts, which the majority of the runs of the HRRR overnight have been producing. && .MARINE... Issued at 333 AM EDT Tue Apr 21 2026 Strengthening south winds early this morning are building waves around 2 to 4 feet around South Haven and 3 to 6 feet north of Holland. Winds begin to relax during the afternoon and the wave field will diminish into the evening and night. There is a chance of a thunderstorm over Lake Michigan this afternoon into early evening. The next chance of small craft advisory conditions is north of Whitehall Thursday afternoon. && .HYDROLOGY... Issued at 245 PM EDT Mon Apr 20 2026 We are seeing most of the rivers now in the falling stage of this last event. The crest Monday afternoon has now just passed Grand Rapids, and is heading toward Lake Michigan. The Muskegon and most of the Grand basins should remain mostly dry through Friday now. This will help to allow the rivers to recover a bit after the recent frequent heavy rains. && .GRR WATCHES/WARNINGS/ADVISORIES... MI...None. MARINE...Small Craft Advisory until 5 PM EDT this afternoon for LMZ844>849. && $$ DISCUSSION...CAS/NJJ AVIATION...CAS MARINE...CAS HYDROLOGY...NJJ

NWS GRR Office Area Forecast Discussion