358 FXUS63 KGRR 121925 AFDGRR Area Forecast Discussion National Weather Service Grand Rapids MI 325 PM EDT Sun Apr 12 2026 .KEY MESSAGES... - Excessive rainfall threat this week - Severe thunderstorms possible Monday night through Tuesday night && .DISCUSSION... Issued at 325 PM EDT Sun Apr 12 2026 - Excessive rainfall threat this week Multiple chances for showers and thunderstorms tonight, Monday night, Tuesday night, and beyond will lead to an increasing flood threat after what has already been an abnormally wet spring since the beginning of March. Rivers have come down from their crests last week but will be prone to go back up again as thunderstorms dump rain onto fairly moist soil. The usual caveats of convective hit-or-miss rain swaths are in play, but any well developed storms that repeat over the same areas will be in an environment with PW values 1.25 to 1.5 inches, near the climatological maximum this time of year, so rainfall production should be efficient. The river forecasts issued Sunday morning contain forecast rainfall through Monday morning. River forecasts are susceptible to change tomorrow once tonight`s actual rainfall is counted and Monday night`s thunderstorm rainfall enters the forecast. An areal flood watch covers portions of central Michigan (and to the north) through Wednesday morning, but could be extended south in the next day or two as the thunderstorm threat eventually migrates that direction. - Severe thunderstorms possible Monday night through Tuesday night This evening... a well mixed warm sector has moved into southern Michigan up to Muskegon-Alma. A convectively enhanced shortwave ejecting out of the south-central Plains today will bring another potential for showers and isolated storms this evening as low- level wind fields increase ahead of the wave. The relatively dry air beneath the showers may facilitate mixing down gusts over 40 or 50 mph in some locations into tonight, as multiple runs of the HRRR have been depicting. Monday evening... Thunderstorms are likely to develop in the vicinity of a frontal wave and surface low over Wisconsin and Michigan during the evening and overnight. CAMs show a variety of solutions regarding if and where storms develop over the area on the warm advection side Monday evening, so confidence is on the lower side for now. If storms develop, elevated supercells with wind and hail hazards are possible. A squall line late in the night may then arrive in west-central Michigan with a wind hazard. Tuesday... a lingering tail of convection during part of the morning can`t be ruled out, but much of the day is likely to be rain-free before the next shortwave arrives in the evening or night. A plume of 1000-2000 J/kg of CAPE will be stretched over much of the Midwest including Michigan, with sfc-500 mb shear around 40 knots, so severe weather hazards are again possible. && .AVIATION /18Z TAFS THROUGH 18Z MONDAY/... Issued at 148 PM EDT Sun Apr 12 2026 Largely VFR conditions are expected until later tonight when rain and thunderstorms expand over the entire area bringing a period of MVFR ceilings with MKG having the best chance for IFR to LIFR ceilings into Monday morning. Southwest winds will be breezy this afternoon into tonight with the potential for some stronger gusts toward 40 knots mixing down later this afternoon into the early evening. Already observing these gusts off of the lake boundary at BIV and MKG. Rain exits the area between 6-12Z Monday with dry conditions through the remainder of the TAF forecast. && .MARINE... Issued at 325 PM EDT Sun Apr 12 2026 Hazardous winds and waves for small craft continue through much of Monday, followed by a slight lull, then reinvigorating again Monday night into Tuesday morning. So far today, the cool and stable marine layer has prevented a lot of gale force winds aloft from mixing down (there have been 40 mph wind gusts crashing to the ground a couple miles inland). Some southwesterly gales are possible late this evening, as have been depicted by the HRRR, as showers and scattered thunderstorms move over the lake. Additional thunderstorm hazards over the lake are possible between Monday evening and Tuesday night. && .GRR WATCHES/WARNINGS/ADVISORIES... MI...Flood Watch through Wednesday morning for MIZ037>040-043>045. MARINE...Small Craft Advisory until 4 PM EDT Monday for LMZ844>849. && $$ DISCUSSION...CAS AVIATION...RAH MARINE...CAS

NWS GRR Office Area Forecast Discussion


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