566 FXUS63 KGRR 021855 AFDGRR Area Forecast Discussion National Weather Service Grand Rapids MI 255 PM EDT Wed Sep 2 2026 .KEY MESSAGES... - Heat Through This Evening, Some Storms Overnight - Periodic Storms Thursday Afternoon into Late Night - Short Break Midday Friday, Then More Rain Chances && .DISCUSSION... Issued at 254 PM EDT Wed Sep 2 2026 - Heat Through This Evening, Some Storms Overnight Surface dew points have overachieved this afternoon compared to most model guidance, and as a result heat indices have nudged higher. Widespread indices of 95 to 100 are occurring and will continue to occur across southern Lower Michigan through this afternoon and evening given air temps in the upper 80s to low 90s and dew points of 70 to 75 making it feel pretty miserable in the sun. Reduce strenuous outdoor activities through the evening. In spite of favorable MLCAPE values of 1500-2500 J/kg this afternoon as well as 0-6 km shear values of 40-50 kts, some MLCIN of 100-200 J/kg is still present north of I-96 where some convection has occurred but has largely been held in check. A LLJ of 30-35 kts is present overhead our region, though it is more convergent over SE Lower Michigan toward the Thumb region. That is where scattered strong to severe storms are most likely to develop into this evening. Much of our region will be storm free through early tonight with the exception of north of M-20 and near/east of US 127. Beyond midnight tonight, the LLJ over Wisconsin increases back to 30- 35 kts and angles over Lake Michigan. Scattered showers/storms may develop especially from Muskegon to Lansing during the 05z-12z time frame. Most are not likely to reach severe levels. Model soundings do support good CAPE through the hail growth region but an inversion around 850 mb will likely (though not with 100% certainty) limit damaging wind potential. Localized downpours may lead to a few minor lowland and/or street flooding occurrences overnight. - Periodic Storms Thursday Afternoon into Late Night There are some mixed signals regarding Thursday`s daytime storm chances, leading to less confidence than desired (eg. NBM POPs are around 30%). Deep layer shear of around 40 kts will be present north of I-96. HREF mean SBCAPE values are less substantial than today but still 1000-2000 J/kg. We do have upper jet divergence moving through which could aid in the development of some organized convection during the midday to afternoon period. The low level jet weakens to 20-25 kts. There is an incoherent signal amongst HREF members as to the placement and intensity of convection tomorrow. The HRRR/RRFS do bring in some organized swaths of storms with 60+ mph gusts during the afternoon, while the other CAMs are not supportive. At this point, the SPC Marginal Risk seems to cover the spatial threat well, with some isolated/scattered instances of strong wind gusts and hail possible. After 00z, a stronger nocturnal LLJ develops to our west and angles in during the late night hours. Very high PWAT values are shown by the 12z ECMWF during this time, with 2.00-2.50" moving in. Very heavy downpours and some street flooding may develop if organized convection gets going on the nose of the LLJ Thursday night into Friday morning. Large hail also looks to be a threat with these potential storms. - Short Break Midday Friday, Then More Rain Chances A decent model signal exists for a break in the rain chances late Friday morning into the early afternoon period. Daytime heating on Friday could bring some pop-up showers and thunderstorms by evening with perhaps additional rain chances overnight along a potential surface boundary in the area. && .AVIATION /18Z TAFS THROUGH 18Z THURSDAY/... Issued at 143 PM EDT Wed Sep 2 2026 VFR conditions will continue into this evening then the chance for showers and thunderstorms increases after midnight. Some MVFR and brief IFR is possible as storms move through. Although timing remains uncertain, the greatest chances are expected to be between 06Z and 10Z. Southwest winds around 10 knots this afternoon will go light and variable overnight, except for higher winds in the vicinity of thunderstorms. && .MARINE... Issued at 245 PM EDT Wed Sep 2 2026 No hazardous winds or waves are expected for the next several days with the highest waves of up to 2 feet occurring this evening. However, mariners will need to be aware of thunderstorm chances through Saturday. && .GRR WATCHES/WARNINGS/ADVISORIES... MI...Heat Advisory until 8 PM EDT this evening for MIZ056>059-064>067- 071>074. MARINE...None. && $$ DISCUSSION...Hoving AVIATION...Ostuno MARINE...Hoving

NWS GRR Office Area Forecast Discussion