157 FXUS63 KGRR 151120 AFDGRR Area Forecast Discussion National Weather Service Grand Rapids MI 720 AM EDT Mon Jun 15 2026 .KEY MESSAGES... - Thunderstorms Likely Tuesday, Marginal Risk for Severe - Soaking Rain and Possible Severe Storms Wednesday Afternoon-Eve && .DISCUSSION... Issued at 350 AM EDT Mon Jun 15 2026 - Thunderstorms Likely Tuesday, Marginal Risk for Severe Not much change from previous thinking as shear and sufficient instability increase during peak heating ahead of a strengthening surface low over Wisconsin on Tuesday. Despite the cooler-than- normal temperatures and dew points in the 50s, lapse rates from the low to mid levels support deep convection with CAPE around 1000 J/kg and a marginal threat for severe wind and hail depending on how much destabilization and organization can occur. The kinematics appear rather messy with models developing multiple convective cells and potentially more than one round of convection between mid day and evening, but one can find spots where 0-1 and 0-3 km bulk shear approaches 30 knots with either straight or somewhat curved hodographs, allowing for some updrafts to become mesocyclonic with a nonzero tornado threat. Some narrow swaths of rain totals greater than an inch are also possible. - Soaking Rain and Possible Severe Storms Wednesday Afternoon-Eve Definitely an unusual June pattern with a large upper low parked over south-central Canada, and a 100 knot upper level jet extending from the Pacific Northwest through the Great Lakes into the Northeast, with robust Clipper-like shortwaves rapidly progressing through the flow. The MSLP of the surface low within the ECMWF and GFS dips below 990 mb as it marches through the Upper Midwest on Wednesday, which is 4 to 5 standard deviations below normal for this time of year. There remain some differences in the track of the low regarding latitude and also timing, however, the chance of rain Wed afternoon-evening is very high, with the middle 50 percent of ensemble members providing widespread 0.5 to 1 inch of rainfall. Instability does appear to be elevated for the most part, though the surface in southern Michigan may not stay entirely stable especially if a northern track verifies. Agree with SPC`s take on a slight risk of severe weather for I-94 and south given a the extreme amount of low to mid level wind shear for the time of year. A hail and/or wind threat may occur even with a somewhat stable surface if gravity wave associated convection develops within this unbalanced upper- level flow regime as the 00Z NAM pressure fields might suggest. && .AVIATION /12Z TAFS THROUGH 12Z TUESDAY/... Issued at 720 AM EDT Mon Jun 15 2026 VFR is expected with a patch of clouds at or above 20,000 feet, and daytime cumulus development above 5,000 feet. Winds will be gusty from the west-southwest during the day. && .MARINE... Issued at 350 AM EDT Mon Jun 15 2026 A moderate swim risk may develop this morning with 15 knot WSW winds. A high swim risk and small craft advisory conditions are likely on Tuesday, and again Wednesday night into Thursday. Thunderstorms may also be present over Lake Michigan on Tuesday and Wednesday. && .GRR WATCHES/WARNINGS/ADVISORIES... MI...None. MARINE...None. && $$ DISCUSSION...CAS AVIATION...CAS MARINE...CAS

NWS GRR Office Area Forecast Discussion