274 FXUS63 KGRR 311033 AFDGRR Area Forecast Discussion National Weather Service Grand Rapids MI 633 AM EDT Mon Aug 31 2026 .KEY MESSAGES... - Multiple Rounds of Showers and Thunderstorms, Warm and Humid && .DISCUSSION... Issued at 320 AM EDT Mon Aug 31 2026 Overall guidance is struggling with the current pattern over the next few days, therefore confidence is not high in the exact timing and placement of rain and thunderstorm chances. However some synoptic features give slightly higher confidence at times. Today: Warm air advection will shift into the area this morning along with the low level jet sparking off showers and thunderstorms. Best chances (greater than 40 percent) will be north of I-96 with a 20 to 40 percent chance along and south of I-96. A brief lull in showers and thunderstorms is expected late this morning into the early afternoon. A push of 700mb warm air advection and a weak low to mid level front may spark off additional showers and thunderstorms this afternoon. Both the HREF and REFS highlight this along and south of I-96 this afternoon. Thunderstorms will be elevated with plenty of dry air toward the surface. This could result in some gustier winds with thunderstorms. Some machine learning guidance highlights this potential for stronger winds. Tuesday: Late tonight into Tuesday additional thunderstorms are possible with the best chances during the day (30 percent) with the low level jet moving over the area and positive vorticity advection. Mean MUCAPE values are around 2000 J/kg with mean sfc-500mb shear values around 30 to 40 knots. Differences within the various solutions bring lower confidence to Tuesday`s forecast. If storms develop there is a marginal risk for them to be severe with winds as the main concern. Tuesday will also be one of the hottest days this week with highs in the mid 80s to low 90s and heat index values in the 90s with the potential for values to approach 100 in southern Michigan. Tuesday night the warm front lifts north with areas along and north of I-96 seeing an additional chance for thunderstorms. Rest of Week: Wednesday will be another hot day with heat index values in the 90s mainly along and south of a Grand Haven to Mount Pleasant line. Areas along and south of I-94 could see heat index values around 100. This will be highly dependent on thunderstorm chances. Some solutions have a weak shortwave moving through during the afternoon and evening and others have a stronger trough and low pressure system along with a stronger jet. If this latter solution pans out there will be a higher risk for severe thunderstorms. Stay tuned. Northwest flow returns to the area Thursday into the weekend. We`ll be on the northeast side of the ridge parked over the central CONUS. Within this flow we`ll need to watch for ridge riders bringing additional rain and thunderstorm chances. High temperatures will be cooler in this flow (70s and 80s), but dew points will largely be in the 60s. && .AVIATION /12Z TAFS THROUGH 12Z TUESDAY/... Issued at 633 AM EDT Mon Aug 31 2026 Thunderstorms have developed in eastern Wisconsin and are tracking east. These storms are expected to reach MKG around 11-12Z, GRR around 13Z and AZO/BTL around 14Z. Guidance suggests that storms should weaken as they approach LAN and JXN, but it isn`t handling the current situation well. For now have Prob30 groups for thunderstorms, but an amendment may be needed if this area of storms holds together. Lower visibilities will be possible with any thunderstorm along with heavy rainfall. Dry conditions return by this afternoon, however pop-up thunderstorms will be possible between 19-00Z this afternoon and early evening. Southwest winds gusting to 20 to 25 knots are expected across much of the area, wind gusts may remain below 20 knots at JXN. At this time have a dry forecast tonight into early Tuesday morning, but thunderstorm chances may need to be introduced. && .MARINE... Issued at 320 AM EDT Mon Aug 31 2026 Another round of breezy southerly winds is expected today causing waves to build to 2 to 5 feet. A Small Craft Advisory and Beach Hazard Statement are in effect. Thunderstorms will be possible this morning and during the afternoon. Additional chances for thunderstorms exist late tonight through Tuesday with the potential for gusts 34 knots or greater with the strongest storms. For the rest of the week winds and waves are expected to remain below Advisory or Statement levels. Thunderstorms will be possible again on Wednesday and Thursday with winds becoming more offshore Friday into the weekend. && .GRR WATCHES/WARNINGS/ADVISORIES... MI...Beach Hazards Statement from 11 AM EDT this morning through this evening for MIZ037-043-050-056. MARINE...Small Craft Advisory from 11 AM this morning to 11 PM EDT this evening for LMZ846>849. && $$ DISCUSSION...RAH AVIATION...RAH MARINE...RAH

NWS GRR Office Area Forecast Discussion