164 FXUS63 KGRR 170801 AFDGRR Area Forecast Discussion National Weather Service Grand Rapids MI 401 AM EDT Wed Jun 17 2026 .KEY MESSAGES... - Chance of Severe Weather and Flooding in Southern Michigan - Quieter Late Week Then Rain and Storms Possible Sunday && .DISCUSSION... Issued at 401 AM EDT Wed Jun 17 2026 - Chance of Severe Weather and Flooding in Southern Michigan Anomalously deep low pressure cyclone for June, featuring two pressure minima at or below 990 mb over Minnesota and southern Lake Michigan, associated with a developing coupled upper-level jet structure supporting significant upper-level divergence, will provide dynamic weather to the Midwest today. A several hour period of rain and thunderstorms between early-mid afternoon and late evening will bring threats of strong winds and flooding. Ahead of the low, a 60 to 70 knot 850 mb jet nosing into southern lower Michigan late in the day will help supply moisture for a large area of moderate to heavy rain. Initially rain-cooled air at the surface over Lower Michigan will keep much of the severe and tornado threat over Illinois and Indiana. However there are signs of a weaker warm frontal boundary advecting into southwest lower Michigan under destabilizing midlevels during the evening. Vertical wind shear in the low and mid levels will become extreme for a time this evening especially in southern Michigan between 5 and 9 PM, including sfc-1 km shear values around 50 knots in the vicinity of an anticipated convective complex embedded within the rain. While surface parcels may not be outright convectively unstable in the vicinity and north of I-94 this evening, a strong to severe wind threat may still manifest in portions of the area. Some runs of the HRRR and other HREF member models develop narrow bands of wake-low enhanced southeast winds at times on the northwest edge of the precipitation shield. Additionally, some runs of the HRRR are producing small scale signatures of Gravity Wave Associated Convection (GWAC) within one of the cells in southern Michigan amid the complex, which if this does indeed occur, may entail an increased hail threat despite meager elevated instability, and a temporary wind direction reversal from southeast to northwest as the stable air in the lower levels is lifted and dropped. As far as precipitation, broadly 1 to 2 inches of rain is supported in the larger mesoscale picture. However, depending on the evolution of the smaller mesoscale convective features, a narrower swath of 2 to 4, perhaps isolated 5 inches of rain may fall in several hours. This is shown in the HREF`s local probability matched mean QPF. A flood watch has been issued to cover this threat of flooding particularly for low-lying and urban areas. - Quieter Late Week Then Rain and Storms Possible Sunday Thursday and Friday look cool and breezy with just very light and scattered rain showers possible at times through Saturday. The next shortwave trough ejecting from the western CONUS may develop another surface low over the Plains to Midwest late in the weekend. Depending on the track, another batch of heavy rain could move in later on Sunday. There is still a wide distribution of QPF amid the ensembles due to differences in the low`s track, so confidence in heavy rain or thunderstorms is not high for now. && .AVIATION /06Z TAFS THROUGH 06Z THURSDAY/... Issued at 151 AM EDT Wed Jun 17 2026 Early this morning, IFR fog is being reported around MOP and north of RQB, expected to persist until 12 or 13 Z. Otherwise, VFR is likely until about 16 Z. Associated with a strong low pressure system by June standards, rain is expected to move in from the west between 16 and 19 Z, gradually becoming heavier with worsening visibility and ceilings between 19 and 21 Z. Enough instability for thunderstorms embedded within the rain will likely be present starting between 20 and 22 Z, persisting for a few hours, then followed by a few more hours of prob30 thunderstorms as the predominant rain ends and showers become the mode. LLWS and gusty surface winds over 30 knots are possible during the hours of most intense rainfall. && .MARINE... Issued at 319 PM EDT Tue Jun 16 2026 Small craft advisory and beach hazards statement remains in place through the rest of the evening. After the frontal system passes the winds and waves will subside. Showers and storms will move through tomorrow, though winds and waves will be offshore. Another round of strong winds and waves are expected tomorrow night into Thursday. && .GRR WATCHES/WARNINGS/ADVISORIES... MI...Flood Watch from this afternoon through late tonight for MIZ064>067-071>074. MARINE...Small Craft Advisory from 2 PM this afternoon to 4 PM EDT Thursday for LMZ844>849. && $$ DISCUSSION...CAS AVIATION...CAS MARINE...Ceru

NWS GRR Office Area Forecast Discussion