666 FXUS63 KGRR 181928 AFDGRR Area Forecast Discussion National Weather Service Grand Rapids MI 328 PM EDT Fri Sep 18 2026 .KEY MESSAGES... - Heavy Rain Likely Across Southwest Lower Michigan This Weekend - Mainly Dry Next Week After Weekend Rain - Cooler Than Normal Weather Much of the 7 Day Forecast && .DISCUSSION... Issued at 310 PM EDT Fri Sep 18 2026 - Heavy Rain Likely Across Southwest Lower Michigan This Weekend We have gone ahead an issued a Flood Watch for Allegan, Barry, Van Buren, Kalamazoo and Calhoun Counties. These are the counties that we have more confidence in seeing the higher amounts (2-4 inches) by Sunday morning. 1 to 2 inch amounts are likely outside of those 5 counties. Aloft, a zonal upper flow will be in place through Sunday with numerous shortwaves working east through the flow over the Great Lakes region. Down at the surface weak low pressure will move from the Central Plains into the Ohio Valley. An overrunning situation sets up as south/southwest flow rides northeast over cooler air at the surface over Lower Michigan. This lift will result in precipitation that lasts the better part of 24 hours from near midnight tonight to midnight Saturday night. The repeated training of echoes over Southwest Lower Michigan will result in heavy rain. Forcing will also be provided by the right front quad of an upper jet to our north that will lead to a low level jet response that stays fairly steady state (aimed at our area) for the better part of 24 hours. PWAT values over Southern Lower Michigan will be in the 1.75 to 2.00 inch range which is very high for this time of year. Surface dew points will not be as high (in the 60s F), but this is still high for this time of year as well. Bottom line...we are looking at rounds of heavy rain that will likely lead to ponding of water in low lying areas and urban centers as well. Rises on area rivers and streams will occur. At this time we are not expecting widespread river flooding in terms of reaching flood stage but we will be monitoring closely. - Mainly Dry Next Week After Weekend Rain We are looking at mainly dry weather next week for the bulk of the area as Canadian high pressure sags in from the north. The question mark will be Southern Lower Michigan which may remain just close enough to low level moisture as we sit beneath an upper trough. - Cooler Than Normal Weather Much of the 7 Day Forecast Cooler than normal weather is expected through the period with highs in the 60s much of the time. Rain this weekend and then cooler air moving in at 850mb will be the reason for the cooler weather. 850mb temperatures in the teens C this weekend will cool into the single digit C range next week. Normal highs are in the lower 70s for the most part and we should be below that much of the next 7 days. It will feel like fall. && .AVIATION /18Z TAFS THROUGH 18Z SATURDAY/... Issued at 156 PM EDT Fri Sep 18 2026 Flight restriction probabilities: - Through 00Z: 90 percent chance VFR all terminals - After 06Z: 30 percent chance MVFR fuel alt ceilings (below 2 kft AGL) at all terminals except LAN/JXN Main additions this update: - extra detail regarding precipitation after 06Z tonight - new PROB30 group for heavy rainfall for western terminals (MKG, AZO, GRR, BTL) centered around 12Z We are seeing growing chances for locally heavy rainfall over parts of southwest Lower MI Saturday morning, so reflected this in the terminals with PROB30 groups featuring +TSRA. As noted previously, we will see a thickening and lowering of cloud bases through the next 12 to 18 hours. It is possible that later updates will need to feature wind gusts more prominently, but that remain highly uncertain at this point as it will depend on exact placement of the heaviest rain bands. && .MARINE... Issued at 310 PM EDT Fri Sep 18 2026 We are entering a period of easterly winds on Lake Michigan from this evening right through weekend. The result will be lower wave heights, 0-2 feet near shore and 1-3 feet out towards the 5 mile mark. Tonight wind will be east at 10 to 20 knots becoming northeast at 10 to 20 knots for Saturday through Sunday. We feel we should remain below Small Craft Advisory wind criteria, but we will be watching gusts at the lakeshore sites for values into the 22+ knot range. It will not be a great weather weekend for those trying to hang on to summer, with a bit of a wind, some chop and plenty of rain. && .GRR WATCHES/WARNINGS/ADVISORIES... MI...Flood Watch from 2 AM EDT Saturday through Sunday morning for MIZ064-065-071>073. MARINE...None. && $$ DISCUSSION...Duke AVIATION...TJT MARINE...Duke

NWS GRR Office Area Forecast Discussion