338 FXUS63 KGRR 172020 AFDGRR Area Forecast Discussion National Weather Service Grand Rapids MI 420 PM EDT Thu Sep 17 2026 .KEY MESSAGES... - Fairly quiet weather tonight and Friday - Heavy Rain expected Friday night and into the weekend - Near normal temperatures through the 7 day forecast && .DISCUSSION... Issued at 320 PM EDT Thu Sep 17 2026 - Fairly quiet weather tonight and Friday Clouds will generally be on the decrease tonight with partly cloudy to mostly clear conditions developing by daybreak. Rainfall and clearing skies leads to the thought of fog development. Winds around 1,000 feet will hold at 10-20 knots which should preclude widespread fog. We are thinking some light fog development is possible especially along the I-94 corridor. On Friday, mid and high clouds will thicken in advance of our next system. Skies will likely be mostly cloudy at sunset. We expect to remain dry through the day on Friday as the system will still be out in the Plains. Fall like temperatures tonight and Friday with lows in the 40s over Central Lower Michigan and highs in the lower to middle 70s in most areas tomorrow. - Heavy Rain expected Friday night and into the weekend The main focus of the 7 day forecast is on Friday night into Saturday and potentially into the weekend as well with the threat of heavy rain. A shortwave will eject out of the Western U.S. trough and move into the Northern Plains tonight. This shortwave will then shear eastward into the Great Lakes region Friday into Saturday. A surface boundary will remain off to our south during this time and provide a lifting and focusing mechanism for the potential of heavy rain. An 850mb low level jet will be focused right at Southwest Lower Michigan late Friday night and Saturday morning. The current forecast is for 1-2 inches of rain with this lead event (Friday night/Saturday morning). Additional rain is possible Saturday night into Monday as the surface boundary is still in fairly close proximity. Additional mid level shortwaves are forecast to ride eastward through the region during this time. Additional rain of an inch or so is possible, but the position of the low level jet is in question. Convection to the south can modulate the area of heavy rain quite a bit so confidence in the Saturday night through Monday rainfall is lower. We are likely looking at some ponding of water in low spots at times of the heavier rainfall as well as rises on area rivers. At this point however we are not expecting widespread river flooding. We could be looking at some short term river flood advisories. We feel the location and magnitude of the heavier rain will come more into focus tomorrow. - Near normal temperatures through the 7 day forecast Temperatures this time of year typically are around 70 for highs and 50 for lows. We are not expected to be far off from that through the period. Lows will generally be in the 50s with highs in the upper 60s to lower 70s. && .AVIATION /18Z TAFS THROUGH 18Z FRIDAY/... Issued at 154 PM EDT Thu Sep 17 2026 The main concern over the forecast period is the IFR and lower cigs and vsbys. The light rain fall will slowly fade through the afternoon, with MVFR and lower vsbys ending by 22Z. The low stratus will persist through much of the period. IFR to LIFR cigs should slowly improve late in the day and through 00Z at the northern TAF sites. IFR cigs should improve to MVFR by 06Z. The southern TAF sites, AZO, BTL and JXN, could linger through 08Z with MVFR until 14Z Friday. && .MARINE... Issued at 420 PM EDT Thu Sep 17 2026 Issued marine dense fog advisory through 8 PM Thu for the nearshore waters of South Haven to St Joseph. Webcam at the South Haven south beach shows visibility dipping to a quarter mile. A couple of this morning`s runs of the HRRR model caught the idea of a narrow strip of fog butting up against the shoreline from South Haven then moving to the south of St Joseph this evening. In general an off shore flow is expected over the course of the next few days, or through the weekend. Low pressure will pass off to our south with high pressure situated over much of eastern and central Canada. This will place the Great Lakes in a easterly flow between the two. The east flow will keep wave heights at lower values, 1-3 feet, but we may see winds at times increase into the 15-25 knot range. So, while waves will not push us to advisory levels winds may get us there. Friday night and Saturday looks to be the time frame of the stronger winds. && .GRR WATCHES/WARNINGS/ADVISORIES... MI...None. MARINE...Dense Fog Advisory until 8 PM EDT this evening for LMZ844. && $$ DISCUSSION...Duke AVIATION...Ceru MARINE...Duke/CAS

NWS GRR Office Area Forecast Discussion