594 FXUS63 KGRR 171045 AFDGRR Area Forecast Discussion National Weather Service Grand Rapids MI 645 AM EDT Mon Aug 17 2026 .KEY MESSAGES... - Showers and storms Tuesday afternoon into Wednesday - Next round of precipitation Friday night into Saturday && .DISCUSSION... Issued at 304 AM EDT Mon Aug 17 2026 - Showers and storms Tuesday afternoon into Wednesday After a period of surface pressure ridging and west-northwesterly flow aloft today, Tuesday will see an upper low progressing to northern Ontario with increasingly cyclonic upper flow over the western Great Lakes. Medium range deterministic guidance is consistent with an upper PV max swinging across the UP and northern Lower MI Tuesday night. This will serve to drive a cold front through our forecast area Tuesday night into Wednesday. Although rain showers seem likely for the area Tuesday night, there is some question about whether there will be sufficient instability for thunderstorms. Guidance such as the ECMWF ensemble is right on the cusp with ensemble mean MUCAPE in the 600-800 J/kg range. The best forcing for ascent (and associated destabilizing effects) appears as if it will remain north of the forecast area. The better chance for thunder over southern Lower MI may be during the day Wednesday when the destabilizing effect of insolation will come into play. Due to the questions about conditional instability, it is no surprise that most guidance, including the Machine Learning models, are downplaying the severe threat with this event. Things could change, of course. Deep layer shear on the order of 30-40 knots would have the potential to organize at least strong multicell clusters if the instability manages to over-perform. What is a bit more concerning is the potential for excessive rainfall across southwest Lower MI, as noted in the WPC Day3 ERO. Forecast soundings do support this possibility with deep unidirectional flow parallel to a slow-moving cold front. This, combined with seasonably high precipitable water and potentially deep warm cloud depths, could produce heavy rain where cell training is able to occur. - Next round of precipitation Friday night into Saturday Medium range guidance is starting to lock into our next precipitation event occurring Friday night and potentially affecting much of the weekend with episodic precipitation and below normal temperatures as a vertically stacked low moves into the western Great Lakes. The upper feature in question is quite sharp and amplified, but being five days out in the forecast, there is much that can change between now and then. && .AVIATION /12Z TAFS THROUGH 12Z TUESDAY/... Issued at 645 AM EDT Mon Aug 17 2026 Main change this update was to minimize or remove fog. This is based on: 1) Somewhat stronger observed near-surface winds that are keeping the remnant nocturnal boundary layer mixed. 2) Scattered to broken cloud layers that are low enough to disrupt the outgoing longwave radiation flux that is typically needed to develop and maintain fog. This leaves just occasional MVFR ceilings being the only threat to otherwise VFR conditions in the short term. To address this, we gradually raise and then eliminate ceilings areawide by 16-18Z areawide. && .MARINE... Issued at 304 AM EDT Mon Aug 17 2026 Winds and waves should remain below advisory thresholds through early Tuesday. The Sable Points area could see a period of near Small Craft Advisory level waves Tuesday afternoon/evening as both the WW3 and the XGB Machine Learning model predict wave heights at the Ludington buoy reaching approximately 3.5 feet. This will be short-lived, however, as winds will veer to a more westerly direction and weaken in response to a looser pressure gradient in the immediate vicinity of the approaching cold front. && .GRR WATCHES/WARNINGS/ADVISORIES... MI...None. MARINE...None. && $$ DISCUSSION...TJT AVIATION...TJT MARINE...TJT

NWS GRR Office Area Forecast Discussion