020 FXUS63 KGRR 170739 AFDGRR Area Forecast Discussion National Weather Service Grand Rapids MI 339 AM EDT Fri Apr 17 2026 .KEY MESSAGES... - Dense Fog through this morning with fair weather today - Strong, possible severe storms Friday night - Cooler, Breezy weekend with cooler and drier weather ahead && .DISCUSSION... Issued at 253 AM EDT Fri Apr 17 2026 - Dense Fog through this morning with fair weather today Bottom Line Upfront: Dense fog continues to overspread the region which has reduced visibilities to 1/4SM or less. This fog will continue through most of the morning with fog slowly lifting and visibilities improving late this morning, early this afternoon. Expect impacts to the morning commute. Have issued a dense fog advisory accordingly. The copious amounts of rainfall this week has brought large amounts of low level moisture across Southern Michigan and northern Indiana. Soundings show strong inversion at the surface through the lowest 1KFT with light to calm winds. Dry and slightly warmer air above will keep the cooler, moisture air trapped this morning. The main question is when will the inversion break and the visibilities improve. With high pressure building overhead, not expecting for any flow to mix down. Due to that, will need to wait for daytime heating to allow for warming the surface and dewpoint depression to drop enough for fog to lift and dissipate. This will thus take all morning, with conditions improving slowly with fog lingering through most of the morning and fog finally lifting between 10 AM and Noon. - Strong, possible severe storms Friday night As stated above, High pressure will dominate the weather pattern today which will bring a welcome reprieve to the wet and stormy pattern that has dominated Michigan this week. However, those conditions will be short lived as another frontal system is expected to move through the region tonight and into Saturday. A large barotropic low will be moving through central Canada and its corresponding low and mid levels will swing through an elongated trough and corresponding front through the Great Lakes Friday night into early Saturday. Latest CAMS show steep lapse rates along with good mixed level CAPE. That will correspond with a strong 45KT LLJ that should provide decent shear along and behind the front. Mode of convection should be QLCS with potentially embedded supercells. With soundings being inverted in the lowest levels there is a potential for strong DCAPE which is an indicator that damaging winds and large hail are the main threat. There is also the presence of a cap in the 2 to 5KFT range. This could be a limiting factor. Another limiting factor will be the time of day, as this front should be trekking through the area during the evening. However, there is a good chance that the LLJ and shear overcomes those factors and that any cap could also allow the strongest storms chance to increase enough to break the cap and mix down to the surface with damaging winds. Due to these concerns southwestern lower Michigan is either in a marginal to slight risk. The storms should move through quickly so not expecting training storms. However given the inundation of the region, any rainfall could bring some runoff. QPF remains in the quarter inch to three quarter inch range. Some overperforming storms could have strong enough rain rates to put down an inch but will any heavy downpours will be short lived. Strongest storms should be along and south of the I 96 corridor but as stated above, additional rainfall is expected across Lower Michigan. For more discussion of the rivers please go to the hydro section. - Cooler, Breezy weekend with cooler and drier weather ahead The above mentioned cold front will then usher in cooler northwesterly flow which will allow for cooler temps through the weekend and into early next week. Strong winds with gusts upwards of 30 mph are possible late Saturday. Lows Sunday morning will be at to below freezing with highs on Sunday struggling to get into the mid 40s. While most would not appreciate the cold, it also means dry which given the latest moisture will allow for rivers to recover. A building high over the central US will keep downsloping flow through the region through at least mid next week. Temperatures should rebound back into the mid to upper 60s mid next week. && .AVIATION /06Z TAFS THROUGH 06Z SATURDAY/... Issued at 213 AM EDT Fri Apr 17 2026 The main aviation concern this morning is the fog. A dense fog advisory will cover all TAF sites. Airport Minimums for visibility are expect overnight. So far MKG, GRR, JXN have already dropped to a 1/4SM LAN and AZO have dropped to LIFR. The widespread recent rainfall, calm winds and inversion will cause these to drop to VLIFR as well. Cigs are expected to drop to airport minimums as well. The main question is when these will lift. Latest model soundings give good credence to fog/mist and LIFR and lower cigs continuing through most, if not all, the morning. There is a chance MKG improves between 12Z to 14Z. Cigs and vsbys should finally improve to VFR between 15Z to 8Z. VFR will continue through the daytime with clouds infiltrating tonight with showers and storms possible Friday night into Saturday morning. && .MARINE... Issued at 313 AM EDT Fri Apr 17 2026 Benign marine conditions are expected through much of the day Friday before winds and waves increase later Friday into the weekend. A small craft advisory will take effect tonight and should continue due to the gusty winds through Saturday into Sunday morning. Marine Dense Fog will continue into this evening. A line of thunderstorms should cross the lake tonight with the potential for winds in excess of 34 knots. && .HYDROLOGY... Issued at 251 PM EDT Thu Apr 16 2026 Bottom Line Up Front: Significant flooding continues on the Muskegon River with minor to moderate flooding expected in the Grand River Basin through the coming days. Overnight last night, 1 to 3 inches of rain fell across the lower Muskegon River basin, with isolated pockets in the Grand River basin as well. This will make its way into the river system resulting in flooding. Major flooding is expected along the Muskegon River at all forecast points, with minor to moderate flooding forecast for the Grand River and its tributaries. The Kalamazoo River basin seeing less rain of late, no river flooding is expected. Our Saginaw River basin points at the Chippewa River at Mt. Pleasant, and the Pine River at Alma are also forecast to reach minor flood stage. Little additional rainfall, outside of thunderstorms, is expected through the daytime hours Friday. Thunderstorms then bring widespread totals of 0.25-0.75" Friday Night into Saturday, highest near I-94. Given the saturated antecedent conditions, urban and small stream flooding is possible with thunderstorms, particularly Friday Night. && .GRR WATCHES/WARNINGS/ADVISORIES... MI...Dense Fog Advisory until 11 AM EDT this morning for MIZ037>040- 043>046-050>052-056>059-064>067-071>074. MARINE...Dense Fog Advisory until 8 AM EDT this morning for LMZ844>849. && $$ DISCUSSION...Ceru AVIATION...Ceru MARINE...Ceru HYDROLOGY...Thomas

NWS GRR Office Area Forecast Discussion