750 FXUS63 KGRR 140001 AFDGRR Area Forecast Discussion National Weather Service Grand Rapids MI 801 PM EDT Sun Sep 13 2026 .KEY MESSAGES... - Greater chances of rain, mainly this afternoon - Showers, thunderstorms, possible heavy rain starts Tuesday - Unsettled into this weekend && .DISCUSSION... Issued at 213 PM EDT Sun Sep 13 2026 - Greater chances of rain, mainly this afternoon Based on radar and satellite trends, combined with model guidance, have raised PoPs to near categorical this afternoon. Thunderstorms are not expected but there should be a fairly extensive coverage of measurable precipitation. Rain should be mostly, if not totally, out of the forecast area by this evening. As noted before, winds should decrease rapidly this evening as the surface high moves into Lower MI. We are still looking at lows in the 40s Monday morning with upper 30s not out of the question in the favorably cooler sandy soil regions to the north. - Showers, thunderstorms, possible heavy rain starts Tuesday A cyclonically breaking upper shortwave trough originating from the northern Rockies with a surface low tracking into Canada by Tuesday. This will amplify the downstream upper ridge and push it eastward across the Great Lakes. The associated eastward progression of the surface high will introduce increasing southerly flow in its wake. The cold front an associated low level theta-e ridging will enter the forecast area early Tuesday morning. With MUCAPE approaching 1000 J/kg, there will be a chance for thunderstorms. As noted previously, the cold front is expected to stall somewhere in the vicinity of the southern CWA. There will be a sharp theta-e and conditional instability gradient with this feature. ECMWF ensemble (ECE) mean guidance indicates precipitable water values approaching 2 inches, which is highly anomalous with respect to both climatology for this time of year and with respect to the propensity of the ECE to forecast values this high for this time of year. WPC is indicating a marginal risk for excessive rainfall, which is quite reasonable. However, accurate placement of the actual threat area will be tricky and highly location-dependent given the sharpness of the boundary. - Unsettled into this weekend Ensemble guidance really speaks to the poor predictability leading into this weekend. Often it`s possible to carve out a particular day that`s likely to be dry. Not so this time. While there is a signal for a dry day happening at some point, which day that is has varied from run to run. Hence, we are featuring persistent 20-40 percent PoPs through the period. && .AVIATION /00Z TAFS THROUGH 00Z TUESDAY/... Issued at 801 PM EDT Sun Sep 13 2026 Surface high pressure is located just upstream of the Southwest Lower Michigan TAF sites over Minnesota and Wisconsin at 00z. The high will shift eastward tonight and be located directly over our area on Monday. No significant aviation concerns are expected the next 24 hours with high pressure providing VFR weather. As for the details... Winds will become light and variable this evening fairly quick, or by 03z. Light and variable winds will continue overnight and through the morning hours of Monday. A light southeast wind will develop Monday afternoon at 4-8 knots. Skies will generally be clear tonight, but there will be some scattered cumulus towards the lakeshore at around 5,000 feet. These clouds are associated with lake instability (colder air overriding warmer water). The cirrus clouds to the south will move away this evening. Cirrus will spread back in from the west after 20z on Monday. Cirrus bases should be at or above 20,000 feet. && .MARINE... Issued at 213 PM EDT Sun Sep 13 2026 No changes to the marine headlines at this time. As noted previously, gale conditions are expected to commence Monday night. However, to prevent confusion, we will wait for current headlines to expire tonight before addressing this next threat window. After the onset of gale conditions Monday night, hazardous conditions should persist through Tuesday and into Tuesday night. && .GRR WATCHES/WARNINGS/ADVISORIES... MI...Beach Hazards Statement until 11 PM EDT this evening for MIZ037- 043-050-056-064-071. MARINE...Small Craft Advisory until 11 PM EDT this evening for LMZ844>849. && $$ DISCUSSION...TJT AVIATION...Duke MARINE...TJT

NWS GRR Office Area Forecast Discussion