766 FXUS63 KGRR 061806 AFDGRR Area Forecast Discussion National Weather Service Grand Rapids MI 206 PM EDT Thu Aug 6 2026 .KEY MESSAGES... - Mainly afternoon scattered showers and thunderstorms today - Showers and thunderstorms Friday night, drier Saturday - Showers and thunderstorms likely Monday or Monday night - Continued chances for thunderstorms into mid-week && .DISCUSSION... Issued at 206 PM EDT Thu Aug 6 2026 - Mainly afternoon scattered showers and thunderstorms today The 17Z surface analysis indicated a nearly stationary front extending through extreme SW Lower MI to north of Jackson and then curving northeast towards the Thumb. Thunderstorms are expected to be restricted mainly south and east of this front with severe convection unlikely. Mid and upper cloudiness partly sourced from an MCS over Missouri is noted on the equatorward side of the polar jet and this area of cloudiness is streaming across much of Lower Michigan at this time. This is limiting insolation and diurnal destabilization. Moreover, local VWP profiles indicate weak shear. This all supports the idea of disorganized and generally short-lived showers and thunderstorms mainly this afternoon. Coverage will probably be limited compared to previous days. As noted in the aviation section of this forecast discussion, we cannot rule out some fog once again Friday morning. - Showers and thunderstorms Friday night, drier Saturday There is agreement amount multiple deterministic models that a shortwave trough will swing through Friday night/early Saturday timeframe with an uptick of convection. Some of the Machine Learning models hint at severe potential during this time. Other guidance shows convection dissipating before reaching Lower MI. That said, guidance like the 12Z HRRR does indicate sufficient thermodynamics (MUCAPE ~2000 J/kg and DCAPE > 800 J/kg) for damaging convective wind gusts even in the absence of organizing shear. If convection manages to cross Lake Michigan relatively intact and move into this favorable environment, that will significantly increase the chances for strong/severe wind gusts. Somewhat implicit in this scenario is a subsequent largely dry Saturday afternoon/evening. That is what our forecast will advertise, but with the same caveat that things can change due to timing. - Showers and thunderstorms likely Monday or Monday night Probably the best chances for severe weather during this forecast cycle exist during this time frame. Medium range guidance generally agrees that a shortwave trough within a band of stronger mid-level westerly flow will traverse the western Great Lakes. Shear will be substantially more favorable for organized convection (for example 0-6km bulk values around 50 kt, 0-3km bulk values around 30 kt). Conditional instability is less certain. It would appear that at least the southern forecast area could see MLCAPE exceeding 1000 J/kg, which would be sufficient for severe weather given the kinematics. This is all subject to change, but certainly one of the most noteworthy things to watch. - Continued chances for thunderstorms into mid-week As noted previously, a persistent upper low in the vicinity of Hudson Bay will keep deep cyclonic flow over the western Great Lakes with several opportunities for precipitation as upper PV lobes rotate through. && .AVIATION /18Z TAFS THROUGH 18Z FRIDAY/... Issued at 152 PM EDT Thu Aug 6 2026 Quite a bit of high clouds are streaming northeast across the region this afternoon along with scattered cu. A weak frontal boundary stretching from the Thumb to the far southwest part of the Lower Peninsula may become a focus for thunderstorm development later this afternoon, but that`s less than certain; CAMs have backed off on storm development. As such, we left them out of the TAFs. Yesterday`s rainfall have left residual moisture near the ground and coupled with light winds, will result in some MVFR fog development overnight...closer to sunrise. The fog will only last a couple of hours before dissipating. && .MARINE... Issued at 206 PM EDT Thu Aug 6 2026 Winds and waves should remain below advisory criteria, but we could see a repeat of marine fog tonight, especially south, given expected air dewpoints close to 70F, water temperatures closer to 60F, and light winds. Winds were almost calm near the lakeshore this morning, which prevented turbulent mixing and allowed for an extensive coverage of fog. That looks quite plausible again tonight. Unfortunately, this is a situation where you often have to wait and see via webcams or surface observations the onset of fog because of the sometimes contributing, sometimes deleterious microscale processes that can occur near the lakeshore. && .GRR WATCHES/WARNINGS/ADVISORIES... MI...None. MARINE...None. && $$ DISCUSSION...TJT AVIATION...04 MARINE...TJT

NWS GRR Office Area Forecast Discussion