050 FXUS63 KGRR 060002 AFDGRR Area Forecast Discussion National Weather Service Grand Rapids MI 802 PM EDT Wed Aug 5 2026 .KEY MESSAGES... - Periods of Rain and Thunderstorms Into Next Week && .DISCUSSION... Issued at 226 PM EDT Wed Aug 5 2026 Broadly, forecast expectations have not changed in any significant way. The most plausible outcome in the short term is for another round of showers and non-severe thunderstorms to move into the area late afternoon/early tonight and exit early Thursday morning. This is far from a certain forecast, however. The challenge continues to be timing rounds of showers and thunderstorms across the area, mostly south of a diffuse quasi- stationary front that extended northeast to southwest through the northwest forecast area this afternoon. Scattered rain showers are already ongoing across the forecast area. More widespread ongoing convection is noted across IA/MO/IL ahead of upper PV forcing depicted by both satellite water vapor imagery and short range model guidance. Convection allowing guidance has shown significant run to run variability, which suggests large forecast sensitivity to antecedent convective trends. It is interesting to note that even the ECMWF deterministic model hints at this; it erodes the PV max (and therefore organized upper forcing) as it moves east towards Lower MI. This is likely due to modeled deep moist convection and attendant mid-level latent heat release that has the effect of decreasing both static stability and cyclonic vorticity (the ingredients of PV) at the upper levels. The substantial influence of diabatic forcing we see here amidst weak synoptic forcing is what really hurts the predictability of this forecast. As noted previously, conditional instability is not favorable for strong storms and will be further limited by ongoing precipitation. 12Z HREF guidance does indicate a continued chance of thunderstorms overnight trailing off closer to sunrise. Another item of concern is locally heavy precipitation towards I-94. This is advertised by HREF LPMM. Having said that, the 12Z REFS is completely different, showing greatest (but far less impressive) precipitation amounts towards Clare County. This all speaks to the low forecast certainty we can expect even in the first 24 hours, but there is the possibility of localized significant rain, leading to at least some nuisance flooding overnight. Thursday afternoon now looks a little drier than before with the focus for most precipitation, including thunderstorms, shifted east towards southeast Lower Michigan. But again, this will depend a lot on convective evolution beforehand. After Thursday, broad upper troughing will slowly migrate from the western Great Lakes to the central Great Lakes with perhaps some flattening by the beginning of next week. As noted before, the timing of convection will remain uncertain, yielding a now familiar repeating theme of 20-40 PoPs in the extended forecast. It appears unlikely that we will be seeing much in the way of strong/severe convection or significant areal flooding. && .AVIATION /00Z TAFS THROUGH 00Z FRIDAY/... Issued at 755 PM EDT Wed Aug 5 2026 Scattered thunderstorms through the first half of the overnight period have the potential to affect the airports. MKG and GRR will see a lull over the next few hours. LAN, JXN, AZO and BTL continue to have a decent chance for having VCTS to TSRA in between 00Z and 04Z. Have adjusted TAF`s accordingly. After another lull, a period of showers and storms could develop from the slowly moving cold front. So have PROB30`s between 06Z and 10Z. Any TAF site that received precipitation should get some IFR cigs and vsbys in fog/mist. This should develop around 10Z and linger through most of the morning. Another round of convection is possible tomorrow afternoon, especially the southern and easter TAF sites of AZO, BTL, JXN and LAN. && .MARINE... Issued at 226 PM EDT Wed Aug 5 2026 Scattered showers and a few thunderstorms are expected through tonight into at least early Thursday. Showers and thunderstorms will continue to be possible this weekend into next week. No severe thunderstorms are expected. Winds and waves should generally remain below Small Craft Advisory and Beach Hazard Statement levels. && .GRR WATCHES/WARNINGS/ADVISORIES... MI...None. MARINE...None. && $$ DISCUSSION...TJT AVIATION...Ceru MARINE...TJT

NWS GRR Office Area Forecast Discussion