372 FXUS63 KGRR 281812 AFDGRR Area Forecast Discussion National Weather Service Grand Rapids MI 212 PM EDT Fri Aug 28 2026 .KEY MESSAGES... - Rain possible Saturday night into Sunday, mainly north - Lesser thunderstorm concerns early next week - Unseasonably hot and humid Monday and Tuesday - Uncertain timing for thunderstorms the entire coming week && .DISCUSSION... Issued at 212 PM EDT Fri Aug 28 2026 - Rain possible Saturday night into Sunday, mainly north Generally light rain will be possible as early as Saturday night thanks to mid/lower tropospheric isentropic ascent and moist advection. There will be a deep and dry residual boundary layer this precipitation will be falling through, resulting in substantial sub- cloud evaporation that will limit accumulations. this is expected to persist into the day Sunday. Rainfall accumulations currently look to be limited to north of I-96. - Lesser thunderstorm concerns early next week At this time yesterday, we were concerned about a heavy rain threat Sunday night. Now we are seeing in the ensemble guidance a distinct northward shift of the precipitation focus, more towards northern Lower MI, and with lesser precipitation amounts overall. Over Lower MI, it appears that instability will be questionable although we will see a gradual deepening of atmospheric moisture by early Monday. - Unseasonably hot and humid Monday and Tuesday The concern for most impactful weather has pivoted from heavy rain to oppressive heat. Heat indices exceeding 90 degrees both Monday and Tuesday afternoons look highly probable (greater than 80 percent chance). While this is not that exceptional when compared to what occurs during a typical summer, it will feel manifestly different compared to the dry and seasonably cool weather we`ve had lately. Additionally, this coincides with the start of the school year for most locations, so there could be vulnerability for those facilities without air conditioning. The greatest threat will be towards I-94 where dewpoints could make a serious run at 70F, which would compound the oppressiveness. - Uncertain timing for thunderstorms the entire coming week Capping will limit afternoon thunderstorm potential at least early in the week. As noted previously, better chances for showers and storms will occur at night when a nocturnal low-level jet can become established and contribute to both conditional destabilization (via low level theta-e advection) and forcing for ascent (via warm advection, used as a proxy to the Laplacian of thickness advection). The magnitude and placement of processes like these, being inherently elevated, are difficult for NWP to model. Moreover, synoptic flow and associated forcing can be significantly altered by the latent heat release effects of previous convection. Thus, predictability decreases rapidly with increasing forecast projection times. The medium range deterministic models certainly bear this out with very different solutions. However, it is safe to say based on latest ensemble guidance that chances for showers and thunderstorms will persist at least through Friday. && .AVIATION /18Z TAFS THROUGH 18Z SATURDAY/... Issued at 108 PM EDT Fri Aug 28 2026 FEW to SCT clouds in the 6-8kft range expected at most sites this afternoon, with intermittent ceiling development in this range at LAN over the next few hours. These then give way to high clouds aoa 10kft tonight which will lower and thicken with an approaching system. With the exception of MKG with a 5-10 knot lake breeze, most sites will see light and variable winds with a predominantly southwest component through the evening. Light winds tonight then give way to increasing winds to 5-10 knots for all sites from the southwest Saturday. && .MARINE... Issued at 212 PM EDT Fri Aug 28 2026 The lake breeze this afternoon will transition to a weakly discernible land breeze tonight with winds adopting a largely shore-parallel, but also somewhat offshore component. With background flow over the rest of the lake being more south-southwesterly, this could introduce a weak convergent boundary with light to calm winds over the nearshore waters. Dewpoints will be close to water temperatures so there is a chance of some marine fog, probably thinking more localized in nature at this point. By Saturday afternoon, southerly winds will have had achieved sufficient duration to grow waves to around 3 feet adjacent to the Sable Points. There is a low chance (20 percent) that this scenario would escalate to a 3 to 5 foot Small Craft Advisory (SCA) situation. Sunday looks like a much higher probability (70 percent) SCA timeframe from Muskegon northward, but as noted previously, there are some questions about downward momentum transfer efficacy and subsequent wave growth in what will be a warm air advection regime. Machine learning guidance is notably lower than the operational WW3 and it has shown some past skill in this type of setup. Thunderstorms will be possible on and off starting Sunday night and well into next week with the best chances (30 to 50 percent) during the overnight hours. && .GRR WATCHES/WARNINGS/ADVISORIES... MI...None. MARINE...None. && $$ DISCUSSION...TJT AVIATION...Thomas MARINE...TJT

NWS GRR Office Area Forecast Discussion