621 FXUS63 KGRR 011829 AFDGRR Area Forecast Discussion National Weather Service Grand Rapids MI 229 PM EDT Sat Aug 1 2026 .KEY MESSAGES... - Rain showers, low chance of thunder today and tonight - Dry Monday and Tuesday, then chances for rain && .DISCUSSION... Issued at 229 PM EDT Sat Aug 1 2026 - Rain showers, low chance of thunder today and tonight Satellite water vapor channels and Airmass RGB product show a deep upper PV intrusion curling around the eastern flank of a vertically stacked low over east-central IL. This intrusion extends into far southwest Lower MI as well. There is some question regarding how much conditional instability we will realize this afternoon and evening. The general consensus is for parts of our forecast area, particularly far SW Lower and south central Lower to make a run towards 1000 J/kg MUCAPE. Insolation should be minimized by extensive mid/upper cloudiness associated with broad upper deformation (and concomitant divergence) over most of southern Lower MI. As noted previously, we are in an environment characterized by seasonably large precipitable water values (greater than 90th percentile). When combined with light flow in the cloud-bearing layer and a warm cloud depth well in excess of 10000ft AGL, we are looking at a setup with very high precipitation efficiency. Thus, any showers or thunderstorms will be able to produce large rainfall rates with a risk of at least some mostly nuisance flooding. Earlier CAM guidance advertised a narrow, generally west to east band of precipitation developing this afternoon/evening across the central CWA. The forcing for this has been a bit uncertain but could possibly be related to a deep wind col over this area between the closed upper low immediately southwest and an open trough immediately northeast. In any case, newer CAM model solutions are collectively less consistent about developing this feature. What seems most plausible is that potentially heavy rain could brush the southeast forecast area, including Jackson County, later tonight into Sunday morning in conjunction with low level fgen oriented northeast of the progressing lower tropospheric low and this precipitation could linger into early Sunday afternoon. This scenario agrees with the WPC excessive rainfall forecast showing a greater flood risk just south of the forecast area. - Dry Monday and Tuesday, then chances for rain Forecast certainty is high (greater than 90 percent) for dry conditions Monday and Tuesday. By Tuesday night, a broad upper trough is expected to be positioned over the northern Plains with a surface front poised to enter the forecast area from the northwest. There is some question regarding the timing of this front reaching southern Lower MI; several of the deterministic solution indicate an upper PV max swinging through Wednesday evening, which in turn would help with the southeastward progression of the front. The coupled forcing for upward vertical motion supplied by the front and PV max will justify greater PoPs. As noted before, we will be in a persistent broad upper troughing pattern, meaning continued chances of showers and thunderstorms with poor predictability regarding timing and coverage. It is worth noting that the latest available medium range machine learning and CFSv2 ensemble guidance point to increasing severe convection chances in the Friday/Saturday timeframe and an active pattern for strong/severe convection that could persist into next week. && .AVIATION /18Z TAFS THROUGH 18Z SUNDAY/... Issued at 149 PM EDT Sat Aug 1 2026 There is a mix of VFR/MVFR/IFR out there with cloud ceilings and showers associated with a slow-moving low pressure system that is centered over Illinois. In general, can plan on prevailing MVFR today and IFR tonight into Sunday morning. The main concern for the rest of this afternoon is the potential for a band or two of heavier rain showers, possibly with an embedded thunderstorm, with IFR visibility. Presently there is one such isolated thunderstorm in the vicinity of OZW (Howell). Models have been favoring the vicinity of LAN for the greatest potential for a heavy rain shower or TS over the next several hours, but other aerodromes could also get in on heavier showers temporarily. && .MARINE... Issued at 229 PM EDT Sat Aug 1 2026 As noted previously, offshore flow associated with gusty northeast winds will grow waves mostly beyond the 5 nautical mile offshore distance that marks the western edge of our nearshore zones. However, recent runs of the WW3 show that our southernmost zone (south of South Haven) may approach 4+ feet at its most southwestward extent, so decided to issue a Small Craft Advisory lasting until 4 AM. Have not issued a beach hazards statement given the suboptimal weather right now for visiting the beach. Winds will likely subside a bit towards sunrise Sunday and we may need to revisit the need for beach hazards given we will be in an advancing surface high setup that can produce locally strong shoreline parallel winds. && .GRR WATCHES/WARNINGS/ADVISORIES... MI...None. MARINE...Small Craft Advisory until 4 AM EDT Sunday for LMZ844. && $$ DISCUSSION...TJT AVIATION...CAS MARINE...TJT

NWS GRR Office Area Forecast Discussion