845 FXUS63 KGRR 190543 AFDGRR Area Forecast Discussion National Weather Service Grand Rapids MI 143 AM EDT Tue Aug 19 2025 .KEY MESSAGES... - Showers and Storms through Tuesday Afternoon - Dry Wednesday through Friday - Cooler with a Few Showers Possible Next Weekend && .DISCUSSION... Issued at 323 PM EDT Mon Aug 18 2025 - Showers and Storms through Tuesday Afternoon 700mb wind and moisture convergence continue to drive the ongoing showers over the lake and along the US-10 corridor. The line of shower and storms in Wisconsin is expected to make it into the lakeshore around 6pm. This line will weaken as instability decreases into western Michigan. Coverage will increase further east when the shortwave and associated lift with the low level jet and warm air advection move in later this evening and through the overnight. While shear is decent around 30 knots overnight, instability is lacking with HREF mean MUCAPE values of only a few hundred J/kg. Therefore do not expect widespread lightning or severe storms overnight into Tuesday morning. Precipitable water values will be around 2 inches however leading to efficient rainfall rates around an inch or more per hour with any thunderstorm. HREF localized probability- matched mean highlights isolated areas along the lakeshore potentially seeing 2 to 3 inches of rain in a 24 hour period. Taking a look at the shorter range 6 hour porbability-matched mean values are generally an inch or less, therefore the current risk for flooding is low. Some showers and storms may linger into Tuesday afternoon for areas mainly south and east of Grand Rapids, otherwise dry conditions move in for the remainder of the day. - Dry Wednesday through Friday High pressure builds in resulting in dry conditions through Friday. Expect afternoon high temperatures in the 80s with lows in the 50s and 60s. - Cooler with a Few Showers Possible Next Weekend A deep upper level trough looks to shift southeast into the norther Great Lakes over the weekend. There has been a slight trend upward in PoPs (20 to 30 percent) Friday night into Saturday morning. Rain chances are limited due to the amount of dry air with this system, otherwise colder temperatures are expected with highs in the upper 70s to low 80s on Saturday dropping into the 70s for Sunday into Monday. Some areas Sunday night into Monday could see temperatures drop into the upper 40s. && .AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/... Issued at 143 AM EDT Tue Aug 19 2025 Changes this update were to refine timing of thunderstorms and to increase southeasterly winds in the short term ahead of the approaching surface low. Thunderstorms have been having a difficult time reaching our eastern terminals before dissipating but chances for this happening should increase later this morning as the surface low and associated isentropic ascent shift eastward into Lower Michigan. We expect showery precipitation to redevelop across much of Lower MI as this happens. Forcing for upward vertical motion will be greatest at the I-94 terminals (AZO-BTL- JXN) and at LAN. This will foster convective destabilization and be supportive of thunderstorm development later today. Model guidance suggests that showers and thunderstorms should be quite unstructured and popcorn-like in coverage; this is common in these types of isentropic ascent regimes. This does make timing at terminals difficult, however, so have elected to use PROB30 quite extensively after 12Z. Another complicating factor is lowering ceilings in the vicinity of the approaching low. IFR seems likely areawide later today. Western terminals such as MKG...AZO...GRR could see a transition to MVFR or even VFR around or after 00Z tonight, but did not include this for the sake of brevity. && .MARINE... Issued at 335 PM EDT Mon Aug 18 2025 The first concern on Lake Michigan is what appears to be a similar setup to last night. A southerly low-level jet with winds around 25 knots just off the deck develops tonight. However, unlike last night, we will be in low-level warm advection instead of cold advection helping to somewhat hamper our ability to mix down any gustier winds. Given that, while isolated small craft advisory level winds are possible, will hold off on an advisory for now given low confidence in duration/coverage and will re-evalaute with the newest short term guidance this evening. The next concern for boaters and swimmers comes tomorrow afternoon. Synoptically a classic "advancing high" setup will develop with short range model guidance showing a healthy burst of wind down the shoreline in the afternoon/evening. Particularly combined with the post-frontal airmass conditions will likely be favorable for a Beach Hazards Statement and Small Craft Advisory. However in coordination with neighbors will let the midnight shift make the final call. Looking at available medium and short range guidance, the signal is there for the advancing high setup to recur Wednesday afternoon and evening bringing the potential for Beach Hazards/Small Craft Advisory conditions once again. && .GRR WATCHES/WARNINGS/ADVISORIES... MI...None. MARINE...None. && $$ DISCUSSION...RAH AVIATION...TJT MARINE...Thomas

NWS GRR Office Area Forecast Discussion