FXUS63 KGRR 031142 AFDGRR Area Forecast Discussion National Weather Service Grand Rapids MI 742 AM EDT Thu Jul 3 2025 .KEY MESSAGES... - Slight chances for showers or isolated storm today and July 4 - Holiday weekend heat - Showers and storms possible Saturday night through Sunday && .DISCUSSION... Issued at 348 AM EDT Thu Jul 3 2025 - Slight chances for showers or isolated storm today and July 4 A weak front becoming oriented NW-SE and flow-parallel is mostly stalled out over Lower Michigan today. It may have a better pronounced moisture difference on either side of it than a temperature difference. Some afternoon surface convergence over southern portions of Mid Michigan today may assist in popping up isolated showers. The threat of storms will be tempered by midlevel height rises and warming temperatures around 10-15 kft / around 0 Celsius which will limit instability at and above that level. For July 4, all of the latest HREF member models show spotty precip over Western/Mid Michigan, worth adding some 20 percent PoPs / slight chance showers or storm during the day. Elevated thunderstorms may be ongoing in the morning over Wisconsin, fed by southwesterly warm air advection. Some models take storms across Lake Michigan during the morning but those solutions may be too aggressive. Additional surface-based development may occur over Mid Michigan during the afternoon within a zone of low-level convergence. The day-by-day increasing low-level heat and moisture may just counter-balance the upper-level ridging, which yields more modest lapse rates and skinny CAPE, offering some chances of localized showers/storms but again with a tempered overall threat. - Holiday weekend heat A building upper-level ridge is expected over the Midwest and Great Lakes Fri-Sat, July 4-5. Temperatures near 90 on July 4 with dew points in the mid 60s will yield a heat index in the lower to mid 90s. On Saturday, temperatures are favored to reach in the lower 90s in most locations away from the Lake Michigan shore, and heat index values may reach the mid to upper 90s. Whether or not this reaches the traditional "heat advisory" criteria, the impacts of heat and humidity will be at least of moderate concern for people active outdoors or without air conditioning. - Showers and storms possible Saturday night through Sunday An upper-level trough and surface cold front passing through late Saturday night through Sunday may touch off areas of thunderstorms. While rain is not a guarantee everywhere, the high PW environment with increasing wind shear may produce some heavy downpours along with a marginal severe wind threat. && .AVIATION /12Z TAFS THROUGH 12Z FRIDAY/... Issued at 742 AM EDT Thu Jul 3 2025 Expecting VFR cloud bases today at or above 5,000 ft. Short term models overnight have been fairly consistent in showing a potential for afternoon showers mainly between 18 and 00 Z, affecting east and southeast of GRR, which would include BTL, LAN, and JXN. Any showers that do develop could produce temporary drops in visibility, along with some LLWS and local surface wind gusts around 25 knots. && .MARINE... Issued at 424 AM EDT Thu Jul 3 2025 Low Swim Risk due to waves and currents today and on July 4 with winds topping out around 10 to 15 knots. At least a Moderate Swim Risk on Saturday July 5, especially near and north of Grand Haven, with south winds 15 to 20 knots and waves 2 to 4 feet, possibly 3 to 5 feet at some beaches with a High Swim Risk. Moderate to High Swim Risk also likely on Sunday with 15-20 knot west-northwest winds and waves 2-4 or 3-5 feet. && .GRR WATCHES/WARNINGS/ADVISORIES... MI...None. MARINE...None. && $$ DISCUSSION...CAS AVIATION...CAS MARINE...CAS