734 FXUS63 KGRR 090802 AFDGRR Area Forecast Discussion National Weather Service Grand Rapids MI 402 AM EDT Sun Aug 9 2026 .KEY MESSAGES... - Thunderstorms this afternoon and tonight with heavy rain - Chances for showers/storms Mon-Tue but focus may be south - Drying out mid to late week && .DISCUSSION... Issued at 402 AM EDT Sun Aug 9 2026 - Thunderstorms this afternoon and tonight with heavy rain High pressure is in place this morning bringing quiet and dry weather to the Great Lakes region. Upstream showers and thunderstorms have formed over portions of Minnesota. The expectation is that these storms will congeal and move our direction through the course of the morning. These showers and storms should move into the area this afternoon, after 200pm. Feel there is more of a heavy rain threat this afternoon and tonight than a severe threat, but cannot rule out severe weather either. Machine learning schemes show all severe weather hazards in play today as a frontal boundary will be draped across our southern forecast area near I-94. Wind looks to be the primary hazard near and south of I-96. MUCAPE values will be in the 1000-2000 j/kg range with deep layer shear on the order of 30-40 knots. We will be monitoring the state of the atmosphere as the activity moves in during the afternoon. There is enough of a moisture surge that comes in though which should keep instability are a moderate level. PWAT values rise from near 1.00 inch to start the day, surging towards 1.7 inches late in the day. A surge of that magnitude in the mid summer can bring both severe weather and hydro/flooding concerns. We are in a day 1 slight risk for both severe weather and flooding potential. The HREF continues to show some rather impressive 48 hour max rainfall totals. It is indicating the potential for some rainfall amounts that may locally push towards 5 inches through the day on Monday. Models continue to show redevelopment/training potential into this evening and tonight at the low level jet provides steady lift over a cold pool that will be continually reinforced. Short term flooding headlines are very possible this afternoon and evening. - Chances for showers/storms Mon-Tue but focus may be south Chances for showers and storms continue into Monday and Tuesday, but the potential does not look as good as it did yesterday. The focus of the low level jet is more off to our south, but the GFS does bring it back in on Tuesday. There is model disparity with regard to the LLJ in this time frame so confidence in forecast particulars is not high. Expecting at least some showers and storms in the south on the northern periphery of thunderstorm complexes rolling from IL to IN/OH. Any additional rainfall during this time could add to issues associated with what falls today. The day 2 (Monday) slight risk is generally south of our area and cannot argue with that given that is where the LLJ is focused. - Drying out mid to late week Surface high pressure builds in for mid to late week which should bring a respite from the active weather of earlier in the week. We actually look to dip back into the 70s for highs Thursday, Friday and Saturday. && .AVIATION /06Z TAFS THROUGH 06Z MONDAY/... Issued at 220 AM EDT Sun Aug 9 2026 This aviation forecast could be presented as the tale of two halves. The first have the forecast is rather quiet, the second half will be much more active. The first half of the forecast (06z-18z) will be quiet with ridging slowly sliding eastward away from the area. Skies will be mostly clear with clouds holding off for the most part until after 18z. Winds will be light and variable becoming south around 10 knots. The activity picks up in the second half of the forecast period, 18z to 06z. A complex of showers and thunderstorms will try to move into the area this afternoon between 18z and 21z. From 21z into the evening hours occasional showers and thunderstorms should occur with gradually lowering VFR ceilings from the mid levels 8k to 10k feet to around 4k feet towards 00z. As we approach 06z ceilings will likely be trying to push into the MVFR and IFR categories near and just north of I-96. && .MARINE... Issued at 402 AM EDT Sun Aug 9 2026 Not expecting much in the way of marine issues the next couple of days. Waves will reach the 1-3 foot range today with south winds of 10-20 knots. Waves are forecast to be 2 feet or less from tonight through Monday and into Monday night. The reason for the lack of waves is the fact that we will be situated in a fairly slack pressure gradient in a Col generally. The time frame to keep an eye on though is today. We have the potential to see higher waves on a short term basis as thunderstorms move through the region. Winds and waves are usually higher in and near thunderstorms and that should be the case today as well. The 3km NamNest shows the potential for a period of 30-40 knot winds centered around 5pm. So, we will need to keep an eye on this potential given its a mid summer weekend and boating/swimming activity should be high. && .GRR WATCHES/WARNINGS/ADVISORIES... MI...None. MARINE...None. && $$ DISCUSSION...Duke AVIATION...Duke MARINE...Duke

NWS GRR Office Area Forecast Discussion