246 FXUS63 KGRR 081854 AFDGRR Area Forecast Discussion National Weather Service Grand Rapids MI 254 PM EDT Mon Jun 8 2026 .KEY MESSAGES... - Rain and Thunderstorms This Afternoon into Tuesday - Hot Wednesday - Storms Wednesday night through Thursday night, severe possible - Dry Friday, thunderstorm chances return for the weekend && .DISCUSSION... Issued at 254 PM EDT Mon Jun 8 2026 - Rain and Thunderstorms This Afternoon into Tuesday No real changes to previous thinking. We are now at a short enough forecast projection time to fully utilize the CAMs for assessing the heavy rain threat. Starting with the SPC HREF, LPMM/PMM QPF guidances advertises localized splotches of 2+ inches tonight into early Tuesday. This looks plausible given that we are looking at anomalously high precipitable water, nearly 13000 ft warm cloud depths, and almost stationary Mesoscale Beta Element (MBE) vectors, indicative of the potential for boundaries to synergize with convection in a way that keeps precipitation slow-moving. All of this being said, the individual CAM solutions point towards only scattered and progressive storms that would reduce the threat for heavy rain. That, when combined with relatively dry antecedent conditions, reduces the threat for flooding somewhat; however, rainfall rates could still be quite significant. Even half an hour of intense rainfall like this could be enough for localized flooding, especially across extensively paved areas. For Tuesday, convection will eventually depart to the east and as noted previously, thunderstorms could redevelop in the afternoon/evening timeframe around US-127 during peak diurnal destabilization. Suboptimal thermodynamics and kinematics will likely keep this convection sub-severe. Additional thunderstorms may develop with the warm front late Tuesday night into Wednesday morning, otherwise chances hold off until Wednesday night. - Hot Wednesday Wednesday looks to be the hottest day of the week with apparent temperatures reaching or exceeding 100F farther south towards I-94. Highs around 90F and dewpoints in the low to mid 70s are rather unusual for this time of year, so people will likely be unaccustomed to these type of conditions. The area will likely remain under partly cloudy skies until Wednesday night when the next round of thunderstorms enters the area. - Storms Wednesday night through Thursday night, severe possible An advancing upper level trough will bring increasingly southwest and slightly difluent flow to the area Wednesday through Thursday. This will provide ample synoptic forcing for multiple rounds of thunderstorms. Precipitable water (PWAT) values will remain anomalously high, approaching two inches, meaning that very efficient rainfall can be expected with quick accumulations even for the more rapidly moving storms. As noted previously, we expect mainly remnant convection from WI Wednesday night. Downstream conditional instability over Lower MI will be quite strong with MUCAPE at or above 3000 J/kg. This suggests an outflow-dominant MCS will be the most likely storm mode, yet one still with the potential to create expansive damaging wind gusts. After a pause in precipitation for the first part of Thursday, dynamic forcing for the Thursday night round of storms will be particularly strong with cyclonic breaking of the upper wave across the area in the 06-12Z Friday timeframe. Although this does coincide with maximal diurnal stabilization, it`s highly plausible that an MCS with an already established cold pool will be upstream and poised to move into the forecast area. Furthermore, the shear and instability parameter spaces look better balanced compared to Wednesday night. This means probabilities are better for greater organization, intensity, and longevity with this particular system with the main threat being damaging winds. - Dry Friday, thunderstorm chances return for the weekend Dry air moves in behind the thunderstorms Thursday night bringing a break in the rainfall along with cooler temperatures. Rain chances return for the weekend with an upper level trough in the vicinity. The best chances (30 to 50 percent) continue to be Saturday night into Sunday as was also noted previously. This consistency in model guidance helps confidence a little, but predictability is simply not great at this time range after a succession of other precipitation events. So, be prepared for a possibly ever-changing weekend forecast. && .AVIATION /18Z TAFS THROUGH 18Z TUESDAY/... Issued at 147 PM EDT Mon Jun 8 2026 No real changes to the inherited TAFs except to refine timing of onset of precipitation tonight based on radar trends and to advertise IFR ceiling restrictions continuing after 12Z Tuesday. These TAFs were already quite verbose due to persistent and hard- to-time probabilities of thunderstorms, so didn`t want to add too much detail after 12Z, but the upshot is that precipitation will exit the area from west to east in the 12Z to 18Z Tuesday with a gradual transition to VFR conditions. && .MARINE... Issued at 254 PM EDT Mon Jun 8 2026 Waves are expected to remain 2 feet or less through Wednesday morning as winds remain light. Southeast winds are expected through tonight before shifting to the southwest behind the low. Rain and thunderstorms are expected late this afternoon into Tuesday. Expect low visibilities within any thunderstorms as they will be capable of heavy rainfall. The increased dew points over the cold waters of Lake Michigan may also pose a threat for fog as well. We`ll need to monitor conditions in case a Marine Dense Fog Advisory is needed. Winds and waves then pick up Wednesday afternoon into Thursday. South to southwest winds gusting to around 25 knots will cause waves to build to 3 to 6 feet especially for areas north of Holland and around the Sable Points. && .GRR WATCHES/WARNINGS/ADVISORIES... MI...None. MARINE...None. && $$ DISCUSSION...TJT AVIATION...TJT MARINE...TJT/RAH

NWS GRR Office Area Forecast Discussion