991 FXUS63 KGRR 130542 AFDGRR Area Forecast Discussion National Weather Service Grand Rapids MI 142 AM EDT Wed May 13 2026 .KEY MESSAGES... - Showers and a few thunderstorms this evening - Small chance for showers on Friday - Higher chances for showers/storms Saturday night through Tuesday && .DISCUSSION... Issued at 240 PM EDT Tue May 12 2026 - Showers and a few thunderstorms this evening A cold front associated with a clipper system will sweep through the area this evening and tonight. The front should clear the forecast by 600am on Wednesday. Forcing and instability should combine to produce a few thunderstorms during the 700pm to 200am time frame. Given the presentation of 4hr reflectivity in the HREF, storms should be isolated/scattered at best. This makes sense given marginal moisture in place at the surface. 50-55F dew points surge in from the southwest ahead of the front. The marginal moisture results in low MUCAPE values at 500 j/kg or less. Where frontal lift combines with MUCAPE values around 500 will be the locations that a few storms develop. If low level moisture overperforms a bit, we could see some small hail or gusty winds to around 40 mph. We get back to dry weather for Wednesday and Thursday. - Small chance for showers on Friday There is a chance for showers that exists on Friday associated with a weakening cold front pressing in from the west. We are carrying 20-40 percent chances for light rain showers across Western Lower Michigan on Friday with dry weather in the east. Some light pops are possible in all areas Friday night. The driver of the precipitation other than the weakening cold front are some embedded shortwaves in zonal flow. Overall what occurs will be on the light side. - Higher chances for showers/storms Saturday night through Tuesday We move into a more active pattern later in the weekend into early next week. Deep southwest flow develops with a couple of systems in play through the Sunday through Tuesday period. A warm front lifts into the area from the southwest on Sunday. We break into the warm sector Sunday night into Monday before a cold front sweeps through Monday night. We will have much deeper moisture in the area during this time frame with surface dew points pushing into the 60s. PWAT values rise to around 1.6 inches which is near daily max values looking at climatology. Sunday and Monday look to be the better chances for convection that may trend towards severe given the combination of moisture and stronger LLJ winds. && .AVIATION /06Z TAFS THROUGH 06Z THURSDAY/... Issued at 139 AM EDT Wed May 13 2026 Showers and thunderstorms have cleared the terminals and the rest of the period will be dry, but cloudy. Latest IR loop shows some breaks in the clouds ahead of a larger area of cloudiness that will move over the terminals within the next few hours associated with cold air advection. We`ll see cigs fall to MVFR prior to 12z and remain there much of the day before clearing and a return to VFR by 00z. && .MARINE... Issued at 240 PM EDT Tue May 12 2026 A clipper system moving through the Great Lakes region today and tonight will bring Lake Michigan stronger south winds today which veer to the northwest tonight. South to southwest winds this afternoon and evening will reach the 20-30 knot range with a few gale force gusts at times. We are thinking the gale gusts will not be persistent enough for an upgrade to a Gale Warning. Winds shift to the northwest after midnight with a similar setup...20-30 knots with a chance for gale force gusts. We will be monitoring both time frames, but will continue with the Small Craft Advisory for now for today and tonight. Winds and waves will decrease to below advisory levels Wednesday night. Will hold off on extending the SCA for now through Wednesday just to make sure we do not need to upgrade. Later shifts will make that decision. && .GRR WATCHES/WARNINGS/ADVISORIES... MI...None. MARINE...Small Craft Advisory until 2 AM EDT Thursday for LMZ844>849. && $$ DISCUSSION...Duke AVIATION...04 MARINE...Duke

NWS GRR Office Area Forecast Discussion