752 FXUS63 KGRR 131902 AFDGRR Area Forecast Discussion National Weather Service Grand Rapids MI 302 PM EDT Mon Apr 13 2026 .KEY MESSAGES... - Severe thunderstorm and flood potential tonight and late Tuesday && .DISCUSSION... Issued at 302 PM EDT Mon Apr 13 2026 - Severe thunderstorm and flood potential tonight and late Tuesday This evening, multiple clusters of storms are favored to develop across Wisconsin, northern Illinois, and Lower Michigan, with low- mid level wind fields and low-level warm/moist advection increasing ahead of a shortwave trough. HREF mean MUCAPE will be around 1000-1500 J/kg with effective bulk shear around 40 knots. Model sounding thermodynamic and kinematic profiles are favorable for some hail- producing cells and scattered swaths of damaging winds especially in the first half of the night, including in southwest Michigan after 6 PM if the storms currently in Illinois maintain strength and organization. Tornado potential is relatively low but not zero, as there could be some curvature in the low-level hodographs and 0-3 km shear near 30 knots, but surface stabilization by later in the evening may tend to be the limiting factor. Tuesday evening, another wave of convection is favored with supercells, clusters, and/or lines in a more potent thermodynamic and kinematic environment. Mean HREF MUCAPE is between 1500-2500 J/kg and curved low-level hodographs and longer upper-level tails (respectable low-level and deep-layer shear) are expected. HREF Significant Tornado Parameter mean is about 2-3 during south of M-46 during the evening before the surface begins stabilizing a couple hours after sunset. Diffuse mesoscale warm-frontogenesis at the surface may also be occurring over the area during the evening. Hail and wind are both expected to be primary hazards. For information on the increasing flood threat, see the Hydrology section below. && .AVIATION /18Z TAFS THROUGH 18Z TUESDAY/... Issued at 205 PM EDT Mon Apr 13 202 Southwest Lower Michigan is in southwest flow at 18z, with a lifting cloud deck generated from last night`s rainfall. The ceilings are in the process of lifting from mainly MVFR to VFR. We await additional showers and thunderstorms that will spread in tonight, likely affecting most of if not all of the TAF sites. Confidence is not high in how storms evolve though. We will see a repeat of lower clouds settling in once again with the rain and cooling of the nighttime. The lower ceilings will persist into the morning hours of Tuesday before trying to lift to VFR once again midday Tuesday. && .MARINE... Issued at 302 PM EDT Mon Apr 13 2026 Areas of dense fog in the nearshore will likely persist into tonight and may or may not dissipate after thunderstorms pass through tonight. After a slight lull this evening, hazardous winds and waves for small craft are expected through Tuesday. Thunderstorms, possibly severe thunderstorms with hail and wind, are possible over Lake Michigan this evening/tonight and Tuesday evening/night. && .HYDROLOGY... Issued at 302 PM EDT Mon Apr 13 2026 The further north you go in the Lower Peninsula, the more significant the flooding situation becomes. Around an inch of rain fell across much of the area along and north of the Muskegon River watershed over the weekend, with a much heavier swath of 2-4 inches across the Manistee River watershed and the headwaters portions of the Muskegon basin. Coming on the heels of one of the wettest springs on record so far, and in the midst of a significantly active (and wet) ongoing weather pattern, the water that`s already on the ground is going to produce renewed flooding on the Muskegon River (and many others rivers further north as well). With another 2-3" of rain over the next few days in the Muskegon, we`re now expecting the most significant flood on the Muskegon River since the flood of 2018. If the next few days of rains overperform (which is entirely possible), the forecasted crests will go even higher. If you live near the Muskegon river, now is the time to prepare for significant flooding. Meanwhile, further south, conditions in the Grand River basin are in slightly better shape, although water is still high from all of the recent rains. We`ve finally dropped below stage at all locations in the basin, but that trend will soon reverse as water levels start to climb again throughout the week. With an additional 2-3" of rain expected in this area as well, it`s beginning to look like the region will generally repeat the flood levels experienced last week. In addition to the river flooding, some of the thunderstorms we`re expecting over the next few days could be very slow moving over the same areas repeatedly. If this happens, some localized rain totals of 4+ inches are possible in the next few days. If this happens over one of the urban areas, significant fast- developing urban flooding is possible. && .GRR WATCHES/WARNINGS/ADVISORIES... MI...Flood Watch through Wednesday morning for MIZ037>040-043>046- 050>052. MARINE...Dense Fog Advisory until 8 AM EDT Tuesday for LMZ844>849. Small Craft Advisory until 4 AM EDT Wednesday for LMZ844>849. && $$ DISCUSSION...CAS AVIATION...Duke MARINE...CAS HYDROLOGY...AMD

NWS GRR Office Area Forecast Discussion