837 FXUS63 KGRR 271835 AFDGRR Area Forecast Discussion National Weather Service Grand Rapids MI 235 PM EDT Mon Apr 27 2026 .KEY MESSAGES... - A Period of Strong Winds Likely This Afternoon and Evening - Turning Progressively Colder This Week; Frosts & Freezes Coming && .DISCUSSION... Issued at 235 PM EDT Mon Apr 27 2026 - A Period of Strong Winds Likely This Afternoon and Evening A deepening springtime low will be moving from IA toward the western U.P. tonight as a shortwave trough swings through the region. Periods of showers and a few thunderstorms will accompany this system. Strong deep layer shear moves in after 20z but with an absence of CAPE. Very little MLCAPE is indicated by the 12z HRRR for the remainder of today and tonight, with perhaps 500+ J/kg of MUCAPE moving in prior to FROPA overnight. While instability will be our main limiting factor for organized convection, the very strong wind fields just off the surface are concerning with 50-60 kts indicated from 925-850mb this afternoon and evening. What we`ve seen with the 12z CAM guidance is a signal for wake low wind gusts in the post-stratiform rain region on the north side of a convective complex moving across IL/MO and that has certainly bore out in reality. Several sites across IA and now IL have gusted in the 45-55 mph range behind the stratiform precip region moving through metro Chicago. This is coincident with 3 hour pressure drops around 8-10 mb. We feel the best way to handle this is with a Wind Advisory as opposed to the less visible SPS. If a small area of higher gusts materialize then perhaps SVRs will be needed, or if a larger scale, a High Wind Warning may be considered. The Advisory will be in place through 8pm but could be extended if the threat goes longer than anticipated. Any additional showers and storms that develop late tonight and into the overnight hours will likely be fairly tame but could put down some swaths of 0.50" or more of rain creating brief ponding of water on roads overnight before that threat diminishes. - Turning Progressively Colder This Week; Frosts & Freezes Coming Ensemble guidance is in good agreement regarding upper troughing settling in this week and below normal temperatures moving in. Mean 850mb temperatures are shown to dip to around -5C by late week. Several nights of frosts and freezes are likely to materialize across most of the region starting Wednesday night and lasting through Saturday night. The worst conditions are anticipated Thursday night and Friday night. Both of those look to be radiational events. && .AVIATION /18Z TAFS THROUGH 18Z TUESDAY/... Issued at 136 PM EDT Mon Apr 27 2026 -------------------- Key Changes ------------------------------ - Sudden increase in winds to 35-50 knots likely behind showers, Added TEMPO Monday afternoon - Adjusted timing and restrictions for overnight showers/storms --------------------------------------------------------------- Upstream wind and pressure observations across Illinois/Wisconsin/Iowa show a wake low has developed behind a line of showers approaching the area. This suggests that a sudden jump in east/southeast wind gusts to 35-50 knots is likely along the back edge of and behind a line of rain showers. This is expected to reach MKG/AZO i the 19-20z timeframe and continue east through the afternoon/evening clearing the area by 00z. Winds then return to the large scale pattern of 15G30KT once this clears. Given decreasing instability with eastward extent, can`t rule out a thunderstorm embedded within the line. However, confidence in thunder at any given terminal is low. A second wave of showers and thunderstorms arrives after 23z with isolated thunderstorms possible. A surge of moisture with this will bring MVFR to IFR conditons to all terminals. Showers and storms clear late tonight as a cold front crosses the terminals with winds shifting to westerly behind the front. && .MARINE... Issued at 235 PM EDT Mon Apr 27 2026 A short duration Gale Warning is being issued to cover gusty winds behind an area of showers and embedded thunderstorms moving over Lake Michigan. These wind gusts are already observed to be in excess of 50 mph across northern IL and would last longer than a typical thunderstorm wind gust would. The wind direction would be from the E/SE. Following the Gale, a Small Craft Advisory goes into effect to cover building waves and elevated winds over the lake into Tuesday. && .GRR WATCHES/WARNINGS/ADVISORIES... MI...Wind Advisory until 8 PM EDT this evening for MIZ037>040-043>046- 050>052-056>059-064>067-071>074. MARINE...Gale Warning until 8 PM EDT this evening for LMZ844>849. Small Craft Advisory from 8 PM this evening to 6 PM EDT Tuesday for LMZ844>849. && $$ DISCUSSION...Hoving AVIATION...Thomas MARINE...Hoving

NWS GRR Office Area Forecast Discussion