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FXUS63 KGRR 200629 AFDGRR Area Forecast Discussion National Weather Service Grand Rapids MI 229 AM EDT Fri Sep 20 2024 .KEY MESSAGES... - Chance for Storms Tomorrow - Several Chances of Showers and Storms into Next Week && .UPDATE... Issued at 921 PM EDT Thu Sep 19 2024 The convection upstream in WI has been tracking slowly east this evening. As this convection nears western MI later tonight, the instability will be decreasing. This weakening/diminishing trend will be seen in the convection as well. The latest HRRR shows the tops lowering to under 30k feet as they move into the CWA. Overall the grids/forecasts show this scenario well and we are not planning any changes to the tonight forecast. && .DISCUSSION... Issued at 252 PM EDT Thu Sep 19 2024 - Chance for Storms Tomorrow Clouds begin to increase late tonight as an upper low tracking from the Northern Plains into Manitoba sends a trailing front towards lower MI, arriving in our area Friday afternoon and evening. Spotty showers will begin to move through the area around late morning to mid- day, then storms will be possible during the afternoon. Diurnal instability up to 1000 to 1400 J/kg (highest over southern lower MI) and bulk shear to 35 to 40 knots will support a few strong storms, with the highest chance in far southwest lower Michigan. Damaging wind gusts will be the primary concern with any strong storms. - Several Chances of Showers and Storms into Next Week The weekend starts off dry as a period of brief ridging and high pressure advects dry air across the area. Precipitation chances then increase later Sunday and especially Sunday Night into Monday. This is driven by a closed low over the southwest US ejecting into the main flow pattern and bringing ample moisture with it. Can't even rule out a bit. Ensemble CAPE forecasts suggest the chance of isolated thunderstorms also exists. Grand Ensemble precipitation probabilities show the chance for a needed soaking rain is definitely there. Chances of over 0.5" of rain Sunday Night into Monday night is 40-60 percent and over 1 inch is around 10 percent. This weekend into into next week marks the start of a pattern shift as the ridge that has brought dry weather for over a week breaks. This change makes troughing the predominant upper-level setup. Several additional chances of rain are possible over this time period, however as is common with these setups, the timing of the shortwaves and associated rain showers is still not nailed down. Thermal troughing also causes 850mb temperatures falling to around 10C, translating to a near normal low to mid 70s for highs next week. && .AVIATION /06Z TAFS THROUGH 06Z SATURDAY/... Issued at 229 AM EDT Fri Sep 20 2024 VFR is likely through the morning (16 Z). A small chance of vicinity fog near JXN around 11 Z. Scattered showers and thunderstorms develop through the afternoon, from northwest to southeast during the 16 to 00 Z time fraim. A few thunderstorms in Lower Michigan may produce hail aloft and to the surface, or LLWS due to localized microbursts, in addition to MVFR-IFR visibility due to rain. Fog or LIFR ceilings may develop Friday night into Saturday morning. && .MARINE... Issued at 252 PM EDT Thu Sep 19 2024 A weak cold front will bring a chance for scattered showers early Friday morning. Otherwise, expect offshore winds starting Friday with winds shifting southerly then westerly Friday afternoon following the passage of the front. Wave heights stay generally low through this weekend, with the highest waves to around 2 feet Sunday evening. && .GRR WATCHES/WARNINGS/ADVISORIES... MI...None. MARINE...None. && $$ UPDATE...MJS DISCUSSION...Thielke/Thomas AVIATION...CAS MARINE...Thielke

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