Area Forecast Discussion
Issued by NWS Grand Rapids, MI
Issued by NWS Grand Rapids, MI
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562 FXUS63 KGRR 070800 AFDGRR Area Forecast Discussion National Weather Service Grand Rapids MI 400 AM EDT Tue May 7 2024 .KEY MESSAGES... - Afternoon-evening thunderstorms, possibly severe - Rain on Thursday and Saturday && .DISCUSSION... Issued at 400 AM EDT Tue May 7 2024 - Afternoon-evening thunderstorms, possibly severe On the southeast periphery of a maturing low over the Dakotas, a 500 mb trough and vorticity maximum will move from the central Plains to the western Great Lakes today. Upper level height falls and an approaching left exit region of an 250 mb jet streak will broadly support ascent and cooling in the midlevels. In the lower levels, the nose of a southerly 850 mb jet will slide through northern Indiana and southern Michigan while a surface warm front is expected to progress northward to roughly a Muskegon-Jackson line during the afternoon. A frontal triple point and possibly a deepening surface pressure field is expected in the vicinity of Chicago/Milwaukee by late afternoon. Ongoing line of thunderstorms marching from the Plains to Mississippi river valley overnight is expected to arrive in southwest Lower Michigan around 11 AM to Noon and progress northeastward toward Clare by 3-4 PM while weakening as it moves into a more stable environment. Severe weather is not likely with this first round, though lightning may still be a hazard for southwest locations. In its wake, rapid moistening of the low levels (through some combination of rainfall and advection) along with cooling mid-upper levels (with the arrival of the 500 mb trough) is likely to support development of scattered thunderstorms (possibly severe) between 4-10 PM, even if there is considerable cloud cover. HREF mean surface-based CAPE in southwest Michigan reaches 1000 J/kg by 5 PM in southwest Michigan, south of a Muskegon-Jackson line, though there may be also be an effective unstable layer above the surface on either side of the warm front. Most of the CAMs show similar character of convection this evening, with a mode of scattered to numerous cells of small to moderate width evolving into clusters across southern Michigan, and perhaps other quasi-linear thunderstorms farther north crossing Lake Michigan. These storms should be strongly sheared, given an expected 0-3 km wind difference of 30 to 40 knots. The magnitude of 0-1 km streamwise vorticity available for the given storm motion is questionable, however. There may be cells which briefly encounter a more favorable environment to attain supercell characteristics, perhaps in vicinity of the warm front, or with less predictable environmental modification from preceding cells or interactions between cells. Can`t really rule out any severe hazard in this environment, though the most likely limiting factors may be if potential instability isn`t realized or if updraft width remains modest. - Rain on Thursday and Saturday A couple shortwave troughs will result in chances for showers peaking Thursday and Saturday. Confidence is above average with good model agreement and ensemble support. One trend that has been noted since yesterday is a further south shift in the track of the low on Thursday which could mean the rain stays south of central Lower Michigan. This is not yet completely reflected in the current grids. An area of mid-level frontogenesis north of a low pressure center tracking across the Ohio Valley will result in rain moving in late Wednesday night and continuing through Thursday afternoon. Northerly flow on the back side of the low will advect in a cool Canadian airmass. We will have to watch for frost potential if skies clear out Thursday night. Shortwave ridging should continue the fair weather for Friday into Friday night but clouds will be moving in after midnight as the next shortwave trough approaches, with rain moving back in for Saturday. The trough cits off into a 500 mb low across our area by Saturday and there could be some diurnal enhancement of the showers Saturday afternoon. Given the steep lapse rates, some thunder is possible and even some hail with any deeper updrafts. The low lifts out gradually early next week but there is low confidence on how quick the showers end, with model differences and ensemble spread increasing after Sunday. && .AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/... Issued at 123 AM EDT Tue May 7 2024 VFR conditions will prevail through Tuesday morning then MVFR conditions could develop by 18Z was a band of showers moves north into Lower Michigan. Strong thunderstorms are possible between about 20Z and 23Z and these storms could bring hail and strong gusty winds. Otherwise winds will be light tonight then southeast to south winds around 10 knots could gust to 20 knots at times on Tuesday. && .MARINE... Issued at 400 AM EDT Tue May 7 2024 East-southeasterly winds today become southerly this evening and southwesterly tonight. Hazards to boaters will come from thunderstorms crossing over and/or developing over Lake Michigan between 11 AM and 9 PM. These may produce lightning, hail, or stronger winds. && .GRR WATCHES/WARNINGS/ADVISORIES... MI...None. MARINE...None. && $$ DISCUSSION...CAS/Ostuno AVIATION...Ostuno MARINE...CAS