Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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000 FXUS63 KGRR 231148 AFDGRR Area Forecast Discussion National Weather Service Grand Rapids MI 748 AM EDT Fri Apr 23 2021 LATEST UPDATE... Aviation .SYNOPSIS... Issued at 330 AM EDT Fri Apr 23 2021 - Milder temperatures and a rainy Saturday - Warm Tuesday then potential for heavier rain mid/late week && .DISCUSSION...(Today through next Thursday) Issued at 330 AM EDT Fri Apr 23 2021 -- Milder temperatures and a rainy Saturday -- Sunshine today will become increasingly filtered through cirrostratus and altostratus. A plume of slightly warmer air at 850 mb will be over us, allowing high temperatures to reach near normal for the first time since Sunday. Rain on Saturday (especially the all-day washout kind) is not guaranteed for everyone, and there remain considerable differences in rainfall amounts among HREF and ECMWF ensemble members ranging from less than 0.10 to as much as 0.75 inches even at any one particular location. The upper level trough is not fully phased as it passes over Michigan, with the two distinct vorticity maxima located over the Ohio Valley and northern Wisconsin/Michigan. The greatest potential for rain will develop amid a plume of mid-level moisture from the southern stream undergoing isentropic ascent over southern Michigan. It`s this area of steady, more stratiform rain where models are most disagreeing with both amounts and northward extent, but this would provide the greatest potential for an all-day gently soaking rainfall. More showery (shallow convection) and spotty rain amounts may develop later in the day farther north in central Michigan associated with the northern upper level trough and surface cold front. -- Warm Tuesday then potential for heavier rain mid/late week -- After a cool Sunday behind the cold front, temperatures rebound on Monday as strong warm-air advection resumes. There is a fair amount of spread among the ensembles for Monday`s high temperature depending on how quickly the warm air can return, though a majority of ECMWF members give areas south of I-96 temperatures well into the 60s, with central Michigan staying a little cooler. Tuesday appears to be the day with greatest predictability next week, with a sharp Rossby wave trough carved out in the western CONUS and a building ridge over the Midwest. Nearly all members of the ECMWF, GFS, and CMC ensembles give Grand Rapids a high in the mid 70s to around 80. Will introduce thunder chances Tuesday night as mid-level lapse rates are steep and an extension of the low level jet may advect moisture underneath. The predictability of exact temperatures and precipitation chances thereafter is lower, but in a broad sense, as the trough moves through the center of the country and deep moisture advects from the Gulf over the Great Lakes, a cluster of ensemble members provides 1 to 2 inch rain swaths over us sometime between Wed and Thu. The evolution of this trough later in the week is completely in question, and while temperatures Thu into Fri may trend cooler and near normal within and behind the trough, there remain a number of ensemble members that bring warmth back for the first weekend of May. && .AVIATION...(For the 12Z TAFS through 12Z Saturday morning) Issued at 748 AM EDT Fri Apr 23 2021 VFR weather is expected to continue both today and through the evening hours. High clouds will gradually lower and thicken this afternoon into tonight. Bases will lower from above 20,000ft this morning to 5,000 to 10,000ft this afternoon and evening. Clouds will begin to dip into the MVFR category overnight ahead of a weather system advancing our way. By 12z on Saturday, there should be fairly widespread MVFR conditions in place. Rain showers will break out overnight, especially for areas south and west of KGRR like KAZO. Southwest winds of 10-20 knots are expected today. && .MARINE... Issued at 330 AM EDT Fri Apr 23 2021 Models are showing 30 to 40 knot winds just 1000 feet above the water prior to sunrise this morning, however, the temperature inversion between that height and the water will prevent the winds from mixing down. Lake water temperatures are about 40 to 43 degrees, insufficient to enhance mixing as temperatures at 1000 feet are about 45 degrees. No plans to issue a small craft advisory this morning, however, in warm advection environments with southwest winds, gusty winds can still occur in the harbors despite the inversion. Can`t rule out some gusts from the southwest at 20-25 knots this morning. Otherwise, conditions hazardous to small craft will hold off until Saturday night into Sunday. && .GRR WATCHES/WARNINGS/ADVISORIES... MI...None. LM...None. && $$ SYNOPSIS...CAS DISCUSSION...CAS AVIATION...Duke MARINE...CAS is the U.S. government's official web portal to all federal, state and local government web resources and services.