Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI
Issued by NWS Detroit/Pontiac, MI
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075 FXUS63 KDTX 201703 AFDDTX Area Forecast Discussion National Weather Service Detroit/Pontiac MI 103 PM EDT Fri Sep 20 2024 .KEY MESSAGES... - Warm weather continues through this weekend with a slight increase in humidity each day. - Scattered showers and storms are possible this afternoon and early tonight with additional rainfall Sunday night into Monday. - Lower confidence in rain chances for the middle of next week while temperatures cool off to seasonable values. && .AVIATION... Weak south-southeast winds with mid to high clouds around this afternoon ahead of a weak cold front. Lower VFR clouds will arrive early this evening, as the weak convergence long the front slowly crosses southeast Michigan. Enough instability around to support scattered coverage of showers late this afternoon into the evening, with isolated thunderstorms not out of the question, but too limited in potential to highlight attm. Border line MVFR/VFR cigs will be possible where showers develop. Clouds decrease late tonight behind the front, which could lead to a period of low stratus or fog by sunrise Saturday as winds will be nearly calm. Held the visibility in the tafs in MVFR for now, as confidence is low in dense fog, as there may be just enough wind around. For DTW/D21 Convection...Scattered showers around this evening with a slight chance of thunderstorm as a weak frontal boundary tracks through. Will continue with the prob30 mention of showers. Confidence in thunderstorms is too low to include at this time. DTW THRESHOLD PROBABILITIES... * Low for ceilings at or below 5000 ft this evening and tonight. * Low for thunderstorm chances this evening. && .PREV DISCUSSION... Issued at 357 AM EDT Fri Sep 20 2024 DISCUSSION... After 13 consecutive days of dry weather tied to longwave ridging that encompassed the northeast quadrant of CONUS, the first viable chance for rainfall commences this afternoon for portions of Southeast Michigan. A protruding occlusion resulting from a mature low pressure system located over northern Manitoba supports the development of a secondary low over Illinois today. Ongoing convection is expected to drift eastward through the morning hours with the translation of the trough axis (and weak cold front). Most deterministic solutions and ensemble members support areas of measurable rain this afternoon and into the evening hours given some degree of integrity and staying power of the underlying dynamics. However, the column is prohibitively subsaturated per the 20.00Z KDTX RAOB, particularly in the mid-levels. Latest thinking supports high-based initial showers after 18Z with perhaps some rumbles of thunder near the Tri-Cities, but SBCAPE is non-existent in a subset of available NWP data. 20.03Z HRRR does suggest an earlier timing and fits well with the 20.00Z ARW/NAM3km/FV3 solutions depicting a stark decay in showers as they approach the Metro area after 22Z given less favorable ThetaE profiles and declining instability. Made some adjustments to PoPs to reflect higher confidence in rain for the northwest part of the CWA and lesser confidence for the southeast localities. Southeast winds veer westerly Saturday morning as the composite frontal boundary heads into southern Ontario. This opens up a brief period of stability as short wavelength ridging builds across Lower Michigan. Residual morning clouds burn off by late morning yielding a similarly warm day in the absence of post-frontal cooling. A more active upstream pattern materializes Sunday as a shortwave speeds across The Cascades phases with a southern stream jet streak over The Dakotas. This effectively kicks the anticyclone into motion and shears out the feature while the cyclonic wave begins to close off over northern Ontario. Corridor of height falls nose into the area with persistent train of CVA from the high plains into the Great Lakes. Ridge of ThetaE traces from The Gulf up to the Ontario low with higher potential for showers and some storms locally Sunday PM and overnight. Timing uncertainty lends only Chance PoPs Sunday evening before the northern system`s cold front approaches offering a more substantial focus for showers and storms. Notable divergence in solutions early next week with defined post-frontal dryness or a stalled frontal boundary that generates rain until mid-week. One thing is clear, temperatures will break from late June readings into more Fall-like ranges with highs in the 70s. MARINE... The long stretch of dry weather and light winds will continue through the morning hours as high pressure remains in control. The dry weather will come to an end for portions of the Great Lakes starting this afternoon and evening as a cold front and upper-level disturbance aids in the production of scattered to numerous rain showers with embedded thunderstorms. Thunderstorm activity will be most likely over northern Lake Huron this afternoon and evening. Slightly elevated gusts nearing 20 knots will be likely during the frontal passage across northern Lake Huron, but otherwise, a weak pressure gradient will maintain lighter winds through the weekend. Additional rain and thunderstorm chances return Sunday evening through Monday. && .DTX WATCHES/WARNINGS/ADVISORIES... MI...None. Lake Huron...None. Lake St Clair...None. Michigan waters of Lake Erie...None. && $$ AVIATION.....SF DISCUSSION...KGK MARINE.......AM You can obtain your latest National Weather Service forecasts online at www.weather.gov/detroit.