Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI
Issued by NWS Detroit/Pontiac, MI
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585 FXUS63 KDTX 211634 AFDDTX Area Forecast Discussion National Weather Service Detroit/Pontiac MI 1234 PM EDT Sat Sep 21 2024 .KEY MESSAGES... - Warm weather continues through the weekend with higher humidity and rain returning Sunday evening. - Lower confidence in rain chances next week, but higher confidence in temperatures cooling off to seasonable values. && .AVIATION... Clear-mostly clear skies around the rest of the day with light west- northwest flow. The surface moisture/higher dew pts will be attempting to diminish as the boundary layer mixing depths increase. This makes for a challenging forecast tonight with the fog potential, as high clouds spilling over tonight also attempts to migitate fog potential as well. Confidence is low in avoiding fog altogher, as there looks to be a light easterly trajectory setting up late, which puts the moisture source of Lake Huron/Lake ST. Clair into play. Ultimatilly, will carry just a MVFR visibility for now and monitor the progress of the high clouds this evening. Increasing southerly flow on Sunday, with gusts expected to top out around 20 knots. For DTW/D21 Convection...No thunderstorms are expected through Sunday morning. DTW THRESHOLD PROBABILITIES... * None && .PREV DISCUSSION... Issued at 348 AM EDT Sat Sep 21 2024 DISCUSSION... Final lingering showers have dissipated/exited stage left into the southern Ontario peninsula early this morning with the departing warm sector (and ThetaE ridge). This gives way to a drier forecast for today as height rises permeate from southern stream longwave ridging, but the overall response will be somewhat muted. The basal portion of an upper level trough brushes North Dakota, shearing the embedded shortwave ridge folding into Lower Michigan which is located along the northern end of the synoptic ridge. Did remove previous low-end PoPs over eastern Lower through the rest of the morning. Latest obs and GOES composite imagery suggest accelerating morning fog development in the wake of nocturnal cloud cover. Will monitor the need for a CWA-wide SPS given the rather isolated nature of advisory worthy sub-half mile visibilities, but those locally dense areas may persist until approximately 9 AM. For the rest of today, weak westerly winds persist with increasingly clear skies as isentropic downglide further dries the column. H8 temps remain elevated today in the absence of post-frontal thermodynamic relief, in addition to some net-positive advective processes. This lends a similar temperature forecast today with highs in the low to mid 80s. Attention then turns upstream to a consolidating PV anomaly over south-central Canada that strengthens through the weekend and eventually drives a frontal boundary (and cooler airmass) across the Upper Midwest. Lots of moving parts to address regarding when the front actually reaches Southeast Michigan while a closed low over the desert Southwest opens up into a belt of seasonably strong southwesterly winds from the Texas Panhandle into northern Ontario. This promotes a steady northeast shove of higher specific humidity values that eventually spill eastward into the local area marked by a two-fold increase in PWATs from 18Z today to 18Z Sunday. Showers and storms will likely be on-going Sunday morning along the dynamically active corridor, arcing into the primary closed/occluded 1000 mb surface low tracking into southern Hudson Bay. This noses the frontal zone eastward Sunday evening with perhaps a secondary surface convergence response over southern Lower which converts hours of top-down moistening into a cluster of pre-frontal showers and non-severe storms. Too soon to call specifics on timing, but NWP support is largely lacking on anything developing before 18Z Sunday. Consensus of deterministic and ensemble QPF for the 24-hour period ending at 18Z Monday averages to around a half to three quarters of an inch of rain with some indications of the better FGEN and/or left- exit region ascent for the northern half of the forecast area which could see closer to an inch. Did lower highs by a few degrees Sunday (low 80s) to align with more restrictive cloud field. Low confidence carries into the early week extended forecast period as the lower portion of the frontal slope stalls out and waffles over the western Great Lakes as evidenced by a quasi-static H8 temperature gradient. Surface winds come full-circle with a northwesterly feed ushering in a more seasonable airmass with highs in the low-mid 70s. Steady feed of CVA and saturated low-levels maintains persistent Chance PoPs through midweek before several wave interactions get underway. This includes an incredibly amplified (and progressive) ridge over the western half of NoAm, the release of the south-central Canadian low, and a potential tropical system traversing the Southeast. MARINE... A ridge of high pressure will hold across the Great Lakes today, bringing the continuation of light winds with the return of dry weather. The ridge of high pressure will hold through tomorrow morning, but rain and embedded thunderstorms will fill in over the Great Lakes starting tomorrow afternoon and evening, initially from a low pressure system with added support from a cold front. Wind direction will veer from SSE to NNW following the passage of the cold front. Dry conditions are expected Monday after the passage of the front, with additional rain chances entering on Tuesday as a second low pressure system pushes from the Ohio Valley into Lake Erie/Ontario. && .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.