Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI
Issued by NWS Detroit/Pontiac, MI
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358 FXUS63 KDTX 251042 AFDDTX Area Forecast Discussion National Weather Service Detroit/Pontiac MI 642 AM EDT Wed Sep 25 2024 .KEY MESSAGES... - There is a low chance for showers today, with dry weather forecast Thursday and Friday. - Daytime highs will mainly be in the 70s. && .AVIATION... Very moist low level conditions left in the wake of an exiting low pressure system this morning. This will maintain an extensive coverage of IFR to lower MVFR stratus throughout the first half of the day. Improvement in boundary layer depth with daytime heating will work to lift ceilings into VFR with time this afternoon. Early day southwest wind becoming westerly with modest speed today. Some widely scattered showers possible. Drier air arrives this evening and overnight as winds become northwesterly. This will bring a decrease in cloud coverage through the night. Some potential for patchy fog toward daybreak Thursday morning. For DTW/D21 Convection...Thunderstorms are not expected this period. DTW THRESHOLD PROBABILITIES... * High for ceiling at or below 5000 feet through this afternoon. Low this evening. && .PREV DISCUSSION... Issued at 239 AM EDT Wed Sep 25 2024 DISCUSSION... Several wave interactions get underway today as an incredibly amplified (and progressive) long wavelength ridge extending from New Mexico to the Northwest Territories releases eastward. This leads to a more defined split in the downstream vorticity field aloft, resulting in the eventual distinction between two cyclonic systems; a closed low over Southeast Missouri and a shortwave trough over Ontario. Locally, waning CVA between these two features still supports a few additional isolated low-topped showers through the morning hours. The occluded (southern) low is forecast to hold position over the Mid-Mississippi Valley today while a secondary height fall response resulting from the merger of yesterday`s departing low and upstream a shortwave troughing the periphery of the inbound ridge brushes the northern half of the forecast area. Cannot rule out some light showers/sprinkles during the afternoon hours today based on steepening boundary-layer lapse rates and a couple hundred J/kg of SBCAPE, but weak capping inversion near 6 kft AGL should suffice in preventing any meaningful precipitation (or convective) response. Latest guidance does favor areas along/north of I-69 which are better situated with respect to the next lobe of vorticity generated from the composite cyclonic wave. Formidable stratocumulus clouds start to scatter out early this evening with the approach of inbound ridging. Given H8 temps of 10- 12C (more in-line with GFS/ECMWF) and a brief late day diurnal boost, highs warm into the low 70s for most with mid 70s more likely for the urban areas. Effective clearing and realized sky fraction will dictate the degree of nocturnal cooling tonight. A modest gradient in overnight lows is expected between the Metro Detroit and the Tri-Cities (upper 50s to lower 50s, respectively) as a bit of low cloud and perhaps some stray cirrus linger later into the overnight period for the Metro area. Lower column anticyclone emerges through H7 over the Upper Midwest Thursday morning while southern Lower Michigan becomes the center point of a narrow split-flow configuration. Surrounding synoptic evolution involves the composite Canadian upper trough closing off as it moves into Quebec, becoming increasingly ageostrophic as the basal curvature interfaces with Mid-Atlantic ridging. To the southwest, the cut-off low retrogrades and turns counter-clockwise back into the northern Gulf states as Hurricane Helene slams into the Florida Big Bend Thursday evening, colliding with the orphaned low. Fortunately, all of these moving parts will be of little impact for the local forecast area Thursday given statically stable thermodynamic profiles and subsidence which yield mostly sunny conditions and highs in the mid-upper 70s. The sequence of the aforementioned wave interactions will have an impact on how conditions unfold over Southeast Michigan Friday night once the post-tropical wave slingshots into the Ohio Valley via the vertically stacked low. Current expectations roll the remnant Gulf system into the Lower Ohio Valley Friday evening marked by a 993 mb surface low quickly rolling northwest across southern Indiana (21Z Fri to 03Z Sat). This lends potential for some light showers, mainly south of I-94, as mass gets shove up adjacent isentropic surfaces. Wide range of outcomes in-play this point given the conditional nature of how the overall pattern unfolds as evidenced by 0.75 inches of rainfall by 12Z Saturday per 25.00Z GDPS versus zero via ECMWF/ICON. Auto-populated WPC/NBM QPF of a few hundredths of an inch of seem reasonable at this time, as long as trends maintain a more southerly track. Given the proximity to the system, areas closer to the Michigan/Ohio border can expect a more notable gust response. East-northeast winds will be capable of gusting to around 30 mph Friday afternoon/evening. Non-zero rain chances continue Saturday and into Sunday for the southern half of the CWA as the coupled lows merge near the confluence point of the Ohio and Mississippi Rivers. There is some longitudinal spread between the ECMWF and GFS as where this unification occurs, but the ThetaE field will be well-established by this point in time with higher likelihood for additional rainfall correlated with areas that received rain Friday night and/or Saturday morning. Temperatures will be a few degrees above climatological averages during this time with highs in the mid 70s and lows in the upper 50s. A progressive Pacific wave-train forces the system eastward Sunday or Monday which precedes a polar airmass by the middle of next week. MARINE... Low pressure clearing the Central Great Lakes this morning, with mostly light northwest winds around today, except over northern Lake Huron where winds look to briefly top out around knots. Winds then dropping at or below 15 knots Wednesday night into Thursday as high pressure builds back into the region. Winds becoming northeast to end the work week as the high moves off into Eastern Canada and low pressure lifts north out of the Southeastern United States. The pressure gradient will be tightest over Lake Erie, and gusts of 25 to 30 knots appear likely Friday-Saturday morning. && .DTX WATCHES/WARNINGS/ADVISORIES... MI...None. Lake Huron...None. Lake St Clair...None. Michigan waters of Lake Erie...None. && $$ AVIATION.....MR DISCUSSION...KGK MARINE.......SF You can obtain your latest National Weather Service forecasts online at www.weather.gov/detroit.