Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI
Issued by NWS Detroit/Pontiac, MI
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397 FXUS63 KDTX 041809 AFDDTX Area Forecast Discussion National Weather Service Detroit/Pontiac MI 209 PM EDT Thu Jul 4 2024 .KEY MESSAGES... - A stray shower or thunderstorm remains possible this afternoon and evening, mainly north of I69 and toward the Ohio border. - A Low pressure system in the Midwest brings a surge of showers and thunderstorms across all of Lower Michigan Friday. - The system lingers over the Great Lakes Saturday keeping a chance of showers in the forecast. - The holiday weekend closes out with dry weather Sunday under partly sunny sky. && .AVIATION... Area of showers lifting into far southern Lower Michigan will need to overcome an abundance of dry air in low-levels before rainfall has any chance at reaching the surface. Forecast soundings continue to advertise large dewpoint depressions below the 5 kft AGL level which suggests an initial virga response. Overall convective picture appears rather muted for this afternoon/evening, therefore only went with a TEMPO for showers as lighting should be sparse in areal coverage and frequency. Initial lake breezes are evident on radar with limited inland penetration and a lack of parcel activation as diurnal heating struggles beneath the cloud canopy. Upstream observational trends, in conjunction with latest CAMs, favor column moistening and subsequent rainfall lifting northeast by 20Z with generally restriction-free ceilings and visibilities. Convective trends will need to be monitored for any necessary amendments related to updraft overachievement and potential isolated storm development through the early evening hours once a vicinity trough moves through the Lower Peninsula over the next several hours. Conditional on evening rainfall/moistening and nearly SKC conditions, some fog is possible early Friday morning, thus TEMPO-ed in a brief MVFR mention. Dry start in-store Friday with increasing VFR clouds. For DTW/D21 Convection...Showers and perhaps rumble of thunder possible this afternoon and evening. Low confidence in thunderstorm development from the current line of showers and with additional activity from an inbound trough, therefore opted for a -SHRA mention. Better chances for storms expected Friday afternoon/evening with scattered activity. Did include a PROB30 for tomorrow. DTW THRESHOLD PROBABILITIES... * Low for thunderstorms today from 20-00Z, then medium for Friday from 22-00Z. && .PREV DISCUSSION... Issued at 1053 AM EDT Thu Jul 4 2024 UPDATE... Mesoscale trends continue to be monitored for any update needs related to showers and/or storms this afternoon and tonight. So far, the slight chance/isolated POP in the going forecast is expected to hold based on morning observational trends. High cloud debris moving across Lower MI originates from the central Plains to Ohio valley convective complex while the associated northern fringe of stratiform rain remains south of the IN/OH border. The rain area shows signs of diminishing with the nocturnal component of elevated instability, but the parent MCV does maintain some organization and will be capable of grinding out a light high based shower while traversing Lower MI this afternoon. The mid level circulation also has potential to interact with a larger scale surface trough/front analyzed across the northern Great Lakes. This trough is projected to settle into central Lower MI this afternoon while blending in with some lake breeze augmentation. This is another favored area for an isolated shower or thunderstorm, although inbound high clouds will limit surface based instability through late afternoon and early this evening. PREV DISCUSSION... Issued at 331 AM EDT Thu Jul 4 2024 DISCUSSION... Despite the "cold" frontal passage last evening, overall environment remains largely the same over SE MI with dewpoints still holding in the 60s and 850mb temps in the mid-upper teens (C). While past model trends prior days suggested the frontal boundary would push far enough south over Ohio to keep associated activity likewise south, latest trends, particularly within the CAMs, suggest a much muddier picture. Shortwave wave feature is progged to ride along this boundary over northern Ohio supporting a complex of showers/storms to our south. The northern periphery of the stratiform component of this complex looks to partially shed north into southern lower MI early afternoon, after ~19Z, resulting in high-based widely scattered coverage of a light shower-drizzle-virga mix. Additionally, some signal exists for an isolated convective development chance along the edge this stratiform `shield`, focused over areas north of I-69, where lesser cloud cover supports the generation of a few hundred J/kg of MLCAPE combined with potential lake breeze interaction increasing low level convergence and upper jet divergence residing directly overhead. Overall, its a messy setup that carries inherit low predictability for any one area actually seeing rain, though aforementioned trends can`t be ignored so have introduced slight chance PoPs (15-20%) across the region for the latter half of the day. Low pressure rapidly develops over the Plains Thursday before reaching the western Great Lakes early Friday. Surface circulation draws the staled frontal boundary over the Ohio Valley back north into southern MI Friday afternoon while also beginning to undergo the occlusion process. Result is a complex frontal setup over SE MI as both the warm/occluded fronts work into the region. Areas under the warm sector- most likely along/south of I-69, will see the return of lower 70 dewpoints as a ribbon of enhanced theta-e is wrapped in from the mid-Mississippi. While this moisture axis would support a respectable instability axis (between 1000-1500 J/kg of MLCAPE), ample cloud cover likely prevents this from being fully realized. Regardless, moisture-rich environment beneath seasonably strong synoptic forcing, both from the mature low/mid-level trough as well as left exit region jet divergence, should support scattered to numerous showers with embedded scattered thunderstorms through the afternoon-evening hours before the arrival of the mid-level dry slot late evening/overnight. Upper trough tracks over the central Great Lakes Saturday allowing thermal troughing to settle into the region and drop 850mb temps to around 10C. Cooling aloft in concert with favorable synoptic difluent flow aloft looking to support the diurnal steepening of lapse rates as well as potentially generate at least of few hundred J/kg of mixed-layer instability. This offers chances for isolated to scattered showers across SE MI for the afternoon-early evening hours. Low amplitude mid-level ridging and accompanying weak surface high pressure then briefly work across southern lower MI for Sunday bringing drier, sunnier conditions. MARINE... Weak high pressure builds into the central Great Lakes through the day with light westerly winds early becoming light and variable. Overall benign weather prevails, but may see an isolated pop-up shower near the nearshore waters in the afternoon or evening. Wind then organizes out of the east by Friday morning as the next low pressure system moves into the western Great Lakes. This system sends a warm front through during the day Friday with scattered to numerous showers with embedded thunderstorms. The cold front will follow late on Friday with winds veering around to southwest by Saturday morning at around 10 to 20 knots. Unsettled weather with light showers remains possible on Saturday as troughing is slow to depart. HYDROLOGY... Low pressure arrives over the Great Lakes Friday generating numerous showers with scattered embedded thunderstorms for much of the daylight hours. Gulf moisture is looking to be wrapped into this system pushing precipitable water values near 2 inches. Basin wide average rainfall totals between 0.25-0.5 inches are most favored given the progressive nature of rain expected. However given the moisture rich environment, locally higher amounts around 1 inch would be possible under any thunderstorms. Minimal flooding potential expected outside the usual low lying areas. && .DTX WATCHES/WARNINGS/ADVISORIES... MI...None. Lake Huron...None. Lake St Clair...None. Michigan waters of Lake Erie...None. && $$ AVIATION.....KGK UPDATE.......BT DISCUSSION...KDK MARINE.......TF HYDROLOGY....KDK You can obtain your latest National Weather Service forecasts online at www.weather.gov/detroit.