Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI
Issued by NWS Detroit/Pontiac, MI
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789 FXUS63 KDTX 041958 AFDDTX Area Forecast Discussion National Weather Service Detroit/Pontiac MI 358 PM EDT Thu Jul 4 2024 .KEY MESSAGES... - Patchy light rain or sprinkles remain possible this afternoon and evening, mainly toward the Ohio border, while a heavier shower is possible toward the Tri Cities and northern Thumb. - A Low pressure system in the Midwest brings a surge of showers and thunderstorms across all of Lower Michigan Friday. There is a Marginal Risk of severe thunderstorms in the afternoon and evening. - The low pressure 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. && .DISCUSSION... Upstream convective complexes changed the mesoscale environment considerably over the last 24 hours as activity grew upscale into a well organized MCV now affecting Lower MI this afternoon and evening. The mid level circulation is weakening with time but remains capable of light showers across the area through early evening. Dry air below 700 mb, shown in the 12Z DTX sounding and in hi-res model soundings, is proving to be an effective obstacle for greater coverage reaching the ground but is expected to be overcome in pockets until the mid level MCV circulation moves into Ontario by mid evening. Hourly mesoanalyses reflect the stabilizing influence of the high cloud cover limiting MLCAPE to sub 500 J/kg farther north toward the hybrid surface front/trough/lake breeze boundary lingering in northern to central Lower MI. An isolated heavier shower materialized there as of forecast issuance suggesting a rumble of thunder is possible through late afternoon and early evening peak heating. A weak mid level short wave ridge follows passage of the evening MCV circulation leading to a few hours of dry weather with some decrease in cloud cover late tonight. A quick transition into the next low pressure system then begins as the leading edge of mid level moisture approaches Lower MI Friday morning. Model trends over the last few forecast cycles indicate slower timing and much less coverage of showers on the leading edge of weaker/less effective moisture transport. While this salvages the morning mostly free of rain in SE MI, it does open the door to greater potential for severe thunderstorms in the afternoon. The upgraded outlook is mostly out of respect for the seasonably strong forcing brought by the mid level wave and upper level jet, and for the potential of surface based instability capitalizing on the lack of morning activity. It would not take much to enable overachievement of MUCAPE from sub 1000 J/kg HREF projections into the 1000-1500 J/kg range combined with the inbound wind profile producing bulk shear in the 40-50 kt range. The other moving part that may be a negative factor is occlusion of the surface system which could pinch the instability axis southward before or during the afternoon to early evening storm window. As usual, timing will then be everything in the determination of severe storm outcomes. The mid level dry slot is projected with good model consensus to punch through southern Lower MI Friday evening while still shrinking the deformation pattern from SW to NE into northern Lower MI. This occurs as the mid level circulation deamplifies rapidly leaving just a stray shower possible and entry level chance POPs maintained for the bulk of Saturday. The in-between mid level ridge then remains on schedule for Saturday night bringing weak high pressure and a chance to close out the holiday weekend with warm and dry weather through Sunday. && .MARINE... A weak trough settles across the central Great Lakes this evening with isolated showers possible mainly near the shoreline areas. Otherwise, weak winds and dry conditions prevail through early Friday. Low pressure arriving from the Midwest on Friday then brings a more active pattern with scattered to numerous showers with embedded thunderstorms expected after noon. Isolated storms may become severe with wind gusts in excess of 35 knots and hail. The low begins to move out Friday night with winds veering to the northwest, but lingering troughing holds through Saturday with additional showers possible. A broad ridge of high pressure then fills in on Sunday with mainly dry conditions and continued light winds. The next cold front moves across the Great Lakes on Monday, bringing another period of unsettled weather to start the week. && .HYDROLOGY... Low pressure moving into the Midwest swings a frontal system through Lower Michigan during Friday while supporting numerous showers and scattered thunderstorms across the area. Locally heavy rainfall could briefly reach rates of 1 inch per hour while basin average totals come in closer to 0.25 to 0.5 inch until activity diminishes and exits eastward toward evening. Steady motion of the system mitigates any serious concerns with flooding potential limited to ponding of water on roads and in areas prone to collecting standing water. && .PREV DISCUSSION... Issued at 209 PM EDT Thu Jul 4 2024 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. && .DTX WATCHES/WARNINGS/ADVISORIES... MI...None. Lake Huron...None. Lake St Clair...None. Michigan waters of Lake Erie...None. && $$ DISCUSSION...BT MARINE.......TF HYDROLOGY....BT AVIATION.....KGK You can obtain your latest National Weather Service forecasts online at www.weather.gov/detroit.