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Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI
Issued by NWS Detroit/Pontiac, MI
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391 FXUS63 KDTX 151957 AFDDTX Area Forecast Discussion National Weather Service Detroit/Pontiac MI 357 PM EDT Thu Aug 15 2024 .KEY MESSAGES... * Showers increase coverage this evening becoming widespread with a rumble of thunder possible late tonight through Friday morning. * A short break occurs late morning and early Friday afternoon followed by another round of thunderstorms from mid afternoon through early evening. This activity will be capable of heavy rainfall and an isolated storm could produce damaging wind. * Broad coverage of low pressure stalls across Lower Michigan this weekend. The system supports showery and unsettled weather with temperatures on the cool side of normal through Sunday. && .DISCUSSION... A well organized but slow moving low pressure system tracks through the Midwest tonight while spreading showers a few thunderstorms across Lower Mi into Friday morning. This occurs along and ahead of a warm front that is still west and south of SE Mi in the morning. The warm front does make some northward progress by afternoon before occlusion allowing a warm sector instability profile a chance to build in during peak afternoon heating. Mid afternoon to mid Friday evening then holds as the primary time window for severe thunderstorm potential. The Day 2 Marginal Risk remains in place in the afternoon update. Early to mid afternoon observations support model depictions of the mid level system becoming closed over the surface low center in northern MN by this evening. The mid level moisture axis extends from the Gulf coast northward and becomes sharply defined into the eastern flank of the MN system. Veered low to mid level wind then sets up an effective pattern of moisture transport that is already underway around the 700 mb level but which reloads and deepens into Lower Mi tonight. This is the primary forcing for showers and possibly a rumble of thunder, although with a minimal amount of elevated instability indicated in model data. HREF mean surface based CAPE is zero through 15Z Friday, as expected with the front to our south, and MUCAPE holds below 1000 J/kg as an elevated instability indication. This matches up with some old school deterministic 850 mb LI right around zero within the Theta-E ridge. Progression of the surface occlusion draws the warm front up into central Lower Mi Friday afternoon. The warm sector then briefly covers the area with good timing for a mid afternoon boost in surface based instability. Consensus of deterministic and HREF CAPE rises into the 1500-2000 J/kg range by 21Z but is strongly dependent on adequate recovery from morning rain activity of generous coverage and duration. There does appear to be a respectable decreasing cloud trend possible during early afternoon judging by mid level model RH fields suggesting the forecast be open to at least low end recovery of instability. The surface frontal features then become a good focus for convergence just as difluent flow crosses into Lower Mi ahead of the mid level circulation. The limiting factor for severe intensity storms may end up being the low end wind profile that barely reaches 30 knots through 500 mb during late afternoon into Friday evening. Mixed pulse/multicell storm mode in an otherwise moist vertical profile favors locally heavy rainfall with a rain loaded borderline severe wind gust as the primary hazards. The mature closed low pressure system continues to occlude and cut off Friday night. A mid level dry slot is projected across model solutions which brings a few hours of dry weather later Friday night. Moisture wraps around the center of the mid level low by Saturday morning setting up a textbook closed low/cold core shower pattern. Coverage is modulated by the diurnal cycle and a thunderstorm is also possible during peak heating Saturday afternoon. The stalled system sets up a repeat shower/storm performance for Sunday and temperatures hover a few degrees below normal throughout the weekend. && .MARINE... Leading wave of showers has already started to impact inland portions of lower Michigan at issuance, and will gradually expand over the waterways this evening-overnight as elevated portions of a warm front pivot into the Great Lakes. Not seeing much instability with this first band of precipitation, limiting widespread thunder chances until daytime Friday. Wind gusts approach 25 knots overnight- Friday morning, but frequent gusts and wave heights fall short of advisory thresholds. Several additional rounds of showers and thunderstorms will be possible Friday and Friday night ahead of a slow-moving upper low. There is a chance that storms Friday afternoon-evening could be strong, producing localized wind gusts in excess of 35 knots along with lightning and heavy rain. Shower and thunderstorm chances last through the weekend as the low drifts slowly overhead, eventually shifting southerly winds to the north by early Sunday morning. Onshore flow and another uptick in wind speeds are then possible early next week in the wake of the low. && .HYDROLOGY... Showers increase coverage tonight with a rumble of thunder possible late tonight and Friday morning. Storms become likely again from mid afternoon into early Friday evening, and this is when flooding has greater potential as storms will be capable of locally heavy rainfall. A broad low pressure system then stalls over Lower Michigan Friday night through the weekend. Total Rainfall over the 3 day stretch is expected to be near 1 inch, but any strong thunderstorms could produce enhanced totals around 2 inches. Localized minor flooding of urban, low-lying, and otherwise flood prone areas remains possible in the strong thunderstorms, but the off and on nature of the rainfall is not expected to cause significant flooding. && .PREV DISCUSSION... Issued at 155 PM EDT Thu Aug 15 2024 AVIATION... VFR conditions will prevail as coverage of high clouds increase into the evening. The increasing mid level moisture will also bring increasing rain chances this evening and tonight while some degree of low level dry air holds. Expecting conditions to remain predominately VFR with FEW-SCT cloud bases below 5,000 feet, though any heavier shower may bring a brief period below VFR. Covered the more likely time frames for pockets of rainfall with TEMPO groups between 00Z and 09Z with potential for a break in activity early tomorrow morning before an uptick in shower activity occurs again around 12Z and beyond. Isolated thunderstorms will be possible, but low probability precludes mention at this time. For DTW/D21 Convection...Low probability for an isolated thunderstorm late tonight and Friday morning. Friday afternoon will hold a slightly better chance for thunder. DTW THRESHOLD PROBABILITIES... * Low for thunderstorms late tonight and Friday. && .DTX WATCHES/WARNINGS/ADVISORIES... MI...None. Lake Huron...None. Lake St Clair...None. Michigan waters of Lake Erie...None. && $$ DISCUSSION...BT MARINE.......MV HYDROLOGY....BT AVIATION.....AA You can obtain your latest National Weather Service forecasts online at www.weather.gov/detroit.