Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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000 FXUS63 KDTX 180259 AFDDTX Area Forecast Discussion National Weather Service Detroit/Pontiac MI 1059 PM EDT Thu Aug 17 2017 .AVIATION... Stratus will filter into area within southwest flow around low pressure crossing through the northern Great Lakes. Expect MVFR cigs late tonight through a good part of Friday with a trend late back to VFR and eventually some clearing. These southwest winds will be a bit gusty from mid/late Friday morning through the afternoon with gusts into the 22-26 knot range. Wind direction will also veer to a more westerly direction during the afternoon. For DTW...Expect generally VFR stratus to work into area tonight, but some potential of MVFR late at night. These conditions will hold on Friday with improvement back to VFR from afternoon into evening. Wind gusts to 25 knots will be possible from 240-250 degrees. //DTW THRESHOLD PROBABILITIES... * High in ceilings AOB 5000 ft late tonight into Friday afternoon. && .PREV DISCUSSION... Issued at 300 PM EDT Thu Aug 17 2017 DISCUSSION... A very moist but debris laden warm sector now overtaking southeast Michigan, the arrival accompanying a secondary low level theta-e surge and the associated arcing line of showers/thunderstorms. The early afternoon timing of the precipitation effectively shrinking the window for recovery. There is a narrow corridor of clearing now migrating into the backside of this cloud debris field, allowing for some degree of destabilization aided low 70s dewpoints. SPC mesoanalysis indicates roughly 500-700 J/KG of MLCAPE within this zone as temperatures bounce back toward the 80 degree range. It will be within this axis where some renewed development remains plausible this evening as a pre-frontal trough sweeps through. Latest hi-res guidance suggests a low coverage, as convective growth continues to fight modest instability, with paltry low level lapse rates. The background deep layer wind field will remain supportive of convective organization should instability allow for more robust updrafts, but certainly a very conditional severe weather risk at this stage. Mid level circulation will lift from lake superior this evening to south-central Ontario by midday Friday. Trailing shortwave pivoting around the southern periphery of this system will track across central lower MI overnight into Friday morning. An increasingly component of dry air advection ongoing through this time becomes a strong limiting factor in sustaining rainfall chances. Isolated potential for a few showers/sprinkles given the accompanying increase in cva with this wave, but certainly nothing organized given the downturn in deep layer moisture quality and unfavorable timing at the diurnal minimum. Period of 850-925 mb cold air advection emerges by Friday morning, likely accompanied by some expansion in lower stratus. Weakly cyclonic mid level flow augmented by the diurnal process likely sustains a high degree of cloudiness through the daylight period, particularly with northward extent. Moisture depth and forcing is lacking, suggesting shower production will be limited. Firm gradient extending out from the system over Ontario will make for breezy westerly conditions. Highs arriving largely within the 75 to 80 degree range. Next shortwave sliding through the broaden background troughing timed to arrive late Friday night into early Saturday. Brief uptick in mid level dynamics with this system may prove sufficient for a quick shot of rainfall/embedded thunder, before increasing mid level stability/subsidence takes a progressive hold from northwest to southeast through the latter half of Saturday. With good prospects for some late day recovery, highs in the vicinity of 80 degrees will be attainable. Surface high pressure centered across the Ohio Valley on Sunday and the majority of Monday will keep conditions dry as south/ southwesterly winds help raise daytime high temperatures well into the 80s for both days. Rain and thunderstorm chances will gradually ramp up late Monday into Tuesday as upper-level troughing with embedded short-waves starts to build across the Great Lakes aloft. At the surface, persistent southerly winds will allow a sufficient amount heat and moisture to build in across the state, producing sufficient conditions for shower and thunderstorm chances. A cold front is also expected to move through late Tuesday, which will provide additional lift for shower and thunderstorm development. Behind the cold front, dry air will filter back in from the northwest, bringing quiet conditions for the late half of next week. The cold front will act to keep temperatures capped in the 70s for a daytime high Thursday, and if the GEM and ECMWF models pan out, highs in the 70s will persist into the weekend. MARINE... South-southeast winds generally in the 15 to 20 knot range this afternoon will shift to the southwest tomorrow behind the cold front which is tracking through this evening. Post frontal cold air advection will boost the over-lake instability profile, and allow for peak wind gusts more in the 20 to 30 knot range. Wind and waves will then decrease over the weekend as high pressure builds into the Great Lakes region. HYDROLOGY... Tropical airmass sliding east of southeast Michigan this afternoon, and drying has commenced. None-the-less, scattered heavy rain producing thunderstorms remain possible through early this evening with leftover daytime instability. With the thunderstorm coverage expected to be limited and progressive nature of activity, not expecting flooding as additional basin averages look to be quarter of an inch or less. However, localized quick half an inch to one inch total remains possible with the strongest storms. && .DTX WATCHES/WARNINGS/ADVISORIES... MI...NONE. Lake Huron...Small Craft Advisory from noon to 8 PM EDT Friday for LHZ443. Small Craft Advisory until 8 PM EDT Friday for LHZ441-442. Small Craft Advisory from 10 AM to 8 PM EDT Friday for LHZ421-422. Lake St Clair...Small Craft Advisory from noon to 8 PM EDT Friday for LCZ460. Michigan waters of Lake Erie...Small Craft Advisory from noon to 8 PM EDT Friday for LEZ444. && $$ AVIATION.....DG DISCUSSION...MR/AM MARINE.......SF HYDROLOGY....SF You can obtain your latest National Weather Service forecasts online at www.weather.gov/detroit.

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