Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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000 FXUS63 KDTX 272304 AFDDTX Area Forecast Discussion National Weather Service Detroit/Pontiac MI 704 PM EDT WED JUL 27 2016 .AVIATION... Gust front from earlier convection north of I-69 will provide a northerly push PTK south early, but generally result in light and variable flow before frontal boundary settles through area later this evening and brings northeast flow. While additional isolated convection cannot be ruled out, gust front has become very shallow with little/no new activity recently. Will maintain dry terminals as a result. For DTW...Northerly flow will become light/variable this evening and then northeast overnight trough Thursday. May have to add VCTS for a short time if additional convection tries to fire as gust front adds convergence to existing lake breeze boundaries. Initial thought is for little/no activity near terminal attm. //DTW Threshold Probabilities... * Low for thunderstorms impacting KDTW this evening. && .PREV DISCUSSION... Issued at 402 PM EDT Wed Jul 27 2016 DISCUSSION... Forecast reasoning provided in the updated forecast discussion earlier today remains in good agreement with the SWOMCD issued at 1747 UTC. The combination of low level equivalent potential temperature content and introduction of midlevel cyclonic flow has allowed convection to initiate and develop along the northern instability gradient. Showers and thunderstorms should continue to fill in with time along the sagging frontal boundary. A strong to isolated severe thunderstorm threat exists primarily with any new or fresh convective updrafts. The main potential impacts from thunderstorm activity will be to 40 mph and brief heavy rainfall with slow storm movement of west to east at 20 mph. Given the overachievement to convection coverage thus far, did bump up POPs into the likely or numerous range across the Saginaw Valley and northern Thumb. A pronounced anticyclonic curvature to the fair weather cumulus streets south of M 46 suggests a continued strong ridge influence. Expecting dry weather for southeastern Michigan south of the I 69 corridor. Shower and thunderstorm activity should push out of the area by 2 to 3Z. Thursday. A relatively high amount of uncertainty has been surrounding this period with little inter-model continuity pertaining to the evolution of midlevel moisture and cyclonic flow that is now over Wisconsin. The ECMWF model which had been very progressive in bringing the potential wave into southeastern Michigan for the daytime Thursday has backed off considerably with this solution. This has always been sort of a tricky period forecasting with a total lack of any clean or pristine potential vorticity advection in the background of very weak 1000-500mb geopotential height falls. Therefore, it has been difficult to go above a chance pop. The weather for Thursday will be governed by a synoptic cold front or equivalent potential temperature boundary that will be in the process of slowly settling southward through lower Michigan. Models agree fairly well the boundary will not totally clear the area until Thursday night. Increased high temperatures for the Detroit Metro slightly more into the upper 80s, but these values could still be a degree or two too cool. With easterly flow in place to the north of the boundary there is a reverse lake shadow look to the thermal and moisture fields. The end result is a better theta e ridge structure developing over western lower Michigan due to increased low level convergence. The 27.12z ECMWF is in agreement with the best precipitation potential occurring to the west of the cwa. Significantly rolled POPS back with low to mid chance POPs, dry in the Thumb. It is appearing there will be a diurnal flare up to the chances as well, and may need total maturation of boundary layer, after 21Z to get activity into SEMICH. MLCAPES in the southern and western sections of the cwa are expected to be limited in the 250 to 750 J/KG range. There is a very low potential for strong thunderstorm development. Friday. There is again a muddled solution with surface high pressure trying to maintain control. The problem is that overall flow pattern aloft is a blocked one and is actually cyclonic. Combine this will some very persistent, weak, ragged potential vorticity of low confidence, a continued non zero chance for showers and thunderstorms exists on Friday. The problem is, cannot justify blanketing a POP for the setup. Decided to go dry and will allow future shifts to reevaluate. With lower dewpoints, temperatures Friday will definitely feel less humid with highs in the low 80s. Broad upper troughing over the Upper Midwest this weekend will give way to rising heights next week. This will result in a near normal temperatures this weekend and a warming trend next week. As for precipitation chances this weekend, medium-range guidance and ensembles continue to differ amongst each other and in a run-to-run sense on whether any consolidation of vorticity can take place. If it does, it could result in a meaningful precipitation event (e.g., 12Z GEM). If the pattern remains ill-defined, it will likely result in no more than a few showers for areas south of M-59. At this point will continue to play the blender approach with low chance PoPs over the south this weekend. MARINE... Light wind in advance of a cold front will turn moderate northwesterly upon its passage late tonight into Thursday. Accompanying frontal passage will be a chance of thunderstorms across the waters. Elevated waves will exist in the nearshore waters, but are not expected to meet small craft advisory criteria at this time. Moderate winds will turn northeasterly as high pressure builds in Friday and Saturday. Small craft advisories will be necessitated during this time due to persistent onshore flow. && .DTX WATCHES/WARNINGS/ADVISORIES... MI...NONE. Lake Huron...Small Craft Advisory from 11 AM Thursday to 4 AM EDT Friday FOR LHZ421-422-441>443. Lake St Clair...NONE. Michigan waters of Lake Erie...NONE. && $$ AVIATION.....DG DISCUSSION...CB/DT MARINE.......JVC You can obtain your latest National Weather Service forecasts online at www.weather.gov/detroit.

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