671 FXUS63 KDTX 240930 AFDDTX Area Forecast Discussion National Weather Service Detroit/Pontiac MI 530 AM EDT Tue Aug 24 2021 .AVIATION... Light southwest winds today will usher in increasing heat and humidity, leading to a build up in instability and strato CU near 5000 feet. However, the mid levels are very warm, and will tend to cap any potential thunderstorm activity. Confidence then diminishes this evening and especially tonight as a thunderstorm complex over the Western Great Lakes attempts to move into Lower Michigan. Uncertainty with how well it holds together to impact the TAF sights by 12z Wednesday. Will introduce a prob30 late tonight for northern sites (MBS/FNT), which should have best shot at thunderstorms. .DTW THRESHOLD PROBABILITIES... * Low for ceiling 5000 ft or less today and tonight. * Low in thunderstorms late tonight into Wednesday. && .PREV DISCUSSION... Issued at 314 AM EDT Tue Aug 24 2021 DISCUSSION... Se Mi currently lies within a region of subsidence as a mid level short ridge is now traversing Lower Mi. This has been sustaining mainly clear skies overnight. While this ridge axis will slide east this afternoon, model soundings suggest fairly robust capping around 800mb into late afternoon/evening. This and an overall lack of forced ascent will suppress convective development. Warmer and more moist low level air will gradually advect into the forecast area during the day amidst light west-southwest flow. Some increase in the cloud cover will occur as moisture increases below the capping inversion. This will offset diurnal heating a bit. So despite 925mb temps forecast to rise toward 25-26C, afternoon highs around 90 seem reasonable. Shower/thunderstorm chances/timing tonight and on Wednesday will be heavily dependent upon upstream convective trends which unfortunately still carry a high degree of forecast uncertainty attm. A forward propagating MCS advancing into Minnesota this morning is forecast to move into the western Great Lakes this afternoon. Overall, conditions farther east over Lower Mi will be less favorable and it is expected that this system will weaken/dissipate prior to reaching Se Mi. Additional MCS development upstream is however expected this afternoon/tonight (very decent EML plume noted across the upper Midwest). The potential for forward propagation of upstream convection into Se Mi will be more favorable overnight and through the day on Wednesday as steep mid level lapse rates and a mid level theta e plume contribute to increasing instability across Lower Mi. In light of the forecast uncertainty and high degree of spread among the hi res suite, a blanket chance type pop will be carried tonight through Wednesday evening. Moisture depth will be adequate enough on Wednesday to limit mixing down of the lower to mid 70s sfc dewpoints in the afternoon, leading to a very humid day. If convection is able to hold off until late in the day Wednesday, afternoon highs will be able to attain upper 80s to possibly lower 90s, resulting in heat indices ranging from the mid 90s to around 100. A mid level wave tracking across far nrn Ontario and James Bay Wed night into Thurs will force a cold front south across the Great Lakes. The latest model suite bring this front into Se Mi on Thursday, with the front stalling over srn Mi by Friday. It will result in a north-south moisture/temperature gradient across Se MI. Some building of the subtropical ridge across the east coast is forecast over the weekend by the GFS/ECMWf. The corresponding mid level height rises and increased south-southwest flow across the srn lakes will drive the front back north as a warm front by Saturday and will support continued hot and humid conditions through the weekend. MARINE... Light and variable winds across the central Great Lakes today as high pressure attempts to hold on downstream of a potential thunderstorm complex over the eastern Plains. Could see some activity survive into the region this afternoon supporting isolated to scattered showers/storms. Better setup for active weather arrives tonight into Wednesday with southwest wind lending concern for locally higher waves and gusts in the stronger storms. Flow turns northerly Thursday with little impact as a cold front drops south. Scattered showers or thunderstorms are also possible with this frontal passage. More energetic northeast flow and increased wave action arrives early Friday with a more pronounced surface gradient. Small Craft Advisories may be needed for the southern Lake Huron waters (tip of the Thumb) if model trends persist with occasional waves near 5 feet. && .DTX WATCHES/WARNINGS/ADVISORIES... MI...NONE. Lake Huron...NONE. Lake St Clair...NONE. Michigan waters of Lake Erie...NONE. && $$ AVIATION.....SF DISCUSSION...SC MARINE.......KK You can obtain your latest National Weather Service forecasts online at www.weather.gov/detroit.