188 FXUS63 KDTX 092333 AFDDTX Area Forecast Discussion National Weather Service Detroit/Pontiac MI 733 PM EDT Thu Jul 9 2020 .AVIATION... Limited window yet late this evening for an isolated thunderstorm in the vicnity of FNT and MBS based on recent radar trends. Otherwise, simply some lingering high based cloud cover within a light south or variable wind into the mid morning hours. A greater potential for thunderstorms will exist Friday afternoon and early evening as a frontal boundary tracks through the region. An initial round of convection may arrive as early as 15-16z, with more widespread activity anticipated for mid-late afternoon. Confidence remains high enough to outlook these periods for all southeast Michigan terminals. DTW THRESHOLD PROBABILITIES... * Medium for thunderstorms Friday afternoon and early evening. && .PREV DISCUSSION... Issued at 356 PM EDT Thu Jul 9 2020 DISCUSSION... Another hot and muggy afternoon and evening to get through on this Heat Advisory stretch across southeast Michigan before a little relief arrives tomorrow. Southwest flow into the area has maintained high moisture with dewpoints in the upper 60s to low 70s. This is both increasing heat indices to near 100 and supporting the continued chances for diurnally driven convection. The main trigger for these showers and thunderstorms are lake boundaries and minor waves rolling through the area. Any initial activity will then kick out outflow boundaries, which could then triggering additional showers and thunderstorms. Environment today is characterized by MLCAPE values of 1000-2000 J/kg, steep low level lapse rates of at least 8-9 C/km with flow below 20 knots up to around 10 km. In this weakly sheared environment, disorganized pulse type showers and thunderstorms will be the main storm mode. High moisture and steep low level lapse rates will support downburst potential from water loading of the more robust updrafts. The Heat Advisory will remain in effect through this evening as warm and muggy conditions persist with overnight lows remaining in the 70s. The good news is a brief pattern shift takes place tomorrow as a shortwave is forecast to track across the Great Lakes. Timing of this shortwave will bring increasing cloud cover early in the day with scattered convection possible during the late morning/early afternoon. This will mitigate further warming and hold high temperatures in the upper 80s to low 90s. High moisture with dewpoints in the upper 60s to low 70s will remain in place tomorrow, so it will still feel pretty muggy. Continued higher moisture will bring heat indices into the low to mid 90s with only localized spots capable of reaching the upper 90s in the urban heat island of Detroit. This moisture will again fuel convection with the best forcing for more widespread activity after 18Z with the approaching cold front and larger scale ascent. Clouds and any earlier activity puts into question the degree of instability that can develop with slightly better shear promoting better organization than previous days. It still appears to be a low severe threat at this time, but it will be something to keep an eye on tomorrow as the day evolves. Northwest post frontal winds bring a much needed break from the highs in the 90s this weekend. Humidity will drop as well with the 70s dewpoints being pushed south of the state with high temperatures forecast to fall back into the 80s. The synoptic pattern over the weekend will have troughing/lower heights over the Great Lakes as a few Pacific shortwaves move through the northern periphery of the Four Corners ridge. This leaves open the possibility for some scattered showers and thunderstorms over the weekend with the stronger shortwave on Sunday bringing the best chance for more widespread activity. Unfortunately, the respite from heat will begin to release through early next week as ridging builds into the Great Lakes and allows southwest flow to bring a return of heat and humidity. By Wednesday, models forecast 850mb temperatures climbing back above 20C helping high temperatures rise back into the low/mid 90s range. MARINE... A weak pressure gradient across the region will persist through tonight while providing light and variable winds across the region. Afternoon thunderstorm chances will once again exist today but most storm activity will remain over land areas where instability will be a little greater. A weak area of low pressure will then move across Lower Michigan on Friday. This system will bring a much better chance for thunderstorms to the marine areas. This system will also result in light south-southeast winds across the marine areas Friday with winds backing toward the northwest by Saturday as the low pressure system moves east of the region. Light offshore winds will keep marine conditions favorable outside of thunderstorm activity. && .DTX WATCHES/WARNINGS/ADVISORIES... MI...Heat Advisory until midnight EDT tonight for MIZ047-048-053-060>062- 068>070-075-076-082-083. Lake Huron...NONE. Lake St Clair...NONE. Michigan waters of Lake Erie...NONE. && $$ AVIATION.....MR DISCUSSION...AA MARINE.......DRK You can obtain your latest National Weather Service forecasts online at www.weather.gov/detroit.