372 FXUS63 KDTX 192259 AFDDTX Area Forecast Discussion National Weather Service Detroit/Pontiac MI 659 PM EDT Tue Oct 19 2021 .AVIATION... Large high pressure situated over the southeastern US will continue to dominate the weather across Se Mi into the day Wednesday. Aside from a few high clouds this evening, skies will generally remain clear into Wed morning. With Se Mi along the northern edge of this high pressure system, light WSW winds will occur through the night. Mid and high level clouds will then increase across Se Mi Wed afternoon in advance of a slow moving upper low. .DTW THRESHOLD PROBABILITIES... * None. && .PREV DISCUSSION... Issued at 405 PM EDT Tue Oct 19 2021 DISCUSSION... Our next leg up on this week's warming trend is fully underway this afternoon with a boost from full sun and light SW surface wind. Highs settle into the lower 70s leading into a classic fall evening in the Great Lakes with just a gradual thickening cirrus cloud pattern before midnight. Clouds thicken and lower with a light SW surface wind, both of which help buoy temperatures for lows only around 50 by sunrise Wednesday. Satellite imagery this afternoon indicates the central Rockies upper level low making a move into the Plains on its way toward the Great Lakes during Wednesday. This occurs while the cold front settles from northern Ontario into the Straits area by Wednesday morning. Clouds over SE MI late tonight and early Wednesday are a result of mid level theta-e advection and establishment of the theta-e ridge around 700 mb along and ahead of the surface front. Below that, dry air lingers over the region from broad high pressure on the Atlantic coast, a factor that still favors slower onset timing for showers during the afternoon before stronger moisture transport Wednesday night. A few sprinkles are possible toward sunset near the Tri Cities while thicker clouds provide more of a challenge for high temperatures. Readings in the lower 70s do seem possible once again with the help of continued SW flow ahead of the Plains low pressure system. The potential for showers and a few thunderstorms expands across Lower MI Wednesday night driven initially by low level jet forced moisture transport ahead of the inbound low pressure system. SW flow around 850 mb is shown in model data increasing toward 40 knots after midnight leading to strong isentropic ascent in advance of the low level theta-e ridge. This transitions to greater dynamic forcing as the upper level circulation moves into the region while interacting with the low level moisture axis. Regional and available hi-res models show surface based instability following a standard diurnal wane late Wednesday night while maintaining some elevated instability judging by 850 mb LI hanging around -2 C into Thursday morning. The strength of the system and plentiful moisture supply add confidence to the categorical forecast for showers with some support for general thunder in entry level instability. The low pressure system continues developing Thursday resulting in a pronounced dry slot sweeping through the region during the morning. Model soundings and low level moisture fields suggest a few hours of drizzle takes the place of showers in SE MI. North flank deformation sets up initially to the north and is then drawn into southern Lower MI Thursday afternoon as the upper level circulation moves over the area. Showers then cut off quickly after the system exits Thursday evening and the associated cold front sweeps southward. There is an initial surge of cold advection post front that returns temperatures to the 50s by Friday. The decidedly fall-like air mass is then provided some reinforcement by additional rounds of upper level height falls which also keep things unsettled heading into the weekend. MARINE... The northern extent of an area of high pressure centered over the southeastern US will keep the local weather quiet through Wednesday. The ridge has shifted slightly eastward allowing for a wind shift to southwesterly which will persist through Wednesday as well. A low pressure system tracking across northern Ontario tomorrow will force a cold front southward across Lake Superior tonight. The front will continue south reaching central Lake Huron by Wednesday evening where it will then stall and become a warm front as a second low over the central Plains then tracks up along it and across the region Thursday. The front will offer chances of showers and thunderstorms starting Wednesday night, which will continue into Thursday night. Winds will be southwesterly on the south side of the front and northeasterly to the north of it. Cold air advection behind the system will bring about gusty northwest flow on Thursday. && .DTX WATCHES/WARNINGS/ADVISORIES... MI...NONE. Lake Huron...NONE. Lake St Clair...NONE. Michigan waters of Lake Erie...NONE. && $$ AVIATION.....SC DISCUSSION...BT MARINE.......DRK You can obtain your latest National Weather Service forecasts online at www.weather.gov/detroit.