Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI
Issued by NWS Detroit/Pontiac, MI
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145 FXUS63 KDTX 241417 AFDDTX Area Forecast Discussion National Weather Service Detroit/Pontiac MI 1017 AM EDT Tue Sep 24 2024 .KEY MESSAGES... - Seasonable temperatures through the early and mid week period. - Showers are likely today and tonight. - Most areas will receive around a half inch of rain or less, but localized totals in excess of 1 inch will be possible. - A few widely scattered showers possible Wednesday. Otherwise, drier conditions through the rest of the week with temperatures trending back above normal. && .UPDATE... Ample moisture transport within broad low to mid level isentropic ascent and mid level height falls from approaching short wave impulses will sustain numerous coverage of showers this afternoon and evening, in line with the current forecast. The attendant sfc low now over south central Illinois will track into southern Michigan this evening, driving the associated sfc warm front into the far southern sections of the forecast area. Mid to upper 60s sfc dewpoints south of the front will offer the potential for some weak to moderate destabilization within the warm sector this afternoon. Some of this instability will advect into far southern Michigan later today. Current model solutions suggest anywhere from 500 to 1000 ML Cape lifting toward the I-94 corridor late this afternoon and evening. 0-6km shear values are forecast to be around 30 knots with respectable low level system relative helicity values. The expectation for low LCL heights combined with the severe parameters seems supportive of a risk of isolated mini supercells late this afternoon/evening. The primary severe weather hazard would be a risk of a weak tornado. The main uncertainty at this point will be the efficiency with which instability is able to advect into Se Mi, especially with the cloud cover and light easterly sfc winds now encompassing the forecast area. && .PREV DISCUSSION... Issued at 609 AM EDT Tue Sep 24 2024 AVIATION... Widespread IFR ceilings to start the morning with a warm frontal boundary and approaching low pressure. Column saturation peaks within the next hour two supporting LIFR at a few terminals. Ceilings then lift to MVFR midday with low-end potential to briefly cross the VFR threshold before descending to IFR (and perhaps LIFR) again tonight. Several rounds of showers are expected throughout the day with varying degrees of rainfall intensity which could result in brief IFR visibility reduction. Additionally, an embedded rumble or two of thunder cannot be ruled out after 18Z, although temporal and spatial uncertainty preclude any PROB30 adjustments at this time. Light northerly winds veer toward the east-southeast today, then southwesterly as the low departs, and eventually west-northwesterly Wednesday morning as the warm front also exits. For DTW/D21 Convection...Showers today and perhaps a few thunderstorms this afternoon and evening, but low confidence on timing and duration as precipitation remains scattered. DTW THRESHOLD PROBABILITIES... * High for ceiling at or below 5000 feet today and tonight. * Low for thunder after 13Z today, then medium from 22-03Z. PREV DISCUSSION... Issued at 321 AM EDT Tue Sep 24 2024 DISCUSSION... Broadening corridor of northward moisture transport underway in the form of extensive cloud cover and some pockets showers early this morning. This process governed by a modest increase in mid level southwest flow immediately downstream of pending height falls tied to an elongating upper trough set to arrive late today. These conditions will largely define much of the daylight period, as interludes of greater isentropic ascent working across the advancing elevated warm frontal zone contribute to intervals of generally light shower production. Insolation potential minimized with a lack of sun, while the low level gradient holds from an easterly direction. This ensures a limited diurnal temperature response, capping afternoon temperatures in the upper 60s to lower 70s. Prospective secondary convective response between 20z and 03z along the east flank of the surface low and warm frontal intersection offers greater potential for a brief uptick in rainfall intensity. General consensus of model sounding data shows meaningful depth to the instability with a skinny cape profile, with upwards of 1000 j/kg of mlcape nosing across the Ohio border. Limited residence time of any clusters will preclude a greater flooding threat. Given the track of the surface low, a localized increase in both magnitude and depth of shear remains plausible across far southern sections. While the lack of greater magnitude of destabilization would seemingly work against establishing deeper rotating updrafts and a corresponding sig wx threat, the pattern from a synoptic level gives pause and remains worthy of monitoring as future CAM cycles and observational trends arrive. Amplifying mid level trough trailing the lead wave arrives Wednesday, effectively drawing the cold front across southeast Michigan through the day. Modest frontal convergence in the presence of sufficient moisture quality and diurnal destabilization points to widely scattered shower development targeting the late day period. No meaningful airmass change this period, as temperatures again push into the 70s. Benign weather conditions for the late week period. Deep column drying with increasing stability Thursday as mid level northwest flow briefly exists atop surface high pressure. A 589 dm upper high then takes residence Friday, ensuring moderating temperatures with continued dry conditions to finish the week. Attention shifts to the south as a large closed system centered over the mid/lower Mississippi valley interacts with a tropical system forecast to make landfill over the eastern Gulf coast sometime Thursday. Southeast Michigan still likely outside the main influence of this height fall center at least early during the weekend period, but with rainfall chances steadily climbing as the system lifts east by the latter half of the weekend. Further revision to precipitation chances expected. MARINE... Light east-southeast winds (mainly 10-15 kts) this morning as an area of low pressure tracks through the Central Great Lakes late today and tonight, maintaining the lighter winds. Numerous showers with a slight chance of thunderstorms will accompany the low. Large area of high pressure building in Wednesday night-Thursday will lead to light northwest winds, becoming northeast to end the work week as the high moves off into Eastern Canada and low pressure lifts north out of the Southeastern United States. The pressure gradient will be tightest over Lake Erie, and gusts reaching at or above 25 knots appear likely Friday-Saturday morning. HYDROLOGY... Low pressure brings scattered to numerous showers and a chance of thunderstorms today and tonight. Basin average rainfall is forecast to be between one quarter and one half inch, but totals in excess of 1 inch will be possible where any training storms develop. Flooding is not likely but there is the potential if any heavier rainfall axis does develop over the urban corridor. && .DTX WATCHES/WARNINGS/ADVISORIES... MI...None. Lake Huron...None. Lake St Clair...None. Michigan waters of Lake Erie...None. && $$ UPDATE.......SC AVIATION.....KGK DISCUSSION...MR MARINE.......SF HYDROLOGY....MR You can obtain your latest National Weather Service forecasts online at www.weather.gov/detroit.