Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI
Issued by NWS Detroit/Pontiac, MI
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115 FXUS63 KDTX 171307 AFDDTX Area Forecast Discussion National Weather Service Detroit/Pontiac MI 907 AM EDT Sat Aug 17 2024 .KEY MESSAGES... * Slow moving low pressure system drifts over lower Michigan this weekend supporting showery and unsettled weather. * Greatest coverage, consisting of numerous showers and scattered thunderstorms, will be focused during the daylight hours each day with lesser rain chances overnight. * Dry conditions return by Monday with below normal temperatures through at least the first half of the work week. Breezier conditions can also be expected Monday as well. && .UPDATE... A subtle capping inversion near 520mb was shown on the 12Z DTX sounding. Weak mid level cold air advection during the day along with diurnal instability will quickly erode any mid level capping. Upper level divergence within the exit region of a cyclonically curved upper jet max now advancing into Southern Lake Mi will overspread Se Mi by early afternoon while a short wave feature pivoting around the upper low arrives late in the day. In combination with a pool of deep layer moisture overhead, occasional showers with scattered thunderstorms can be expected through the afternoon. An earlier forecast update was issued to increase pops during the afternoon. Weak deep layer shear and Modest ML Cape values around 1k J/kg today will limit the risk of severe thunderstorms. && .PREV DISCUSSION... Issued at 640 AM EDT Sat Aug 17 2024 AVIATION... Showers associated with an incoming vorticity center will dot areas from KPTK north early this morning with additional activity firing this afternoon with some isolated thunderstorm potential. Ceilings will jump around a bit this morning, but settle into MVFR mainly and possible IFR overnight tonight at times. Southwesterly flow will veer to the west tonight and eventually northwest late tonight into Sunday morning. For DTW/D21 Convection...There is another minor chance for thunderstorms after 19z, but mainly expect SCT-BKN coverage showers. DTW THRESHOLD PROBABILITIES... * Low for thunderstorms this afternoon. * High for cigs at or below 5000 ft today and tonight. PREV DISCUSSION... Issued at 308 AM EDT Sat Aug 17 2024 DISCUSSION... Overnight broken line of showers and isolated embedded thunder finishes crossing SE MI early this morning as the parent mature, closed low drifts into lower MI. A brief break in rain chances then follows for much of this morning as associated vort max swings into southern Ontario. Lapse rates steepen by late morning as the mid- level thermal trough axis reaches the area, which in combination with broader background upper divergence, supports the generation of scattered showers. Coverage is expected to increase through the afternoon as daytime heating leads to several hundred J/kg of MLCAPE, which also offers isolated to scattered thunder potential as well. Column wind profiles remain weak, largely aob 20kts, precluding any severe potential from any storm that manages to develop. Loss of diurnal heating by the latter part of this evening will lead to a significant reduction in shower coverage with most locales likely staying dry all night, particularly in the south. Exception to this is areas along/north of I-69 where widely scattered showers look to linger due to the closer proximity to the low center/vort max that will be sliding over central lower MI. Similar day for Sunday as the low center will have only reached southern Lake Huron by that point. Greatest persistence and coverage of showers likely resides over the Thumb given the low positioning though daytime heating will once again support at least scattered showers and isolated thunder over the rest of SE MI (if not greater). Coverage once again decreases Sunday evening with rain chances completely ending by early Monday morning as low pressure finally pushes into the Northeast and height rises build back into southern lower MI. Upper troughing lingers over the central Great Lakes Monday allowing northerly flow in the wake of the low to draw a cooler Canadian airmass south into our area. 850mb temps fall to around 10C, give or take a degree, limiting high temps to the lower 70s with upper 60s possible in the Thumb. Improved mixing depths within this cold advective regime are able to partially tap into the 25-30kt flow aloft to support a breezier day with gusts generally in the 20-25mph range across SE MI; though stronger gusts around 30mph will be possible, especially over the Thumb. Upper midwestern surface high pressure becomes fully established by Tuesday bringing weaker, though still north/northeasterly, winds for the midweek period. Upper trough is slow to depart however resulting in highs holding below average in the lower to mid 70s for this timeframe. Moderation in temps back to around normal looks to occur by late week as upper ridging over the central CONUS gradually creeps into the Great Lakes. MARINE... First of several rounds of showers and thunderstorms is underway at issuance as the upper low inches eastward this morning. The low is expected to track directly overhead today, leading to more numerous coverage of showers and storms especially as daytime heating begins. A few storms could produce strong downdrafts, supporting localized potential for thunderstorm gusts to exceed 35 knots. On the broader scale light southerly prevails for most of the day, shifting to the north late tonight as the low exits into Ontario. Northwest flow increases on Sunday as the western flank of the pressure gradient gains influence. Gusts increase further Sunday night-Monday as a 35 knot jet wraps around the low, which in combination with onshore flow will likely necessitate Small Craft Advisories early next week. A quieter pattern returns by mid-week as high pressure builds into the Great Lakes. HYDROLOGY... A broad low pressure system moves very slowly across Lower Michigan this weekend. This system will support periods of numerous showers and scattered thunderstorms with each day seeing an uptick in coverage during the daylight hours. Total additional forecast rainfall between this morning and Sunday evening will range from a general 0.25-0.5" south of I-69 to 0.5-1" north. Localized higher amounts possible dependent on strength and frequency of thunderstorms. Localized minor flooding of urban, low-lying, or otherwise flood prone areas remains possible from thunderstorms, but the off and on nature of the rainfall is not expected to cause significant flooding. && .DTX WATCHES/WARNINGS/ADVISORIES... MI...None. Lake Huron...None. Lake St Clair...None. Michigan waters of Lake Erie...None. && $$ UPDATE.......SC AVIATION.....DG DISCUSSION...KDK MARINE.......MV HYDROLOGY....KDK You can obtain your latest National Weather Service forecasts online at www.weather.gov/detroit.