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Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI
Issued by NWS Detroit/Pontiac, MI
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146 FXUS63 KDTX 170405 AFDDTX Area Forecast Discussion National Weather Service Detroit/Pontiac MI 1205 AM EDT Sat Aug 17 2024 .KEY MESSAGES... * Isolated to scattered thunderstorms possible this afternoon and early evening. This activity will be capable of heavy rainfall and an isolated storm could produce damaging wind. * Broad coverage of low pressure stalls across Lower Michigan this weekend. The system supports showery and unsettled weather with temperatures on the cool side of normal through Sunday. && .AVIATION... Organized convergence associated with parent vorticity maximum will result in Wisconsin thunderstorm activity to push into the Saginaw Valley and northern Thumb region tonight. Tail of the vorticity forcing with the best low-mid thetae advection will also result in chances for showers and thunderstorms at the Detroit terminals between 08-11Z. Relatively quiet conditions are expected across much of the area for a brief time Saturday morning before a longer duration of shower and possibly some scattered thunderstorms Saturday afternoon. Winds will trend southerly today to westerly Saturday. For DTW/D21 Convection...Chance for thunderstorms exists 08-11Z Saturday morning. Included a prevailing for showers after 19Z Saturday. DTW THRESHOLD PROBABILITIES... * Medium for thunderstorms early this morning. Increasing potential Saturday afternoon but low confidence. * High for cigs at or below 5000 ft this tonight and Saturday. && .PREV DISCUSSION... Issued at 1046 PM EDT Fri Aug 16 2024 UPDATE... Updated the gridded forecast to increase PoPs overnight (60-70% range) out of respect for the line of showers and embedded thunderstorms now pushing onshore of western Lower Michigan. Model signal exists for well organized 925-850mb convergence tracking through the northern cwa north of I 69 after 09Z associated with the parent vorticity max. The best 900-700mb thetae advection response appears set for areas south of I 69 including metro Detroit for the same time period. Mesoanalysis and forecast soundings suggest MUCAPES of over 1000 J/kg is likely tonight which will support thunderstorm activity. A skinny CAPE profile and a forecasted stable layer between 2.5 and 4.0 kft agl will limit the strong or severe thunderstorm threat. Southeast Michigan has a General Thunderstorm designation in the SWODY1 outlook. PREV DISCUSSION... Issued at 344 PM EDT Fri Aug 16 2024 DISCUSSION... An occluded frontal system ahead of a mid level closed low will progress across Michigan through the remainder of the day. The early day showers with isolated embedded thunderstorms has worked to stabilize the atmosphere early this afternoon bringing a period of dry conditions and some sunshine. The big question coming into today was how much instability we were able to build back into southeast Michigan for the mid-late afternoon and early evening period. Earlier day satellite showed an area of clearing across southwest Michigan in the relatively small warm sector zone ahead of an advancing weak cold front and an area of sustained convection that developed near the triple point in west central Michigan just off the shoreline around 15-16Z. So far, the west central Michigan activity has been weakening and isolated showers near the I-69/I-94 intersection have struggled to gain any strength thus far. If an isolated strong to severe thunderstorm weather threat is to materialize, it will likely be within the 20Z to 01Z window and be focused more across the southern portions of the CWA in the better warm sector air and instability (~500-1000 J/kg MLCAPE per RAP). Other environmental conditions are not all that impressive though. Left exit region upper jet dynamics help a little with larger scale ascent, but lower level flow will be fairly weak with 850-700 mb winds of largely less than 20 knots. This yields effective shear values of around 20 knots and mid-level lapse rates will be weak at 5.5 C/km per SPC Mesoanalysis. Also of note are that many of the latest CAMs are offering limited convection during the best destabilization time. All that said, a Marginal Risk remains in place today for isolated strong to possibly severe thunderstorms that can develop and maintain strength across Lower Michigan within the higher humidity airmass. Strong downburst winds will be the main threat along with brief heavy downpours supported by dewpoints in the upper 60s to low 70s. Any evening convection wanes with the loss of daytime heating leading to a brief period of dry conditions through about 06Z when the stacked low pressure system moves into central Lower Michigan offering renewed scattered showers and isolated thunder. Highest PoPs focused more towards the Saginaw Valley closer to the surface low pressure. The stacked occluded low will linger over the region this weekend bringing continued rainfall. Saturday will see a diurnal uptick in shower and isolated thunderstorm activity, but even less shear expected over the weekend will preclude any mention of organized thunderstorms. Isolated to scattered precipitation lingers Saturday night as the low slowly moves east into Ontario. Northwest flow then takes hold for Sunday, but shortwaves rotating around the low will maintain precipitation chances through Sunday. Diurnal heating Sunday afternoon will again bring potential for the diurnal increase in coverage of showers and general thunderstorms. Average rainfall totals over the course of this afternoon through Sunday will be up to 1.00-1.50 inches with the highest totals focused north of I-69. Gusty conditions pick up Sunday night and Monday as the northwest to north flow ushers in cooler air aloft and mixing depths tap into 25- 30 knots of flow. Wind gusts during this time will be largely between 20-25 mph, but gusts to around 30 mph will be possible across the Thumb. High pressure will build into the Midwest bringing dry conditions by Monday with daytime highs on the cooler side of normal in the low to mid 70s. Similar weather conditions with mainly weaker winds will carry into the mid-week period as the high pressure tracks across the Great Lakes. MARINE... Ongoing band of showers will continue to impact Lake St. Clair and the majority of Lake Huron for the next couple of hours. Behind this band is a corridor of clearing, although new showers and storms have started popping up near the warm front in SW lower Michigan. So far these cells have struggled to hold together for very long, but with clearing skies and increasing instability there is still a chance (30-40%) that a stronger storm may impact the waters after about 00z (8pm EDT) tonight. Stacked low pressure system responsible for today`s unsettled weather will remain in control through the weekend, maintaining shower and thunderstorm chances for both Saturday and Sunday. The surface low passes through early Sunday morning to shift winds from southerly to northerly. Onshore wind direction and a belt of enhanced low level winds will increase winds and wave heights toward Small Craft Advisory thresholds Sunday night and Monday. Quiet and dry weather is then anticipated for middle of next week. HYDROLOGY... A humid airmass will support locally heavy rainfall with any shower or thunderstorm that is able to develop and maintain strength across Lower Michigan this afternoon and early evening. Activity is trending towards being very isolated, but any storm that can maintain any strength could be capable of a quick half inch to an inch of rainfall. A broad stacked low pressure system moves very slowly across Lower Michigan during the weekend. This system will support several periods of numerous showers and scattered thunderstorms with each day seeing an uptick in coverage during the daylight hours. Total rainfall from this afternoon through Sunday evening is expected to be 1-1.5 inches on average and focused mainly north of I-69. Localized totals between 2-3 inches possible dependent on strength and frequency of thunderstorms. Localized minor flooding of urban, low-lying, and otherwise flood prone areas remains possible in the strong 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. && $$ AVIATION.....CB UPDATE.......CB DISCUSSION...AA MARINE.......MV HYDROLOGY....AA You can obtain your latest National Weather Service forecasts online at www.weather.gov/detroit.