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Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI
Issued by NWS Detroit/Pontiac, MI
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083 FXUS63 KDTX 161944 AFDDTX Area Forecast Discussion National Weather Service Detroit/Pontiac MI 344 PM EDT Fri Aug 16 2024 .KEY MESSAGES... * Isolated to scattered thunderstorms possible this afternoon and early evening. Any activity that can develop 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. && .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. && .PREV DISCUSSION... Issued at 149 PM EDT Fri Aug 16 2024 AVIATION... Morning band of showers held longer into the afternoon complicating the forecast for this evening. A cold front remains well to the west over southern Lake MI and should pass over the area this evening with a couple hours of destabilization ahead of it in a narrow warm sector. Computer models have all backed off a bit from generating broader coverage of showers and storms ahead of the front lowering confidence. But it remains warm and humid with the warm front and cold front still to pass through so will hedge away from convection and go with a tempo for showers this evening. If/when convection can develop, will amend the impacted airports as needed. Better support for showers comes tonight and the ongoing tempo from 06-10Z still looks good. Yet another round of showers and possible storms will come Saturday afternoon as the upper low drifts overhead. Ceilings will lower on Saturday with the low in place. For DTW/D21 Convection...Low probability exists for thunderstorms this afternoon/evening as early day showers held longer into the day reducing the amount of instability that could rebuild into the area. The cold front will pass through this evening with a couple hours to try to reorganize ahead of it so still a chance of activity between 20-00Z. Took out thunder and left showers as confidence in widespread thunderstorms is low. Additional rounds of showers will exist tonight after 06Z with a trough swinging through and again on Saturday afternoon with the upper low in place. DTW THRESHOLD PROBABILITIES... * Low for thunderstorms through the evening. * Medium for cigs at or below 5000 ft this afternoon and evening. && .DTX WATCHES/WARNINGS/ADVISORIES... MI...None. Lake Huron...None. Lake St Clair...None. Michigan waters of Lake Erie...None. && $$ DISCUSSION...AA MARINE.......MV HYDROLOGY....AA AVIATION.....DRK You can obtain your latest National Weather Service forecasts online at www.weather.gov/detroit.