textproduct: Detroit/Pontiac

KEY MESSAGES

- A weak cold front brings scattered showers and thunderstorms this afternoon-evening, focused north of I-69.

- A little cooler Friday and Saturday, then a chance for showers Saturday night.

- Warmer Sunday with increasing storm chances, then cooler and wetter next week.

DISCUSSION

A pattern shift gets underway today after a prolonged period of dry weather; the last day with measurable rainfall at one of the three DTX climate sites dates back to September 5th (at MBS). Scattered afternoon convection has initiated over central Lower Michigan and is tracking towards the Tri-Cities and Thumb regions. This activity precedes a cold front attempting to drop south across the Lower Peninsula and Lake Huron. Moisture progs aren't very impressive given ambient dryness and weak pre-frontal ThetaE convergence, therefore deep-layer saturation and instability will be lacking. Nonetheless, still expect some convective persistence along with additional clusters of showers and thunderstorms through the evening hours, predominantly north of I-69 where FGEN and dewpoints are more supportive of rainfall. The threat for severe weather is low given MLCAPEs below 250 J/kg and bulk shear of 20 knots or less, but the dry boundary-layer could lead to a few stronger gusts (less than 40 mph). Activity should diminish by late evening as the storms collapse within an area of higher CIN, south of M-59.

More seasonable weather expected for most locations on Friday as the front eventually stalls near the MI/OH/IN border. 850 mb temperatures drop into the low teens (Celsius) capping highs in the upper 70s for most areas. An upper-level ridge axis, wedged between a longwave trough over Quebec and a closed upper low over The Plains promotes column stability across Southeast Michigan Friday, providing mainly dry conditions. Surface high pressure builds southeastward into central Ontario offering northeasterly gradient flow that delivers cooler readings within the Huron lakeshore. Morning clouds eventually give way to increasing sun by the afternoon hours.

The cut-off upper low pivots into the Upper Midwest on Saturday, interfacing with the northern stream jet. This closed low aloft then phases with a shortwave trough digging across the High Plains. Glancing energy off the eastern fringe slides in atop the stalled frontal boundary retreating northeastward as a warm front. This combined forcing brings renewed shower/storm chances late Saturday into Sunday. The perturbed mid-level flow and stronger LLJ winds support periods of thundershowers Sunday PM after an uptick in dewpoints (lower 60s) settle in. A rather sharp drop in geopotential heights arrives late Sunday into Monday as the vicinity upper low ejects into eastern Canada. This sets up a cooling trend for next week with additional opportunities for rain.

MARINE

A cold front drops southward across the region today, reaching the southern basin of Lake Huron by early afternoon and western Lake Erie this evening. This will be marked by an increase in north to northwest wind to around 10 to 15 kt. Funneling effects on Saginaw Bay will result in a localized northeast wind direction and locally higher gusts of around 20 kt. There will also be chances for scattered storms late this afternoon into the evening, mainly across the Saginaw Bay region. Strong high pressure then builds in across northern Ontario which produces northeast flow across the region Friday with sustained speed of 15 to 20 kt and gusts near 25 kt across Saginaw Bay and southern Lake Huron. Wind veers to east and southeast by Saturday, with the higher magnitude of wind shifting toward the northern Lake Huron basin.

PREV DISCUSSION

Issued at 101 PM EDT Thu Sep 18 2025

AVIATION...

The cluster of showers and isolated thunderstorms over central Lake Mi are tied to a weak mid level short wave impulse. This feature is set to move across central Lower Mi and the Tri Cities region mid to late afternoon at the same time enhanced low level convergence emerges as a result of the inland advancement of the Saginaw Bay marine layer. The forcing combined with weak diurnal instability should lead to additional convective development in the MBS region late this afternoon. Convection will then push southward during the evening along a southward advancing cold front. The loss of daytime instability and some potential lingering mid level capping across metro Detroit is expected to result in a rapid decrease in the coverage and intensity of convection as it pushes south, likely dissipating as it moves into the north Detroit suburbs.

The cold frontal passage this evening will be marked by a wind shift to the east-northeast. Low level moistening under a deepening frontal inversion will increase the chances for MVFR based clouds during the course of the night. MVFR probabilities will be higher from PTK northward where the frontal inversion will be deeper. Some IFR ceilings are possible toward daybreak Friday, the probabilities are not quite high enough to include in the terminals at this time.

For DTW/D21 Convection...A weakening trend is expected this evening as convection attempts to push into the far northern portions of the D21 airspace. It is expected that this weakening trend will keep convective probabilities too low to include in the DTW TAF.

THRESHOLD PROBABILITIES...

* Low in thunderstorms this evening.

DTX WATCHES/WARNINGS/ADVISORIES

MI...None. Lake Huron...None. Lake St Clair...None. Michigan waters of Lake Erie...None.