textproduct: Detroit/Pontiac

KEY MESSAGES

- Light rain chances gradually rise from west to east today.

- Muggier Thursday as the coverage and intensity of rainfall increases; locally heavy rain is possible.

- Turning cooler and drier over the weekend.

DTW THRESHOLD PROBABILITIES

* Low for ceilings at or below 5,000 feet through 18z. Medium 18z through the end of the TAF period.

* Low for thunderstorms this evening and overnight.

PREV DISCUSSION

Issued at 404 AM EDT Wed Sep 30 2026

DISCUSSION...

The blocked/stable synoptic pattern over the Great Lakes has begun to break down early this morning. Prior ridging continues to flatten and fold into northeastern CONUS, trailing the early week coastal system's departure. Meanwhile, much longer wavelength disturbances upstream are tracking zonally across central Canada and southern CONUS. Southeast Michigan will be positioned between these two waves through the rest of the week, eventually leading to periods of rainfall. At present, leading geopotential height falls have overspread Lower Michigan, marked by a thickening canopy of cirrus aloft. The southern stream wave/closed-low absorbed what remains of eastern Pacific Hurricane Polo leading to the advection of an equatorial airmass from the Four Corners region up to James Bay.

The distribution of saturation is currently skewed toward the upper levels as top-down moistening ramps up over the next 12 hours. Given that the column has been abnormally dry since last Thursday (see KDTX RAOBs since 24.12Z), it will take time to prime the low levels and achieve adequate saturation depths before rainfall becomes more prevalent. Latest mosaic radar imagery shows a sharp cutoff to the rain edge, currently confined to areas west of a Chicago to Gaylord line. The swath of rainfall should migrate eastward throughout the day as the theta-e ridge rolls southeast by this afternoon as embedded shortwave energy gets deflected away from the higher amplitude northern stream wave. The majority of 00Z/06Z solutions depict some degree of measurable precipitation for most areas by 00Z this evening. The lowest confidence in this initial round of diurnal showers continues for locations along/south of I-94 where saturation processes are the most delayed.

The axis of persistent tropical moisture advection drifts over Lower Michigan Thursday amidst a shifting deformation zone and reinforcing CVA. This unfolds as the southern stream low starts to open/phase with the deeper continental wave to the north. Expect longer and more continuous periods of rainfall as Thursday progresses with notably muggier conditions. Surface dewpoints easily climb into the mid 60s, if not upper 60s, while 850 mb temperatures rise into the upper teens (Celsius). PWATs respond accordingly, climbing above 1.75 inches for some areas, which raises the prospect for localized heavy rainfall (see Hydrology Discussion). From a stability perspective, the thunder threat appears quite low given the warm advection scheme, before an approaching cold front extending from the Ontario surface reflection slowly tracks through the region.

Aggressive post-frontal system-relative isentropic downglide leads to a quick conclusion to showers, before the midday hours Friday. N northwest flow arrives downstream of 1027 mb high pressure over the Upper Midwest. Seasonably cool and dry though the weekend.

MARINE...

South to southwest winds will slowly increase ahead of the next cold front, which arrives and stalls over southern Lower Michigan on Thursday. With warm advection leading to more stable profiles and showers moving through during this timeframe, wind gusts are expected to top out only around 25 knots. With the front hung up into Thursday night, the post frontal cold advection now looks slower; thus, the potential for northwest wind gusts reaching or exceeding 25 knots looks to be late in the day on Friday as 850 MB temps lower into the mid single digits by Friday.

Another large area of high pressure building in on Friday night and Saturday will promote light winds for the first half of the weekend. A stronger low tracking through the northern Great Lakes for the second half of the weekend brings another opportunity for winds to reach or exceed 25 knots over Lake Huron.

HYDROLOGY...

The transport of tropical moisture into southern Michigan and an approaching cold front will lead to expanding rainfall today into Friday. The heaviest rain is forecast to occur between Thursday afternoon and Friday morning. There is medium confidence in total rainfall between 0.5 and 1.5 inches. A band of more persistent rain is likely to become established across the area, with positioning currently favored north of I-69. Within the model probability space, there is a chance for rainfall to exceed 2 inches in areas which experience the more persistent rainfall, suggesting at least a chance of minor flooding.

DTX WATCHES/WARNINGS/ADVISORIES

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