textproduct: Detroit/Pontiac

KEY MESSAGES

- Dry weather today with increasing clouds late in the day.

- Several rounds of showers and embedded thunderstorms are likely Saturday, Saturday night, and Sunday.

- A cool and cloudy pattern is expected through early next week.

DTW THRESHOLD PROBABILITIES

* Medium for ceiling at or below 5000 ft Saturday morning.

PREV DISCUSSION

Issued at 421 AM EDT Fri Sep 18 2026

DISCUSSION...

A stalled frontal boundary remains active with convection across the Ohio Valley this morning but SE MI will see increasing drying influence from high pressure passing through northern Ontario today. Northeast flow allows remaining stratus to push south of the stateline with a good amount of sunshine for the area through midday. Subtle negative thickness advection within the southern extent of thermal troughing allows high temps to arrive right around seasonal average in the lower 70s and upper 60s (downwind of Lake Huron).

Low pressure emerges out of the High Plains today in response to a shortwave and upper jet entrance region running the ridge axis in place, bolstered by a feed of monsoonal moisture from the Four Corners region. Will see the first signs of this system locally later this afternoon as high cloud begins to spill in from the west. Dry weather is most likely to prevail overnight as persistent easterly flow maintains a dry pocket within the lowest 8 kft or so. Upstream, organized moisture transport into the developing warm frontal slope will produce a corridor of steady and at times heavy rainfall. The LLJ driving this rain is shown to veer overnight before beginning to target parts of Lower MI Saturday morning. There are still some discrepancies noted in the strength and northeastward extent of this feature among 00z guidance, which appears to govern how far north and east the swath of rain reaches into the local area on Saturday.

Model cross sections show a sharp 850-700mb frontal structure and attendant fgen within deep layer saturation, suggestive of banded precip with inherent uncertainty as to where the frontal slope will activate and for how long at any given location. This is evidenced by a 1+" spread between the 25th and 75th percentile QPF in LREF grand ensemble membership. QPF output in coarser global model solutions is likely overdone spatially and prefer the look of hi-res solutions that can better account for the initial dry easterly flow and some of the mesoscale processes involved. Overall flavor of these solutions shows the heaviest rain sinking southeast across Lake Michigan and Chicagoland into western Lower MI and northern IN Saturday morning before the LLJ weakens and rates/coverage begin to taper off midday. What's left for SE MI is a cool, cloudy, and showery pattern with amounts most probable to hold below a half inch through 00z Sun. Rates could become moderate at times as PWAT rises above 1.50", but the threat for widespread heavier rain is low. Weak lapse rates and marginal elevated instability offer low potential for an embedded thunderstorm or two in the south. Conversely, portions of the area north of I-69 may see little to no rain given the farther proximity from the feed of moisture and lift.

Additional showers are likely, especially south of M-59, Saturday night into Sunday. The stalled frontal slope remains overhead while additional moisture continues to feed along it. As the nocturnal LLJ becomes active again over the northern Ohio Valley, this will be a window for training showers that have potential to produce locally over 1 inch of rain. Placement of the axis of heaviest rain still carries high uncertainty and may yet be displaced south of the state line. Additional clarity should come into focus today and tonight as additional hi-res runs begin to include this period.

High pressure builds back into northern Ontario on Sunday, offering another period of dry northeast flow that should shunt any lingering showers southward by late in the day. A tightening gradient brings breezy conditions with temperatures capped in the mid 60s. Cool and mostly cloudy conditions carry into the early week as the stalled front remains active over the Ohio Valley. Showers will be possible at times, especially in the south. An upper low looks to develop overhead midweek which brings higher likelihood for showers Wednesday and Thursday.

MARINE...

A secondary cold front finishes clearing the region this morning resulting in a modest uptick in northeasterly winds (toward 20- 25kts) over the southern half of Lake Huron. Small Craft Advisories are in effect along the Thumb due to the moderate onshore flow. This front stalls over northern Ohio before attempting to lift back north Saturday as Plains low pressure arrives. Front looks to reach the southern Great Lakes before the system tracks over lower MI resulting in predominant east-southeast turning northeasterly winds this weekend. Widespread showers and scattered thunderstorms are possible, mainly focused over the southern half of the region. A moderate gradient, supporting NE/E gusts 20-30kts, lingers behind the low Sunday into the new work week as the area holds between the stalled boundary over the Ohio Valley and high pressure over northern Ontario.

HYDROLOGY...

A low pressure system approaching from the Plains with increasing deep moisture will bring showers and isolated embedded thunderstorms to the area this weekend. Forecast rainfall totals have decreased with the latest guidance, but relatively high confidence still exists for total rainfall ranging between a half inch and 1.5 inches. The highest amounts are likely to occur mainly south of a line from Lansing to Detroit. A period of moderate rainfall rates and potential for training/repeated showers is most favored to occur from Saturday night into Sunday morning, especially for areas south of M-59. The longer duration rainfall event will mitigate widespread flooding concerns, but ponding will be possible where any repeated rounds of showers lead to localized totals in excess of 1 inch.

DTX WATCHES/WARNINGS/ADVISORIES

MI...None. Lake Huron...Small Craft Advisory until 4 PM EDT this afternoon for LHZ421-441.

Small Craft Advisory until midnight EDT tonight for LHZ442-443.

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