textproduct: Detroit/Pontiac

KEY MESSAGES

- Multiple rounds of showers and a few embedded thunderstorms are forecast Saturday and Sunday. There is considerable uncertainty in the rainfall forecast, especially Saturday night and Sunday. There is a low chance (10-30% chance) that a narrow axis of heavy rain could develop and bring flooding concerns, especially over urban areas.

- Dry weather returns early next week with seasonable temperatures.

DTW THRESHOLD PROBABILITIES

* Moderate for ceiling 5000 ft or less late tonight and Saturday morning.

PREV DISCUSSION

Issued at 401 AM EDT Fri Jul 31 2026

DISCUSSION...

An upper ridge axis passes over the Great Lakes today, shunting surface high pressure off to the south and east. Despite increasing coverage of high cloud this afternoon, 1000-850mb thickness advection will result in an uptick in high temperatures into the upper 80s and lower 90s. Can't rule out a stray late day shower in the Saginaw Valley and Thumb where a weak surface trough may be able to capitalize on some weak instability; otherwise dry conditions prevail with little organized ascent expected.

The upper air pattern amplifies this weekend in response to a Pacific jet streak arriving over the PNW. A shortwave meandering over the upper Midwest today reaches the western Great Lakes Saturday then interacts with a trough deepening overhead on Sunday. This process sets off multiple rounds of ascent that pass overhead as a surface low tracks across northern/central IL into the Ohio Valley. As deep layer moisture wraps around the north side of the low, a wet pattern is in store for the weekend. Scattered showers and a few thunderstorms are forecast to arrive late tonight and Saturday morning as the initial arc of upper height falls and PVA pivot through.

Scattered showers will remain in the vicinity through the day Saturday as the low begins to work into the lower Ohio Valley and deeper moisture spreads in. A complete washout is not expected as several solutions still hold onto some low-level dry air, especially around Metro Detroit, which may limit coverage during the day. A few thunderstorms are also possible but in general the near moist adiabatic lapse rates will limit instability. Increasing coverage of showers seems reasonable by Saturday evening and overnight into Sunday as the upper forcing begins to consolidate overhead, favoring an increasing fgen and deformation response. The environment will support moderate to heavy downpours as PWAT rises to around 1.75" (90th percentile) with warm cloud depth on the order of 12 kft noted in soundings.

There remains a high amount of uncertainty regarding QPF magnitude and placement during this period owing to the scale of the forcing as well as uncertainty in the track/intensity of the low. In general, the ensemble solution space indicates high probability for at least 0.25" of rainfall with reasonable potential for up to 1.50". Will note that the last several runs of the NAM are high outliers within the grand ensemble envelope, but do indicate the low probability high end outcome for storm totals over 3" in the event a deeper low drives a targeted corridor of training convection along a nearly stationary fgen axis. WPC has highlighted all of SE MI within a marginal risk for excessive rainfall Saturday and Sunday.

The system slowly migrates east by Monday as an upper ridge quickly advances into the Great Lakes. There is increasing potential for the upper circulation to become orphaned over the Ohio Valley early in the week, which could keep some showers in the vicinity of Metro Detroit and south. High pressure building in from the north should otherwise maintain relatively benign and seasonable conditions into the midweek period.

MARINE...

High pressure tracks through the region passing just to the south across the Ohio River Valley today. Wind fields will respond accordingly and become southerly to westerly. A low pressure system will then follows and also tracks across the Ohio Valley over the weekend. Winds turn northeasterly across Lake Huron Saturday night, which will bring favorable flow that will lead to increased winds and waves through Saginaw Bay. This low and associated frontal features will bring a chance of showers and thunderstorms starting late Friday night and into Sunday.

HYDROLOGY...

Scattered to numerous showers with embedded thunderstorms are forecast late Saturday into Sunday as a low pressure system tracks into the Ohio Valley. The increasingly moisture laden environment will support moderate to heavy rainfall rates. There is relatively high confidence in rainfall totals between 0.25" and 0.75". Considerable uncertainty exists regarding placement and magnitude of the heaviest rainfall, but low probability (<30%) exists for rainfall totals in excess of 2.00" by late Sunday. This may lead to flooding concerns this weekend, particularly in urban areas.

DTX WATCHES/WARNINGS/ADVISORIES

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