textproduct: Detroit/Pontiac

KEY MESSAGES

- Multiple rounds of showers and a few thunderstorms are forecast today and Sunday.

- Heavy rainfall is likely somewhere over northern Indiana, southeast Lower Michigan or northwest Ohio tonight and Sunday morning. There is still uncertainty in where this axis will develop. It could reach as far north as Ann Arbor to Port Huron and bring flooding concerns, especially over urban areas.

- Dry weather returns on Monday, but it may become unsettled again for parts of next week with chances of showers. Temperatures will be close to average for early August.

DTW THRESHOLD PROBABILITIES

* Low for ceiling 5000 ft or less late today. Moderate to high tonight and Sunday morning.

* Low for thunderstorms late this afternoon and this evening. Moderate tonight and Sunday morning.

PREV DISCUSSION

Issued at 357 AM EDT Sat Aug 1 2026

DISCUSSION...

This weekend's weather will be dictated by a low pressure system tracking from central IL this morning into the Ohio Valley on Sunday. This occluded system is currently being driven by a closed 500mb low slowly drifting across the Midwest. By Sunday, an amplifying upper air pattern sends a deepening Canadian trough southward across the Great Lakes which interacts with the wave and causes it to stretch toward the eastern Great Lakes. As the system works through, expect increasing humidity and coverage of showers that peak tonight into Sunday before waning by Sunday evening as the moisture is pulled east.

The 01.00z DTX raob revealed a deep layer of dry air up to 470mb that has prevented much more than virga thus far as upper moisture continues to wrap around the low upstream. However, there is a more concerted push of low to mid level moisture convergence working into the Saginaw Valley that is bringing scattered showers early this morning. This area will remain favored for additional showers through the rest of the morning, with radar presentation indicating transient moderate to heavier rates at times. Areas farther south and east should should generally see fewer showers until better moisture advection begins to arrive later this afternoon.

Several waves of ascent will spread across the area this afternoon through tonight, then into Sunday as the interaction of the two waves squeezes a narrow corridor of PVA overhead. This forcing paired with increasing low level moisture and convergence will bring the highest coverage of showers and embedded storms. There will be an increasing threat for localized heavy rainfall as the environment will support efficient rainfall processes. Deep saturation and moist adiabatic lapse rates, warm cloud depth around 12 kft, elevated instability of 500 to 1000 J/kg, and PWAT near the 90th percentile at 1.75" all support pockets of heavy rain where showers occur. Weak veering flow through much of the column produces Corfidi upshear vectors of 15 kt or less, indicating slow storm motion and potential for training/backbuilding convection.

Timing and placement of convection continues to carry high uncertainty, but signal is emerging for a flare-up near the I-69 corridor this afternoon. This occurs along a nearly stationary trough developing on the northeast side of the low, and may pose a threat for rainfall over 1 inch within a 1 to 3 hour window. The higher probability for a subtle heavy rainfall (SHARS) type event currently looks to target areas closer to the low center in northern Indiana, with support from latest hi-res QPF output. However, this potential may migrate northeast toward the Metro Detroit to Port Huron vicinity overnight into Sunday morning as the northeast quadrant of the system and the associated mesoscale deformation/fgen forcing and moisture convergence track through. With the shifting/stretching character of the low on Sunday, confidence remains low in these finer details which still may not be resolved well in guidance. Debated the need for a Flood Watch for the urban corridor which would be the most susceptible to heavy rainfall, but preference is to allow 12z and 18z guidance and morning trends to be evaluated in the decision. Latest ensemble QPF probabilities through Sunday remain supportive of rainfall on the order of 0.25" to 0.75" for much of the SE MI, with potential for 2+" in locations that see training convection. That being said, there is also a non-zero chance that certain areas receive very little rainfall given the scale of convection.

Drier air begins to arrive on Sunday as high pressure builds in from the north, but showers may continue well into the afternoon for the eastern forecast area before tapering off by Sunday evening. The high pressure takes residence overhead on Monday, promoting dry and seasonable conditions. The southern portion of the weekend system will get pinched off into a cutoff low early next week, with latest guidance carrying it farther south into the Tennessee Valley and southern Appalachians. This supports dry conditions into Tuesday as well. Moisture with this low eventually begins to migrate northeast, possibly clipping the area on Wednesday with diurnally driven showers. The next cold front works across the region late Thursday into Friday with additional chances of showers and storms.

MARINE...

Lighter winds through the early morning will slowly increase and turn northeasterly Saturday as a low pressure system over the upper Mississippi River Valley shifts eastward towards the Ohio Valley. Northeast winds will increase through the day, which will lead to building waves through the Saginaw Bay during the afternoon and into Sunday. A Small Craft Advisory may be needed during this time period for Saginaw Bay and nearshore zones of the Thumb. Chances for rain and thunderstorms will also continue through the remainder of the weekend as the low and its associated frontal features push across the region.

HYDROLOGY...

Scattered to numerous showers with embedded thunderstorms are forecast today into Sunday as a low pressure system slowly tracks into the Ohio Valley. The increasingly moisture laden environment will support moderate to heavy rainfall rates within any showers, and there is potential for training or backbuilding showers. Relatively high confidence exists in rainfall totals between 0.25" and 0.75". There is the potential for localized areas to receive in excess of 2.00", but confidence is low on the placement of these higher amounts. 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.