textproduct: Detroit/Pontiac

KEY MESSAGES

- A Flood Watch is in effect today for the area generally along and east of a line from Port Huron to Pontiac to Adrian.

- A band of showers and embedded thunderstorms becomes focused over the Watch area this morning. It brings potential for heavy rainfall and flooding until diminishing mid to late afternoon.

- Dry weather returns Monday through Tuesday with temperatures around normal for early August.

- The next chance of rain arrives with a front on Wednesday. This front lingers around the region during the late week period.

DTW THRESHOLD PROBABILITIES

* High for ceiling at or below 5000 ft today.

* Low for thunderstorms through 16z.

PREV DISCUSSION

Issued at 357 AM EDT Sun Aug 2 2026

DISCUSSION...

The Ohio valley/southern Great Lakes low pressure system remains the forecast focus as it still presents a heavy rain threat today. A positive development is that evolution of the system helps confine heavy rain potential to about the southeast half of SE Mi, more so the counties along/near the Canadian border. The area is narrowed down by the northern stream mid/upper level trough digging from northern Ontario into the central Great Lakes while forcing strong contraction of the Ohio valley trough. This results in very good model agreement on a much sharper deformation zone overhead that is capable of heavy rainfall during the morning. The larger scale environment supports several hours of strong dynamic forcing within an unstable thermal profile, weak but still unstable, with a deep moisture supply. Bulk parameters for elevated instability are weak but favorable, such as 850 mb LI slightly negative in a moist adiabatic profile. Dewpoint at 700 mb in the 5-7 C range is also impressive and reflects the warm rain efficiency suggested by PW hovering around 1.8 inches through the morning. The deformation band solidifies through sunrise and is slow to move off a SW to NE axis over far SE Mi presenting a lengthy period of time for heavy rain and potential flooding. Consensus of regional and global models has event total QPF in the 1-3 inch range while HREF 3-6 hr PMM mean QPF exceeds 3 inches. REFS PMM mean is less bullish and farther east to remind us there is still some uncertainty on the final band location. Hi-res versions of the NAM offer much higher QPF to also remind us there is some uncertainty on magnitude as well. Bottom line here is that small changes in the band location can result in big changes to the rainfall outcome. Please see the earlier issued Flood Watch and the Hydro section below.

The rain band exits eastward by late afternoon as the mid/upper level trough and surface low shift to over Lake Erie/Ontario. Moderate northerly wind trails the system across the central Great Lakes which brings cooler and less humid air into SE Mi tonight and Monday. Guidance low temperatures drop into the mid 50s outside of metro Detroit and return to around 80 Monday afternoon, right around normal for early August.

A large area of high pressure is shown to set up and dominate conditions across the Great Lakes for the early week period. Dry weather holds through Tuesday until the next low pressure system approaches from the Plains and central Canada. It brings a cold front that stays in the picture from Wednesday through late week.

MARINE...

Northeast flow picks up across the central Great Lakes today as a low pressure system lifts across the Ohio Valley. The strongest flow will channel across Saginaw Bay leading to Small Craft Advisory conditions, with elevated winds and waves expected for the rest of the Lake Huron nearshore. The Small Craft Advisory was expanded to account for the remaining southern Lake Huron zones, slightly later in time. The chance for additional showers and thunderstorms persists until this evening. An isolated storm or two could produce locally strong winds and elevated waves, more so along the southern waterways. High pressure influence then builds into the region tonight and into Monday offering drier weather and weaker winds. No significant weather is expected the the mid-week time-frame.

HYDROLOGY...

A Flood Watch is in effect today for the area generally along and east of a line from Port Huron to Pontiac to Adrian. A slow moving band of showers and embedded thunderstorms solidifies from southwest to northeast over far SE Michigan during the early morning. Rain will be heavy at times and be capable of producing high rainfall totals in a short period of time. Flood Watch area rainfall totals of 1 to 2 inches in 2 to 3 hours are likely, with a chance for locally higher amounts. Rainfall totals could reach or exceed 3 inches within the Watch area from 6 am to noon. Flooding may occur in poor drainage and urban areas. There is a chance for extensive road flooding and flooding of creeks and rivers are possible. A diminishing trend begins after 2 pm this afternoon.

DTX WATCHES/WARNINGS/ADVISORIES

MI...Beach Hazards Statement through this evening for MIZ049.

Flood Watch until 5 PM EDT this afternoon for MIZ063-069-070-075- 076-082-083.

Lake Huron...Small Craft Advisory from noon today to 4 AM EDT Monday for LHZ441>443.

Small Craft Advisory until 10 PM EDT this evening for LHZ421-422.

Lake St Clair...Small Craft Advisory from noon today to 4 AM EDT Monday for LCZ460.

Michigan waters of Lake Erie...Small Craft Advisory until 4 AM EDT Monday for LEZ444.