textproduct: Detroit/Pontiac

KEY MESSAGES

- There remains an isolated risk for strong to marginally severe thunderstorms into early this evening with the arrival of a cold front.

- Mostly dry conditions with temperatures dipping below average during the mid to late week period.

- Next chance at precipitation Saturday night into Sunday.

DISCUSSION

Surface cold front begins the process of dropping through the region this evening maintaining scattered convection across the north for the early evening before shifting south for the latter half. Available instability decreases through the rest of the day (MLCAPE falling from ~2000 J/kg towards 1400 J/kg) resulting in lessening coverage as the evening progresses. Coverage is expected to be more isolated to widely scattered in nature once the front reaches the southern half of the area late evening- partly from arrival late in the diurnal window and partly from drier air in place (dewpoints in the lower 60s vs the northern upper 60s). Any strong to severe storm chance remains isolated at best as MLCAPE falls sub 1500 J/kg with shear values holding around 25-30kts.

Frontal passage ushers in a markedly cooler airmass as 850mb temps fall from mid teens to mid single digits (C) overnight dropping lows toward the lower 50s. Upper troughing and associated thermal troughing settles across the Great Lakes for the midweek period leading to seasonably cool temperatures. Coolest day of the week is Wednesday where highs only reach the lower 70s with overnight lows around 50, some 10-15 degrees below normal for late July. Trough axis shifts east over southern Ontario by Thursday allowing for slight warming (850mb in the upper single digits C) supporting high temps closer to the mid/upper 70s.

Warm advection eventually redevelops Friday as quasi-zonal mid-upper flow expands across the Great Lakes allowing highs to return around 80. Building upper ridging over the central CONUS this weekend drives a weak periphery shortwave into the Great Lakes late Saturday night-Sunday offering the area's next shot seeing some scattered rain. Upper 80s also look to make a return late weekend/early next week as upper ridging builds east into the central Great Lakes.

MARINE

Upper level disturbance tracking across northern Michigan has led to numerous thunderstorms across Lake Huron and Saginaw Bay. A few of these storms have exhibited strong to severe potential, with damaging winds of 35 to 50 knots and frequent lightning the main concerns through early evening. A cold front tracking through this evening then clears out remaining instability to bring decreasing thunderstorm coverage, while also shifting winds to the northwest. A quick pop in wind gusts toward gale force arrives this evening, centered around 00z (8pm local) just along/behind the frontal passage. Still thinking the duration of gales will be short enough to fall below warning criteria, but will monitor observations closely for a short-fused product. Strong cold advection will continue through Wednesday, maintaining breezy northwest flow through the day and keeping Small Craft Advisories in effect. Beach Hazards Statements are also likely again Wednesday. High pressure then fills in for the latter half of the week, resulting in a quieter pattern for the Great Lakes.

PREV DISCUSSION

Issued at 110 PM EDT Tue Jul 21 2026

AVIATION...

Pockets of deep convection are ongoing across northern portions of the airspace. The greatest coverage and duration will remain in a the corridor from MBS to PHN. Additional modest shower development is occuring within in an expanding agitated cumulus field across central and western Lower Michigan. Expect that this activity will simmer through the afternoon and offer a few opportunities for a shower or storm at each of the terminals - albeit brief at any given location. The best chances are north of the immediate Detroit airspace - but given the lengthy time window for development prior to sunset - the forecast affords coverage to increase.

Plenty of MVFR clouds upstream over Northern Lower Michigan. Expect some lower-level drying to raise the bases some by the time that moisture field arrives over Southeast Michigan. Winds will swing to northwest behind the incoming cold front. Gusts will be prevalent through the daylight hours today (up to 25-kts) and again Wednesday morning (around 20-kts).

D21/DTW Convection...scattered deeper showers with a thunderstorm or two will move across D21 this afternoon with the most frequent activity over the northern half of the airspace. Terminals reflect these chances via a prob30 group given the scattered nature.

DTW THRESHOLD PROBABILITIES...

* Low in thunderstorms this afternoon.

* Low for westerly crosswinds this afternoon.

DTX WATCHES/WARNINGS/ADVISORIES

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

Lake Huron...Small Craft Advisory until 10 PM EDT Wednesday for LHZ421-441>443.

Small Craft Advisory until 4 PM EDT Wednesday for LHZ422.

Lake St Clair...Small Craft Advisory until 10 PM EDT this evening for LCZ460.

Michigan waters of Lake Erie...Small Craft Advisory until 10 PM EDT this evening for LEZ444.