Area Forecast Discussion

KEY MESSAGES

- There remains an isolated risk for strong thunderstorms early this evening with the passage of a cold front.

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

- Next chance of precipitation Saturday night into Sunday.

AVIATION

The primary cold front, crossing the I-69 corridor at forecast issuance, moves quickly through the rest of SE Mi this evening. Convection consists of a few struggling showers out ahead of the front with a broken line of showers along the front itself. Surface based CAPE of 1000-1500 J/kg generally south of I-69 still requires monitoring for an isolated stronger storm along lingering outflows and before the front exits into Ontario and Ohio before midnight.

Dry weather post front turns the forecast toward cloud trends for the late night and Wednesday morning. Early evening observations show scattered to broken MVFR coverage immediately post front where strong cold and dry air advection maintains a well mixed boundary layer. MVFR ceiling solidifies farther north in northern Lower and the U.P. which has a chance to make downslope into the Saginaw Valley after midnight and then down toward PTK before sunrise. Daytime heating and cooler/dry NW flow gusting near 20 knots then quickly mix out remaining clouds by afternoon leaving scattered/shallow cumulus until evening.

D21/DTW Convection... The evening time window for thunderstorms closes as the primary cold front exits the DTW area from 02Z-03Z. Thunderstorms are then not in the forecast for the rest of tonight and Wednesday.

DTW THRESHOLD PROBABILITIES

* Low for thunderstorms this evening.

* Low for ceiling 5000 ft or less late tonight and Wednesday morning.

PREV DISCUSSION

Issued at 330 PM EDT Tue Jul 21 2026

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.

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.