textproduct: Detroit/Pontiac

KEY MESSAGES

- A chance of thunderstorms this evening into tonight. Much of the forecast area is in a Marginal or Slight risk designation for Severe weather with large hail, strong wind gusts, and a brief weak tornado potential threats.

- Low confidence exists in the timing and duration of rain Tuesday and Wednesday.

- Much cooler temperatures return Tuesday and last through the end of the week. A Frost Advisory or Freeze Warning may be needed.

DTW THRESHOLD PROBABILITIES

* High for thunderstorms this evening. Low tonight.

* Low for ceiling at or below 5000 feet this evening and tonight; medium Tuesday.

PREV DISCUSSION

Issued at 328 PM EDT Mon May 4 2026

DISCUSSION...

Well mixed southwesterly flow has developed over all of Southeast Michigan this afternoon with 2pm wind gusts at a few of the ASOS sites reaching and exceeding 45 mph. Surface moisture has mixed out over much of the northern cwa with many observing platforms reporting relative humidity between 19-25 percent. With dry southwest flow persisting this afternoon and with collaboration considerations, decided to go with a Red Flag Warning in effect until 7pm this evening.

Atypical, in-phase squeezing event of northern stream Canadian trough and Southwestern United States ridge will result in a strengthening baroclinic zone over portions of Lower Michigan and northern Ohio River Valley by Tuesday. Two boundaries reflect this competing geopotential height pattern: the first is the deep cold front progressing through the Lake Superior basin, the second is a thetae/CAPE gradient that will emerge and lift into far southern portions of Southeast Michigan between 21-03z this evening.

Very subtle forcing mechanism at work across the forecast area this afternoon with a lack of upper level jet dynamics and synoptic lift locally. Differential loss of the anticyclonic flow trajectories over Southeast Michigan is expected to allow for a region of height falls, inducing northward moisture transport of 900mb thetae content. Remnant midlevel dry adiabatic lapse rates with help from the Evening Transition will result in the development of mixed layer CAPE exceeding 1000 J/kg with the CAPE gradient lifting northward through Metro Detroit. CAM output is quite bullish on convective initiation/development along the boundary 22-01z. 0-6mb bulk shear of 30 to 35 knots will be adequate for storm organization, although hiresolution data shows little signal for storm right movement. CAPE gradient boundary does become parallel to the 850-300mb mean flow suggesting a potential for training of activity. Therefore, large hail and localized heavy rainfall are the main threats this evening. Forecast soundings do suggest higher static stability developing with the loss of daytime heating from the surface to 4.0 kft agl suggesting strong wind gusts may be more difficult to observe outside of any organized cold pool or forward propagation. The threat of a tornado appears to be very low although a boundary is overhead with 0-1km SRH that reaches 150 m2/s2.

The Canadian trough will encroach on the area tonight driving the deep cold front through the area. Differences exist on how generous the coverage of showers and embedded thunderstorms will be overnight highlighted by the latest forecast with PoPs at generally less than 60%. Models show a dead zone of both the 700 and 850mb frontogenesis over Southeast Michigan but with deep southwest flow the potential exists for upstream activity to track through the area. MUCAPE of 250-500 J/kg in place ahead of the cold may support some small hail between 02-05z. The latest SWODY1 has areas south and west of a line from Chesaning to Grosse Pointe designated as a Marginal Risk for Severe weather and a Slight risk across southern Lenawee and Monroe counties.

Uncertainty exists with regards to precipitation timing and coverage Tuesday midday through late Wednesday morning. Low confidence arises from stark differences in the occurrence and subsequent timing of a wave of low pressure set to track northeastward along the baroclinic zone. Latest data of system relative flow on isentropic surfaces supports the surface cold front clearing the area 15-18z with lingering lower column RH overhead. CAM simulated Z supports decaying nocturnal showers pushing through Metro Detroit southward between 15-21z Tuesday which is supported by likely PoPs. The solution space then diverges greatly Wednesday with the NAM the most bullish on an organized low pressure circulation tracking along the Ohio River. This would bring a fairly lengthy period of midlevel isentropic ascent to the southern cwa Wednesday. The current forecast is very lean on PoPs for the Wednesday period.

Low geopotential heights are forecasted to persist over the central Great Lakes through the end of the week and upcoming weekend. Lowered midlevel temperatures will likely support typical afternoon instability shower chances. The biggest item will likely be the need for Frost and/or Freeze headlines particularly Thursday morning. Some moderation in temperatures to near normal is possible by this weekend but the forecast will likely be determined by the amount of clouds over the region.

MARINE...

Breezy southwest winds prevail across the area this afternoon, though the warmer air mass overriding the cooler Great Lakes waters has allowed for a more stable environment with wind gusts mostly remaining below 25 knots. Fetch across the Saginaw Bay remains favorable for locally stronger winds, so have maintained the ongoing Small Craft Advisory through the evening hours. Isolated to scattered showers and embedded thunderstorms will be possible later this afternoon and evening, most favorable across Lake Erie and Lake St. Clair. Any stronger storm development will be capable of producing small hail and wind gusts of 34 knots or stronger.

A cold front dropping south across the western half of Lake Superior this afternoon will gradually work south across the region late tonight and into Tuesday morning. Winds from the north briefly dominant behind the cold front, but then lessen in speed and become more variable during the day Tuesday. Tuesday night through Thursday, winds will back from the northeast to north and eventually from the west, as another area of low pressure organizes south of the region and moves over the Northeast.

DTX WATCHES/WARNINGS/ADVISORIES

MI...None. Lake Huron...Small Craft Advisory until 10 PM EDT this evening for LHZ421-422.

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