textproduct: Detroit/Pontiac

KEY MESSAGES

- A Severe Thunderstorm Watch remains in effect for parts of SE MI until 7 PM. Damaging wind gusts of 60 to 75 mph and torrential rainfall will be the main threats, but potential also exists for large hail and tornadoes.

- Additional showers and thunderstorms are forecast overnight into Friday morning with potential for another round late Friday afternoon or evening. Isolated storms may be strong to severe.

- More seasonable temperatures and lower humidity are slated to return this weekend.

DISCUSSION

An organized MCS prompted Severe Thunderstorm Watch 648 which remains in effect until 7PM. This complex is tracking southeast along a stalled frontal boundary north of a subtropical air mass. Sfc dew points south of the boundary sit in the lower to mid 70s, with steep mid-level lapse rates fueling 2000 to 3000 J/kg of SBCAPE. 35 to 40 knots of effective shear has been more than sufficient for bowing segments and transient embedded supercellular structures. A few stronger updrafts will have the potential to produce large hail, but the current storm mode favors wind gusts of 60 to 75 mph as the primary threat. A tornado or two also can't be ruled out, especially where any segments deviate S to SSE to capitalize on enhanced SRH that exists near Metro Detroit.

Heavy rainfall and localized flooding will be a threat through the event as PWAT sits 2 inches and a deep warm cloud layer depth to 12- 13 kft supports efficient rainfall processes. While most convection has been relatively progressive which limits residence time of heavier rates, the environment supports rates of 1 to 2 inches per hour. Corfidi vector magnitude weakens to 10 kt suggesting potential for backbuilding and training convection that may produce pockets of rainfall in excess of 2 inches within a few hours. If this occurs over urban areas and/or locations that have received heavy rain in the past 24 hours, flooding will be a concern. After the complex exits, a period of subsidence supports a lull through the late evening with potential for fog development overnight.

The next shortwave spills over the upstream ridge axis overnight before passing over Lower MI between 09z and 18z Friday. This wave pulls the nearby reservoir of higher theta-e back NE and toward the area, tightening the instability gradient in place. This offers a favored corridor for nocturnal convection to go upscale and track ESE through the early to mid morning, with a veering LLJ supplying a feed of subtropical moisture into the elevated frontal boundary. Should an organized cold pool capable of overcoming developing CIN emerge upstream, available hi-res ML guidance does support potential for isolated to scattered strong to severe wind gusts. Given unfavorable diurnal timing and limited surface-based instability, the favored scenario is for a weakening trend and relatively muted severe potential. The environment remains supportive of localized torrential rainfall with any convection overnight into Friday morning.

The frontal zone migrates slightly southward on Friday, its placement and the subsequent orientation of the instability axis dictating where additional convection may become favored by Friday afternoon and evening. A majority of CAMs place this corridor across western Lower MI, intersecting areas as far north as M-59. A similar environment in place offers potential for another round of strong to severe storms if convection materializes. The SWODY2 highlights a marginal risk for severe weather tomorrow with support from hi-res ML guidance.

The upper air pattern evolves into a large scale omega block by Saturday, likely holding in place through the weekend with an amplified ridge extending from Texas into the northern Plains. Our placement within the northwest flow downstream of the ridge axis keeps some potential for showers/storms in the south on Saturday, but expansive high pressure is then shown to dominate the Great Lakes by the latter half of the weekend and into the early work week. This will bring the return of a more seasonable air mass with lower humidity.

MARINE

A stalled frontal boundary remains draped across the region through this evening offering potential for additional severe weather, particularly from the St. Clair River down to western Lake Erie. Prevailing winds and waves stay below threshold criteria as the gradient remains weak, but the very unstable/muggy air mass favors rapid development of strong to severe storms. Such storms will be capable of gusts in excess of 40 knots and/or hail in excess of 0.75 inches. MWSs and SMWs will continued to be used to address short- term local watercraft concerns. Hi-Res models indicate the waterways should begin to stabilize by approximately 00Z this evening. A lull in activity is expected for part of the overnight period with winds becoming light and variable, if not weakly easterly. A low confidence forecast continues into Friday with additional rounds of convection likely for portions of the central Great Lakes. The first round of storms could reach Saginaw Bay as early as 10Z Friday, followed by a secondary round further south Friday evening. No long- fused headline concerns expected through the weekend once the ridge over central CONUS eventually dislodges. Gusts should remain just below Small Craft Advisory criteria Saturday, albeit with increasing wave energy given onshore flow. Drier weather is forecast for Saturday and Sunday.

HYDROLOGY

A mature thunderstorm complex is tracking southeastward across the area this afternoon. A subtropical air mass in place allows for torrential rainfall rates of over 1 inch per hour, but relatively brief residence time of these rates has mitigated a more widespread flooding threat thus far. There is the potential for some training or backbuilding of storms this afternoon which would result in pockets of amounts in excess of 2 inches. This would support a localized flooding threat especially if this occurs over any urban areas or locations that received heavy rain already in the past 24 hours. There is a signal for another round of showers and thunderstorms to develop overnight into Friday morning, which will be capable of producing up to an additional 1 to 2 inches of rainfall for localized areas.

PREV DISCUSSION

Issued at 153 PM EDT Thu Sep 3 2026

AVIATION...

A hot and humid airmass has led to blossoming convection along a convergence axis that stretches from roughly Ludington to Detroit. This boundary has and will continue to be the focus for thunderstorms today and tonight, with a thunderstorm complex ongoing near KMBS at issuance. This feature will ride the boundary southeast and impact most terminal sites between now and roughly 22z this evening. Light and variable flow persists outside of thunderstorm activity, but quickly ramps up to 40-50 knots in thunderstorms with additional hazards including large hail and an isolated tornado threat. Once convective activity clears out this evening, a clearing trend is expected overnight and, depending on the extent of this clearing, could result in low stratus/fog development tonight-Friday morning given the moist boundary layer. Thunderstorm activity continues into Friday in which severe weather continues to be possible. D21/DTW Convection... A thunderstorm complex currently near KMBS will track southeast along a convergence axis through the afternoon, bringing potential for severe thunderstorms to all terminals between now and 22z. Damaging winds in excess of 50 knots are the main threat, but large hail in excess of 1 inch and an isolated tornado are possible as well.

DTW THRESHOLD PROBABILITIES...

* Low for ceiling 5000 ft or less early this afternoon. Moderate after 00Z tonight.

* High for thunderstorms between now and 22z this evening.

DTX WATCHES/WARNINGS/ADVISORIES

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