textproduct: Detroit/Pontiac

KEY MESSAGES

- Numerous thunderstorms are likely this afternoon and early evening. Isolated storms may be strong to severe.

- Widespread rain and thunderstorms are expected Wednesday evening and Wednesday night. Heavy rainfall is likely which may lead to localized flooding, especially in urban areas. The potential also exists for severe weather, mainly south of I-94.

- Slightly below normal temperatures are expected through the week and next weekend.

DISCUSSION

Active weather pattern the next two days with multiple rounds of thunderstorms likely and potential for flooding rains, especially Wednesday evening.

A look at the visible satellite early this afternoon shows the system responsible over northern WI with the attendant cold front extending to the south while a prefrontal fgen band exists over eastern Lake MI producing showers. Locally over SE MI some high based showers initiated early with the lack of a cap but little morning CAPE and shear prevented much from developing. Heading into the afternoon, a 30 knot low level jet will lift ahead of the prefrontal band while a pocket of 500mb vorticity pivots around reaching southern MI at the same time. This mostly seems to come together over SW MI, hence SPC upgraded that region to a Slight Risk while leaving part of SE MI in a Marginal Risk. SBCAPE is forecast to top out around 500 J/kg with some bulk shear around 30 knots. We'll have to see how upstream convection can hold together as it approaches for increased severe chances. Dry low levels showing up in the soundings as an inverted V, supporting the higher bases convection will also support wind gusts of 40 to 60 mph as the most probable severe threat this afternoon mainly from 20-00Z.

Wednesday still garners much attention with the unseasonably strong low driving through the region. Some changes/questions of note to the setup in terms of location of the low/occlusion which will dictate severe chances in SE MI. Models still support a strong mid level trough over MN/WI with a sub 990mb surface low under it. A strong mid level wave will rotate through the base of the trough across southern Lake MI while a very strong 60+ knot low level jet racing northward just ahead of the wave through IL and into southern MI just before Wednesday evening (around 00Z Thursday). The strong lift ahead of the surface low will cause the system to occlude and refocus a center over southern MI or northern IN. At this point, most of the support is to push it east near the state line. A portion of the low looks to get rotated up through the Thumb, while a potential MCS diving E-SE helps drive the instability axis southward, keeping the bulk out of SE MI. Strong lift with impressive southwesterly moisture advection will bring PWATs around 2 inches into SE MI on the lead isentropic arm, with the low and associated thunderstorms passing through mainly in the 21-06Z window. Severe weather chances remain limited as we expect the bulk of the instability and surface features to remain to our south. SPC Day2 outlook has remained relatively unchanged focusing on the better setup south of the border. There remains a window around 00- 04Z when a narrow wedge of instability slips north through the area which would be our best chance for severe storms. Main threat will occur mainly south of I-94 but location will be adjusted as we see where the surface boundaries set up. Confidence in flood risk remains elevated Wednesday afternoon into the overnight. WPC Day2 ERO continues to include all of SE MI in a Slight Risk for flooding. The wealth of moisture, strong fgen and EPV, with strong deformation developing just north of the low center, should produce a wide swath of heavy rain across portions of SE MI. Highest totals should occur close to the Ohio Border with decreasing amounts heading north. As we get deeper into the suite of CAMs, precip totals have had a slight downward trend, possibly owning to the shorter duration with such a fast moving system. Regardless, confidence remains fairly high for a swath of 1 to 2 inch rainfall totals, with locally higher amounts exceeding 3 inches possible.

We remain in the broad upper level trough on Thursday with the remnant surface low from MN/WI tracking across central MI Thursday afternoon. Steepening low level lapse rates look to be able to produce a round of low topped showers early Thursday. Mid level trough feature should pass to the east, ending the support for showers overnight but could see another window of diurnally forced showers on Friday, more limited in coverage.

MARINE

A low pressure system and its attendant cold front gradually work through the central Great Lakes from west to east, supporting continued showers and thunderstorms into the evening and early overnight hours. An isolated threat for hazardous wind gusts and large hail remains. A brief break in activity emerges during the daylight hours Wednesday as ridging aloft quickly passes through. A secondary stronger low pressure system follows in quick succession late Wednesday into Thursday. More intense convection is possible with this system, capable of damaging winds, and to a lesser extent, waterspouts and large hail. Prevailing southwesterly gradient winds accompanying the low climb above 25 knots, once the center approaches Lake Huron late Wednesday night. This eventually backs flow northwesterly, based on the northeast trajectory of the system across the Huron basin. Gusts to gales remain possible, mainly for the southern waterways, as the strongest LLJ winds of 40-50 knots pass through the lowest 3 kft. Shallow mixing profiles appear to exhibit only meager growth, maintaining uncertainty with the potential/duration of gales. Opted to forego a Gale Watch for Lake St. Clair and western Erie, but will reevaluate during the evening update. Also, cannot completely rule out gales extending into southern Lake Huron, too. Post-frontal winds decline a bit and organize out of the west-northwest on Thursday, remaining so through at least Saturday. Additional chances exist for showers and a few thunderstorms over the weekend as cyclonic flow persists overhead.

HYDROLOGY

Numerous showers and thunderstorms are forecast this afternoon and evening. Most areas will receive a quarter inch or less of rainfall, but training of storms over the same areas will lead to totals nearing an inch. Flooding is not likely today.

A seasonably strong low pressure system will then arrive late Wednesday, producing widespread rainfall with numerous heavy thunderstorms through Wednesday night. Rainfall totals ranging between 1 and 2 inches are likely for much of SE MI, and isolated areas may receive over 3 inches. Most of this rainfall is expected to occur in 6 hours or less, currently centered between 8pm and 2am. This rainfall may pose a flash flooding threat, mainly in urban areas, and rises on rivers and streams can be expected.

PREV DISCUSSION

Issued at 131 PM EDT Tue Jun 16 2026

AVIATION...

Initial 700-600mb thetae/warm air advection was sufficient to generate numerous high based showers and thunderstorms across much of the central sections of the forecast area early this afternoon. With little convective inhibition, additional outflow boundaries could provide additional forcing for more development the next couple of hours. Additional thunderstorm redevelopment is anticipated along a cold front sometime in the 21-02z time window. There is a better potential for strong to severe thunderstorms this evening. MVFR ceilings are expected this evening and tonight with high ambient moisture from todays rainfall and radiative cooling. Clouds scatter with diurnal heating Wednesday with thunderstorm activity holding off until Wednesday evening.

D21/DTW Convection...There is a chance for thunderstorms to persist this afternoon in the airspace with a more organized line of thunderstorm activity expected to approach D21 between 22-02z.

DTW THRESHOLD PROBABILITIES...

* Moderate for ceiling at or below 5000 ft this afternoon.

* High for thunderstorms this evening.

DTX WATCHES/WARNINGS/ADVISORIES

MI...None. Lake Huron...Small Craft Advisory until 10 PM EDT this evening for LHZ421-422- 441.

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