textproduct: Detroit/Pontiac

KEY MESSAGES

- Hot and humid today. A Heat Advisory is in effect for portions of SE MI.

- There will be a Slight Risk for severe weather today. Most favorable timing for severe weather will be this afternoon and evening.

- Additional shower and thunderstorm chances Thursday and Friday.

DISCUSSION

Ongoing convection across southern Minnesota and central Wisconsin will continue to progress east-northeast through the morning, with primary convective concerns holding across northern Lake Huron, northern lower MI, and the U.P. Very moist low-level conditions remain in place across SE MI with surface dewpoints in the low 70s, which will support intermittent stratus and isolated showers through the morning. Showers or an isolated thunderstorm will be most favorable along or north of M59 this morning.

For this afternoon, any lingering low clouds are expected to erode quickly with enhanced boundary-layer mixing. With the projected convection and thicker cloud canopies holding north of the cwa, h850 temperatures aoa 20C, and some early day sunshine, daytime highs peak in the low 90s across a greater portion of the cwa. Mixing may temper the dew points slightly during peak heating, but the high boundary layer moisture should still support heat indices aoa 100 degrees across southern Michigan. This will be most favorable from the MI/OH border and up through the M-59 corridor, noting the usual enhanced potential across the urban Metro corridor. As a result, a Heat Advisory is now in effect from Noon through 8 PM EDT. Conditions still remain hot and muggy north of the advisory zones with heat indices ranging in the mid to upper 90s. The higher potential for mid to high level clouds and earlier arrival of potential showers and storms (more below) precludes the issuance of heat advisories farther north.

Attention then turns to the potential influence of a convectively generated outflow from the activity that is projected to traverse across northern Michigan this morning. This boundary would be slated to arrive across the Tri-Cities and Thumb late this afternoon and evening, coincident with peak daytime heating. MLCAPE values between 2500-3000 J/kg along with strong kinematics characterized by bulk shear values of 40-45 knots will be likely by the afternoon. Any CI generated off an outflow/pseudo cold front could result in organized convection capable of generating strong to severe thunderstorms. The greatest confidence for this scenario resides across the Thumb where several hi-res solutions depict a more pronounced southward-moving boundary. This potential may extend west into the Tri-Cities for initial development before progressing southward toward the I-69 corridor and Port Huron through the evening.

If strong to severe thunderstorms develop, storm mode may briefly start as semi-discrete but would likely quickly amalgamate to multi- cell clusters or a linear mode. Strong 1-3 and 1-6km shear values of 30-40 knots could support supercell development with any initial discrete formation under modest streamwise vorticity within the midlevels. The likely hazards would likely be damaging wind gusts with any bowing segments from cold pool generation or microbursts given deep layer CAPE. Large hail is also possible noting the CAPE values and strong mid-level lapse rates. Last, ML output does highlight chances for a tornado, most favorable within the Thumb. Uncertainty is higher with tornadogenesis. Again any initial discrete development would would make this possible noting 0-1km shear around 20 knots, but with current thinking hedged towards multi- cell/linear, ambient shear profile would be altered. Some guidance suggests secondary boundaries capable of propagating farther south into the evening through the Metro region. However uncertainty is higher in southern extent of any convection as timing of this coincides with loss of daytime heating. Overall, SWODY1 places all of SE MI in a slight risk for severe weather with all storm hazards possible.

Slightly cooler temperatures projected for Thursday but very humid conditions remain as dew points hold in the 70s. Quasi-zonal flow ushers in warm and humid conditions into the Great Lakes from the Plains, which will again support the chance for showers and thunderstorms tomorrow. SWODY2 clips a portion of southern Michigan for a Marginal risk for severe weather. Persistence forecast holds into Friday, with the overall pattern starting to breakdown into the weekend as ridge amplification commences across the Plains.

MARINE

A low pressure system over Ontario will control the weather across the Great Lakes through mid week. The warm front is now east of Lake Huron with the cold front approaching Lake MI leaving us in the hot and humid warm sector. Though the airmass is mostly stable at this time, it hasn't taken much mid level energy to spark clusters of storms the last few days. The cold front will slowly enter lower MI and drift SE today leading to additional rounds of storms possible through tonight and even into Thursday if the front stalls over the area. The weak gradient will keep speeds down through the week though with max wind gusts rarely touching 20 knots. The slightly cooler air over the waters brings chances of waterspouts with any of the storms over the lakes this week. The very humid airmass over the lakes will also lead to fog development over the waters, mainly the north half of Lake Huron which will need to be watched for another possible Dense Fog Advisory through this morning.

HYDROLOGY

A humid air mass will bring a chance of heavy rainfall within any strong thunderstorm that develops. However, areal coverage and exact timing remains elusive across Lower Michigan. Periodic chances of storms will exist right through Friday. Any strong thunderstorm will have the potential to quickly produce 1-2 inches of rain, which can lead to localized minor flooding of roads and poor-drainage areas.

PREV DISCUSSION

Issued at 1157 PM EDT Tue Sep 1 2026

AVIATION...

Scattered showers and isolated thunderstorms are developing from SW Lower Mi back across the Lake and into WI leading up to midnight, and will continue moving eastward across SE Mi late tonight through about mid morning. Current trends indicate activity grazes slightly farther south toward PTK while also still affecting FNT to MBS. The showers/storms are preceded by mid and high cloud remnants while visibility drifts down toward MVFR in light fog as the warm and humid air mass remains in place over the region. After a break during late morning to early afternoon, instability then builds for another round of thunderstorms mid afternoon into Wednesday evening, again mainly toward MBS and FNT. Storms develop overhead and/or move in from storms ongoing around the MN/IA border region.

D21/DTW Convection... The best chance for thunderstorms is north of DTW toward the northern fringe of D21 late tonight and again Wednesday afternoon.

DTW THRESHOLD PROBABILITIES...

* Low for ceiling 5000 ft or less Wednesday afternoon.

DTX WATCHES/WARNINGS/ADVISORIES

MI...Heat Advisory from noon today to 8 PM EDT this evening for MIZ068>070-075-076-082-083.

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