textproduct: Detroit/Pontiac

KEY MESSAGES

- Hot and humid conditions through tomorrow.

- A Marginal Risk for severe weather remains in place through tonight, with a Slight Risk for tomorrow and tomorrow night.

- More shower and thunderstorm chances Thursday and Friday.

UPDATE

Forecast evidence on thunderstorm trends continues to be monitored for tonight through Wednesday morning. Storms in NE Lower Mi finally broke through at the very end of daytime peak heating. These stay to the north in the absence of any new development toward Saginaw Bay. Greater potential for storms in central and SE Mi is tied to the remnants of the broken band of showers and isolated storms that was percolating all day toward the OH/IN border. The band remnants mark the west to east moisture axis that is also roughly along the 10 C 700 mb cap, and was activated along the south flank of the upper level jet. The moisture axis/theta-e ridge now migrates northward tonight as the jet streak shifts farther into northern Ontario. Continued development of ongoing convection across southern WI is likely, along with a chance of new development from there into central Lower Mi, as 850- 700 mb flow backs slightly SW with the help of the diffuse 500 mb vort max. Downstream MLCAPE settles into the 1000-2000 J/kg range and bulk shear diminishes closer to 30 knots, however both are adequate to sustain thunderstorm clusters across the area during the late night and early morning. The main predictability challenge for severe potential is tied to the tendency for storms to become elevated after midnight. The Marginal SPC outlook remains in place for an isolated instance of damaging wind or large hail in scattered storm coverage mainly along and north of I-69 until shifting into southern Ontario before noon Wednesday.

PREV DISCUSSION

Issued at 715 PM EDT Tue Sep 1 2026

AVIATION...

VFR is being reported over all of southern Lower Mi extending toward the Ohio valley and upper Midwest, including within the broken band of showers near the OH/IN border. There is an isolated thunderstorm in this pattern leading to a brief restriction, however mostly VFR showers continue as the band moves slowly northward with a similar character this evening. The window also appears to be closing for any additional storm development to the north toward MBS and Thumb, however this will be monitored as the southern band moves northward.

Focus turns upstream toward the upper Midwest late tonight and Wednesday morning where new clusters of showers/storms are projected to develop and move into central Lower Mi. The Prob30 remains in the forecast for MBS and FNT with timing refinement and/or southward expansion added in later updates. Conditions otherwise trend toward a few hours of MVFR fog around sunrise as the warm and humid air mass remains in place over the region. Instability then builds for another round of thunderstorms Wednesday afternoon, again mainly toward MBS and FNT.

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 this evening.

PREV DISCUSSION... Issued at 303 PM EDT Tue Sep 1 2026

DISCUSSION...

A challenging forecast continues to plague the Great Lakes Region. A hot and humid air mass with a high degree of instability is in place this afternoon. Temperatures have reached into the upper 80s to 90 degrees as of 2 PM, with heat indices well into the 90s. A couple more hours left to increase those values as well. The broken line of convection which was just south of the border this morning within the northern edge of the 850-700 MB Theta-E ridge has crept northward into southwest lower Michigan and will continue to drift north, presenting a opportunity for showers and thunderstorms this evening and tonight. The highest instability and wind shear are actually located over center/northern Lower Michigan this afternoon per SPC mesoscale analysis, making areas north of M-46 probably the greatest concern for isolated severe storm, but the entire CWA has a Marginal Risk through tonight. Once again, hi-res solutions continue to underestimate the coverage of showers and thunderstorms.

A couple opportunities exist for ridge runners as upper level impulses eject out of the central/northern Plains both Wednesday and Thursday. Growing consensus points to a shortwave tracking through northern lower Michigan tomorrow (15-19z), with a surface front/reflection along or north of I-69 providing the focus. Even with this potential earlier timing, there will most likely be sufficient instability around to support severe storms alongside high 0-6 KM bulk shear values aoa 50 knots. A Slight Risk of severe weather remains in place for both large hail and damaging winds. Machine learning even highlights the northern Thumb region for a tornado threat, which is supported by 0-1 KM bulk shear values around 25 knots. If this faster timing materializes, max temperatures could come up short of the outgoing forecast in the 90- 94 degree range; thus, confidence is not high enough to issue a Heat Advisory, as peak values in the upper 90s to around 100 degrees could be short-lived once again.

Yet another upper level impulse will race through Wednesday night or Thursday morning. Coupled with nocturnal low level jet enhancement, strong thunderstorms producing heavy rain become a concern. Although, at this time, there is no indication of training. None-the- less, PW Values aoa 1.75 inches and 700 MB dew pts still locally nearing 5 C will certainly be supportive of locally heavy rain, with another opportunity or two between Thursday night and Friday night. Confidence remains low, however. Longwave pattern guidance for the weekend indicates upper level confluent flow and subsequent high pressure building in, trending the forecast in a drier direction.

MARINE...

A low pressure system now entering Ontario will steer 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 and tomorrow leading to additional rounds of storms through Wednesday night 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 tonight.

HYDROLOGY...

A humid air mass will bring a chance of heavy rainfall within any strong thunderstorm that develops. However, areal coverage and exact timing remain elusive across Lower Michigan. Periodic chances of storms will exist right through Friday. Any strong thunderstorm will have the potential to produce a quick 1-2 inches of rain, which can lead to localized minor flooding of roads and poor-drainage areas. There are currently no indications of training storms, though more extensive flooding would become a concern if a mature MCS develops.

DTX WATCHES/WARNINGS/ADVISORIES

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