textproduct: Detroit/Pontiac

KEY MESSAGES

- Becoming more humid today with afternoon/evening showers and thunderstorms likely. A few strong to severe storms are possible.

- An isolated shower is possible Thursday afternoon, then drier and less humid conditions are expected through Friday and Saturday.

- A warming trend brings above normal temperatures early next week.

DISCUSSION

Last vestiges of upper shortwave ridging are departing this morning, with remnant warm mid levels maintaining stable conditions through noon. A closed upper low over the arrowhead of MN is on track to pass across Upper MI later today, opening into a trough in the process. Weak moisture transport ensues on modest S to SW wind, culminating in a more humid day with dew points creeping into the mid to upper 60s by this afternoon. As the upper trough erodes the remnant stability, showers and storms should percolate along a surface trough into west MI late this morning. Relatively weak divergence aloft and gradual height falls provide support for ascent as convection spreads eastward along the lead edge of the inbound low-level theta-e ridge, bringing fairly high confidence (60-70%) for showers and thunderstorms this afternoon and evening. Timing in hi-res guidance continues to favor arrival into the Tri-Cities around noon to 3pm, then the rest of SE MI between 3pm and 10pm.

HREF shows mean SBCAPE of 1000-1500 J/kg building in but with convection generally outpacing the higher instability as the evening goes on. Bulk wind shear on the order of 25-30 kt will offer potential for marginal updraft organization and a window for isolated stronger to severe storms. A band of slightly higher SW 850mb flow will accompany the trough, enlarging low level hodographs to some degree especially if surface wind remains backed/southerly. Damaging wind gusts will be the primary threat in the strongest storms, but large hail and a weak tornado will also be possible if any supercellular structures emerge. Hi-res ML guidance continues to favor areas south and west of SE MI for this threat, and will note that neighborhood probabilities for all severe hazards have decreased in 00z HREF. The SWODY1 slight risk area south of I-96 is driven by 15% wind probs, with hi-res reflectivity suggestive of transient linear features in this area. Localized heavy downpours will be possible as PWAT flirts with the 90th percentile around 1.50", but ensemble QPF data do not indicate a broader excessive rainfall threat.

A lull in convection is anticipated by late evening as the initial surface trough works eastward, but lingering instability ahead of the cold front will offer potential for additional spottier showers and perhaps a thunderstorm into the early overnight. The cold front then passes through early morning to end precip chances. The inbound air mass is not much different in temperature but slightly less humid. Weak instability develops on Thursday as longwave troughing induces steeper lapse rates aloft - this fuels ~20% probability for isolated late afternoon and evening showers.

A secondary cold front drops through ahead of high pressure that builds in through Friday and Saturday. This brings pleasant and less humid conditions with lots of sunshine. By early next week, the subtropical ridge over the southern Plains is progged to migrate into the Tennessee Valley. Warmer southwest flow riding in north of the ridge will bring a warming trend next week with the latest NBM advertising a 40-50% chance for parts of SE MI to see highs in excess of 90 degrees Monday and Tuesday.

MARINE

Southwest flow becomes well established across the Great Lakes today as a high pressure system departs into the continental northeast. An approaching cold front will strengthen the pressure gradient ahead of this front, increasing wind speeds through the day. Wind gusts of 20 to 25 knots will be likely across Saginaw Bay given the favorable fetch, where Small Craft Advisories are in effect.

Showers and thunderstorms will also become increasingly likely with the passage of the front, including a low chance for isolated strong thunderstorm development capable of producing wind gusts aoa 34 knots. Winds veer to the northwest after the frontal passage with northwest flow holding on through tomorrow. High pressure fills back in Friday and Saturday.

PREV DISCUSSION

Issued at 1142 PM EDT Tue Aug 25 2026

AVIATION...

Departing high pressure will slowly drift across the eastern Great Lakes tonight, with the influence of they system sustaining VFR conditions through Wednesday morning. A late day cold frontal passage on Wednesday will sustain a chance of convection Wed afternoon/evng.

D21/DTW Convection...Thunderstorm chances will increase late Wednesday afternoon and will continue into the evening in advance of a cold front. Current indications are fore scattered to numerous convection across the airspace late Wed afternoon and evening.

DTW THRESHOLD PROBABILITIES...

* Medium for thunderstorms Wednesday afternoon-evening.

DTX WATCHES/WARNINGS/ADVISORIES

MI...None. Lake Huron...Small Craft Advisory from 10 AM this morning to 10 PM EDT this evening for LHZ421-422.

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