textproduct: Detroit/Pontiac

KEY MESSAGES

- Dry and cool again Friday.

- Rain returns by Friday night into Saturday morning. Locally heavy rainfall of an inch or more is possible from Monroe to Port Huron.

- Milder this weekend, with chances for thunderstorms Saturday and Sunday.

- Warming trend into early next week with highs climbing back above normal.

DISCUSSION

The southern flank of 1034 mb surface high pressure has governed conditions over SE Michigan today, leading to mostly sunny skies and light east-northeast flow. This shallow layer (~5.0 kft) of easterly flow has kept daytime highs about 10 degrees below average. SE CONUS ridging takes control of flow above ~700mb, allowing mid-upper level cloud to stream into the Saginaw Valley and Thumb regions this evening.

High pressure influence wanes Friday as cyclogenesis occurs upstream over the Dakotas. There are several implications of this locally, with the first being a strengthening pressure gradient. East winds thus become breezy Friday with gusts up to 30 mph. Second is the release of the theta-e gradient northward into SE Michigan Friday evening. The existing subsidence inversion (noted between 850- 700mb in the 12z KDTX RAOB) will remain in place until this occurs, capping mixing depths around 5.0 kft agl and allowing temperatures to climb just a few degrees higher than today.

Meanwhile, a second low forms over the Ohio Valley Friday as a low amplitude shortwave ripples over the unstable warm sector. This low establishes its own frontal structure and draws plenty of moisture northward due to its origins near the Gulf. Trajectory of the surface low either directly over SE Michigan or just to the east leaves us on the edge of the strongest moisture transport, with the highest QPF axis following suit. Nonetheless, about 20% of ensemble members continue to generate over an inch of QPF along the eastern border of the cwa (roughly Monroe to Port Huron, including urban areas of Detroit). These higher qpf solutions anchor to a strong 925-850mb frontal circulation, set to lift through SE Michigan between 09-15z Saturday (5am-11am). This is the window where we are likely to see the heaviest rainfall rates, before a mid-level dry slot arrives and strips away column moisture for the afternoon. Rainfall chances and storm total amounts decrease with westward/northward extent for Saturday morning. While there is potential for FGEN to enhance rainfall rates, lapse rates are unimpressive (< 5.5 C/km) for convective enhancement Saturday morning. A rumble of thunder however cannot be ruled out.

Increased variability in the solution space regarding rain chances Saturday afternoon-evening. This will depend largely on the pace at which the low occludes and if any moisture can wrap back around the low. Outgoing forecast is for a dry Saturday night period. Additional rain chances Sunday are tied to the arrival of the broader, northern stream trough, driven eastward by a 70 knot jet streak. There is a better chance of thunderstorms Sunday as mid- levels cool invof the trough. Warming temperatures then expected for the holiday week, climbing back above normal. This is supported by good agreement in the days 5-7 cluster analysis which depict an upper ridge overhead.

MARINE

High pressure centered over the northern Great Lakes will continue to hold through tonight before shifting off to the east. Northeast winds this afternoon will persist with gusts staying below 20 knots for most marine areas. The two areas that are seeing higher gusts have been western portion of Lake Erie, as well as inner Saginaw Bay. Given the fetch down the long axis of the lake which is causing a local increase in wind and waves, Small Craft Advisories are currently in effect for these areas. Winds will continue to increase as the pressure gradient tightens ahead of the next low pressure system lifting north into the region from the south Friday into Saturday. This will bring a chance for showers and thunderstorms to the area, along with Small Craft Advisories going into effect for the rest of the nearshore zones through the day on Friday. Northeast to easterly winds will continue to be gusty through Saturday, with potential for gusts to reach up to 30 knots.

HYDROLOGY

A round of steady soaking rainfall is forecast to spread across southeast MI from south to north Friday evening into Saturday morning as low pressure lifts in from the Mid Mississippi Valley. Additional showers and some thunderstorms are possible Saturday afternoon but carry less certainty at this time. Forecast rainfall totals range generally between a half inch and 1 inch during this event. Locally higher rainfall totals of an inch or greater are possible from Monroe to Port Huron. Overall dry conditions for much of the area over the past 30 days suggests the potential for flooding is low.

PREV DISCUSSION

Issued at 1253 PM EDT Thu May 21 2026

AVIATION...

A large area of surface high pressure centered on the northern Great Lakes extends well south into the Ohio valley tonight. This setup supplies a dry NE wind into SE Mi that is proving more than adequate to keep conditions free of low clouds, despite trajectories across Lake Huron. That leaves just varying amounts of cirrus coverage spreading west to east across Lower Mi tonight which solidify and lower during Friday morning. Surface wind becomes moderate easterly with gusts near 20 knots while ceiling holds above 5000 ft all areas until closer to or after 18Z Friday.

D21/DTW Convection...No thunderstorms this afternoon through Friday morning.

DTW THRESHOLD PROBABILITIES...

* None

DTX WATCHES/WARNINGS/ADVISORIES

MI...None. Lake Huron...Small Craft Advisory until 10 AM EDT Saturday for LHZ422.

Small Craft Advisory from 4 PM Friday to 10 AM EDT Saturday for LHZ421-441.

Small Craft Advisory from 10 PM Friday to 10 AM EDT Saturday for LHZ442-443.

Lake St Clair...Small Craft Advisory from 4 AM Friday to 10 AM EDT Saturday for LCZ460.

Michigan waters of Lake Erie...Small Craft Advisory until 10 AM EDT Saturday for LEZ444.