textproduct: Detroit/Pontiac

KEY MESSAGES

- Light rain south of I-96/I-696 tapers off later this morning.

- A Frost Advisory for part of Southeast Michigan expires at 8 AM.

- Scattered rain showers are possible this afternoon.

- A Freeze Watch remains in effect for all of Southeast Michigan tonight.

- Drier conditions expected Saturday and early Sunday before rain and thunderstorm chances rise early next week.

DISCUSSION

The merger of the northern and southern stream waves has supported an expansive jet streak, occupying a significant portion of eastern CONUS. Southeast Michigan resides along the northern fringe of the longwave feature as a mid-/upper-level low slows over central Ontario, deflecting against northward progress of the composite wave. However, perturbed flow aloft sustains a series of shortwave ejections over the Great Lakes during the next 48 hours. The initial trough is currently transiting northern Indiana/Ohio, shoving its surface reflection eastward toward the southern Lake Erie shoreline. Early morning radar and surface observations reveal a weak precipitation response from the vicinity disturbance, largely inhibited by a lack of deep-layer moisture as evidenced by the 01.00Z KDTX RAOB. Showers should continue for several more hours, predominantly south of the I-96/I-696 corridor, until about 15Z. For the rest of today, a lobe of vorticity extending from the resident closed low offers a boost to ascent, and an opportunity for scattered narrow-diameter showers. Forecast soundings identify surface-based mixing depths maximizing some 8-10 kft AGL this afternoon, extending into the DGZ. Hydrometeor paths face warm and dry air within the final third of descent. This suggests melting and evaporation concerns, which should limit QPF and afford areas of virga. Showers wane in the absence of a diurnal boundary-layer.

The current configuration reinforces lower column NNW flow with thermal troughing sending 850 mb temperatures dropping into the -3C to -7C range tonight. Regarding the Freeze Watch, uncertainty regarding cloud trends precludes any upgrades to the inherited watch. The latest model data indicates areas to the north/west face a higher likelihood to experience sufficient clearing and subsequent longwave cooling. Additionally, gradient winds appear to trend toward calm soon for western locations given the emergence of surface ridging from the Upper Midwest.

Whether or not Freeze criteria is reached, Saturday will get off to a chilly start, by early May standards. Temperatures at sunrise will be in the 30s for most locations, or 10F to 15F below normal. A mostly dry day is forecast as the next round of CVA moves overhead with limited fanfare. PWATS decrease, dropping below 0.30 inches, with just a 2-4 kft saturated layer, centered near 7 kft AGL. A few isolated showers/sprinkles are possible during peak heating, but profiles look too lean (excluding the operational GFS) to include even Slight Chance PoP adjustments. High pressure builds across Lake Michigan during the day, followed by height rises aloft. Temperatures recover into the low 50s, boosted by a rising sun angle while rather light gradient flow holds from the northwest.

Mid-level winds back zonally on Sunday with an uptick in ThetaE while surface winds flip southwesterly. Surface ridging centers much further south, well into The Southeast as a more active period gets underway across Michigan. Confluent low-level flow leads to breezy and possibly some showery conditions. A progressive Alaskan wave descends on the region Monday producing a deepening surface low, to 982 mb. An elongated cold front extends from central Ontario into the southern Plains which will focus a large swath of showers and thunderstorms Monday night into Tuesday. Pre-frontal warm sector brings a brief reprieve from seasonably cooler weather. Timing of the evolution of these features leaves many questions as to how active the mid-week time-frame will become.

MARINE

A slow moving upper-level low pressure system will continue to impact the Great Lakes through the end of the week. This will bring periodic rain shower along with lighter winds given the diffuse pressure gradient. Wind direction holds from the north to northwest through the end of the week with localized higher winds speeds found along northern Lake Huron, with gust potential up to around 20 knots. Wind direction back to the southwest Sunday and holds through early next week as a warm front passes through the Great Lakes. Things brings additional chances for rain showers and a low chance for an embedded thunderstorm.

PREV DISCUSSION

Issued at 1152 PM EDT Thu Apr 30 2026

AVIATION...

Compact low pressure is on schedule with light rain production toward the Ohio border but still south of DTW leading up to midnight. The system grazes the DTW terminal corridor with light rain and borderline MVFR/VFR ceiling after midnight but is only able to produce some virga toward PTK and then just scattered to broken mid clouds farther north. MBS is far enough north to hold in a pocket of mostly clear sky until closer to sunrise.

The low pressure system exits eastward during the morning followed by a renewed supply of cool air in the afternoon from Ontario high pressure. Light NW wind reinforces the air mass that is subject to another round of VFR clouds and widely scattered showers enhanced by daytime instability. Shower coverage and duration are again too limited for inclusion in the forecast while the lower range of VFR ceiling builds into a mid level layer above 5000 ft Friday evening.

D21/DTW Convection...No thunderstorms expected through this TAF period.

DTW THRESHOLD PROBABILITIES...

* High for ceiling at or below 5000 feet late tonight and Friday, then low Friday evening.

DTX WATCHES/WARNINGS/ADVISORIES

MI...Frost Advisory until 8 AM EDT this morning for MIZ047>049-053>055- 061>063-069-070.

Freeze Watch from late tonight through Saturday morning for MIZ047>049-053>055-060>063-068>070-075-076-082-083.

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