textproduct: Detroit/Pontiac

KEY MESSAGES

- Periods of rain today through early Sunday with some embedded heavier rainfall possible at times.

- A cool and cloudy pattern is expected through early next week.

DTW THRESHOLD PROBABILITIES

* Medium in ceilings aob 5000 feet this morning. High this afternoon through tonight.

* Low in thunderstorms through tonight.

PREV DISCUSSION

Issued at 350 AM EDT Sat Sep 19 2026

DISCUSSION...

The large scale pattern is set up for multiple rounds of rainfall over the next 48 hours. A broad anticylone over the lower Mississippi Valley and a cutoff low drifting eastward over the Intermountain West work together to steer a plume of deep moisture across the Midwest. Meanwhile an upper jet entrance region tightens a elevated baroclinic zone over the Great Lakes, ensuring plenty of synoptic and mesoscale support for ascent as the inbound moisture interacts with the steepening isentropic surfaces. A 35 to 40 kt LLJ has been fueling a narrow axis of convection over MN/WI this morning with signs of southeastward propagation as the southern flank begins to tap into the elevated instability gradient situated from eastern IA into the Ohio Valley. The resulting MCS is forecast to follow this gradient as the LLJ veers to westerly over the next few hours, presenting a window for thunderstorms and heavier rain to affect areas south of I-94 later this morning. Otherwise, periodic light to moderate rain is anticipated for much of SE MI through the morning and early afternoon with potential for a lull mid to late afternoon into this evening. Most areas are favored to see up to around a quarter to half inch through this evening, but a few areas could receive in excess of 1 inch depending on the track and evolution of the MCS this morning.

The next target for higher coverage of rain arrives overnight and into early Sunday as the LLJ reestablishes just downstream of a 850- 700mb trough. A couple factors highlight potential for heavier rates to occur between midnight and 8am Sunday. 1) PWAT maximizes in the 1.50 to 2.00" range while warm cloud depth builds to 12+ kft and MUCAPE increases on the order of 250 to 500 J/kg, indicating an environment supportive of efficient downpours more characteristic of mid-summer. 2) Corfidi vector magnitude falls below 10 kt indicating potential for slow-moving and training showers. This is supported by several hi-res runs that develop a surface low and a narrow deformation band in the vicinity. If this band develops, potential exists for additional localized rainfall totals in excess of an inch overnight, much of which could fall over a 1 to 3 hour window. HREF neighborhood QPF probabilities offer a 30% chance for rates over 1"/hr. Worth noting there remains a separate camp of solutions that favor development of a more organized MCS over northern IL. This is shown to track southeast along the CAPE gradient and consume much of the moisture feed, shunting the heavier rain threat to our south. So potential exists for an axis of heavier rainfall early Sunday but confidence remains low on how the frontal zone evolves and the placement of any localized focused ascent.

Strong subsidence emerges over northern Ontario on Sunday, producing a broad surface ridge that expands across the northern Great Lakes with increasing influence of dry northeast wind. Showers may linger near/south of Metro Detroit early in the day but should eventually retreat across the stateline leaving mostly dry but breezy and cool conditions late Sunday through Monday. A chance of unsettled showery weather returns late Tuesday into Wednesday and possibly Thursday as the previously mentioned cutoff low arrives from the west. Dry weather looks to return by late week as a low tracks up the east coast, favoring a ring of subsidence over the Great Lakes.

MARINE...

High pressure has drifted east and is now centered over the Ontario/Quebec border but is trying to hold on across a portion of the eastern Great Lakes. Winds will be southeasterly and largely under 20 knots today as we are positioned just north of a frontal boundary with a low pressure system riding along the front across southern MI. Widespread showers and scattered thunderstorms will accompany the system today and tonight, mainly focused over the southern half of the region. Tightening of the gradient on the backside of the exiting low will ramp northeast winds back up on Sunday reaching 25 to 30 knots. These increased winds will carry into Monday as well which may necessitate additional headlines.

HYDROLOGY...

A low pressure system approaching from the Plains brings periodic showers and isolated embedded thunderstorms to the area this weekend. The first round of rainfall spreads across the area this morning with most locations seeing light to moderate rainfall through early afternoon. Potential exists for a brief period of heavier rain and some thunderstorms later this morning, mainly along or south of the I-94 corridor. Rainfall totals to around 1 inch will be possible if this develops. A relative lull in coverage is then forecast later this afternoon and evening.

The next round is expected to move in late this evening and continue into the early morning on Sunday. This period carries a higher probability for moderate to heavy rates with potential for training/repeated showers, especially for areas south of M-59. Additional rainfall in excess of 1 inch will be possible, much of which may fall within a 1 to 3 hour window. Additional lighter showers may continue through early Sunday with precip chances ending by late Sunday.

Weekend total rainfall amounts are forecast to range between a half inch and 2 inches for most of the area. A conditional threat for over 2 inches exists for any area that sees repeated rounds of heavier showers or storms. This would be most likely to occur south of a line from Lansing to Detroit. The longer duration of the rainfall will mitigate widespread flooding concerns, but ponding and urban flooding will be possible where any localized heavier rainfall occurs.

DTX WATCHES/WARNINGS/ADVISORIES

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