textproduct: Detroit/Pontiac

KEY MESSAGES

- Widespread rain and thunderstorms are expected this evening. Heavy rainfall is likely which may lead to localized flooding, especially in urban areas. A Flood Watch is in effect for areas south of M-59. The potential also exists for severe weather in the same area tonight, with damaging wind gusts, large hail, and isolated tornadoes possible.

- Breezy on Thursday with westerly wind gusts of up to 35 mph.

- Slightly below normal temperatures are expected through the week and this weekend.

DTW THRESHOLD PROBABILITIES

* Low for ceilings aob 5kft today. High tonight.

* High for thunderstorms this evening.

PREV DISCUSSION

Issued at 350 AM EDT Wed Jun 17 2026

DISCUSSION...

A seasonably strong shortwave will ride into the Great Lakes today at the nose of a 130+ kt upper jet streak. Remarkable deep layer ascent will spread in ahead of this wave, producing a deep surface low that swings across the Midwest this afternoon and Lower MI tonight. Model progs continue to advertise a sub-990mb low which is 4 to 5 standard deviations below normal for mid June, and if it verifies will challenge low MSLP records for the month. Ahead of this system a powerful 60+ kt LLJ will spread east across the southern Great Lakes, rapidly advecting in deep moisture and strong isentropic ascent into the elevated warm frontal boundary taking shape through the day. The result will be widespread rain from late this afternoon into the late evening hours, with a threat for flash flooding and severe weather.

Elevated showers and storms will move in late afternoon with heavier activity culminating during the late evening as the strongest ascent drives a line of convection through ahead of the cold front. Model soundings continue to showcase deep saturation and warm cloud layer with PWAT rising to the 1.50 to 1.75 range during the evening. This will allow for efficient, heavy rainfall processes in any convection and instantaneous rates within the strongest cores will likely exceed 1"/hr. The bulk of the rainfall is expected in a relatively brief window between 8pm and 2am and brings a heightened threat for flash flooding especially across urban areas including Metro Detroit. Bulk of ensemble QPF output supports widespread 1 to 2 inch amounts, and 30 to 40% probability for 2+ inches exists in the latest HREF, REFS, and NBM ensemble suites. Placement of the axis of highest QPF continues to carry uncertainty, but signal appears to be most focused along the eventual track of convection near the triple point. Confidence is high enough to issue a Flood Watch for areas along and south of M-59. Will note there is a subset of hi-res guidance that bifurcates the convection and actually places the higher QPF north of the current watch. This remains a possible outcome but carries less probability than the farther southern solution.

The late evening period also brings concern for severe weather as we sit near an upstream environment supportive of supercells and bowing structures given high amounts of low-level shear and SRH. Instability remains the limiting factor locally as we sit north of the CAPE gradient until a brief window during the late evening where the occlusion/surface warm sector attempts to nudge in from the south as the low begins to pivot off to the north. Afternoon/evening convection is likely to ride ESE along the CAPE gradient from northern IN into OH which should prevent the higher surface based instability from advancing north, but trends show the dynamic system still capable of advecting in a narrow axis of marginal instability just as the LLJ begins to depart during the late evening. Hi-res reflectivity hints at potential for bowing and/or supercellular structures to survive into the local area with perturbed MSLP fields indicating capability to produce severe wind gusts even if tenuous amounts of CIN otherwise suggest instability remains elevated. The environment also support a QLCS tornado threat. The cold front quickly passes through shortly after midnight which brings an end to the severe threat. SPC expanded the slight risk farther north to include Metro Detroit to account for the trend in guidance.

Low-level wind veers around to westerly Thursday morning as the system departs, with a secondary LLJ keeping windy conditions in place through the morning as the system departs. Gusts of 30 to 35 mph will be common early with wind gradually subsiding through the afternoon. Lighter and more scattered showers will continue behind the cold front early Thursday morning and lead into a slight increase in coverage mid Thursday morning as a weak secondary cold front tracks through. Seasonably cool conditions continue Thursday and Friday with just a 15% chance of showers on Friday as cool, cyclonic flow aloft maintains weak diurnal instability.

The jet stream lifts northward and introduces some upper confluence on Saturday, though additional PVA may still support a diurnal flare- up of weak convection. The next notable wave in the flow arrives from the Midwest Sunday evening with another relatively deep low for June standards somewhere in the vicinity of the Great Lakes. Local impacts are uncertain as there are still many moving parts, but something to monitor through the late week.

MARINE...

A brief break in convective activity is underway this morning as ridging aloft quickly passes through. A strong low pressure system follows in quick succession leading to evening and overnight thunderstorms capable of damaging winds, and to a lesser extent, waterspouts. Prevailing southeasterly gradient flow accompanying the low climbs above 25 knots across the southern waterways, after the center approaches Lake Huron tonight. Gusts to gales remain possible, mainly for Lake St. Clair and western Lake Erie, as the strongest LLJ winds of 40-50 knots pass through the lowest 3 kft. Shallow mixing profiles appear to exhibit only meager growth, maintaining uncertainty with the potential/duration of gales. Opted out of gale headlines for Lake St. Clair and western Erie given the anticipated sporadic nature of the gale-force gusts. A Small Craft Advisory was issued for all nearshore zones with high confidence in wind and wave criteria being met. Post-frontal winds decline a bit and organize out of the west-northwest on Thursday, remaining so through at least Saturday. Additional chances exist for periods of showers and thunderstorms through the rest of the workweek and over the weekend.

HYDROLOGY...

A strong low pressure system will arrive this evening, producing widespread rainfall with numerous heavy thunderstorms through late tonight. Rainfall totals ranging between 1 and 2 inches are likely for much of SE MI, and isolated areas may receive over 3 inches. Most of this rainfall is expected to occur in 4 hours or less, currently centered between 8pm and 2am, and rainfall rates will likely exceed 1"/hr at times. This rainfall may pose a flash flooding threat, mainly in urban areas, and rises on rivers and streams can be expected. A Flood Watch is in effect for areas along and south of M-59. Additional scattered showers are possible overnight and into Thursday, but these will be much lighter.

DTX WATCHES/WARNINGS/ADVISORIES

MI...Flood Watch from 4 PM EDT this afternoon through late tonight for MIZ068>070-075-076-082-083.

Lake Huron...Small Craft Advisory from 8 PM this evening to 8 PM EDT Thursday for LHZ421-422-441>443.

Lake St Clair...Small Craft Advisory from 8 PM this evening to 8 PM EDT Thursday for LCZ460.

Michigan waters of Lake Erie...Small Craft Advisory from 8 PM this evening to 8 PM EDT Thursday for LEZ444.