textproduct: Detroit/Pontiac

KEY MESSAGES

- A Flood Watch is in effect for far southeastern MI including Metro Detroit until 8 AM.

- Dry but windy conditions expected Thursday with gusts approaching 40 mph during the day

- Additional periods of rain likely Friday night through Sunday morning with potential for an additional inch of rainfall by the end of the weekend.

- Colder conditions ensue early next week with an opportunity for some rain/snow showers before high pressure settles in mid-week.

DTW THRESHOLD PROBABILITIES

* High for ceiling 5000 ft or less late tonight.

PREV DISCUSSION

Issued at 857 PM EDT Wed Apr 2 2025

UPDATE...

Rapid growth of elevated convective clusters across SE Mi have shown signs of organization capable of 1 inch severe hail. Hourly mesoanalysis and 1-3 hr HRRR/RAP estimates indicate elevated MUCAPE in the 1000-1500 J/kg range in effective bulk shear upwards of 50 kts through the elevated portions of the warm front. This is creating an impressive elevated supercell environment that is expected to continue through late evening. Meanwhile, the warm front remains stalled near the west end of Lake Erie on a lingering easterly wind component while observations indicate it is finally gaining ground to the north into SW Lower Mi as the parent surface low reaches western WI. Surface based thunderstorms then still have an opening to reach the fringes of Lenawee and Monroe counties as the main northern IN line arrives on a nearly 70 mph cell motion. Both the elevated hail threat and all hazards at the surface front are being actively monitored for severe intensity.

The Flood Watch also remains in effect through tonight as prospects for heavy rainfall continue with the inbound band of showers and thunderstorms. The mid afternoon activity was focused more toward the northern and western fringe of the watch area which is set to get another round in the 1-2 inch range. The Detroit metro area is also more in line for similar amounts until the pattern exits eastward around 2 AM.

PREV DISCUSSION... Issued at 418 PM EDT Wed Apr 2 2025

DISCUSSION...

Over the next several hours, should start to see the broken line of convection over Illinois reinvigorate as instability builds ahead of linear forcing. This has already started happening over Arkansas/Missouri, and with a strong SW-NE oriented cloud-bearing wind, expect this line to grow upscale quickly. Further east, the zonal warm front has released slightly further north since the mid- day forecast update. Considering these observations and the latest model guidance, the warm front will struggle to lift much further north than the MI-OH border. The meeting of these two features occurs between roughly 00z-06z tonight (8pm-2am EDT) and will be the focus for both the severe and flooding threats.

Starting with severe: the all mode severe threat will be tied closely with the positioning of the warm front this evening. The front itself is denoted by a sharp rise in surface T/Td, with temperature readings in the 70s and dewpoints in the 60s. Adjusted boundary layer conditions along/south of the front would afford surface-based inflow into any thunderstorms that develop, and large curvature to the hodographs aligns with an elevated tornado risk. This would be exacerbated by enhanced surface vorticity that exists along the front. Strong winds aloft and potential updraft/downdraft strength support a damaging wind threat/hail threat as well. That said, this scenario is highly conditional and confidence is decreasing in the front being able to reach much further north than Lenawee/Monroe county, if that. Storms are likely to remain elevated north of the front where temperatures still in the upper 30s-low 40s maintain a strong stable layer.

The flooding threat is much less conditional and carries more confidence for a wider footprint of impacts, and a Flood Watch remains in effect for the Detroit Metro region. As has been noted in previous discussions, PWAT values around 1.5 inches will exceed the daily maximum and a tall wedge of marginal MUCAPE (1000-2000 J/kg) will support convective enhancement to rainfall amounts. Fast SW-NE cell motion in excess of 50 knots will be nearly parallel to the pre- frontal forcing, equating to a several hour period of heavy rainfall potential after 00z/8pm EDT tonight. Global/hi-res models alike continue to support rainfall rates in excess of 0.5" per hour, which the environment has already proven supportive of with the latest bout of rainfall along the M-59 corridor. A more detailed discussion of flooding concerns can be found in the Hydrology section.

The cold front eventually tracks across lower Michigan early Thursday morning, initiating a period of strong isentropic downglide and ending the heavy rainfall threat early. Strong static stability develops above a dry, well-mixed boundary layer to support mostly sunny skies but breezy conditions as the low's pressure gradient is slow to depart. Westerly winds will gust 30-35 mph for most of SE Michigan, but locally higher in the Thumb region.

Friday through early next week...The upper level pattern remains amplified with a ridge of high pressure parked off the Florida Atlantic coast and a long wavelength trough working slowly across the Desert Southwest. This maintains an elongated southern stream jet corridor from The Plains to the Upper Midwest. Southeast Michigan will waver between the warm return flow side of the airmass divide (quasi-stationary front) and the colder flow brought in by upstream disturbances. A northern stream trough sheared off a ridge over the Gulf of Alaska digs across the Canadian Prairies and interfaces with the height field encompassing the northern Great Lakes over time. This acts to increase the anticylonic curvature of upper level flow trajectories. 1030 mb surface ridge influence and poor omega profiles limit potential for shower activity most of the day, but a low-end chance exists (less than 30 percent) for a few late afternoon showers along/south of I-94 with near-normal temperatures.

Lower column moisture parked over the Ohio Valley is expected to rotate into southern Lower Friday evening as the magnitude of southwesterly moisture transport vectors increases through the Mississippi Valley. PWATs are expected to rise to between 1.00 and 1.25 inches Friday night as highly perturbed geopotential heights induce a broad region of ascent. This should suffice in the generation of numerous showers across much of the region with variable rainfall rates. Convective depths could be hampered by tenable mid-level dry air with poor lapse rates aloft. Still, saturated column between 2 kft and 10 kft AGL should support repeat showers with periods of higher UVVs through the layer. Limited thunder potential in the absence of CAPE density.

The central Canada trough deepens and starts to close off into Sunday with minimal forward progress. This maintains shower and thunderstorm chances Saturday night through Sunday as the right entrance region passes overhead. Medium-range QPF indicates a swath of rainfall totals in the 1.5 inch to 2.0 inch range by Sunday night (48 hour total). Should models consolidate in positioning the highest rainfall axis over southern Lower MI (see 12Z ECMWF and GEM), flooding concerns would be heightened due to increasingly saturated soils and runoff drainage concerns, but the rest of the extended-range model data supports a more limited warm sector intrusion residence time. The latter favors 48 hour QPF in the 0.25 inch to 0.75 inch range. Drier air briefly filters with time on Sunday before another low pressure system and attendant cold front drops through the Great Lakes early next week. Thermal troughing sends 850 mb temperatures plunging down into the negative Celsius range with sub-freezing thermodynamic profiles suggesting a period of snow. Accumulations will be dictated by the onset time of snow and whether or not high temperatures can break out into the upper 30s to lower 40s.

MARINE...

A deep low pressure system over Iowa this afternoon will lift northward over Lake Superior overnight. A secondary low will develop along the warm front near Chicago and lift northeast over Lake Huron during this time. Winds across Lake Huron in advance of this front are reaching Gale force so the Gale Warning continues through this evening. Additionally, farther south across Lake St Clair and Lake Erie, wind gusts are topping out around 30 knots with building wave heights leading to a period of Small Craft Advisories for those lakes. Showers and thunderstorms will persist through much of the night as this system works across the lakes. Best chance of strong to severe storms would be for Lake St Clair and Lake Erie. The front will then stall across the Ohio River Valley through the end of the week as high pressure builds back over the Great Lakes keeping gusty westerly flow Thursday and lighter northerly flow Friday. Winds on Thursday may get up to 30-35 knots for several hours and may need a Gale Warning, mainly across the Straits and Saginaw Bay into Central Lake Huron but being a marginal setup will hold off for now.

HYDROLOGY...

A second round of showers and thunderstorms will develop over Illinois and Indiana before lifting into Southeast Michigan after approximately 8 PM this evening. Heavy rainfall is anticipated, with rainfall rates possibly approaching an inch per hour. Average additional rainfall amounts are likely to exceed one inch in the Flood Watch area, but if a line of storms repeatedly impacts the same area, localized amounts could be much higher. Rises on area rivers are likely to occur considering today's rainfall has already approached one inch in many spots. 6-hour flash flood guidance in the Detroit Metro area is between 1.5 to 2 inches, which will be attainable in the 00z to 06z window. Thus there is increasing concern that flash flooding may occur, especially in the urban areas of Detroit. A Flood Watch is in effect this evening into tonight. There is potential for another round of heavy rainfall this weekend.

DTX WATCHES/WARNINGS/ADVISORIES

MI...Flood Watch until 8 AM EDT this morning for MIZ063-069-070-075-076- 082-083.

Lake Huron...Gale Warning from 4 AM early this morning to 4 PM EDT this afternoon for LHZ361.

Small Craft Advisory until 10 PM EDT this evening for LHZ421-422- 441>443.

Lake St Clair...Small Craft Advisory until 4 PM EDT this afternoon for LCZ460.

Michigan waters of Lake Erie...Small Craft Advisory until 4 PM EDT this afternoon for LEZ444.