Expert Forecast Opinion

862
FXUS63 KDTX 030057
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
857 PM EDT Wed Apr 2 2025

.KEY MESSAGES...

- Severe thunderstorms remain possible this evening. Large hail
around 1 inch is a hazard across the area while damaging wind and
tornado hazards are confined to locations near the Ohio border.

- A Flood Watch is also in effect for far southeastern MI including
Metro Detroit through Thursday morning as heavy rainfall moves into
the area between 8 PM and 2 AM tonight.

- 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.

&&

.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 703 PM EDT Wed Apr 2 2025

AVIATION...

After a short break, the next round of showers/storms continues to
fill in from IN/IL into Lower Mi this evening. The band expands in
coverage and intensity first toward the MBS/FNT area and then
eastward toward the DTW corridor by mid evening. This occurs as the
warm front strains to gain northward progress before the cold front
advances into an occlusion later tonight. The warm front is still
positioned south of the Ohio border at forecast issuance but is so
shallow that there are breaks in the typical IFR/LIFR ceiling in the
DTW area while more fully entrenched farther north. The DTW pockets
of VFR fill in as the next round of rainfall occurs and as any
milder air makes it northward prior to the cold front sweeping
activity eastward by about 06Z. Rapid ceiling improvement occurs
post cold front bringing VFR scattered coverage to MBS before
sunrise and reaching DTW early in the morning. WSW wind becomes the
weather highlight as modestly colder air balances early April
daytime heating for boundary layer growth into 30 knot gusts that
last through Thursday afternoon.

For DTW... Thunderstorms remain likely this evening as the next
round expands into SW Lower Mi and then northward/eastward through
midnight. Timing refinements will continue with the current estimate
of 02Z-06Z for greatest coverage in the DTW area.

DTW THRESHOLD PROBABILITIES...

* High for ceiling 5000 ft or less tonight. Low late tonight through
Thursday morning.

* Moderate for thunderstorms this evening.

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 Thursday for MIZ063-069-070-075-076-082-
083.

Lake Huron...Gale Warning until 10 PM EDT this evening for LHZ361>363-421-422-
441>443-462>464.

Lake St Clair...Small Craft Advisory until 4 AM EDT Thursday for LCZ460.

Michigan waters of Lake Erie...Small Craft Advisory until 4 AM EDT Thursday for LEZ444.

&&

$$

UPDATE.......BT
AVIATION.....BT
DISCUSSION...MV/KGK
MARINE.......DRK
HYDROLOGY....MV/TF


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