Expert Forecast Opinion

490
FXUS63 KDTX 020157
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
957 PM EDT Tue Apr 1 2025

.KEY MESSAGES...

- A Winter Weather Advisory is in effect along/north of M-46 late
tonight through Wednesday morning for minor snow and ice
accumulations; periods of rain mixed with melting mixed
precipitation expected further south.

- A conditional threat exists for severe weather Wednesday evening
and early Wednesday night with damaging winds, hail, an isolated
tornado, and localized flooding all possible.

- A Flood Watch is in effect Wednesday evening through Thursday
morning.

- Abnormally warm conditions expected Wednesday afternoon through
Thursday with highs in the 60s.

&&

.UPDATE...

Upstream observations indicate inbound mid level moisture transport
is on schedule to reach central Lower Mi shortly after the 2 AM
Advisory start time. The leading band initially works against a deep
layer of low level dry air shown in the 00Z DTX sounding which is
steadily saturated top down into an all snow thermal profile in model
soundings. This process keeps accumulation in the 1 to 2 inch range
in a few heavier bursts. ENE surface wind maintains Td in the teens
and 20s for a sustained wet bulb cooling influence most pronounced
along and north of M-46 to sustain sub freezing temperatures at least
through early morning. A snow to showery freezing rain transition
then occurs toward daybreak as warm air aloft gains ground in the
profile. Road and soil temperature observations in the 30s this
evening suggest just a minor obstacle for snow accumulation and icing
while elevated road surfaces become most prone to hazardous
conditions for the morning commute. Icing is nudged up toward 0.10 in
this update due to stubborn sub freezing surface air temperature
until closer to the 10 AM Advisory expiration when early April
daytime warming combines with veering surface wind to lift readings
above 32 F.

Farther south, conditions will be monitored along the I-69 to M-59
corridors where surface air temperature wavers closer to 32 F and is
vulnerable to similar wet bulb cooling influence. A shorter period
of snow to wintry mix to all rain is expected in this area for now.
A later onset toward metro Detroit helps readings hold above
freezing there.

&&

.PREV DISCUSSION...
Issued at 715 PM EDT Tue Apr 1 2025

AVIATION...

Ontario high pressure quickly gives up control of conditions across
Lower Mi as dry weather under high clouds this evening transitions
to wintry mixed precipitation from PTK northward late tonight and
Wednesday morning. VFR then drops rapidly into MVFR and briefly into
IFR visibility toward MBS during peak snow intensity near sunrise
through early morning. Ceiling and visibility reduction are slower
south of PTK where precipitation type holds as all rain and before
the Ohio valley warm front moves toward the southern Mi border
toward noon Wednesday. Mixed precipitation changes to all rain while
expanding in coverage through Wednesday afternoon. An elevated
rumble of thunder is also possible with an early afternoon Prob30
representing the best time window at this point in the forecast.
There is just enough surface wind gust component to hold off on LLWS
mention for now. Of greater consequence is IFR/LIFR ceiling and
visibility associated with the warm front as it moves from the Ohio
border into northern Lower Mi by Wednesday evening.

For DTW... VFR above 5000 ft transitions quickly below 5000 ft and
into MVFR early in the morning as rain showers also increase from
the west. Precipitation type holds as all rain through the morning
leading into a period of IFR/LIFR ceiling and visibility as a
moisture laden warm front moves through the region.

DTW THRESHOLD PROBABILITIES...

* Moderate for ceiling 5000 ft or less Wednesday morning, high
Wednesday afternoon.

* Low for thunderstorms Wednesday afternoon.

PREV DISCUSSION...
Issued at 357 PM EDT Tue Apr 1 2025

DISCUSSION...

Aggregate surface ridging, supported by anticyclonic flow aloft,
acts to minimize diurnal boundary-layer cloud development through
the rest of today. Easterly flow prevails during the evening and
early overnight hours as the ridge structure responds to potent
upstream longwave trough with a rapidly evolving shortwave feature.

Mid-level moisture plume moves into Lower Michigan ahead of the
arrival of an elevated warm front migrating northward from the Ohio
Valley. This leads to a notable isentropic response as vapor
condensation occurs within a broad region of system-relative ascent
along the sloped material surface. Forecast soundings reveal some
precipitation type uncertainty, especially between M-59 and I-69
where the duration of cold air will be shorter. Overall QPF with
this lead forcing is generally pretty limited, but light snow
accumulations are anticipated along/north of I-69. Duration of sub-
freezing temperatures will vary greatly between M-59 and the
northern edge of the forecast area, with The Thumb holding below
freezing the longest. This favors some minor snow accumulations with
potential for light icing late tonight through Wednesday morning.
High degree of variability exists with the 12Z NWP suite regarding
temperature profiles, and therefore precipitation
type/accumulations. Opted to issue an initial Winter Weather
Advisory along/north of M-46 for wintry weather leading to nuisance
impacts for the morning commute. Highest confidence in 2+ inches of
snowfall is mainly for Midland/Bay Counties, whereas the icing
threat (later in the Advisory period) favors higher accretion
efficiency over the drier/colder Thumb region.

A changeover to rain should be complete across all of Southeast
Michigan by 16Z as the warm front becomes firmly entrenched. This
reflects a quick jump in 850 mb temperatures from the minus single
digits (Celsius) to positive double digits, leading to a very sharp
inversion between 1.5 kft and 5 kft AGL. Meanwhile, a shortwave
trough closing off through 300 mb tilts strongly negative over the
Missouri River basin featuring an impressive ageostrophic response
and 150 knot jet streak at 250 mb. In spite of the formidable lower
column inversion layer, confluent downwind sub-2 kft winds result in
60 knot LLJ winds passing overhead until late Wednesday evening.
This ties into a highly conditional convective scenario late
Wednesday as tremendous ThetaE advection pumps PWATs into the 1.20
to 1.50 inch range, exceeding April 2 12Z and April 3 00Z DTX
sounding climatology maxima. Instability carries the highest
uncertainty in that surface-based CAPE will struggle to materialize,
but elevated/MUCAPE values of 1200-1500 J/kg are possible, based
near the top of the inversion layer. From a shear/helicity
perspective, hodographs/hodomaps profiles are extremely broad with 0-
1 km SRH values in excess of 1000 m2/s2. Fortunately, these values
occur during the midday hours, before instability has time to build
up.

The main convective threat window, approximately 22Z Wednesday to
04Z Thursday, does lend a considerable reduction in SRH values, but
still affords some low-level rotational threat. With EBWDs of 45-55
knots and 0-6 km bulk shear of 60+ knots, storm speed should be
fast. Convective organization will likely lead to squall line
activity with damaging winds, hail, an isolated tornado, and
localized flooding all possible. Given the later timing and multiple
convective axis, integrity for the norther portion of the line
carries more uncertainty than the southern areas. SPC has included a
portion of Southeast Michigan within an Enhanced Risk area while the
northern half to two thirds carries a Slight Risk designation. Note
that the primary threat is for damaging winds, with a hatched area
for wind.

991 mb surface low tracks from Lake Superior, through Ontario, and
into central Quebec Thursday with a slow recovery in southwesterly
gradient winds. This ensures another mild day with highs in the
upper 50s to mid 60s. Gusty during the day with mixing depths to
around 5 kft AGL which translates to peak gusts in the 35-40 mph
gusts once the boundary-layer interfaces with 40-50 knot flow.
mainly dry conditions expected through the day with a low-end shower
threat Thursday night, dependent on the northward extent of induced
rainfall along a wavering stationary boundary. Split-flow emerges
below 700 mb Friday with a shift to light northeasterly prevailing
winds. This brings highs more in line with seasonal averages to
close out the work before renewed rain chances over the weekend.

MARINE...

Area of high pressure has slide just to our east but still well in
control of the weather through this evening. Winds emerge out of the
southeast tonight and ramp up quickly ahead of the next low pressure
system that will affect the region on Wednesday. Winds across Lake
Huron will likely reach gales tonight through Wednesday so the watch
has been upgraded to a warning. Additionally, farther south across
Lake St Clair and Lake Erie, winds look to hold around 30 knots with
building wave heights leading to a period of Small Craft Advisories
for those lakes. The cold front will sweep through late Wednesday
evening which will flip the winds around to the southwest and though
they will remain elevated, will top out around the mid 20s. There
will also be potential for strong to severe thunderstorms across the
lakes late in the day on Wednesday. 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 lighter westerly
flow Thursday and northerly flow Friday.

HYDROLOGY...

Several rounds of showers and thunderstorms begin Wednesday morning
and continue through late Wednesday night. Daytime rainfall is not
expected to cause widespread flooding as amounts will generally hold
under an inch, with highest amounts across the northern tier
counties where Flash Flood Guidance is generally above 2 inches. A
second wave of thunderstorms, some strong to severe, will lift into
the metro Detroit area Wednesday evening and overnight ahead of a
cold front. Cell motion will be nearly parallel to the cold front in
a high moisture environment characterized by PWAT values near daily
maximum values of 1.5 inches, conducive to training storms with
heavy rainfall. Coverage ahead of the front remains a point of
uncertainty in the guidance, but a high coverage scenario would lead
to rainfall amounts of at least 2 inches with localized higher
totals. 6-hour Flash Flood Guidance in the Detroit Metro area is
generally between 1.5 to 2 inches, supporting increased flash
flooding concerns especially for the urban areas of Detroit and
rises in area rivers. A Flood Watch has been issued as a result.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...Winter Weather Advisory from 2 AM to 10 AM EDT Wednesday for
MIZ047>049-053>055.

Flood Watch from Wednesday evening through Thursday morning for
MIZ063-069-070-075-076-082-083.

Lake Huron...Gale Warning from 4 AM to 10 PM EDT Wednesday 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...KGK
MARINE.......DRK
HYDROLOGY....MV


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