385
FXUS63 KDTX 091055
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
655 AM EDT Wed Apr 9 2025

.KEY MESSAGES...

- A mix of rain and snow is expected across much of Southeast
Michigan this evening and through Thursday; minor snow accumulations
of less than an inch.

- Warmer temperatures ensue this weekend with mainly dry conditions
holding Friday through Sunday.

- Well-above average temperatures are likely early next week.

&&

.AVIATION...

A developing low pressure system and associated elevated frontal
boundary will move into Se Mi from the west this evening and
overnight. The system will drive a region of precipitation across
the area. This and low level moisture advection will support
steadily lowering ceiling heights during the evening, eventually
leading to widespread IFR ceilings by late evening/overnight. Ample
low level dry air will continue circulating around departing surface
high pressure through the afternoon. This will keep the moisture
advection elevated through late afternoon, thereby maintaining just
high based clouds above 5k feet. While the onset of the
precipitation this evening is expected to be rain given the
relatively warm sfc temps. The residual dry low level air will
support ample wet bulb cooling tonight. This will cool the column
enough to support a rain/snow mix across the terminals, likely well
into the night.

For DTW...The lead edge of the light rain is forecast to arrive
between 22Z and 00Z this evening. Low level cooling will support
some wet/melting snow mixed in at times with the rain. At this
point, minimal if any snow accums are expected. As the sfc low moves
into nrn Indiana and Ohio overnight, warmer low level air will nudge
northward into metro Detroit. This suggests rain will be the more
dominate precipitation type later tonight into Thursday morning.

.DTW THRESHOLD PROBABILITIES...

* High in ceilings aob 5000 feet tonight and Thursday morning.

* Moderate for snow as precip type late this evening. Low overnight.

&&

.PREV DISCUSSION...
Issued at 356 AM EDT Wed Apr 9 2025

DISCUSSION...

Scattered lake effect snow showers that lingered over Southeast
Michigan through the early overnight hours have finally dissipated
early this morning as a result of synoptic-scale adjustments. A
longwave trough axis has been slow to transit the Great Lakes and
Northeast since the start of the week due to a large upper-level low
parked over Quebec. Geopotential height rises are underway this
morning as an inbound ridge axis shears into an embedded shortwave
feature diving into the basal portion of the parent wave,
effectively dislodging the upper low with time today. However, clear
skies to start the day will be short-lived as a Pacific wave
amplifies aggressively. Highs below average, in the mid 40s. The
cyclonic portion of the feature spills across The Plains, eventually
digging into the Ohio Valley overnight. As a result, the surface low
positioned further downstream drifts closer to Southeast Michigan
with time, organizing winds from southwest before backing eastward
into Thursday.

Given that low-level flow will generally be oriented from the south,
gradual warming is expected through the column. Warming rate will
not exceed the rate of saturation in spite of flow generally holding
from WSW (above 7 kft AGL). System-relative isentropic ascent-driven
precipitation shield out ahead of the system activates this
afternoon and expands eastward reaching the forecast area by this
evening. Subset of forecast soundings still indicate some
uncertainty regarding precipitation type as most of the column holds
AOB the 0C mark, but the lowest 2 kft to 3 kft AGL layer stays above
freezing for most locations. Latest forecast reflects the belief
that give surface dewpoint depressions of between 4C and 5C, the
wetbulb cooling will preserve snowflakes heading into final
hydrometeor descent, moreso than melting to the point of liquid
rain. In spite of this expectation for more snow than rain
(initially), surface temperatures are anticipated to broadly remain
above freezing which prevents any accumulating snowfall before 00Z.

Precipitation likely continues through much of the overnight period
as the low progresses across the Ohio Valley, but the system`s
surface warm front holds just south of the Michigan/Ohio border.
Meanwhile, the 925 mb and 850 mb temperature gradients also hold
firm up against the state-line into Thursday resulting in a quasi-
isothermal layer within the lowest 7 kft AGL. Small shifts across
the 0C isotherm will impact precipitation type late tonight into
Thursday morning before moisture gets stripped out of the column, in
top-down fashion. This lends a rain/snow mix with varying weighting
of either precipitation type for most locations. Should a cooler
column emerge, perhaps from stronger wetbulb cooling or an earlier
flip to northeast flow within the near-surface layer, more
accumulating snow could develop, although it would require moderate
to heavy snowfall rates to overcome ground-induced melting
processes, even on some grassy/elevated surfaces. Noticeable
differences found in the handling of this event when comparing each
ensemble forecast system (e.g. at DTW, PTK, and FNT) with very few
accumulating snow members for the GEFS, (large) majority membership
for the EPS, and a mixed-bag for the GEPS. Outgoing forecast cedes
snow accumulations to areas north of Metro Detroit, generally
delineated by the Irish Hills and glacial ridge. Overnight lows only
drop into the low or mid 30s.

The low closes off through 300 mb by 12Z Thursday and passes over
southern Lower Michigan midday. This maintains additional
precipitation, trending more showery in nature. Precipitation type
questions persist Thursday before drier conditions return Thursday
night. Thermal moderation is in store over the weekend with static
stability ensuring a precipitation-free forecast. Medium confidence
in temperatures exceeding 60F this weekend before a more pronounced
warm-up gets underway early next week. This will be marked by 850 mb
temperatures in the upper single digits to around 10C as energetic
southwest flow taps into a Gulf moisture feed. Severe weather
potential may eventually require closer monitoring early next week
depending on the phasing and trajectory of the next Pacific wave.

MARINE...

High pressure in place early this morning will slide to the east
today, allowing light southerly winds to develop as a surface low
tracks through the Ohio Valley Wednesday night-Thursday bringing
snow/wintry mix and rain to the Central Great Lakes. The system will
track into the eastern Great Lakes late in the day, allowing winds
to become northeast, persisting right into Friday.

Although northeast winds to not look to top out much past 20 knots,
the prolong nature and unstable low level profiles could be enough
for waves building above 4 feet across the southern Lake Huron basin
and impacting the nearshore waters.

Surface high pressure expected to slide through the Great Lake
region over the weekend, promoting generally light winds and low
waves.

HYDROLOGY...

A low pressure system tracking along the Ohio Valley will bring
widespread rain and melting snow across southern Lower Michigan late
today into Thursday. Storm total precipitation amounts between a
quarter and half inch are most likely for areas south of I-69 with
less to the north. These amounts may cause minor rises on area
streams and rivers but the longer duration of precipitation is not
expected to bring flooding concerns. Note that several model
solutions do indicate higher rainfall and snow water equivalent
precipitation totals, in excess of three quarters of an inch, for
areas along/south of I-94. However, confidence is low in these
higher-end solutions, namely the FV3, NAM, and NAM 3km.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...None.
Lake Huron...None.
Lake St Clair...None.
Michigan waters of Lake Erie...None.
&&

$$

AVIATION.....SC
DISCUSSION...KGK
MARINE.......SF
HYDROLOGY....TF/KGK


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