072
FXUS63 KDTX 232350
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
650 PM EST Mon Dec 23 2024

.KEY MESSAGES...

- Snow persists north of M-46 with a changeover from wintry mix to
snow expected between I-94 and M-46 through the evening. A light
glaze of ice and snow accumulations will impact the evening commute.

- 2 to 4 inches of snow expected north of M-46, and 1 to 2 inches
  between I-94 and M-46. South of I-94 will see mainly rain.

- Warmer weather expected Christmas Day on through the rest of
  the week with increasing rain chances by the weekend.

&&

.AVIATION...

Loss of daytime heating and subtle wind shift to the southwest over
southern sections of Southeast Michigan has been allowing for
precipitation type to slowly change over to snow for many areas. The
holdout to this point has been the city of Detroit and Downriver in
the lower elevation areas. A 850-700mb dry slot now pushing through
the area is expected to decrease precipitation rates moving forward.
Still anticipating another 0-3 hours of light precipitation mainly
in the form of snow as the upper level low pressure center needs to
track across Lower Michigan. MVFR ceilings are anticipated
throughout tonight and into the day on Tuesday. A southwest wind
tonight will become north northeasterly after daybreak Tuesday.

For DTW...The potential exists for light snow activity through
approximately 03Z this evening.

DTW THRESHOLD PROBABILITIES...

* High for ceiling at or below 5000 feet.

* High for snow this evening.

&&

.PREV DISCUSSION...
Issued at 355 PM EST Mon Dec 23 2024

DISCUSSION...

Several hours of wintry mix are already under our belt this
afternoon, though rates have been light with minor ice accumulations
of a Trace to 0.03" observed. Given the light rates, initial
dewpoints in the low 20s have been hard to overcome which has easily
allowed a light glaze of ice to form on paved surfaces. As is
typical with mixed precip events, models struggled to nail this one
down with freezing rain/sleet still ongoing across most areas
between I-94 and M-46. The column is reliant on some sort of cooling
mechanism to switch over to snow, but with rates still struggling in
the dry air, thinking the transition will struggle until the
upstream vort max and surface low reach western lower Michigan.

Model soundings show the wintry mix to snow transition occurring
between 22z-00z for the I-94 to M-46 corridor. The late changeover
will eat into snow totals, so now expecting about 1 to 2 inches of
snow accumulation with additional ice accumulations of a couple
hundredths of an inch. North of M-46, the p-type is already snow and
will continue to be through the event. Not much of a change to
accumulations in these locations, with storm totals of 2 to 4,
locally 5, inches expected. Finally, the decision was made to not
expand the Winter Weather Advisory to Wayne/Monroe/Lenawee Counties.
Temperatures in these locations are still above freezing with
limited evidence of impacts so far, but will continue to monitor
(especially Wayne County) through the evening. By midnight, drier
air will work in aloft to shut off the more intense precipitation.

The surface low and vort max will track across lower Michigan late
tonight, with the trailing dynamics focusing strong isentropic
downglide overhead and a subsidence inversion down to the 900-850 mb
level by Tuesday morning. Precipitation rates will decrease,
although boundary layer saturation persists in typical fashion to
support light snow showers transitioning to flurries or drizzle as
the day wears on. The Thumb may see an additional dusting of snow
accumulation Tuesday morning as northerly flow activates Lake Huron,
although the inbound cold advection is not very impressive (850mb
temperatures around -10 C). Two limitations here as a result: (1)
lake-air delta T`s will just barely meet rule of thumb 13 C
thresholds for lake effect snow, and (2) ice nucleation will be
questionable. Some slick spots will again be possible Tuesday
morning as temperatures start below freezing, but daytime highs
rebound to 32-36 F by afternoon. In different fashion than today,
evaporative cooling potential will be minimal as dewpoints will also
be in the low 30s keeping wet bulb temperatures aoa 32 F. This
should minimize winter precip impacts.

Rest of the holiday week will be characterized by a high amplitude,
short wavelength height field across CONUS. The first of several
shortwave couplets tracks into the Ohio Valley Christmas Day, with
an organized conveyor of isentropic ascent setting up to our south.
As the moisture lifts across the state line however, the forcing
reorients and the moisture field stretches which ultimately leads to
a low PoP/low QPF scenario toward the state line with dry conditions
north. If precip materializes, wet bulb temperatures above freezing
favor rain as the main p-type Christmas Day, with daytime highs in
the mid 30s for all of SE Michigan. A warmup is then anticipated
through the rest of the week as surface high pressure stalls over
the mid-Atlantic, and southerly flow organizes a push of warm
advection into SE Michigan. By the end of the week, high
probabilities (80+ percent) of 50+ degree high temperatures push up
against the MI-OH border. Deterministic guidance shows numerous
waves ejecting into the Great Lakes through the weekend, although
the ensemble signal is still quite muted implying low confidence.
Nonetheless, warm temperatures ensure that precip is primarily rain
during the day 4-7 period.

MARINE...

Chain of surface lows migrates eastward across the broader Great
Lakes region tonight sustaining wintry precipitation into the early
morning hours on Tuesday. Relatively mild south-southwesterly flow
will veer northwesterly once the trailing composite cold front works
through. Resultant cold advection should be rather weak, therefore
not expecting standard gust enhancement behind the boundary. This
keeps winds AOB 15 knots and waves AOB 4 feet. Afternoon wind trends
are cooperating, thus today`s Small Craft Advisory will be allowed
to expire as scheduled. Lingering pockets of light rain and/or snow
exit by Tuesday afternoon with a pair of high pressure centers
building southeast from the Upper Midwest and south-central Ontario.
This veers flow northeasterly with lighter speeds as the surface
pressure gradient further relaxes. The ridge takes better shape over
The Northeast through midweek which maintains lighter east-
southeasterly flow until Friday with mainly dry and seasonable
conditions.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...Winter Weather Advisory until 1 AM EST Tuesday for MIZ047>049-
     053>055-060>063-068>070.

     Winter Weather Advisory until 10 PM EST this evening for MIZ075.

Lake Huron...None.
Lake St Clair...None.
Michigan waters of Lake Erie...None.
&&

$$

AVIATION.....CB
DISCUSSION...MV
MARINE.......KGK


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