641
FXUS63 KDTX 231740
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1240 PM EST Mon Dec 23 2024

.KEY MESSAGES...

- Precipitation expands in coverage and intensity for the evening
  commute.

- Highest snowfall amounts will be north of M-46, with lighter
  amounts and the increased chance of a period of sleet and/or
  light freezing rain south into parts of the Detroit Metro
  region.

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

&&

.AVIATION...

Winter precipitation is developing across the Southeast Michigan
airspace early this afternoon. Shower activity in the first wave of
forcing has been predominantly sleet and light freezing rain - with
some snow across the Saginaw region. This first wave will exit the
region in the next hour or two - to be replaced by the expanding
precipitation shield over central Lower Michigan. The column will
cool some as the precipitation erodes it via evaporation - which
will work to convert the type over to snow from north to south
through the late afternoon and early evening hours. The greatest
rates for snow will at Saginaw. Otherwise, MVFR to occasional IFR
conditions can be expected. The MVFR clouds will linger into Tuesday
after the system exits the region late tonight. The current south
winds will veer around to the northwest and become rather light
overnight.

For DTW...Shower activity producing sleet and light freezing rain
will glide through the airspace during the first few hours of the
forecast. As colder air aloft settles in from the north the
precipitation will transition to light snow - roughly around sunset.
Precipitation will quickly end after 03z.

DTW THRESHOLD PROBABILITIES...

* High for ceiling at or below 5000 feet.

* Medium for precipitation type as light freezing rain and/or sleet
  before 21Z, then high for predominantly snow after 22Z. Low for a
  brief transition to rain after 01Z.

&&

.PREV DISCUSSION...
Issued at 1106 AM EST Mon Dec 23 2024

UPDATE...

A Winter Weather Advisory goes into effect this afternoon for most
of SE Michigan with the exception of Wayne, Lenawee, and Monroe
Counties. 12z KDTX sounding shows a roughly 2.0 km (6.0 kft) layer
of dry air near the surface, with maximum dewpoint depressions of 23
C in the column and surface dewpoint depressions of 9 to 13 degrees
F locally. This dry layer is gradually shrinking as a band of light
precipitation is moving into the cwa, driven by ongoing warm
advection and 30-35 knot LLJ forcing. KMBS is reporting visibility
down to 2 SM and cloud bases down to about 3.0 kft agl within the
wintry mix, with both ASOS and MPING reports wavering between ice
pellets, freezing rain, and light snow. Still do not think the
intensity and duration over any one spot will be enough to overcome
the dry layer, so really only looking at a few hundredths of an inch
of liquid at best.

Once this lead wave comes through in the next 1-2 hours, will have
to rely on additional sub-cloud layer moisture advection to generate
precipitation later this afternoon-evening. 12z guidance is still on
track to bring this moisture in between 18-22z (1pm-5pm) as the
surface low migrates eastward, although the magnitude of this
moisture axis has been underperforming so did tweak snowfall grids
down at most by a half inch (mainly for the Saginaw Valley/Thumb on
the front end of this event). Discussion of p-types across the cwa
(snow to the north, wintry mix south) is still valid, with impacts
still anticipated for the evening commute.

PREV DISCUSSION...
Issued at 312 AM EST Mon Dec 23 2024

DISCUSSION...

Well developed shortwave now working from the Northern Plains into
the Upper Midwest has slowed notably over the past 24 hours with
precipitation associated with this wave now focused from mid/late
afternoon into the evening hours. Model trends of shifting the best
forcing with this system to the north and muting it somewhat have
remained in place for the 00z model cycle. Only minor adjustments
seem necessary in the precip/snow accum/ice accum fields.

Generally expected snow to overspread the northern forecast area
early this afternoon with the progression of lighter precipitation
then working south and east into the mid/late afternoon. Warm wedge
of air overriding the western edge of modified arctic air mass will
lead to a wintry mix at times, especially south of I-69.

Will issue a Winter Weather Advisory for portions of the forecast
area. Areas from the Saginaw Valley into Thumb will receive mainly
snow with 2 to 5 inches expected before precipitation ends with the
passage of the system by around midnight. The I-69 to M-59 corridor
region should receive 1 to 3 inches of snow with more sleet and
possibly patchy light freezing rain mixing in at times. Further
south, higher elevations of Washtenaw County may remain cold enough
for minor ice accumulations mid to late afternoon with light snow
accumulations under an inch into early evening before precipitation
shifts east by mid/late evening. Otherwise, most locations south of
I-96/I-696 will receive only minor snow and/or ice accumulations.

The western edge of this cold high pressure system centered well to
the east holds fast into the middle of the week with highs only edge
up in the mid to upper 30s after reaching the lower/mid 30s later
today. Another weak impulse within west to northwest flow will bring
sporadic rain showers Christmas Day. Otherwise, dry conditions are
expected once today`s system passes east of the area later tonight.

A more pronounced warming trend will ensue later this week into the
weekend as temperatures climb into the 40s (to around 50 in some
locations by the weekend). Plenty of clouds, however, remain in play
through the week with increasing rain chances heading into the
weekend as a series of southern stream shortwaves parade into the
area and eventually break down the western portions of persistent
high pressure anchored from the northeast CONUS back into the Great
Lakes.

MARINE...

Low pressure arrives over the Great Lakes this morning bringing
widespread snow showers that persist into late tonight. In advance,
moderate to strong south winds (25-30kts) are expected across the
bulk of Lake Huron with 20-25kt flow likely over the remainder of
the region. Small Craft Advisories remain in effect around the Thumb
due to 3-5+ ft waves developing in this offshore flow. Low pressure
exits the region by early Tuesday tapering off any lingering snow as
well as shifting winds to the NW. Flow then veers to the NE by late
Tuesday morning as high pressure quickly becomes established. These
northerly winds hold on the lighter side, at or below 15kts, due to
the aforementioned high quickly weakening the local gradient.
Influence of high pressure maintains lighter winds through the
midweek period.

&&

.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...Small Craft Advisory until 4 PM EST this afternoon for LHZ421-
     441>443.

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

$$

AVIATION.....Mann
UPDATE.......MV
DISCUSSION...DG
MARINE.......KDK


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