838
FXUS63 KDTX 230506
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1206 AM EST Mon Dec 23 2024

.KEY MESSAGES...

- Forecast confidence continues to increase for wintry weather on
Monday from midday into the early evening.

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

- Warmer weather expected Christmas Day through the end of the week.

&&

.AVIATION...

Southeast Michigan is caught in a setup tonight that will favor both
midlevel moisture return with southerly gradient trajectories but
mid to upper level subsidence and stability due to confluence aloft.
The net result for the next 12 hours through midday Monday is VFR
conditions with midcloud based at approximately 8.0-10.0 kft agl.
Modest warm air/moisture advection in advance of Pacific shortwave
will then support precipitation chances Monday afternoon and Monday
evening. Latest trend continues in delaying the peak of the
precipitation rates. Northern taf sites will support all snow,
whereas southern taf sites including DTW may see a period of RAPLSN
mix at the front end. Uncertainty continues on when precipitation
will begin and low confidence exists on the duration of any mixed
precipitation..

For DTW...Precipitation onset is set at 19Z today. Will have the
potential for mixed precipitation at the beginning of the event
before precipitation changes to snow. Total snowfall will be
dependent on the duration of snow which remains lower confidence.

DTW THRESHOLD PROBABILITIES...

* Low for ceiling at or below 5000 feet tonight, then high after 19Z
Monday.

* Low for precipitation type as light freezing rain and/or sleet
around 19Z Monday, then high for a changeover to snow after 20Z.

&&

.PREV DISCUSSION...
Issued at 300 PM EST Sun Dec 22 2024

DISCUSSION...

This is the coldest day of this young winter season thus far - with
many locations start from near, if not slightly below, 0F this
morning - and temperatures struggling to climb through the teens.
The early sun has giving way to the mid clouds staged over Western
Michigan. These clouds will thin and move out of the area only to
give way to more mid/high clouds by sunset as the upstream system
approaches.

A well defined shortwave trough over the Northern Plains within the
broader west-northwest flow will spin through the Great Lakes Region
on Monday. The system is currently moisture starved - with limited
precipitation coverage. Expectations are for the low-level moisture
poised over east Texas north into southern Missouri to get pulled
into the warm conveyor belt as the wave crosses the region.

Recent guidance trends on both translation and development sequence
is slower. This is especially noted with the RAP sequence slowing
slightly each cycle. The net impact is a weaker system with a shift
northward of the primary forcing. Furthermore, the NWP suite is now
collapsing on a common solution picture, with the CAM offerings more
muted with the precipitation production compared to prior cycles.
This slower solution timeline places the bulk of the precipitation in
the afternoon and early evening hours - with a little longer runout
time across the Thumb along the near surface track.

The column warms substantially over the next 24 hours with a nose of
above freezing air crawling in around 850-mb from around M-59 and
southward during the midday hours. The integrity of the warm air
will be rather tenuous with any higher precipitation rate quickly
cooling the layer. The vacillation of the thermal profile will be
enough to support a few rapid precip type transitions between light
freezing rain, sleet, and snow - depending on intensity. Only minor
glazing would occur with any light freezing rain, as surface
temperatures will be hovering around freezing. The boundary layer may
warm enough by late afternoon across Metro Detroit and south to
support rain.

There appears to be two to three waves of forcing with moisture
transport. The lack of continuous inflow of moisture will lead to
rather erratic coverage with initial saturation layers in the
isentropic ascent generating a frontogenetic response, only to quell
rather quickly due to lack of sustaining moisture. The net result
will be widely varying snowfall amounts north of I-69. Additionally,
the warming column will push the DGZ deep into the cloud layer -
removed from the peak forcing. Therefore, expecting near to slightly
lower than climatological snow ratios. The net result is a base of 1-
3 inches, with pockets of 4-5 inches where the forcing can maintain
longevity, most likely along and north of M-46.

The western flank of the cold dome will be somewhat resistant to
leave the Lakes Region on Christmas Day - offering another round of
clouds and a small chance of precipitation as another pulse in
moisture transport glides up the isentropic slope. The near surface
layer will be another couple clicks warmer - so rain appears to be
the dominant type for anything that falls.

The column then will continue to warm supporting above average
temperatures for the remainder of the week - probably with more low
clouds and fog again.

MARINE...

Southerly flow strengthens through the night as low pressure
approaches from the upper Midwest. Peak winds occur Monday morning
with gusts between 25-30kts possible across the bulk of Lake Huron
and 20-25kts possible over the rest of the region. Despite the
offshore flow, winds will be strong enough to support 3-5+ft waves
in the Thumb nearshore waters- small craft advisories are in effect
until Monday afternoon. Widespread snow tapers off by early Tuesday
as low pressure vacates to the east setting up northerly winds
(veering from NW to NE) in its wake. Gradient rapidly weakens on the
backside of the system as high pressure is quick to build in
resulting relatively light northerly flow (at or below 15kts). Weak
influence from this high persists through midweek maintaining
lighter flow.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...None.
Lake Huron...Small Craft Advisory from 4 AM early this morning to 4 PM EST this
afternoon for LHZ421-441>443.

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

$$

AVIATION.....CB
DISCUSSION...Mann
MARINE.......KDK


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