205
FXUS63 KDTX 310935
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
435 AM EST Fri Jan 31 2025

.KEY MESSAGES...

- A band of rain showers will lift up to the I-69 corridor early this
  morning, followed by a break in rain showers until mid-day.

- A second wave of rain showers is anticipated to begin early this
  afternoon. These rain showers will transition from rain to snow
  late this afternoon and evening, resulting in up to an inch of snow
  accumulation.

- There is a chance for light snow, sleet and freezing rain Saturday
  night and Sunday.

&&

.DISCUSSION...

Several items to note in the observational data this morning, the
first being the band of rain showers that is impacting the southern
half of the forecast area at issuance. This band has formed in the
vicinity of a developing deformation axis that will continue to
generate precipitation throughout the day. Liquid amounts so far
have been generally between 0.1-0.2", with the band expected to
continue steady progress toward the I-69 corridor over the next
couple of hours. A lull in precipitation activity is anticipated as
upstream radar echoes are quite lean, until the surface low lifts
north around mid-day which will renew higher precipitation chances
for the afternoon-evening timeframe. Anticipated liquid from this
system still falls between 0.5 to localize 1" storm totals.

The other item to note is the very sharp thermal gradient between
Midland and West Branch, with current temperatures of 34 degrees and
16 degrees, respectively. The arctic airmass to our north, which is
not only much colder but also drier, will begin to sag south once
the surface low exits into Ontario. This initiates a changeover from
rain to snow, as early as 2pm near Flint and after 4pm for the
Detroit metropolitan area. The period in which sub-freezing
temperatures coexist with sufficient moisture will be limited to a
few hours, with accumulations struggling initially as temperatures
will be in the mid to upper 30s for most of the day. That said, a
rapid drop in temperatures tonight into the 20s will freeze any
residual moisture that remains on roadways. Did reduce snow
accumulations as the arrival of sub-freezing temperatures is
slightly slower than previous model runs, and the frontal passage
will quickly strip the deep layer moisture from the column. There is
still a low chance for over an inch of snow accumulations between
roughly I-96 and I-69 where the strongest forcing occurs along the
deformation axis. This will likely occur in the Irish Hills where
the elevation factor may be just enough to change p-type to snow
earlier.

High pressure builds into the region on Saturday affording a quieter
period. Temperatures will be much cooler as Canadian air filters in,
with daytime highs close to 10 degrees colder than today. Most areas
will hold in the upper 20s to low 30s. Quiet weather will be brief
as the next clipper system tracks into the Great Lakes Sunday, with
precipitation beginning early Sunday morning. Northern stream
origins of this system mean it will lack deep layer moisture, with a
sturdy dry layer falling right within the dendritic growth zone. So
not only will this cut down on precipitation totals, but also raises
concerns about ice nucleation. There does seem to be an elevated
moist layer that may seed the low levels with ice crystals, although
that is contingent on these ice crystals surviving through the
aforementioned dry layer. Otherwise, the lack of ice nucleation
would result in more of a freezing drizzle scenario Sunday morning
before winds shift to the southwest and advect 32+ degree
temperatures into the area. Should there be sufficient ice aloft and
p-type is mostly/all snow, locations north of I-94 could see a half
inch to over an inch of new snow accumulation on Sunday. Will take a
look at thermodynamics in the hi-res models as they become available
in the next few forecast cycles.

A closed upper low parks off the coast of the Pacific Northwest by
early next week, holding the eastern half of CONUS within a longwave
zonal pattern. This setup will be favorable for glancing shortwaves
to ripple across the Great Lakes region in the Monday-Wednesday
timeframe, but with low predictability this far out. Based on both
ensemble mean 850mb temperatures and the full envelope of ensemble
members in the LREF suite, southeast Michigan will be on the cusp of
the 0 C isotherm which introduces uncertainty for temperatures and
resultant precipitation type forecasts for early next week. At this
point can only really speak to the forecast trends, which tend to
draw a colder airmass into the region briefly on Tuesday.

&&

.MARINE...

Low pressure moving into the western Ohio Valley early this morning
will allow for winds to veer around to the north in response, but 15
knots or less to start the day. North-northeast winds will then only
increase through the day on Friday as the low continues to track
east through the Ohio Valley. Bulk of precipitation will fall as
rain, but a changeover to snow likely occurs before precipitation
tapers off in the evening. With prolonged northeast winds reaching
around 25 knots by Friday evening, large waves will build up over
the southern Lake Huron basin, and a small craft advisory continues
for the nearshore waters of Lake Huron, outside of Inner Saginaw Bay
which remains frozen.

Another surface ridge arriving for Saturday will allow winds to
become light before moderate southerly flow develops for the second
half of the Weekend. Enough of a southeast component for large waves
to impact nearshore marine zones.

&&

.HYDROLOGY...

Light rain has begun over southeast Michigan this morning and works
steadily north through the early morning hours. This band will be
capable of producing a tenth to two tenths of an inch of liquid
before daybreak. A second wave of precipitation moves into the area
early this afternoon, with an additional half inch to three quarters
of an inch of liquid possible. The highest amounts are still
centered between roughly I-94 and M-59. With rain falling on top of
existing snow pack and frozen ground, excessive runoff and ponding
of water will be possible.

&&

.PREV DISCUSSION...
Issued at 1152 PM EST Thu Jan 30 2025

AVIATION...

Wave of low pressure and associated axis of midlevel deformation
will lift northward into Southeast Michigan tonight and impact the
area through the afternoon hours on Friday. Have observed a fairly
abrupt arrival of deeper low to midlevel moisture that may
eventually result in IFR to potentially LIFR ceilings after 07Z
tonight. Rainfall rates are now expected to more prevailing through
daybreak then increase in the 10-19Z timeframe at the Detroit
terminals. A fairly sharp northern edge to the moisture and
precipitation is expected. Did include prevailing -RA at KFNT after
12Z but confidence is low in the timing of onset and in the
duration. As the column steadily cools, there could be a brief
period that precipitation changes over to snow before ending.

DTW THRESHOLD PROBABILITIES...

* High for ceiling 5000 ft or less tonight and Friday.

* High for ptype of rain tonight and Friday morning, Medium to low
  for switching over to snow late Friday afternoon.


&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...None.
Lake Huron...Small Craft Advisory from 11 AM this morning to 10 AM EST Saturday
     for LHZ421-441>443.

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

$$

DISCUSSION...MV
MARINE.......SF
HYDROLOGY....MV
AVIATION.....CB


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