Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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000
FXUS63 KDTX 091037
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
537 AM EST Sat Dec 9 2017


.AVIATION...

Occasional light snow today will make for variable aviation
conditions, but predominantly in MVFR category with brief IFR. Total
snowfall accumulations of half an inch to two inches, with highest
totals across northern TAFS (MBS/FNT), and lowest totals across
southern TAFS (YIP/DTW/DET). Snow should be over by early evening,
but there is uncertainty with the amount of clearing that will take
place, as cold northwest flow shifts due westerly off Lake
Michigan late tonight. Likely scenario is skies will partially clear
out, with transient high mvfr/low VFR clouds returning by Sunday
morning. A few flurries are also not out of the question.

For DTW...Occasional light snow today, dipping visibilities in the 1
1/2 - 4 SM mile range at times. Total accumulation of half an inch
to one inch expected. Light southwest winds this morning shifting to
the northwest this evening. The northwest winds will likely be in
the 10-15 knot range before decreasing slightly and shifting to the
west toward Sunday morning.

//DTW THRESHOLD PROBABILITIES...

* High for ceilings at or below 5000 feet today, low tonight.

* High confidence in precip type being snow.

&&

.PREV DISCUSSION...
Issued at 251 AM EST Sat Dec 9 2017

Discussion...

Area of ascent in advance of digging shortwave continues to steadily
expand eastward into the forecast area early this morning. Initial
plume(s) of light snow within southwest flow in advance of trough
axis will work into the area during the rest of the morning. Rather
dry low levels below H85 within this west-southwest flow (using the
KGRB and KILN soundings as proxies) are certainly cutting into snow
rates to some extent on the leading edge of this area of light snow
as better visibility restrictions have been slow to expand east from
the shores of Lake Michigan where low level moisture is maximized.

Still, low levels over the area should continue to slowly saturate
and allow for light accumulating snow before sunrise. Trajectory of
the moisture plume(s) within this southwest flow will largely bypass
areas of southeast MI along and south/east of I-94 through 10z-12z.
As was noted by previous shift, moisture quality with this system,
aside from the mesoscale influences of the Great Lakes, is very
poor. So, as vorticity max at the base of this shortwave trough digs
southeast into the Ohio Valley by early afternoon, and better
synoptic scale lift is lost, expect the area of light snow across
the region to become very splotchy in character with time (thus
rather inefficient in producing accumulating snow).

Overall, this suggests that snow within areas that receive steady
light snow within southwest flow may total 1 to 2 inches of snow by
late this afternoon. Meanwhile, far southern portions of the area
will probably fall short of an inch as start of accumulating snow is
delayed several hours. Hires models still depict an interesting
period over the Thumb by early evening as northerly flow around
secondary low pressure that develops as upper trough axis reaches
this area of relative "warmth" over the lake waters. Essentially,
lake effect banding over the lake today is forecast to buckle and
pivot into area within some form of mesoscale low feature. Will
maintain a peak of near 3 inches, but must admit the confidence in
eventually achieving that much is low. With all that said, an
advisory is not needed today.

Otherwise, the deep eastern NOAM upper longwave trough is expected
to remain in place for the remainder of the forecast period, perhaps
flattening slightly heading into next weekend. This will maintain
this cold period of weather, which will intensify somewhat as cold
airmass deepens over area in wake of the next clipper system. At
this time expect, temperatures to bottom out in the Tue/Wed time
frame with highs in the lower/mid 20s and lows in the single digits
to teens.

This aforementioned clipper low will bring the next chance of
widespread light snow from Monday into Monday night. This will cross
the Gulf/Alaska (as opposed to the current system which dove sharply
SSE from the northern Alaska) so moisture quality, while likely on
the low side, should be a bit better. Current indications suggest
several inches of snow will be possible during this time frame.

MARINE...

Low pressure tracking through lower Michigan today will lead to
increasing northerly winds over Lake Huron, with gusts of 25 to 30
knots late today into this evening. These stronger northerly winds
will lead to large waves over the southern Lake Huron basin,
impacting the nearshore waters as well where small craft advisories
have been issued. Cold airmass will promote snow showers over the
lake, along with winds remaining elevated into Sunday out of the
northwest-west. Another low tracking through the area on Monday will
allow for light winds, but northwest winds on the backside of the
low on Tuesday will be strong, potentially reaching Gale force over
Lake Huron.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...Small Craft Advisory from 3 PM this afternoon to 7 AM EST Sunday
     for LHZ421-441>443.

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

$$

AVIATION.....DG
DISCUSSION...DG
MARINE.......SF


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at www.weather.gov/detroit.



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