Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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949
FXUS63 KDTX 110454
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1154 PM EST Sun Dec 10 2017

.AVIATION...

Some fluctuation in ceiling heights anticipated during the overnight
and morning periods, as modest northwest wind provides some low
level drying but may prove insufficient to completely clear MVFR
restrictions. Attention then turns to advancing low pressure that
will bring a period of snowfall to the region Monday and Monday
night. Initial period of lighter intensity snowfall expected to
develop during the midday period 16z-20z, where conditions largely
remain at MVFR restrictions outside of the heaviest bursts. Snowfall
intensity will increase during the evening period, with peak
intensity and accumulation potential centered between 7 pm and
midnight. There remains some uncertainty yet in the placement of the
heaviest snowband, but generally expect all locations to witness a
period of IFR during this time.

For DTW...A period of chaotic ceiling heights likely to define the
overnight period, with some residual MVFR restrictions giving way to
generally low VFR stratus...with some pockets of clearing below 5000
ft still not out of the question during the period. Initial burst
of light snow expected to lift through during the early afternoon
period, providing some minor accumulation of up to an inch.  A
second period of potential heavier snow will focus during the late
evening period /00z-05z/, where an additional inch or two of
accumulation will be possible under IFR conditions. Winds remain
modest from the south through tomorrow evening.

//DTW THRESHOLD PROBABILITIES...

* Medium confidence for ceilings aob 5000 feet overnight through
  Monday morning, then high Monday afternoon in developing light
  snow.

* High confidence in precip type being snow this evening and
  again Monday.

&&

.PREV DISCUSSION...
Issued at 345 PM EST Sun Dec 10 2017

DISCUSSION...

Several items of interest in the coming week across SE MI. First we
are monitoring a developing lake effect band coming off central Lake
Michigan this afternoon. Next up is a clipper that will track
through the souther Great Lakes Monday afternoon and evening
bringing the next round of accumulating snowfall. Coldest airmass of
the season then moves in through mid week dropping lows into the
single digits and holding highs around 20F. Additional light
snowfall could come at the end of the week with the next clipper
followed by a brief warm up next weekend.

First up is the potential convergent lake band this afternoon. Winds
have just flipped to the NW at Big Rapids showing the convergence
seen at the shoreline is now making its way inland but GRR radar is
less than impressive thus far showing the moisture plume enhancing.
In addition, hires models less impressed with the setup today as
they were previously. Lowered pops, qpf, and snowfall north of I69
this afternoon to mainly flurries or light snow showers with little
accumulation. The trough dropping into northern lower will reach the
Saginaw Valley around 00Z, which will then break up the convergent
feature as it continues southward through the state.

Models continue to come better in line in regards to the clipper and
associated snowfall on Monday. In addition to agreements in hires
models as they now begin to reach out far enough to forecast the
clipper, there are similarities between medium and long range models
as well. Bulk of the forecast remains unchanged as we still expect a
compact clipper to dig southward into the Ohio Valley Monday
afternoon before getting pulled back NE into a stronger PV anomaly
on the nose of a 150+ knot jet right on it`s heals. The speed of the
second wave seems to be difference in how the event plays out thus
far. A faster and stronger wave will phase the two earlier, which
will start ingesting the smaller clipper faster which pulls the
fgen/deformation forcing northeastward earlier. A slower or weaker
solution will allow the clipper to stay more compact and further
south keeping the main deformation band across the southern part of
the state. Adjustments have been made for location of highest
snowfall with qpf/snow amounts not much different. Still looking at
a wide swath of 1-2 inch snowfall across SE MI with a band of
heavier snow around 3 inches likely with the deformation band on the
northeast flank of the low. Currently this band looks to set up
between M59 and I94. A lead leaf of broad isentropic ascent ahead of
the broader trough will bring light snow showers to the area
starting 15-18Z with the heavier band occuring between 21-03Z.
Remnant light showers will taper off around 12Z Tuesday when drier
air moves in.

Northwest flow ramps up behind the departing wave which will usher
in the coldest airmass of the season thus far with 850mb
temperatures falling to -20C by Tuesday afternoon and -25C located
just north of Lake Superior. Tuesday will not be a pleasant day as
highs reach the low to mid 20s but winds gusting to 20-30 mph will
result in wind chill values in the single digits most of the day.

Cold arctic airmass remains over the Great Lakes on Wednesday,
preventing highs in our area from climbing out of the upper teens to
low 20s with wind chills hovering in the single digits. Surface
ridging between systems will keep the area quiet and allow for some
sunshine through the day. Temperatures Thursday begin a slow rebound
as the coldest air aloft moves off to the east. Models diverge on
the placement and strength of a potential shortwave/clipper pivoting
through on Thursday into Friday, but looking at another chance for
light snowfall across the region.

Above freezing temps to return to Southeast Michigan on Saturday;
jet energy will help flatten the ridge over the western CONUS and
allow for the trough over the eastern CONUS to ease up. Warm air
advection will set up ahead of a developing low in the northern
plains, with highs reaching a few degrees above normal on Saturday
and Sunday. The low pressure system is currently progged to move
over the northern Great Lakes on Sunday with the best of the forcing
displaced to our north; no significant precipitation amounts are
expected though some gusty winds will be possible with the
heightened pressure gradient.

MARINE...

Diminishing winds this evening will transition to light southerly
for most of Monday. Colder air will spread across the waters Monday
night with sustained near-gales forecast to gust to strong gales
early Tuesday morning through Tuesday afternoon. Significant wave
heights will reach 12 feet during this time with maximum wave
heights of 20 feet or higher. Gales will diminish late Tuesday.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...Gale Watch from late Monday night through Tuesday evening for
     LHZ361>363-421-441>443-462>464.

     Small Craft Advisory until 4 AM EST Monday for LHZ441.

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

$$

AVIATION.....MR
DISCUSSION...DRK/TF
MARINE.......JVC


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