Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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822
FXUS63 KDTX 121515
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1015 AM EST Tue Dec 12 2017

.UPDATE...
As of 1015 AM...Earlier band of embedded snow squalls impacting
portions of Shiawassee, Genesee, Oakland, Livingston, and Wayne
counties continues to slowly wane in intensity as the morning
progresses. Radar returns from earlier in the morning suggest that
accumulations within the band may have ranged from a coating to half
of an inch. This activity will continue to weaken through the
remainder of the morning hours, with another few tenths possible in
spots. For the rest of the day, snow shower activity will transition
to sporadic flurries across most of southeast Michigan, with
scattered lake effect snow showers possible along the immediate
nearshore waters of Lake Huron, especially across Huron County where
a Winter Storm Watch remains in effect beginning at 4pm. Will
continue to monitor lake effect development and trajectories, but the
potential still exists for several inches of accumulation at this
time, thus will keep the watch going for now.

The other main weather story today will be the falling temperatures
and gusty northwest winds. Strong cold air advection and tightening
pressure gradient from departing strengthening clipper across the
Northeast will lend to gusty winds of 30-40 mph across most areas
throughout the day. The exception will be Huron County, where gusts
closer to 45 mph especially near Lake Huron will be possible with the
more effectively mixed marine boundary layer. Temperatures will fall
throughout the day, into the lower and mid teens by this evening.
Coupled with the winds, wind chill values will generally range from 5
above to 5 below zero.

&&

.PREV DISCUSSION...
Issued at 611 AM EST Tue Dec 12 2017

AVIATION...

Remaining lake effect snow showers in cold northwest flow will
slowly diminish early this morning, leaving sct-bkn MVFR to lower
VFR stratus/strato-cu. Wind gusts will reach 30 knots at times
throughout the day. Loss of daytime heating/mixing will lead to few-
sct clouds and lighter WNW this evening, but clouds thicken again
late tonight as next clipper low encroaches rapidly from the west.

For DTW...Aside from stray shsns early this morning, generally
expect sct-bkn stratus/strato-cu in the 3500-4500 foot range. NW
wind gusts to 30 knots can be expected today. Clearing/lighter W
flow tonight, but clouds thicken late tonight into Wednesday with
snow quite possibly beginning before 18z on Wednesday as the next
clipper low bears down on the central Great Lakes.

//DTW THRESHOLD PROBABILITIES...

* High confidence for bkn ceilings at or below 5000 feet at times
  today.

* Moderate confidence in northwest crosswind thresholds being
  exceeded today.

PREV DISCUSSION...
Issued at 252 AM EST Tue Dec 12 2017

DISCUSSION...

Arctic air surging into to the Great Lakes Region this morning, as
northern stream upper level trough axis arrives over the Central
Great Lakes, with 500 MB low (510 DAM) tracking across northern Lake
Huron today. 00Z Raob out of Pickle Lake Ontario revealed an 850 mb
temp of -24 C. Some modification of the airmass will take place as it
crosses the northern Great Lakes, but 850 mb temps settling down in
the -18 to -20 C range over southeast Michigan seems reasonable,
which should hold temperatures in the upper teens to around 20
degrees during the day. The Phasing/rapid deepening of the low
tracking into the Eastern Great Lakes has been a bit slower to
occur, and think peak wind gusts topping out in the 35 to 40 mph
should hold, with the exception of the northern Thumb Region as
winds/deeper mixing from Saginaw Bay comes into play, supportive of
winds 45+ MPH. With water temperatures over much of Lake Huron still
in the mid 40s, still have concerns for heavy snow showers impacting
the northern tip of the Thumb region (Grindstone City-Harbor Beach-
Richmondville). Once low pressure releases from Georgian Bay, there
is a meso-high/pressure rise occuring late in the day, which should
help push the lake band(s) over Lake Huron toward the Thumb Region,
with snow showers likely scrapping the eastern shoreline of Huron
County this evening. Right now, the best support is from the 00z
HIRes-ARW, which is indicating almost two tenths of an inch of QPF
00-06Z Wednesday. And unfortunately, it indicates this band will
stay uncomfortable close to the shoreline right into Wednesday
morning. Expectations at this time is for localized 2 to 4 inches,
but there is enough concern to issue winter storm watch for
potentially higher amounts. Obviously with Lake Effect, the majority
of the county will see little to no accumulation, with highly
localized higher amounts right near the shoreline. Outside of the
eastern Thumb region, looking for skies to become mostly
clear/partly cloudy with the northwest flow. Any streamers from the
Lake Superior-Michigan connection likely very narrow and struggling
to make it into southeast Michigan. Thus, favoring lows in the
single numbers to around ten degrees tonight, and with modest
surface wind (5 knots) by Wednesday morning, wind chills of zero to
-10 F are expected.

A bit of a surprise for Wednesday, although the lead upper wave
tracks into the western Ohio Valley as expected, an even stronger PV
anomaly races down from northern reaches of Northwest Territories,
and it is this feature and moisture flux/lower static stability over
Lake Michigan which will help draw the clipper and FGEN band farther
north, now progged to be stretched across southern Lower Michigan
(along/south of I-69). With what looks to be ideal thermal profiles
to work with, 850-700 mb average temp in the negative low/mid teens,
coupled with 2-2.5 G/KG of specific humidity, it appears 2 to 5
inches will fall across the south half of the CWA Wednesday
afternoon-evening. If the system is a little slower to exit
Wednesday night, could even see a little bit more. This solution has
unanimous support from NAM/GFS/Canadian and European, but exact
placement of highest snow/fgen band varies a little bit.

Additional upper level energy embedded within the northwest flow to
bring another chance of snow Friday-Saturday, with some semblance of
another clipper tracking through, but the upper level flow trending
more confluent leading to a weakening low pressure. The baroclinic
zone/low track is also uncertain, with big differences in the
solutions by Saturday between Euro/GFS. Very low confidence in the
weekend forecast.

MARINE...

Strong northwest gales will persist today with gusts approaching
storm force over the exposed nearshore waters of the Thumb and the
open waters of central and southern Lake Huron. Gale warnings
will remain in effect into tonight before winds subside by early
Wednesday morning. Heavy Freezing Spray Warning will remain in place
as well as temperatures fall into the teens and significant wave
heights range between 10 to 15 feet. Maximum wave heights will
exceed 20 feet.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...Wind Advisory until 4 PM EST this afternoon for MIZ049.

     Winter Storm Watch from 4 PM EST this afternoon through Wednesday
     morning for MIZ049.

Lake Huron...Heavy Freezing Spray Warning until 4 AM EST Wednesday for
     LHZ361>363-421-441>443-462>464.

     Gale Warning until 5 AM EST Wednesday for LHZ362-363-462>464.

     Gale Warning until 4 PM EST this afternoon for LHZ422.

     Gale Warning until 4 AM EST Wednesday for LHZ421-441>443.

     Gale Warning until 11 PM EST this evening for LHZ361.

Lake St Clair...Gale Warning until midnight EST tonight for LCZ460.

Michigan waters of Lake Erie...Gale Warning until 4 PM EST this afternoon for LEZ444.

&&

$$

UPDATE.......IRL
AVIATION.....DG
DISCUSSION...SF
MARINE.......DG


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