Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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000
FXUS63 KDTX 120752
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
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.

Additonal upper level energy embedded within the northwest flow to
bring another chance of snow Friday-Saturday, with some semblence 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.

&&

.PREV DISCUSSION...
Issued at 1147 PM EST Mon Dec 11 2017

AVIATION...

Arctic frontal boundary expected to surge across the southeast
Michigan airspace during the early morning period. This frontal
passage will bring the potential for snow shower development,
potentially resulting into some intermittent disruption of
visibility /2-5SM/. Otherwise extensive MVFR stratus deck holds firm
overnight. Increasing post-frontal northwest winds expected to
emerge by mid-late morning. Gusts into the 30 knot range expected
throughout Tuesday. The drier arctic air will work toward lifting
ceiling heights during the morning, leaving predominately lower VFR
stratus for the daylight period.

For DTW...Improving conditions during the early morning period as
the heavier snow exits to the southeast. Some additional light
pockets of snow showers remain possible prior to daybreak, but
additional accumulation will remain minimal. Arctic front surges
through by mid morning /~12z/, with strong northwest winds
developing in it`s wake. Wind gusts to 30 knots will bring crosswind
threshold considerations throughout Tuesday.

//DTW THRESHOLD PROBABILITIES...

* High confidence for ceilings at or below 5000 feet through
  Tuesday.

* High confidence in precip type being snow this forecast.

* Moderate confidence in northwest crosswind thresholds being
  exceeded Tuesday.


&&

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

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

     Winter Weather Advisory until 5 AM EST early this morning for
     MIZ063-068>070-075-076.

Lake Huron...Heavy Freezing Spray Warning from 7 AM this morning to 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 4 PM EST this afternoon for LCZ460.

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

&&

$$

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


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