Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 112037
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
337 PM EST Mon Dec 11 2017

.DISCUSSION...

Winter Weather Advisory has been issued for a portion of southeast
MI for 2-4 inches of snow this evening. More information below.

Active pattern remains in place across the eastern conus as very
energetic long wave trough sends a series of shortwaves down through
the region this week. There will be a couple opportunities for snow
Wednesday and Saturday as clippers slide through the region. In
addition to snow chances, the coldest airmass of the season will
arrive mid week dropping low temperatures to single digits with wind
chills at or below zero Tuesday and Tuesday night.

Current surface analysis is matching up well with guidance over the
last 24 hours with the tightly wound, but open shortwave diving
southeast through the Midwest states toward the Ohio Valley. One
detail we`ve been trying to peg was how the surface low would shape
out as the initial phasing processes begin with a strong vorticity
lobe that is fast on its heals rotating around the parent low.
Combination of shearing height falls along the trough axis with lake
aggregate troughing over the lakes, is leading to a pseudo split
low. The surface reflection that will track through the Ohio Valley
is the dominate feature, with the height fall center over the
straits the secondary center. This southern low has already
generated a thin fgen band extending well east of the low which will
strengthen as it lifts up into SE MI. This will be the item of
interest for snowfall accumulations this evening. Models trends
through this afternoon have increased confidence in the placement of
this band. Latest runs of the RAP, HRRR, and ECMWF all suggest a
slight shift back southward, although most models key in on the area
between I69 and I94 generally between 22-02Z.

It appears the fgen band will lift up across the MI border late this
afternoon through the Detroit Metro before stalling around the M59
corridor. This will last a few hours as the low slide south of the
MI border but snowfall will continue as the trailing deformation
band then pivots through the area tonight. In the end we are still
looking for a widespread 1-2 inches of snow across most of the area
with a band of 2-4 inches setting up where the Advisory is. Note
that there could be a decent gradient of accumulations across Wayne
and Washtenaw with locations across southern portions of those
counties likely not seeing 3+ inches. Similarly for counties along
the I69 corridor, higher chances to see 3+ inches will occur across
southern portions of those counties.

By Tuesday morning the trough will have swung through the area
opening the door for strong northwest flow and good cold air
advection. Thermal trough drops over the Great Lakes with a broad
area of 850mb temps of -20C. With winds gusting to around 30 mph and
highs only around 20-25F, winds chill values will range from -5 to
5F. This will continue through the overnight before winds fall off
enough Wednesday to allow some recovery. Other potential concern for
Late Tuesday is the threat of a lake effect snow band to brush the
eastern Thumb. A difficult feature to ever forecast, but hires
models are starting capture the time period starting around 20Z.
Confidence remains high that the band will develop but models have
been trending east with the meso low over northern Lake Huron
dropping southward. A further west track would act to push the band
closer to shore. So potential is there a quick accumulation of
snowfall along the eastern shore of the thumb.

Wednesday is looking quite similar to today with a clipper tracking
southeast through the Ohio Valley. NAM. GFS, and ECMWF are all
showing around 0.2 inches of qpf across the southern half of the
CWA. With the thermal profile in place, another 2 inches of snowfall
wouldn`t be out of the question.

The longwave trough over the eastern CONUS will remain in place and
allow for a series of shortwaves to pivot over the area Thursday
through Saturday. The active pattern will result in a small chance
for snow showers each day, with the better chance on Friday and
Saturday as the longwave trough takes on a negative tilt and a
surface low develops over the northern Great Lakes. Temperatures
will begin a slow upward trend as the coldest dome aloft moves off
to the east, though still remaining below average with highs
Thursday and Friday reaching the mid to upper 20s. Saturday will see
the return of above freezing temps with highs pushing the mid 30s.

Model guidance is suggesting a shortwave and low pressure system
will move across the central CONUS and eventually to our area late
Saturday into Sunday, though the placement and timing have not been
well resolved this far out. A more southern track like the ECMWF
currently suggests could lead to another light snowfall for the
area, while a more northern track per the GFS would leave SE
Michigan mostly out of the best chance for precip. Will monitor
trends in this system over the next week.

&&

.MARINE...

A strong cold front will transit the waters late tonight. Strong
northwest gales will develop with gusts approaching or briefly
reaching storm-force over the exposed nearshore waters of the Thumb
and the open waters of central and southern Lake Huron. Gale
warnings are now in effect for all marine zones. Temperatures
falling through the 20s into the teens and significant wave heights
of 10 to 15 feet also warrant issuance of a heavy freezing spray
warning. Maximum wave heights will easily exceed 25 feet.

&&

.PREV DISCUSSION...
Issued at 1252 PM EST Mon Dec 11 2017

AVIATION...

Initial burst of isentropic ascent snow continues to exit to the
east, with lingering light snow possible the next few hours before a
brief lull in activity later this afternoon. Clipper system
currently located over northern Illinois will continue to approach
the region this evening, with FGEN band of peak snow late this
evening into the early overnight hours. Hi-res model guidance
continues to struggle with placement of banding, leading to a
relatively low confidence forecast, although trends have been most
consistent in more persistent banding setting up between I-96 and I-
69, impacting the KPTK and KFNT sites the greatest. Timing of band,
which looks to scrape KDTW, KYIP, and KDET for at least a few hours
in a more transient nature looks to be between 23z-01z, and 01z-05z
for KPTK, KFNT, and KMBS.

Flying conditions will briefly improve to mainly MVFR ceilings
through roughly 22z with MVFR/VIS ceilings after lingering morning
snow exits by 19z (IFR conditions possible in lingering snow). IFR
vsby restrictions return with FGEN snowband (down to as low as 1/2
SM at times), with low-end MVFR ceilings and possible IFR ceilings
in heaviest snow. FGEN forcing exits east by 06z with a 3-6 hr
window of wrap-around light deformation snow before drier air moves
in by 12z. Winds will initially be south/southeasterly 5-10 knots
this afternoon before trending light and variable as the clipper low
passes by to the south evening/early overnight. After 06z, winds
quickly pick up out of the northwest in the wake of the clipper,
topping out at around 20 knots with gusts to 30 knots especially
after 12z.

For DTW...Initial surge of light snow ends shortly after TAF
issuance, with a brief 2-4 hr window lull in activity before light
snow returns heading into the evening at MVFR levels. FGEN moderate
snowband looks to clip the terminal in the 23z-01z timeframe with a
quick shot at an inch or two additional accumulation before activity
transitions to wraparound light snow through roughly 10z. IFR vsbys
and low-end MVFR ceilings will be prevalent in the FGEN snow, with
possible worse conditions if hi-res guidance trends further south
with the band as the afternoon progresses. After 12z, gusty
northwest winds to 30 knots may pose risk of crosswind thresholds
being exceeded.

//DTW THRESHOLD PROBABILITIES...

* High confidence for ceilings at or below 5000 feet through
  much of period.

* High confidence in precip type being snow this forecast.

* Moderate confidence in northwest crosswind thresholds being
  exceeded.

* Low for visibility 1/2 SM or less in snow this evening.


&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...Winter Weather Advisory until 2 AM EST Tuesday for MIZ060-061-
     068>070-075-076.

     Winter Weather Advisory from 7 PM this evening to 2 AM EST Tuesday
     for MIZ062-063.

Lake Huron...Heavy Freezing Spray Warning from 7 AM Tuesday to 4 AM EST
     Wednesday for LHZ361>363-421-441>443-462>464.

     Gale Warning from 1 AM Tuesday to 5 AM EST Wednesday for LHZ362-363-
     462>464.

     Gale Warning from 4 AM to 4 PM EST Tuesday for LHZ422.

     Gale Warning from 4 AM Tuesday to 4 AM EST Wednesday for LHZ421-
     441>443.

     Gale Warning from 1 AM to 11 PM EST Tuesday for LHZ361.

Lake St Clair...Gale Warning from 4 AM to 4 PM EST Tuesday for LCZ460.

Michigan waters of Lake Erie...Gale Warning from 4 AM to 4 PM EST Tuesday for LEZ444.

&&

$$

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


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at www.weather.gov/detroit.



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