Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 130035
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
735 PM EST Tue Dec 12 2017


.AVIATION...

Short wavelength surface ridge axis in addition to differential
height rises and anticyclonic vorticity advection will cause
inversion heights to fall tonight. Dry arctic airmass should then
allow lower tropospheric stratocumulus to strip out for tonight. IR
imagery suggests this has been occurring early this evening,
particularly over the eastern half of Lower Michigan. An easterly
wind component will emerge out of the NW by morning.

A clipper system is then forecasted to descend into the central
Great Lakes region on Wednesday bringing a widespread accumulating
snowfall. High cloud will overspread the state late tonight before
the lower column moistens before MVFR cigs/vsbys develop in light
snow by 15Z. Peak of the event is expected between 20-01Z over
southeastern Michigan with periods heavy snow possible. VLIFR to
LIFR conditions will be possible in heavy snow during the evening.
East to north winds are forecasted during the event with windspeeds
in the 10 to 15 knot range.

For DTW...MVFR light snow developing after 15Z Wednesday, with IFR
light snow after 18Z. Peak of the event is expected between 22-02Z
with VLFIR/LIFR in heavy snow possible.

//DTW THRESHOLD PROBABILITIES...

* High confidence in cigs below 5000 ft agl after 15Z Wednesday.

* Moderate to high confidence in heavy snow with visibilities less
  than 1/2sm and cigs at or less 200 ft agl 22-02Z

&&

.PREV DISCUSSION...
Issued at 305 PM EST Tue Dec 12 2017

DISCUSSION...

Deep longwave trough in place over the eastern US today with
embedded 850mb temps around -18C containing highs to upper teens to
around 20 degrees today. A fresh snowpack will contend with an
elevated gradient wind for influence on low temperatures for much of
the night, increasing uncertainty in the potential to achieve
temps much lower than about 10 degrees. Transient ridge axis
building behind departing low pressure will put SE Michigan in a low
gradient col pattern late, but with high clouds arriving it may
occur too late to have a significant impact on temps. Maintained 8
to 12 degrees for most areas with any potential to drop toward zero
expected to be confined to the Saginaw Valley area.

Attention for Wednesday turns to high latitude energy over Nunavut,
which is shearing into south-central Canada, and a low amplitude
lead wave which is over Alberta at this time. Confidence in the
track of the clipper as it approaches the Great Lakes is higher than
normal since it will have virtually nowhere else to go given the
amplitude and strength of the background flow. A very deep layer of
lead isentropic ascent will lift from SW to NE into the area during
the morning. Virga will transition to light snow after about 15z
with intensity becoming moderate as forcing increases. The layer of
active FGEN as this occurs is impressively deep, yet overrunning is
not cutoff by strong vertical motions given moderate static
stability above most of the frontal slope. This will keep the
ageostrophic frontal circulation from becoming so dominant that it
begins to affect areal coverage. Thus, despite the prominence of
fgen forcing, expect a fairly widespread swath of snow to develop.
Moisture will not be particularly impressive with 850-700mb specific
humidities hovering between about 1.7 g/kg and 2 g/kg, but a depth
of 10kft will be outstanding. Dendrite production within the deep
DGZ zone should be very efficient. Given the lack of a riming layer,
except perhaps near the immediate surface, snow ratios will likely
settle near 15:1, which is in line with in-house guidance. As the
wave begins to pivot, snow may briefly come to end somewhere south
of I-94 with 3 hour accumulations by 18z of T to 0.5 east of I-75
and 1-2 inches west of US-23.

Isentropic ascent will support a continuance of this pattern into
the afternoon for the northern half of the area. The focus will turn
toward the intense FGEN band being depicted by the EC/NAM/GFS as
well as the GEM (although the GEM strays from the consensus with a
slightly more southerly track the dynamics are identical). The
momentum field will respond with help from the lead wave, allowing a
strengthening low-level jet to surge into the southern half of the
CWA by afternoon. Intense convergence along the lead edge of the LLJ
as it rams into the frontal zone will combine with developing
conditional instability to yield a 2-3 hour period of high-end
snowfall rates of 2"/hr or higher. Intense upward motion supports an
upward nudge to 20:1 ratios within this band, careful not to go too
high given compaction during heavy snow and the fact that surface
temperatures will be at their peak during this period, potentially
leading to minor riming closer to ground level. Modifications to the
forecast track will affect the placement of this band, but the
general consensus at the moment is for this to bear down on the
Detroit Metro area and points west (i.e. Ann Arbor, Howell) roughly
20-23z, or in the middle of rush hour. The potential for high impact
timing, briefly high-end snowfall rates, and storm totals of 5 to 8,
perhaps locally more, warrants the issuance of a winter storm watch.
It will begin at the expected timing of the onset of light snow
accumulation around 15z.

At this time, it appears the northern third of the CWA will see
prolonged light to moderate snow worthy of only an advisory. In
fact, the northern Thumb may only see daytime accumulations of
around an inch, but developing onshore flow after 00z will very
likely force one or more LES bands into Huron County 00-06z
Wednesday night as snow intensity begins to diminish elsewhere.
Convective depths of at least 5kft and a respectable DGZ depth of
2000 feet within that layer yield high confidence in an LES
response. The forecast problem after 00z Wed may thus become one of
convective mode in the Thumb in addition to inland penetration. For
now, painted part of the Thumb with a generic swath of 1-2" during
this this period with the expectation that duration will be limited
by continually backing flow leading to a steady change in band
orientation throughout this time. After briefly being forced north,
today`s airmass will filter back in behind the winter storm,
allowing lows to fall back into the single digits for Wednesday
night.

A pair of PV anomalies in the NW flow will drop from the Canadian
prairies and swing through the Great Lakes Friday, resulting in a
developing low over the northern part of the region and snow showers
for our area. Snow squalls will not be out of the question with the
level of instability and 30-40 kt winds at 850mb that shallow
convection could tap into, leading to quick bursts of heavy snow and
gusty winds at the surface. Highs will remain below normal in the
mid 20s.

Some lingering lake effect bands may persist into Saturday morning,
though modest surface and upper level ridging look to move in and
quiet things down for the rest of the weekend. Southerly flow on the
western periphery of this ridge will help heights rise and
temperatures rebound to above freezing for the early part of next
week. Active pattern looks to continue in the long term with a
series of upper waves passing over. Confidence is quite low in
timing/location of the resultant systems given model uncertainty at
this point.

MARINE...

Strong northwest gales will gradually subside by Wednesday morning.
In the mean time, heavy freezing spray will continue with the gales.
A storm system will bring moderate snow from across southern Lake
Huron on south Wednesday during the day. Winds will increase from the
northeast behind the system Wednesday night while backing to the
northwest on Thursday. Cold air will help produce bands of lake
effect snow on Lake Huron Thursday. Another system will help back
the flow to the southwest for Friday while bringing more snow
activity to the lakes. After tonight`s gales, winds are expected to
remain below gale force for the rest of the week and weekend.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...Winter Storm Watch from Wednesday morning through late Wednesday
     night for MIZ047-053-060>063-068>070-075-076-082-083.

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 AM EST Wednesday for LHZ421-441>443.

Lake St Clair...Small Craft Advisory until 10 PM EST this evening for LCZ460.

Michigan waters of Lake Erie...Small Craft Advisory until 10 PM EST this evening for LEZ444.

&&

$$

AVIATION.....CB
DISCUSSION...JVC/TF
MARINE.......DRC


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