Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 090242
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
942 PM EST Fri Dec 8 2017

.UPDATE...

Steady southeast progression of an amplifying pv anomaly evident on
water vapor and upper air analysis currently digging across
northwest Wisconsin. Lead wing of narrow mid level isentropic ascent
preceding the advancing height fall gradient generating an elongated
swath of mainly virga over Wisconsin within the attendant moisture
axis. Lake Michigan moisture plume firmly entrenched locally
/exception being near the Ohio border/, thanks to persistent WSW low
level flow trajectories. Simply an extensive stratus in place at
this stage, moisture depth and forcing lacking to generate anything
more than a passing flurry prior to midnight. As the upstream moist
axis and weak ascent starts to augment the existing lake moisture
flux, anticipate an expansion of light snow shower activity into
central lower MI around or just after midnight. Latest probabilistic
guidance and hi res model output maintain support for further
expansion into the existing lake moisture plume residing locally,
particularly northwest of an Ann Arbor/Pontiac/Port Huron line.
Points southeast of this line appear more tenuously positioned in
terms of snowfal potential during morning period, given greater
displacement from the lake moisture contribution and lackluster
moisture quality with the inbound system. Will go ahead and apply a
slight nudge downward in expected snowfall totals along this
corridor for the morning period.  Points to the north will benefit
from both greater moisture availability and longer residence time,
with accumulation in the vicinity of an inch plausible by daybreak
across the Tri-cities.

&&

.PREV DISCUSSION...
Issued at 653 PM EST Fri Dec 8 2017

AVIATION...

Approaching low pressure system on track to bring a period of
widespread light accumulating snowfall to the SE Michigan airspace
late tonight and early Saturday. Extensive MVFR stratus preceding
this system holds firm from PTK northward early tonight, as
prevalent lake Michigan moisture plume remains nearly stationary
under continued SW flow. Light snow will then increase in coverage
from northwest to southeast through the early and mid morning hours.
Window of greatest snowfall intensity, resulting in IFR visibility
restrictions and some accumulation, now centered between 10z and
17z. Snowfall totals ranging from 3 inches at MBS to around an inch
at DTW/YIP. Improving conditions throughout the afternoon, although
with the potential yet for some additional pockets of higher
intensity snow showers as a low level front sweeps through. Modest
southwest wind into Saturday, turning to northwest late Saturday
with the frontal passage.

For DTW...Window of VFR expected to last through much of the
overnight period. Latest trends suggest onset timing of light
snowfall within the 12-14z window Saturday morning, with a several
hour period of light accumulation in IFR/low MVFR thereafter. Total
accumulation upwards of an inch.  Winds will remain modest from the
southwest through Saturday.

//DTW THRESHOLD PROBABILITIES...

* High for ceilings at or below 5000 feet after 12Z Saturday and
  continuing during Saturday.

* High confidence in precip type being snow.

PREV DISCUSSION...
Issued at 318 PM EST Fri Dec 8 2017

DISCUSSION...

The Lake Michigan moisture plume has supported ample cloud cover
generally north of the M 59 corridor, while locals to the south have
had an abundance of sunshine today. Some subtle veering of the winds
across the lake have been resulting in a slow expansion of the
strato cu field farther south. Earlier snow shower/flurry activity
across Central Michigan has since diminished as a mid level
subsidence bubble has reduced the convective cloud depths. This
subsidence should continue to suppress chances for snow showers into
the evening. There will be some slight boundary layer decoupling
this evening to ease the gusty SW winds a bit.

A fairly robust mid level short wave trough now rotating toward nrn
Minnesota will rapidly drop into the wrn Great Lakes by 12Z Saturday
before tracking across nrn Indiana/Ohio Sat afternoon/evening. The
low level flow will undergo backing toward the southwest again
tonight. The increasing mid level ascent will lift inversion bases,
allowing light snow showers to develop across the Saginaw Valley and
thumb region late this evening/overnight. Light snow will then
slowly expand across the rest of Se Mi during the course of the
overnight through Sat morning with the approach of the upper wave.
The better mid level height falls/positive pv advection will slide
across the Ohio Valley on Sat. There will be some degree of mid
level deformation present across Se Mi (strongest across the Saginaw
Valley and thumb) as the wave slides across the area. Given that
this wave is originating from northern Canada, moisture quality will
be quite poor (700mb specific humidities at or below one g/kg). This
is reflected in the 12Z model suite qpf fields which keep total
liquid equivalent precip from around two tenths of an inch
along/north of the I-69 corridor, tapering to under a tenth around
Detroit/Monroe. With expected snow ratios on the order of 14-16 to
one, total forecast snowfall from early Sat morning through Sat
evening will be two to threes inches in the tri cities/thumb,
tapering to an inch or less Detroit/Monroe.

With temperatures in the 20s through Sat morning, the light snow
should have no difficulty sticking to untreated surfaces. Perhaps
the more impactful portion of this event will be late Sat afternoon
and evening associated with a sfc trough axis forecast to drop
across the area from the north in the wake of a departing sfc low.
Very cold air aloft with the mid level trough overhead will result
in steep sfc to 600mb lapse rates. This may yield a brief period of
high intensity snow showers, especially across the thumb where Lake
Huron enhancement will be very good. The brevity of the high
intensity snow across the thumb region before the flow backs more
northwest Sat evening will preclude headline concerns for
Huron/Sanilac counties attm.

Good mid level subsidence and low level drying in the wake of the
upper trough Sat night will result in clearing skies with cold temps
(lows down in the teens). An arctic cold front will slide across the
northern Great Lakes on Sunday. West-southwest low level flow will
develop across srn Lake Mi south of this boundary. The moisture flux
off the lake and a plume of mid level moisture will support some
flurries/light snow showers, especially Sun afternoon and evening.

Model guidance is reaching better agreement on the strength and
placement of the next clipper on Monday. The 12Z GFS and ECMWF both
have the surface low moving over SE Michigan Monday evening at
around 1000 mb, then quickly moving off toward New England and
continuing to deepen. This system will provide light snowfall (1-3")
across all of SE Michigan with the best chance for accumulating snow
to occur during Monday afternoon and into the evening. Highs will
remain in the upper 20s to low 30s.

The clipper will usher in a wave of colder air that will bring highs
Tuesday and Wednesday down to the low to mid 20s. Lake effect snow
showers off Lake Huron will be possible in the eastern Thumb on
Tuesday as north-northwesterly flow takes over behind the departing
system. This flow will weaken and back to westerly by Wednesday,
diminishing the chance for the lake effect to continue. The coldest
temps of the season so far will be likely Wednesday morning as
impressive cold pool aloft allows surface lows to fall to the teens
or even single digits overnight.

Models diverge on the next system, though some signal exists for
another clipper to move through on Thursday evening into Friday,
potentially resulting in another region-wide accumulating snowfall.
Longwave troughing then looks to ease up a bit next weekend,
allowing for a slight increase in temperature.

MARINE...

Winds will continue to decrease through the evening hours as a weak
trough stalls across central Lake Huron. Gusts will remain below 20
knots as low pressure tracks through southern lower MI on Saturday.
Winds will flip to the northwest on the back side of the low
Saturday evening and will increase back to near gale force for a few
hours. Winds will remain elevated through Sunday evening until an
area of high pressure builds over the region decreasing winds below
20 knots. A period of snow showers will occur as the low passes
through the eastern lakes. In addition we are watching the potential
for a fairly heavy lake effect band to set up in the northeast flow
on the backside of the low Saturday afternoon and evening.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...NONE.
Lake St Clair...NONE.
Michigan waters of Lake Erie...NONE.
&&

$$

UPDATE.......MR
AVIATION.....MR
DISCUSSION...SC/TF
MARINE.......DRK


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