Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 082018
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
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.

&&

.PREV DISCUSSION...
Issued at 100 PM EST Fri Dec 8 2017

AVIATION...

Mostly dry through the afternoon outside of a few possible flurries
at MBS. A trough/boundary has dropped across mid MI which will
provide some borderline VFR/MVFR CIGs the rest of the day for MBS
and FNT, possibly PTK as well. A low pressure system will drop into
lower MI tonight bring a period of snow to the region into Saturday.
Accumulations will range from around 1 inch at DTW up to 3 inches at
MBS. Winds will be largely out of the southwest but will start
making the switch to northwest behind the low as it exits eastward
Saturday afternoon. Gusts will be highest this afternoon reaching 20
knots, but will diminish overnight and remain under 10 knots through
the rest of the forecast.

For DTW...VFR through the day as clear skies gradually fill in with
mid clouds from the low that will move in tonight. Light snow
developing early Saturday and continuing into the afternoon.
Snowfall accumulations around 1-2 inches expected before activity
tapers off Saturday afternoon.

//DTW THRESHOLD PROBABILITIES...

* High for ceilings at or below 5000 feet after 10Z Saturday and
  continuing during Saturday.

* High confidence in precip type being snow.


&&

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

$$

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


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