Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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339
FXUS63 KDTX 012256
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
556 PM EST Thu Dec 1 2016


.AVIATION...

Westerly flow across Lake Michigan is sustaining the more widespread
showers and MVFR ceilings generally along/south of the I-96/M 59
corridor, impacting mainly PTK and the Detroit terminals. This
region of convergence will persist through the evening before slowly
sliding south late tonight into Fri morning, resulting in some
improvement in conditions at PTK overnight. The depth of colder air
will increase tonight, mainly at FNT and MBS. This should support
prevailing VFR conditions at these terminals.

For DTW...Low level convergence will focus periodic rain showers off
southern Lake Mi through the rest of the evening. Mid level
subsidence will reduce convective cloud depths during the course of
the night, likely causing remnant showers to transition toward
drizzle. Sfc temps will remain above freezing. The low level flow
will veer slightly more northwesterly across Lake Huron Fri morning.
This will transition the more widespread lower clouds and
showers/drizzle south of metro Detroit by late Fri morning.

//DTW Threshold Probabilities...

* High in ceilings below 5000 ft through tonight. Medium on Friday.

* High in precipitation type being all rain tonight and Friday
  morning.

&&

.PREV DISCUSSION...
Issued at 347 PM EST Thu Dec 1 2016

DISCUSSION...

The large scale pattern will become less favorable for lake effect
precipitation during the late afternoon and evening. Mesoscale
features moving through the west flank of the upper low are forecast
in model data to shear eastward by early evening. This will leave
the western Great Lakes more under the influence of large scale
subsidence and reduced convective depth through tonight. At the same
time, model soundings and plan views continue to show a plume of
substantial moisture and instability below the subsidence inversion
extending eastward from Lake Michigan originating from considerable
stratus coverage shown on satellite all the way back into the
northern Plains. While large scale subsidence will lower convective
depth, surface delta theta-e near -3 K shown in model time sections
could enable shallow rain showers enough staying power to make into
SE Michigan roughly along the I-94 corridor. In the absence of
legitimate showers, patches of drizzle will be possible in lingering
deep cyclonic flow with magnitude around 30 knots through the
inversion layer. This will be easily sufficient to maintain forcing
within the low level moist layer given that lapse rate profile as
the moist layer is forced below the -10 C level. The good news is
that surface temperatures are expected to approach but remain above
freezing in the moderate gradient wind and under the extensive
stratus coverage through Friday morning.

SE Michigan will enter into a cloudy but otherwise tranquil early
December pattern by Friday afternoon through Saturday, in between
the upper low departing the Canadian east coast and the upper ridge
centered near Hudson Bay. A broad area of surface high pressure will
be associated with this ridge and the large scale blocking pattern
developing across North America. This will consist of the above
mentioned upper ridge anchored by the Pacific coast trough and the
cut-off low over northern Mexico. The cut-off low will maintain a
Gulf moisture supply over the southern Plains that will be available
to the northern stream as a lead wave moves into the northern Plains
by Saturday night. There remains good agreement in the 12Z model
runs that show this lead short wave cutting into the blocking ridge
over the Great Lakes by Sunday, tapping that Gulf moisture, and
bringing our next shot of precipitation Sunday afternoon and night.
At this point, it looks like a routine higher POP/lower QPF
isentropic ascent scenario with model QPF around 0.1 inch liquid.
Partial thickness favors all snow while daytime onset with highs in
upper 30s make it possible for an above freezing layer deep enough
for a mix before wet bulb and diurnal cooling take over by Sunday
evening. A slushy/grassy/elevated surfaces accumulation of 1 inch or
less is the most likely outcome given the liquid QPF expectations.

As Sunday`s low pressure system exits the region a weak ridge of
high pressure moves in behind it for Monday. Current models indicate
low pressure over Texas making a fast track northeast late Monday;
meanwhile a north Pacific low tracks into western Ontario Tuesday.
These two lows eventually merge together Tuesday afternoon. Chances
of rain or rain and snow are expected to persist in the forecast
through most of the extended period. Precipitation has the potential
to change over to all snow by Thursday. It should be noted that
there are still plenty of opportunities, this early out, for the
forecast to change as systems develop and timing is refined.

MARINE...

Extended period of cold cyclonic flow will sustain gusty conditions
through Tomorrow.  Westerly gusts generally peaking in the 25 to 30
knot range today.  This will maintain small craft conditions for
waves from outer saginaw bay through the tip of thumb.  Winds become
more northwesterly on Friday.  High pressure for the weekend will
allow winds to become light and variable. Light winds early next
week giving way to increasing southerly winds for the mid week
period.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...Small Craft Advisory until 4 PM EST Friday FOR LHZ421-441.

Lake St Clair...NONE.
Michigan waters of Lake Erie...NONE.
&&

$$

AVIATION.....SC
DISCUSSION...BT/SS
MARINE.......SF


You can obtain your latest National Weather Service forecasts online
at www.weather.gov/detroit.



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