Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 092055
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
355 PM EST Fri Dec 9 2016

.DISCUSSION...

In the near term, cellular lake effect snow showers are expected to
dissipate over SE Michigan with the loss of diurnal heating.
However, continued WNW flow over the moderately unstable lake will
ensure LES remains active on the west side of the state. A weak
upper wave embedded in cyclonic flow aloft will help deepen the
nocturnal convective layer such that snow showers may extend east
again overnight. Primary focus for convergence given existing flow
regime will be around the I-94 corridor. Accumulations left at a
minimum for now pending observational evidence of organized banding.

LES continues into Saturday with a stronger wave progged by the
suite of models to pivot across the area during the first half of
the day. Boundary layer flow backing to westerly will force the
convergent corridor to focus north of 8 mile. Strong signal exists
in the NAM12/ARW/NMM for organized banding and see no reason to
argue given favorable thermodynamic profiles and fetch off the lake.
Coverage during the afternoon is expected to wane as flow further
backs to SW in advance of low pressure approaching from the west.
Nonetheless, could envision a narrow band of light accumulation of
at least an inch in Livingston County and the northern suburbs prior
to 21z.

Attention then turns to winter storm late Saturday night into Sunday
night. Strong consensus exists in a pop sense, warranting an
increase to 90 to 100 pops area wide. NWP struggles with regard to
lead energy have largely been resolved as GFS/NAM/GEM trended toward
the more consolidated look of the EC and UKMET, although noteworthy
initialization errors appear to plague the 12z NCEP suite per Suny
Stony Brook ensemble sensitivity analysis.

Expectation of system evolution remains unchanged with lead
isentropic ascent within a strengthening right entrance region
drives the first half of the event followed by a transition to more
dynamic lift upon the approach of the wave aloft. 12z NAM cross-
sections reveal high static stability throughout the column, calling
into high question its stronger surface low and high QPF output.
Largely for this reason, preference is given to EC/Canadian
solutions which depict a weaker cyclone overall.

Light snow is expected to be begin late Saturday night as lead
energy enhances the jet streak overhead. Aforementioned high static
stability will effectively limit the vertical response during this
time, although the initially cold column favor will still maximize
efficiency. Expect At least one half to one inch by 12z Sunday
morning across the southern half of the CWA. Peak snowfall will
occur as the low-level jet slides by to the south. Substantial
moisture advection and increasing dynamic support aloft will favor
profilic hydrometeor production. However, model progs of the
resultant warming column elevate the dendrite growth zone well above
10,000 feet. Nonetheless, 1 inch per hour rates over a 4 to 6 hour
period will be plausible during the day Sunday along the nose of the
jet.

The more phased Canadian and GFS suggests a lower snow ratio
scenario courtesy of a stronger LLJ warming the column. The
resulting habitat would support a mix of crystal types that would
then undergo notable riming upon decent, particularly across the
southern CWA. The primary change noted in the 12z EC was a less
phased solution over the northern Great Plains. Snow density in that
case would be much more favorable for overachieving. The official
forecast calls for snow ratios of 12 to 15:1 across the north and
falling to 8 to 10:1 across the south. The going forecast therefore
3" to 7" north of I-69 and 5" to 9" to the south. Confidence is
highest across the central CWA between Detroit and Flint which are
less likely to be impacted by small fluctuations in track. Assuming
no major changes, the primary considerations for tonight will likely
be snow density and potential for headline issuance.

Monday through Saturday Extended Forecast: Scattered snow showers
will be possible through mid-week in the wake of Sunday`s low
pressure system. Frontal passage on Monday will open the door for
cold arctic air to begin filtering into the region. Temperatures
will drop steadily through the week and by Wednesday/Thursday, high
temperatures will be in the teens with overnight lows projected to
drop into the single digits. On Friday, weak trough of Canadian Low
pressure is expected to drop through southeast Michigan.

&&

.MARINE...

Modest northwesterly winds this evening will become westerly
overnight while strengthening.  This will result in renewed
gustiness into the 20 to 25 knot range throughout southern Lake
Huron lasting through Saturday.  This pattern will maintain
favorable conditions for lake effect snow squalls into early
Saturday.  A stronger low pressure system will lift across the
Southern Great Lakes Sunday night into Monday.  While this will
result simply result in a modest increase in winds, this system will
bring a widespread accumulating snowfall during the latter half of
the weekend.

&&

.PREV DISCUSSION...
Issued at 234 PM EST Fri Dec 9 2016

AVIATION...

Existing northwest flow will be slow relinquinsh through the
evening, suggesting the favored placement of the primary lake
moisture plumes will remain west-southwest and northwest through
this time.  With that said, the potential remains for lighter
pockets of lake effect snow shower activity to infiltrate the region
as winds do gradually back with time. Outside of any snow showers,
conditions at VFR conditions. Increasing low level convergence as
winds eventually become westerly tonight will then favor greater
eastward expansion of snow showers through the morning hours,
particularly across the DTW/YIP/PTK corridor. Potential for these
snow showers to intermittently carry greater intensity, providing a
corresponding reduction to visibility and the possibility of a quick
accumulation.

FOR DTW...Potential for a few segments of the ongoing lake effect
activity just upstream to funnel over the terminal yet today,
although trajectory favors placement of the more intense and
persistent activity to the southwest.  Greater potential for locally
intense snow showers overnight into Saturday morning.

//DTW Threshold Probabilities...

* Moderate for ceilings below 5000 feet through this evening, then
  High tonight.

* High for snow as precipitation type.


&&

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

$$

DISCUSSION...JVC/DE
MARINE.......MR
AVIATION.....MR


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