Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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004
FXUS63 KDTX 091932
AFDDTX

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

.AVIATION...

Northwest flow will continue to draw moisture off Lakes Michigan and
Superior, allowing lake enhanced clouds and lake effect snow showers
to impact the area again today. Coverage and intensity is expected
to be less than yesterday however. Best potential for showers will
be within two periods for most of the area. The first will be in the
afternoon (after about 19Z) as upper energy crosses Michigan and we
hit the peak of the diurnal cycle. This should drop conditions
across the area back to MVFR, with most of the shower activity
expected to occur south of FNT. The second period of enhanced
activity is expected late tonight (after about 08Z) as another upper
wave works into Lower Michigan and provides a boost to low and mid-
level moisture. This round looks more likely to drop ceilings below
2000 feet, mainly from PTK southward.

FOR DTW...Early morning activity should wane around TAF valid time
as drier air works into the area from the north. Expect to see a
short period of ceilings above 5000 feet before diurnal cycle begins
to drop ceilings. Upper energy arriving during the late afternoon
will provide a better chance for snow showers after about 19Z.

//DTW Threshold Probabilities...

* Moderate for ceilings below 5000 feet after 14z. High during the
  afternoon and this evening.

* High for snow as precipitation type.

&&

.PREV DISCUSSION...
Issued at 406 AM EST Fri Dec 9 2016

DISCUSSION...

A low amplitude mid level ridge axis will traverse Lower Michigan
today. The large scale subsidence will be fairly weak, only taking
lake modified equilibrium levels down to roughly 9k ft per model
soundings, given the degree of cold air enveloping the region. So
the lakes will remain quite active in terms of generating snow
showers. The low level wind field is currently in the process of
backing toward the northwest, not typically a wind direction which
brings much lake effect snow showers into Se Mi. More probable
locations for snow showers will be southwest of a Howell to Detroit
line. Hi res solutions do continue to suggest remnant snow showers
from NW Lower Mi impacting the Saginaw Valley and thumb region at
times today. Some enhancement off Saginaw Bay may support a few
higher intensity snow showers across the thumb region. The lake
modified airmass and winds have held nighttime temps in the 20s.
Despite this, daytime temps will struggle to surpass 30 with 925mb
temps around -8C overhead.

The low level wind fields will then back more westerly this
afternoon into this evening, bringing better chances of light snow
showers into the forecast area. A short wave impulse will race from
the upper Ms Valley across Lower Mi late tonight into Saturday
morning. This will increase mid level moisture and boost equilibrium
levels, thus increasing lake effect snow shower intensity. Enhanced
low level convergence is expected to set up from Lake Mi along
either the I-96/M59 corridor and/or I-94 corridor. Steep low level
lapse rates and good lift in the dendritic growth zone may lead to
some intense snow showers. The strongest convergence is expected late
tonight into early Sat afternoon. Some minor snow accumulations
certainly seem possible.

While there is still some timing and amplitude differences among the
models, there is growing agreement among the 00Z suite in a
potential long duration snow event beginning either Sat night or Sun
morning and possibly persisting into Monday morning. A low amplitude
short wave is forecast to eject out of the Pacific northwest, racing
into the northern and central high plains on Sunday. A broad region
of warm air advection is forecast to overspread the Great Lakes well
in advance of this feature. Isentropic ascent within the ascending
branch in the entrance region of an upper jet will be focused
across srn Lower Mi by Sun afternoon. Moisture advection into the
region of ascent will actually be quite good with 850mb mixing
ratios fcst to rise to 3 g/kg by Sun evening. The ascent on Sunday
will be rather weak and the mid levels will be rather stable. The
degree of moisture and forcing should still support some minor
accumulations through the daytime period. Stronger large scale
ascent will then arrive Sun evening as the short wave pushes into
the western Great Lakes, giving the potential for additional
snowfall accumulations. The Sun night/Mon morning aspect of this
system still carries a good deal of uncertainty. A stronger system
will likely drive warmer low level air into at least the southern
portion of the area, possibly changing precip over rain. Deepening
of the upper trough over the Great Lakes may also drive a mid level
dry wedge into the area more quickly, thereby marking an early end
to the snow. In light of these uncertainties, a 12 to 18 hour
snowfall total of 4 to 7 inches looks possible attm.

This system will depart to the east on Monday. A large upper low is
then forecast to rotate from central Canada to eastern Canada during
the work week. This will drive arctic air across the Great Lakes and
will support well below normal temperatures during the latter half
of the forecast period.

MARINE...

Winds and waves will continue to decrease over the Central Great
Lakes today as low pressure continues to move through Eastern
Canada. Northwest winds will continue to funnel cold air into the
area today and tonight, generating lake effect snow squalls over
Lake Huron through early Saturday. Winds over the northern half of
Lake Huron will be fairly light tonight and Saturday as weak low
pressure becomes established, but west winds will increase over
southern Lake Huron, Lake St Clair and Western Lake Erie during this
timeframe as high pressure builds south of the area. Wind gusts look
to remain below 25 knots, however. A stronger low pressure system
will lift across the Southern Great Lakes Sunday night into Monday,
bringing snow and perhaps some rain to at least portions of the area.

&&

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

$$

AVIATION.....MR
DISCUSSION...SC
MARINE.......HLO


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