Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 092006
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
306 PM EST Sat Dec 9 2017

.DISCUSSION...

The light snow diminished across the area during early this
afternoon with waning large scale ascent. Steep low to mid level
lapse rates and remnant synoptic moisture has sustained some fairly
intense lake bands across the Great Lakes. Recent satellite data
indicate a meso vortex just south of Alpena within the weak flow.
The departure of the upper trough and associated surface low/trough
axis to the east this evening will lead to northerly flow across
Lake Huron, which will likely push the heavier snow showers with
this meso feature into the thumb region early this evening. One to
two inch per hour snowfall rates are possible within this snowfall.
The low level flow will back toward the northwest during the course
of the evening, keeping the period of heavier snow showers in the
thumb brief. Thus no winter wx headlines for Huron/Sanilac counties
seem needed attm. The rest Se Mi will remain largely dry. The
gradient flow and periodic cloud cover will hold min temps in the
20s.

There is fairly good agreement among the NAM and ARW solutions in
the development of an axis of low level convergence focused along
roughly the I-69 corridor Sun afternoon within low level westerly
flow. This will funnel the lake effect plume into Se Mi. Model
guidance suggests the potential for steepening low level theta e
lapse rates Sun afternoon, raising concerns for a period of higher
intensity snow showers. This will warrant a narrow region of higher
pops with at least a mention of some light accums possible. Veering
of the low level flow toward the northwest Sun evening should
fracture any lingering snow band that releases off Lake Mi, ending
the snow shower chances.

Another mid level short wave is forecast to rotate across the nrn
Ohio Valley Mon afternoon/evening. This feature will emerge in the
lee of the Canadian Rockies on Sunday and quickly rotate into the
Upper Midwest Mon morning. The wave is forecast to dampen as it
slides toward the Mid Atlantic late Mon night as a strong positive PV
anomaly drops into nrn Ontario from northern Canada. A broad region
of mid level moist isentropic ascent is forecast to spread across Se
Mi Mon aftn/evn. Moisture will be a little better with this system
in comparison to today (700mb specific humidities around 2 g/kg).
Lowering mid level static stability may lend itself to some regions
of enhanced mid tropospheric fgen. Given the fast movement of the
forcing, a general 1 to 3 inches of snow accums will be maintained
in the forecast. It should be noted that there remains some model
uncertainty with respect to the strength of this system, thus later
adjustments to current forecast snow totals are certainly possible.

The aforementioned PV anomaly dropping into nrn Ontario Monday will
rotate across the ern Great Lakes Tues into Wed, forcing an influx
of arctic air across the region. By Tues night, 850mb temps are
forecast to flirt with -20c across Se Mi. This airmass along with
gusty northwest winds will support single digit and sub zero wind
chills Tues through Wed. Another mid level short wave feature is
forecast to slide across the Great Lakes on Thursday along the back
edge of the arctic air as it begins to retreat to the east. This
will be the next chance for light snow. Building mid level heights
across the area late in the work work will provide a modest
moderation in temperatures by next weekend.

&&

.MARINE...

A sfc trough over srn Lake Huron this afternoon will depart to the
east this evening, opening the door for strengthening north-
northwest winds across Lake Huron. Increasing mixing depths will
support gusts topping out in the 25 to 30 knot range during the
evening. Winds will then back toward the west overnight as a cold
front drops into northern Lake Huron from Canada. The gradient flow
and ample mixing will support gusts around 25 knots through the day
Sun. The front will weaken as it drops south across Lake Huron Sun
afternoon and evening. The passage of the front will be marked by a
veering of the winds to the northwest.

Winds will weaken Sun night into Monday as low pressure advances
pushes into the region from the west. Deepening of the surface low
to the east of the Great Lake Mon night and Tuesday and an influx of
arctic air will lead to a high probability of northwest gales on
Tuesday. In addition, the degree of cold air will also support high
probabilities of freezing spray. Winds will undergo a gradual
weakening trends Wednesday as high pressure attempts to expand into
the area from the west.

&&

.PREV DISCUSSION...
Issued at 1220 PM EST Sat Dec 9 2017

AVIATION...

Widespread light snow and fog is expected to persist through the
early and mid portions of the afternoon as long as southerly
component to the flow exists ahead of surface low/trough now coming
onshore in far southwest lower Michigan. Bulk of heavier
precipitation should be closely associated with forcing near the
surface low as it moves southeastward across northern Indiana and
into Ohio during the afternoon. At the same time, surface
trough/front extending from southwest lower Michigan to near Saginaw
Bay will gradually move southeastward as well passing MBS around
19z; FNT around 20z; and the far southeast Michigan TAF sites around
21z. As precipitation will continue ahead of the trough, but weaken
this afternoon.  Also, expect visibilities to gradually become MVFR,
before going VFR shortly after frontal passage.  Likewise cigs
generally in the low MVFR range will gradually go VFR later this
afternoon and evening.

//DTW THRESHOLD PROBABILITIES...

* High confidence for ceilings at or below 5000 feet this afternoon
  and evening, low for ceilings after 04z tonight.

* High confidence in visibility below 3 miles through mid afternoon.
  High confidence for visibility aoa 6 statute miles aft 00z this
  evening.


&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...Gale Watch from late Monday night through Tuesday evening for
     LHZ361>363-421-441>443-462>464.

     Small Craft Advisory until 7 AM EST Sunday for LHZ421-441>443.

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

$$

DISCUSSION...SC
MARINE.......SC
AVIATION.....DW


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