Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 130438
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1138 PM EST Mon Nov 12 2018


.AVIATION...

Snow will rapidly diminish through 07 as forcing lifts north and
east of the area. Additional accumulation no greater than one half
inch. Cold advection will quickly improve vsbys and IFR cigs with
prevailing MVFR likely improving to low VFR during the predawn
hours. Redevelopment of VFR straocu will then be accompanied by
stout northwest wind around 12-15kts with gusts upwards of 20 kts
possible as the boundary layer deepens during peak heating.
Potential for stratocu to erode by late Tuesday evening as
increasingly anticyclonic flow builds in from the west.

.DTW THRESHOLD PROBABILITIES...

* High for cigs aob 5kft.

* High for ptype of snow.

&&

.PREV DISCUSSION...
Issued at 958 PM EST Mon Nov 12 2018

UPDATE...

Short-lived residence time of right entrance dynamics will be
sufficient to produce a few hours of respectable mid-level fgen,
mostly due to tilting, centered in the 700-500mb layer per recent
cross-section analysis. Light snow is currently spreading into the
area from the southeast yielding vsbys between 1 and 2 miles and is
oriented along this axis of ascent. Supersaturation with respect to
ice in the favored section for dendrite growth centered within a
respectably deep vertical motion field atop a rather thin riming
layer should be sufficient to yield event average snow ratios of 10
to 12:1 with up to 15:1 briefly possible during peak intensity
before midnight, highest along the glacial ridge where overlap of
quality moisture and shallowness of riming layer will be most
favorable. Dry northwest flow will be an inhibiting factor to the
northwest while the Detroit area loses some initial QPF contribution
to snow due to diabatic cooling. Updated forecast totals around 1
inch between Flint and Detroit with up to 2 inches along the terrain,
including the northwestern suburbs of Detroit - a mostly cosmetic
update.

PREV DISCUSSION...
Issued at 357 PM EST Mon Nov 12 2018

DISCUSSION...

A band of snow continues to organize over Missouri and southern
Illinois during mid afternoon and is expected to develop into Lower
Michigan tonight. Total snowfall is held in the 1-2 inch range while
the footprint of that accumulation is broadened slightly northward.
Ending time just before the morning peak travel time still looks
good.

Mid afternoon surface analyses indicate the weak cold front or
prefrontal trough extending over southern Ontario into the Ohio
Valley. The reinforcing surge of colder air/primary front also
continues to make up ground from the upper Midwest on the way to
merging with the prefrontal trough over the eastern Great Lakes
during the evening. This frontal progression provides a path for a
band of snow to move from the mid Mississippi valley into Lower
Michigan tonight. Not far behind the surge of colder air in the low
levels is the northern stream upper trough advancing across the
upper Midwest. This feature is steepening the height gradient across
the Great Lakes as the southern stream upper jet moves downstream of
the long wave trough axis. The result will be a strengthening of the
jet which produces enhancement of mid level frontogenesis and
corresponding ageostrophic vertical motion response over the central
Great Lakes. This is expected to continue powering the band of snow,
which already looks respectable in upstream observations, as it
migrates northward from the mid Mississippi Valley into Lower
Michigan tonight.

As mentioned above, the QPF axis is broadened a bit farther north
based on the latest model guidance, however without subtracting from
areas near the Ohio border just yet. This is due to uncertainty on
the precise zone of activation within the elevated front. It is also
possible that bands of heavier snow will waver in a north to south
fashion and appear striated on radar due to frontal activation at
different times up and down the frontal slope. This areal and
temporal uncertainty also supports holding total snowfall
accumulation in the 1-2 inch range in the absence of greater
persistence of stronger bands. Outside of persistence, the speed of
motion of the pattern is also expected to be a limiting factor for
total accumulation while the stability profile and moisture supply,
along with presence of mesoscale forcing and favorable microphysics,
all remain solid within the mid level frontal zone. A brief period
of lake enhancement is also possible as the wind increases from the
northeast across southern Lake Huron and then backs toward the
northwest as the system exits eastward prior to sunrise Tuesday.
Expect this timing to bring an end to the snow before the morning
peak travel period.

A broad area of high pressure builds into the Great Lakes Tuesday and
maintains dry weather and well below normal temperatures through mid
week. A compact, closed southern stream circulation will then lift
out of the lower Mississippi Valley Thursday interacting with
northern stream energy diving southeast from the upper Midwest. These
two features will phase and result in east coast cyclogenesis for
the end of the work week. There continues to remain some uncertainty
regarding the timing of the phasing, with recent model trends
suggesting a slightly slower solution, which would bring
precipitation across much of southeast Michigan Thursday night on the
northwest flank of the system. Diurnal timing will drive ptype, but
light snow accumulations certainly look to be in play Thursday night,
especially south and east of Flint. Northwest flow in the wake of
the departing east coast low heading into the weekend and early next
week will lead to the potential for lake effect activity. This
activity may possibly be enhanced to a degree by additional pieces of
northern stream energy moving through. Temperatures will continue to
run well below normal through the weekend, and be more reminiscent
of mid-December than mid-November.

MARINE...

Cyclogenesis will occur along the east coast of the United States
tonight and Tuesday. Cold advection over the Great Lakes and
organized gradient wind over the long fetch will support an increase
in north winds tonight and Tuesday. The likelihood for gusts in the
25 to 30 knot range will be high, especially Tuesday afternoon
through Tuesday night. A Small Craft Advisory is now in effect
tonight and Tuesday for the nearshore waters of Lake Huron from
Port Austin to Port Sanilac.  Winds will weaken on Wednesday with
high pressure expanding across the Ohio Valley. The direction will
back to the southwest by Wednesday night, with a southwest gradient
increasing into Thursday.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...Small Craft Advisory until 4 AM EST Wednesday for LHZ441-442.

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

$$

AVIATION.....JVC
UPDATE.......JVC
DISCUSSION...BT/IRL
MARINE.......CB


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