Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 151512
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1012 AM EST Fri Dec 15 2017

.UPDATE...
As of 1010 AM EST...Current forecast largely remains on track, with
just a few tweaks based on latest trends in obs. Latest water vapor
imagery depicts midlevel impulse nicely now moving into western
Michigan, and this feature will continue to trend eastward across
southeast Michigan through the early/mid afternoon hours. Ahead of
the impulse, associated with a weak clipper passing by to the north
across Lake Superior, large-scale lift and increasing moisture plume
downwind from Lake Michigan has continued to result in periods of
light snow showers and flurries, especially across northern areas but
now starting to fill in across most of the region.

Still watching the potential for snow squall activity this afternoon,
with activity already in full force across western Michigan, with
convective profile showing glaciation in the satellite imagery
depicting enhanced snowfall rates. Latest hi-res guidance has become
a little more bearish with the eastward propagation of these squall
this afternoon into southeast Michigan, as the post-midlevel impulse
environment is not quite as favorable as previously thought. Still,
the potential is certainly there for snow squalls, especially in the
1pm-6pm timeframe, with guidance focused on a corridor primarily
between I-69 and I-94. Will continue to watch closely evolution of
streamline trajectories heading into the early afternoon, as this
will play a critical role in where the possible bands set up. Will
continue with a persistence forecast and not make any significant
changes to the afternoon forecast concerning snow squall potential.

A limiting factor that may also damper snow squall potential is the
boundary layer capping inversion, which increasingly does not appear
to be as deep as previously thought, limiting the depth of available
omega and supersaturation with respect to ice in the DGZ. A plume of
50-100 J/kg of SBCAPE advecting downstream into the corridor
mentioned above, coupled with winds becoming westerly behind the
midlevel impulse (with southwesterly ahead of it) providing a low-
level convergence zone should be enough to override some of the
boundary layer issues noted above.

Snow totals through the day are still expected to generally be under
an inch, but higher amounts are possible in snow squalls as snow
rates will be maximized, especially closer to the US 23 corridor
where the better moisture convergence looks to set up. Gusts up to 30
mph will be possible in the squalls as well, leading to localized
hazardous and treacherous travel conditions with near whiteout
conditions and quickly snow covered roads as well.

&&

.PREV DISCUSSION...
Issued at 548 AM EST Fri Dec 15 2017

AVIATION...

Low pressure system over Lake Superior will bring another period of
active weather across SE MI today and tonight. A swath of light snow
will work across the area this morning with winds turning westerly
and increasing later this morning in its wake. This will bring lake
effect snow showers east from Lake MI which may cause rapid
fluctuations in vsby and cigs. A weak cold front will then drop
through the area turning winds to the northwest this evening pushing
the lake effect bands south of the area. All in all it looks like a
low VFR type day with periods of MVFR or IFR in the heavier showers.
Best chance of the heaviest snow bands will be in the 16-21Z
timeframe. MBS will be in the 16-19Z window with Detroit area
terminals around 18-21Z. Snow showers may linger through the night
across the Detroit terminals as the front slows its progress.

For DTW...Varying conditions through the period and especially this
afternoon as lake effect snow bands or snow squalls set up over
lower MI. Amendments to timing will be made as the bands start to
present themselves, but best timing looks to be from 18-21Z. VSBYs
could drop to IFR or possibly even LIFR in the most intense showers.

//DTW THRESHOLD PROBABILITIES...

* High confidence in cigs aob 5000 ft agl through the period.

* Low for ceilings below 200 ft or visibilities below 1/2SM this
  afternoon in the heaviest snow showers.

PREV DISCUSSION...
Issued at 423 AM EST Fri Dec 15 2017

DISCUSSION...

Some perpetuation of very light snow showers/flurries ongoing early
this morning, the result of renewed eastward penetration of the lake
Michigan moisture plume. This process a response to a period of
backed west/southwest low level flow occurring in advance of an
inbound shortwave now translating across northeast Wisconsin.
Attendant surface trough leading in the main height falls with this
system, with an extensive region of light snow ongoing tied to both
an increase in mid level ascent and low level convergence. This
forcing on track to lift across southeast Michigan during the
daylight period. A noted increase in moisture quality as respectable
synoptic moisture taps the resident lake Michigan moisture plume
will yield a period of deep layer saturation with respect to ice.
This improvement in microphysics under the background of increasing
large scale forcing will subsequently support a fairly widespread
region of light snow/flurries locally. Highest coverage centered
late morning and early afternoon. Accumulation of less than an inch.

Late day transition toward potential lake effect snow showers/
squalls, as large scale support sweeps east and westerly low level
flow deepens. The overlake thermodynamic profile certainly
supportive, yielding a convective depth near 10 kft and upwards of
100 J/KG of cape within the outward projecting lake bands. The
underlying mean low level wind field will favor some degree of
downstream penetration of this lake convection. This setup favors
multi-bands or segments focusing somewhere between the I-69 and I-94
corridors, but with no clear convergence signal to suggest one
particularly will be more susceptible in this case.  Inbound lake
bands likely be quite squally, given expected gusts to 30 mph under
modest late day mixing.  This will lead to brief, localized high
intensity snowfall rates, with potential for a quick dusting to an
inch of additional accumulation.

Weak frontal boundary will remain in the vicinity of southeast
Michigan tonight and Saturday. The combination of weak fgen forcing
and subtle mid level warm air advection will maintain a narrow
ribbon of ascent over or near the region during this time. Mixed
signal across the model spectrum in terms of placement and magnitude
of this forcing, but recent probability guidance suggests at least
modest potential for another period of light snow to develop locally
during this time. Existing boundary will provide a slightly greater
thermal gradient on Saturday, as the resident air mass begins the
slow moderation process under increasing mean thicknesses. Highs
ranging from lower 20s north to lower 30s south.

Pattern of warm air advection will strengthen Saturday night and
Sunday, in advance of a southern stream wave ejecting out of the
plains.  High degree of uncertainty yet in terms of the possible
track and strength of this system, which will ultimately determine
potential coverage/timing and ptype of any associated precipitation.
Forecast will continue to conservatively call for a rain/snow mix at
this stage.  Potential for all locations to inch above the freezing
mark on Sunday.

MARINE...

High pressure is in place across the southern lakes while the next
clipper begins to track across Lake Superior. This clipper will
bring gusty southwesterly winds and elevated waves back to Lake
Huron this afternoon. Though gusts should stay below gale force they
may reach 30 knots this afternoon before turning northwest behind
the departing system this evening. The winds will result in elevated
waves around the tip of the Thumb this afternoon through tonight
necessitating a small craft advisory for a portion of the nearshore
waters. Winds decrease and turn out of the east this weekend as a
high pressure system drifts just north of the Great Lakes.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...Small Craft Advisory from 1 PM this afternoon to 4 AM EST Saturday
     for LHZ421-441.

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

$$

UPDATE.......IRL
AVIATION.....DRK
DISCUSSION...MR
MARINE.......DRK


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