Area Forecast Discussion
Issued by NWS Gaylord, MI

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FXUS63 KAPX 140446

Area Forecast Discussion
National Weather Service Gaylord MI
1146 PM EST Wed Dec 13 2017

Issued at 954 PM EST Wed Dec 13 2017

Went ahead and cancelled the remaining winter weather advisories
as the snowfall has significantly tapered off as much drier air
has filtered in on winds backing more northerly. Focus now turns
to 1000-850mb NE flow off Lake Huron which is bringing generally
light snow showers into NE lower and interior nrn lower due to
upslope. Other snows were seen working across Leelanau county.
These snows showers will eventually turn directly out of the
north, then NW into Thursday morning. Although light, the concern
is for earlier reports of the snowflakes being dendrite, large
dendrites that resulted in significant snowfall earlier today.
Saint Ignace with 3 inches, Mackinac Island with 5 inches. Despite
quite shallow inversion heights of 4kft or less, the cloud layer
lies within the dendritic growth zone for continued dendrites and
easily stackable fluff. Already increased snowfall amounts
earlier, and am leaning toward increasing totals to 1 to 2 inches
(we`ve seen this before, could be higher and wouldn`t be
surprised). Actual QPF may not top 0.01, but snow ratios in these
situations can be astronomical. Otherwise, as flow turns out of
the north, skies clear in the interior. Eastern upper is already
clear, per downslope over land flow out of Ontario.


.NEAR TERM...(Tonight through Thursday)
Issued at 315 PM EST Wed Dec 13 2017

...Widespread snow end quickly this evening...

High impact weather potential...Lingering accumulating snow for the
remainder of this afternoon and early evening.

Pattern synopsis/forecast: True Alberta clipper racing southeast
into southern Lake Michigan this afternoon, expected to reach
northern Ohio by early this evening. Broad area of snow to the north
of this low within isentropic upglide/deformation zone, with
transient enhanced banding structures driven by localized fgen
dynamics. Heaviest snows have targeted areas along and south of M-
72, especially in the Traverse City/Houghton Lake/Manistee corridor
where several inches of snow has already been reported. Further
north, continuous low level drying and removal of better dynamics
have kept snow amounts light, with most of eastern upper remaining

In true clipper fashion, this system will race southeast, ending the
widespread snow threat in the same fashion as it does so. Ridge of
high pressure arrives overnight into Thursday, relegating any
additional snows to the light lake effect variety near the big
waters, while drying out the rest of the region.

Primary forecast concerns/challenges: Ending of accumulating snow
this evening.

Details: Widespread snow will continue to steadily push southeast,
with the focus for heavier snows trending more south with time as
fgen/deformation forcing does the same. Heaviest snows, on the order
of a few more inches, will center along and south of the M-55
corridor, with amounts tapering off rather quickly as one goes
north. Earlier issued advisories will remain, with 00z end time
looking to coincide nicely with the departure of the heaviest snows.
Atmosphere aggressively drys out as that high builds in overnight,
but impressive shallow instability more than enough to kick off some
light lake effect snow showers in favored northeast to north flow
areas along the Lake Michigan and Lake Huron coasts. Shouldn`t be
too big a deal, and expecting any lake snow accumulation to remain
under an inch. Expect a bit of clearing across inland areas
(especially across eastern upper Michigan), which will help
temperatures drop back into the single digits, perhaps even a few
degrees below zero for areas north of the big bridge.

Light northerly flow lake effect continues into early Thursday
morning. That begins to change heading through the second half of
the morning and afternoon as departure of high pressure and approach
of next weak shortwave trough backs winds quickly to a westerly
direction. This will help punt those snow showers further inland,
although again with only minimal accumulation. A bit less cold than
the last few days, but still well below normal with highs Thursday
afternoon in the upper teens to middle 20s.


.SHORT TERM...(Thursday night through Saturday)
Issued at 315 PM EST Wed Dec 13 2017

High Impact Weather Potential...Heavy lake effect snowfall possible
late Friday.

Pattern Synopsis/Forecast...Upper low drops into the eastern
US/Canada trough through the Great Lakes Friday night. Surface low
precedes the upper low tracking through the northern lakes Friday
with some heavy lake effect snow conditions possible Friday night.
High pressure moves in for Saturday along with a fast moving upper

Primary Forecast Concerns...Clipper-like system moves across Lake
Superior Friday with lake/air deltaT`s around 20C so any synoptic
snow will be lake-enhanced. Fast moving system but has descent
amount of moisture below 500mb in W-NW low level flow early on.
Concern is late Friday when shortwave energy from upper low crosses
the region. Increase in lift with inversion heights around 10kft in
NW-NNW low level flow, looks like heavy snow is a good bet across
the usual parts of eastern Upper and northern Lower. Problem is that
low level flow is on the light side and does not get locked in for
long duration. Surface high moves in quickly Saturday morning
dropping inversion heights and disrupting low level wind

.LONG TERM...(Saturday night through Wednesday)
Issued at 315 PM EST Wed Dec 13 2017

High Impact Weather Potential: Perhaps accumulating lake snowfall by
Wednesday of next week.

Shift in polarity in the PNA/EPO teleconnection indices...especially
in the PNA from strongly positive to negative during the extended
period (EPO trending negative with time).  The upshot of this is a
breakdown of the current strong ridging over western North America
as troughing over the eastern Pacific gets kicked inland into the
intermountain west this weekend (the downward trend in the PNA).
This should provide a brief respite from the influx of cold air into
the Great Lakes as upper flow becomes more southwesterly...but
renewed height falls/troughing on or around the 19th with ridging
into Alaska (the downward trend in the EPO).

Looking at weak anticyclonic flow/short wave ridging over the Great
Lakes Sunday with some low level warm advection.  Consensus forecast
focusing PoPs across Upper Michigan/tip of the mitt Saturday night
while guidance mass fields would suggest more of a Lower Michigan
focus.  At the very least not comfortable with having no precip
mention across much of northern Lower Saturday night into Sunday.
Energy getting dumped into the desert southwest/northern Mexico this
weekend is a forecast issue early next week in terms of how quickly
this short wave troughing lifts out...preference is toward the
slower side (more in line with the North American guidance). like the idea of turning colder again or around the
19th (Tuesday)...with lake effect snows returning for midweek.


.AVIATION...(For the 06Z TAFS through 06Z Thursday night)
Issued at 1140 PM EST Wed Dec 13 2017

Drier air will continue to filter in via northerly winds.
Indications are that SCT-BKN MVFR CIGS and light snow/flurries
will sporadically impact the airports for awhile through Thursday
as winds continue to back around to the west over this time. A
shortwave trough arrives late in the TAF period with deeper
moisture to potentially enhance the snow, but just some light snow
is all that is expected. Maybe 1 to 3 inches through the night.
BKN MVFR/low end VFR deck expected Thursday, becoming OVC MVFR as
the prevailing sky condition for Thursday night.


Issued at 315 PM EST Wed Dec 13 2017

Another round of small craft advisories needed for much of
Lake Michigan and Lake Huron as gusty north to northeast winds
develop behind departing southern Lakes low pressure. Winds back
quickly westerly Thursday, decreasing in speed some in the process.
Winds/waves will go below advisory criteria on Lake Michigan, with
some lingering gusty conditions expected on Lake Huron through much
of the day.


LH...SMALL CRAFT ADVISORY until 5 PM EST Thursday for LHZ347>349.
LM...SMALL CRAFT ADVISORY until 5 AM EST Thursday for LMZ323-342-


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