Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 102151
AFDBUF

Area Forecast Discussion
National Weather Service Buffalo NY
451 PM EST Sat Dec 10 2016

.SYNOPSIS...
A flow of cold air will continue to generate areas of lake effect
snow east and southeast of the lakes into tonight before finally
weakening Sunday morning. An area of low pressure will then move
through the Great Lakes region Sunday afternoon and Monday and bring
a widespread general snowfall to the region...which may end as a
little rain or sleet before ending. Bitterly cold air will then build
in behind this system for the latter half of next week with
additional lake effect snow expected.

&&

.NEAR TERM /THROUGH SUNDAY/...
Lake effect snow will continue to be the main concern through Sunday
morning before our attention turns to a synoptic system by Sunday
afternoon. From a synoptic standpoint, mid level northwest flow will
remain in place across the Great Lakes and New England through
Sunday morning and maintain a cold airmass. Meanwhile surface high
pressure will build from the Ohio and Tennessee Valleys to the Mid
Atlantic coast. This will force anti-cyclonic flow in a cold airmass
over Lakes Erie and Ontario with flow slowly backing to the west and
then southwest overnight. Lake instability remains impressive, with
equilibrium levels around 15k feet early this evening dropping to
around 8k feet by early Sunday morning. Outside of lake effect areas
it will remain dry through early Sunday morning.

Off Lake Erie...

Through the rest of the afternoon several bands of moderate to
briefly heavy snow will continue from southern Erie and western
Wyoming counties into the western Southern Tier. This evening
boundary layer flow begins to back more westerly and eventually WSW.
This will re-orient another band of snow currently over NE Ohio, and
will likely merge with our bands of snow to produce one, more well
defined band of snow over Chautauqua County by early evening. This
band of snow will then begin to move steadily northward and reach
Southern Erie county by mid to late evening. Snowfall rates will
reach 2 inches per hour at times from this afternoon through this
evening.

As we go through the overnight, the band of lake effect snow will
continue moving north as boundary layer flow backs to the southwest.
This will move the band farther north into portions of northern Erie
County and the southern half of the Buffalo Metro area overnight. The
low level flow will be anti-cyclonically curved, and past cases
such as this have confined the heavy snow to very close to the
lakeshore. This would target areas from Hamburg and Orchard Park to
Lackawanna and South Buffalo for late tonight. Expect a quick 3-6
inches in these areas. With this in mind, we have added an LES
advisory for Northern Erie County.

Sunday morning the band of snow will continue northward and may
drop another inch or two from Buffalo to Tonawanda and Fort Erie
right along the lakeshore as flow will be weak and rapidly backing
southerly. Any remaining lake effect will move into Canada by mid
morning.

Off Lake Ontario...

An area of persistent snow showers will continue through late
afternoon to the west of Rochester, including the Niagara and
Orleans county shorelines to near Brockport. Another band of snow
will continue from Sodus Bay to Cato. Snowfall rates will remain
fairly light through late afternoon at less than an inch per hour.

Through the evening boundary layer flow will begin to back westerly
and become more anti-cyclonic. This will force most of the snow
from Rochester westward to retreat back over the lake, with snow
ending on land. The eastward extent of this snow will continue to
brush the Wayne and Northern Cayuga shorelines with several inches
of accumulation through the evening, mainly from Sodus Bay eastward.

Overnight into early Sunday morning the situation becomes very
complex. All of the mesoscale model guidance develops a lake
induced meso-low late this evening over central Lake Ontario, then
carries this east to the east end of the lake overnight. On the
south and west corner of this meso-low, some model guidance
including the Canadian GEM, high resolution 3km EMC WRF, and to
some extent the operational NAM develop a more intense single band.
The meso-low will act to enhance low level convergence, such that
heavy snow may develop right along the lakeshore from Oswego to
Mexico Bay overnight into early Sunday morning. These events are
rare and very subtle, but can bring very heavy snowfall rates
within a few miles of the lakeshore in a cold, anti-cyclonically
curved flow regime.

With the above in mind, have upgraded Oswego county to an LES
warning mainly for tonight and early Sunday, with local amounts of 7-
10 inches possible right along the Lake Ontario shore. There is
some uncertainty as to exactly where this will setup given the
presence of the meso-low, so it is not out of the question that
heavier amounts may also effect areas near Fair Haven or far
southern Jefferson County. For now will leave advisories in place
for those areas.

What remains of the Lake Ontario band will tea kettle over the
eastern end of the lake Sunday morning, and may produce a few more
inches from Mexico Bay to Cape Vincent before lifting into Canada
by late morning.

Right on the heals of the lake effect snow event, warm advection
will overspread the area from southwest to northeast on Sunday.
This will bring widespread light snow, starting in the morning
across the western Southern Tier, then reaching the Niagara
Frontier and western Finger Lakes by early afternoon, and Central
NY by late afternoon. Accumulations will be light through late
afternoon, with an inch or less in most places and 1-2 inches
across the western Southern Tier. More on this event in the short
term section below.

&&

.SHORT TERM /SUNDAY NIGHT THROUGH TUESDAY/...
Sunday night a widespread light to moderate snowfall will overspread
the region, brining a few to several inches of snow accumulation to
the entire region. While snow will be tapering off from west to east
by sunrise Monday, snow accumulations from overnight may result in
slippery travel conditions for the Monday morning commute.

Digging into the details, a shortwave trough moving into northwest
Montana late this afternoon will track along the established
baroclinic zone across the northern tier of the US and across the
Great Lakes Sunday night. The upper level wave and surface low
reflection are expected to track just north of Lake Ontario during
the day Monday. This is not an ideal low track for a significant
widespread snow for the region, as the best forcing and dynamics
with the system will pass across southern Ontario province. Instead,
the forecast area will benefit from increasing warm advection along
the baroclinic zone by Sunday afternoon, which will peak Sunday
night as we end up near the nose of the low level jet and enhance
area of moisture advection. This will be sufficient to produce a
roughly 12 hour period of snowfall, with the heaviest rates around
1/2 inch per hour through the middle of the night. Latest guidance
continues to keep the above freezing air aloft south of the forecast
area, across central PA, and thus this will remain almost entirely a
snow event for the forecast area. The areas most likely to see 4 to
7 inches of snow accumulation, sufficient for a winter weather
advisory are across the higher terrain of the western Southern Tier
and across the North Country. Otherwise, anticipate the southerly
low-level jet will downslope enough north of the higher terrain
(south of Lake Ontario) and along the northern Finger Lakes to
erode snowfall amounts, keeping accumulations in the 2 to 4 inch
range. No matter the total accumulation, every one should be
prepared for a potentially slipper Monday morning commute, and allow
extra time to get to work.

Monday we`ll lose the moisture within the snow growth zone as a
wedge of drier air aloft follows the cold front. Point soundings
suggest we`ll remain nearly saturated in the lower levels near the
surface through the day. This will likely bring some drizzle to the
area, and possibly light snow. Temperatures will likely be too warm
for lake effect early Monday morning, though some cooling through
the day may bring a little lake effect snow late in the day, this to
follow the light rain/drizzle/orographic lift precipitation earlier
in the day.

Behind the cold front Monday, winds will become gusty. Through the
day Monday the surface low will remain steady state- averaging
around 1010 mb and not overly deep. Mixing behind the cold front
will likely bring winds gusting within the 30-40 mph range across
the lake plains Monday and into early Monday evening.

Later Monday and Monday night cold air advection will lower the
dentritic snow growth layer, such that it will now cool to areas
that are with more moisture, leading to likely a band of lake effect
snow east of Lake Ontario, but still questionable moisture east of
Lake Erie, which may continue to produce light snow showers or
drizzle. Lapse rates will not be as great as with prior lake effect
events this season, with temperatures at 850 hPa dipping to around -
8 to -10C at 850 hPa...this over lake waters that are now cooling to
around +5C. Expect bands to remain weak...and for now will have just
a couple inches in the forecast, with the higher totals east of Lake
Ontario.

Temperatures Monday and Tuesday will likely climb above the freezing
mark. Overnight lows will drop below freezing, with the coldest
night Tuesday night where temperatures will drop into the teens to
lower 20s.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/...
As was advertised nearly a week ago...the displacement of the ever-
present polar vortex will send our temperatures to well below normal
levels for the bulk of this period. Not only will the mercury
average some 10 to 15 deg f below normal mid December values...but
the mid winter cold will also support frequent lake snows in the
traditional snow belts east and southeast of the lakes.

The arctic gyre...which had been oscillating around the Pole for the
past six weeks...will continue to settle southward through Thursday.
This equatorward migration will team up with an Alaskan ridge during
the start of this forecast period to establish a cross polar flow...
one that will at least temporarily drain Siberia of the bitter cold
air that it has been hoarding since the start of the cold weather
season. In fact...for a short time the coldest air in the Northern
Hemisphere will end up on this side of the Pole over the northern
Canadian prairies. The good news for us is that the core of this
glacial airmass will remain north of our forecast area...and
generally poleward of 50N. That is certainly not to say that we will
not get a taste of the cold though...as H925 temps are forecast to
bottom out in the vcnty of -18 to -20c. Another bit of good news is
that without a downstream Greenland block...the deep mid level
vortex will sweep through the base of the trough and eventually exit
off the Canadian maritimes. As far as the day to day details...

The much discussed mid level vortex will steadily push southeast
Tuesday night and Wednesday to about James Bay...while high pressure
at the surface will extend from the Central Plains to the Ohio
Valley. This scenario will support a deep cyclonic flow over the
Lower Great lakes where H85 temps will lower from about -12c to -16c
by Wednesday evening. While this will support ample lake induced
instability...the synoptic environment will be relatively dry. This
should preclude a significant lake response with less impressive
lake snows being directed across the Southern Tier and over sites
near and just north of the Tug Hill. Elsewhere...it will be partly
to mostly cloudy with Wednesday afternoon temps in the 20s f.

As the large mid level vortex crosses from James Bay to central
Quebec Wednesday night and Thursday...the steering flow over the
Lower Great Lakes will veer to about 280-290 deg. This will push the
lake snows a little further south. While the Lake Erie activity is
still not expected to be overly impressive during this 24 hour
period...there could be a little more synoptic moisture to accompany
the long fetch so that the snows near and just south of the the Tug
Hill could be more impactful. Otherwise...this should be the coldest
day of the forecast period with afternoon temps only forecast to be
in the teens away from the lakes.

Ridging over the northern plains will then amplify and build across
the Great Lakes region Thursday night through Friday night as the
Polar vortex will exit the Canadian mainland. While this will
promote a gradual modification of the cold air...lake snows will
persist within a 290-300 flow. In fact...multi-lake upstream priming
at this time could enhance the lake snows...especially off Lake
Ontario. Temperatures in many areas on Friday should climb back
above 20f.

Finally for Saturday...a complex frontal system is expected to cross
the region with some widespread snow anticipated. Given the lack of
significant surface development...snowfall from this vantage point
is not expected to be significant. Temperatures on Saturday ahead of
the front should climb back towards the freezing mark.

&&

.AVIATION /22Z SATURDAY THROUGH THURSDAY/...
Outside of lake effect snows, expect mainly VFR conditions. A WNW
flow will result in the most persistent snows across the Western
Southern Tier and points along the south shore of Lake Ontario.
Expect frequent IFR conditions within the bands of lake effect
snow, with KJHW the only terminal significantly impacted. At KROC,
most of the snow will remain just north along the lakeshore,
although occasional brief snow showers may bring a few periods of
IFR. Wind direction becomes more westerly and eventually
southwesterly tonight, which will carry lake effect snow northward
off both lakes. This will carry the Lake Erie snow northward
towards KBUF and away form KJHW overnight. The Lake Ontario snow
will reach KART for a few hours Sunday morning. The lake effect
snow will finally end by mid to late morning Sunday.

A general light snow will then develop Sunday morning across the
western Southern Tier, spreading northeast across the rest of the
area during the afternoon. Expect widespread IFR VSBY to develop
with the onset of light snow, along with MVFR CIGS.

Outlook...
Sunday night...Widespread IFR in light snow.
Monday...improving to MVFR with leftover light snow or drizzle.
Tuesday...MVFR with a chance of snow showers.
Wednesday and Thursday...VFR/MVFR with a chance of snow showers, IFR
east of the lakes in lake effect snow.

&&

.MARINE...
A moderate westerly flow will continue to require SCA headlines
through tonight for Lake Erie. Winds will briefly diminish on Lake
Ontario, but then slowly pick up again from west to east this
evening. Winds may not ever pick up on far eastern portions of Lake
Ontario, but westerly winds on the rest of the lake should be ample
to build waves and eventually require small craft headlines for all
of the Lake Ontario nearshores.

Winds drop off on Sunday between systems, then increase ahead of the
next system Sunday night when more small craft headlines may be
needed.

&&

.BUF WATCHES/WARNINGS/ADVISORIES...
NY...Lake Effect Snow Advisory from 1 AM to 9 AM EST Sunday for
     NYZ007.
     Winter Weather Advisory from 4 PM Sunday to 10 AM EST Monday
     for NYZ005>008.
     Lake Effect Snow Warning until 9 AM EST Sunday for NYZ006.
     Lake Effect Snow Advisory until 4 AM EST Sunday for NYZ004-005.
     Lake Effect Snow Advisory from 11 PM this evening to 9 AM EST
     Sunday for NYZ010.
     Winter Weather Advisory from 1 PM Sunday to 7 AM EST Monday
     for NYZ019>021.
     Lake Effect Snow Warning until 10 PM EST this evening for
     NYZ019-020.
     Lake Effect Snow Warning until 1 AM EST Sunday for NYZ085.
     Lake Effect Snow Advisory until 1 AM EST Sunday for NYZ012.
     Lake Effect Snow Advisory until 10 PM EST this evening for
     NYZ021.
MARINE...Small Craft Advisory until 7 AM EST Sunday for LEZ040-041.
         Small Craft Advisory until 7 AM EST Sunday for
         LOZ043>045.
         Small Craft Advisory until midnight EST tonight for LOZ042.

&&

$$

SYNOPSIS...HITCHCOCK
NEAR TERM...HITCHCOCK
SHORT TERM...CHURCH
LONG TERM...RSH
AVIATION...HITCHCOCK
MARINE...HITCHCOCK


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