Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 100041

Area Forecast Discussion
National Weather Service Buffalo NY
741 PM EST Fri Dec 9 2016

A west-northwesterly to westerly flow of cold air will continue to
generate areas of lake effect snow east and southeast of the lakes
through Saturday. An area of low pressure will then move through
the Great Lakes region Sunday and Monday and bring a widespread
general snowfall to the region...which may mix with a little rain
or sleet before ending.


During this period...a general west-northwesterly flow of cold air
will continue through much of tonight...before turning a little more
westerly late tonight and Saturday. This will continue to generate
areas of lake effect snow downwind of the lakes...with the activity
generally tending to shift a little further north later tonight and
Saturday as winds become a little more westerly.

Off Lake Erie...the lake snows are largely confined to Chautauqua and
Cattaraugus counties at the moment...and continue to take advantage
of an upstream connection to southern Lake Huron. Expect these to
largely remain in their current position into the early overnight
hours...before edging a bit further northward as winds back a bit
during the remainder of the night and Saturday. Expect the bands to
produce up to another foot across the Southern Tier tonight and
another 3-6 inches during Saturday...for which Lake Effect Snow
Warnings remain in effect as outlined below. As the bands migrate
a little to the north...some of the snows will leak back into far
southern portions of Southern Erie County as well...which may require
the issuance of another Lake Snow Advisory for this latter area
at some point tonight.

Meanwhile off Lake Ontario...somewhat more disorganized lake snows
extend from Orleans county to Oswego county...and inland into portions
of the Finger Lakes Region as of this writing. With the loss of diurnal
influences and a potential connection to Lake Huron redeveloping...
these should tend to reorganize through the evening and once again
focus from Monroe county to southern Oswego county...where another
5 to 10 inches will be possible in the most persistent snows tonight.
Late tonight and Saturday...a temporary weakening of the wind field
in conjunction with the aforementioned backing flow should cause this
band to weaken and at least partially retract back over Lake Ontario
for a time...though some additional accumulations of at least 2-4
inches will still be possible over the aforementioned area. For now...
lake snow headlines for areas downwind of Lake Ontario remain in
effect as outlined below.

As for temperatures...these will drop into the teens and low 20s
overnight...with single digits found in areas of clearing skies and
fresh snow across the North Country. Highs on Saturday will rise to
the mid 20s to lower 30s.


Lingering lake effect snow will continue east of the lakes Saturday
night, and will slowly lift northward toward Buffalo and Watertown
by Sunday morning while weakening and decreasing in inland extent.
While a few more inches of snow accumulation are expect across the
western Southern Tier and Tug Hill region, the weakening snow bands
may only result in an inch or two of snow across the Buffalo and
Water town areas Sunday morning.

Our attention then turns to the widespread snowfall event that is
setting up for Sunday night, which may bring some slow downs to
morning commutes across the region Monday morning. This weather
system evolves out of the low pressure system currently coming on
shore in the Pacific Northwest. The shortwave trough, and
accompanying surface low reflection, will track along the baroclinic
zone across the Great Lakes Sunday night and Monday. Ahead of the
wave and surface low, widespread snowfall will develop in the warm
air advection and along the band of frontogensis, with the region
also in a favorable jet-quadrant for enhanced lift. Model consensus
is in good agreement, supporting 100 PoPs Sunday night.

Model guidance is improving with the track of this system, with more
guidance now pointing to keeping the above freezing air aloft south
of the forecast area, across PA. This means there is a decreasing
chance of a wintry mix of precipitation Sunday night into Monday,
with most of the precipitation falling as all snow Sunday night. With
the track of the surface low still just north and west of the
forecast area, we will see warmer surface temperatures pushing into
the mid 30s during the day Monday. Precipitation will also end from
west to east Monday morning through mid-day as mid and upper level
dry air moves in across the area, which will result in snow ending
and changing over to a light drizzle on Monday. The the increasing
likelihood of all snow Sunday night, snowfall amounts will likely
end up warranting an advisory, with most locations picking up 4 to 6


The active and very wintry pattern will continue through the long
term, and even intensify by late next week. Looking at the longwave
pattern, a sharp upper level ridge will build over Alaska early next
week and allow for a period of cross polar flow, with Siberian air
making the trip across the Arctic and into northern Canada. The
Alaskan ridge will remain in place through much of the week and
force a deep longwave trough to develop over central and eastern
North America. A large piece of true arctic air will move south
through the Canadian Prairies and reach the Great Lakes and New
England later next week. This may set the stage for another round of
significant lake effect snow for Thursday and Friday. As the cold
air comes in, another baroclinic wave may bring some widespread
general snow Wednesday.

Looking at the details, on Tuesday and Tuesday night a weak trough
will pass through the Great Lakes and may produce a few light snow
showers of little consequence. There may also be some limited lake
effect snow east of the lakes, but marginal instability will keep
this light.

On Wednesday the longwave trough will begin to dig into the Northern
Plains and Upper Great Lakes. Ahead of the trough, medium range
model guidance develops a baroclinic wave over the Ohio Valley and
Northeast. The GFS and most of it`s ensemble members along with the
Canadian GEM are more aggressive with this, and would produce a
general light to moderate snowfall across the entire area. The 12Z
ECMWF is much weaker with this feature. The ECMWF has shown a good
deal of inconsistency with this system, as previous runs looked more
similar to the current 12Z GFS. With this in mind, went closer to
GFS based guidance with snow likely for Wednesday.

In the wake of this system cold air will flood into the Great Lakes
Thursday and Friday. This will be the coldest air of the season,
with 850mb temps plummeting to -20C or colder. This will translate
into highs in the teens and lows in the single numbers away from the
immediate lakeshores. The deep cold air crossing the lakes and the
overall pattern is suggestive of more significant lake effect snow
downwind of the lakes for Thursday and Friday, however there are
significant differences in wind direction, not to be unexpected
given this is still 6-7 days out.


Through the TAF period...a general west-northwesterly flow of cold
air will continue through much of tonight...before turning a little
more westerly late tonight and Saturday. This will continue to generate
areas of lake effect snow and attendant LIFR to IFR conditions southeast
and east of the lakes...with these generally tending to shift a little
further north later tonight and Saturday as winds back. Off Lake Erie
the lake snows will impact the KJHW terminal...while off Lake Ontario
the bulk of the lake snows will affect areas from the vicinity of KROC
to KFZY. Outside of the lake snows...general VFR conditions will be the
rule through the TAF period...with just a chance of MVFR in passing
scattered light snow showers.

Saturday night...Lake snows and attendant areas of IFR/MVFR weakening
and lifting northward across areas east and northeast of the lakes...
with VFR conditions predominating elsewhere.
Sunday...Deterioration to MVFR/IFR with widespread light snow developing.
Sunday night and Monday...MVFR/IFR in widespread light snow.
Tuesday...MVFR with a chance of snow showers.
Wednesday...MVFR/IFR with snow likely.


Across Lake Ontario...winds should tend to diminish from west to
east tonight before increasing again for a time across the western
half of the lake Saturday afternoon and evening. While this should
allow the current advisories for that lake to be dropped as tonight
progresses...additional advisories may become necessary for areas
from Sodus Bay westward during Saturday.

On Lake Erie...a stronger west-northwesterly to westerly flow will
persist through Saturday night...necessitating a continued SCA
through that time frame.


NY...Lake Effect Snow Warning until 6 AM EST Saturday for
     Lake Effect Snow Advisory until 6 AM EST Saturday for NYZ002.
     Lake Effect Snow Warning until 6 PM EST Saturday for NYZ019-
MARINE...Small Craft Advisory until 7 AM EST Sunday for LEZ040-041.
         Small Craft Advisory until 4 AM EST Saturday for
         Small Craft Advisory until 10 PM EST this evening for



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