Area Forecast Discussion
Issued by NWS Buffalo, NY

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000
FXUS61 KBUF 110948
AFDBUF

Area Forecast Discussion
National Weather Service Buffalo NY
448 AM EST Sun Dec 11 2016

.SYNOPSIS...
Lake effect snow will linger across the Eastern Lake Ontario Region
before tapering off late this morning. Then low pressure will move
through the Great Lakes region this afternoon through Monday
bringing a widespread general snowfall...which may mix with a little
rain or sleet before ending later tonight and Monday. Colder air
will then build in behind this system during the latter half of next
week...with additional lake effect snow expected downwind of the
lakes.

&&

.NEAR TERM /THROUGH TONIGHT/...
There are two forecast concerns during the period, with lingering
lake effect snows to be immediately followed by a synoptic event.
Radar shows a healthy lake effect band extending from Lake Ontario
into Oswego county early this morning. This band is producing 1-2
inches of snow an hour, with early morning spotter reports
suggesting at least 7 inches of snow has fallen with localized
amounts likely to reach a foot near Oswego. Mesoscale guidance has
handled this meso-low very well, with the heaviest snow focused just
south of it. The modest 15-20kt 925mb flow which has been pushing
snow onshore is forecast to back from W-SSW through early afternoon.
This will push the snow north of Oswego, while consolidating even
closer to the lakeshore as it approaches Jefferson county. No
changes to headlines here, noting that the advisory amounts in
Jefferson county should be confined to extreme southwestern
portions of the county.

The Lake Erie band has weakened considerably as equilibrium levels
lower to around 8k ft. A band of convergence across the lake will
lift north, and perhaps become a bit better organized as it lifts
north toward the city of Buffalo. Following both radar trends and
latest mesoscale guidance it appears any additional amounts will be
sub-advisory. Based on this, will drop all lake effect headlines of
Lake Erie. This said, there still may be some light lake effect snow
in the Buffalo metro area, but only amounting to an inch or two.

There will be virtually no break between the events, as a broad
area of weak mid-level warm air advection enters the region this
afternoon. Snow with this should be quite light initially, but
will pick up in intensity this evening as the upper level trough
moves through and helps enhance lift. Precipitation should taper
off from west to east late tonight as the trough exits into New
England. It will be cold enough for all snow in most areas,
however BUFKIT sounding show a potential for sleet or rain mixing
in across the Western Southern Tier late tonight.

Consensus QPF is modest, generally a quarter to half inch with
the variance in this mainly due to upsloping and downsloping in
the southerly flow. This seems reasonable, however mesoscale
guidance may be overdoing the impact of the upsloping a bit with
the forecast a consensus of all guidance given the synoptic
nature of the system. Amounts may struggle to reach advisory
criteria across downslope regions, while other areas should easily
reach criteria. Taking this into consideration, the amounts should
run in the 3 to 5 inch range for most of the area, supporting a
winter weather advisory for the entire cwa. Totals will be a bit
more in Oswego and Lewis counties, especially the south face of
the Tug Hill which may get 7 inches or so due to upsloping. Again,
this will be a synoptic event so it is unlikely that any areas
will escape without getting at least some measurable snow from
this system.

&&

.SHORT TERM /MONDAY THROUGH TUESDAY/...
The feature of our next weather system, a shortwave, is now bearing
down upon the Dakotas this early Sunday morning. This feature will
advance from the Central Great Lakes Monday morning to southern
Quebec Monday evening, while sliding a surface low across Southern
Canada. A cold front from this low will cross the region Monday.
Ahead of the front synoptic snow/ and a little rain will be
advancing across the northern Finger Lakes and Eastern Lake Ontario
region. Mid level drier air behind the cold front will greatly ease
the snow potential across WNY through the day. Point soundings
suggest we`ll remain nearly saturated in the lower levels near the
surface through the day though. 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. 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 bands of lake effect snow
east of both lakes. Moisture profiles are more conducive for snow
east of Lake Ontario, and will carry higher pops here than off Lake
Erie. 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 few inches in the forecast east of Lake Ontario.

Tuesday moisture will surge ahead of a mid level trough, with such
moisture passing across mainly Lake Ontario, but also brushing by
Lake Erie. This increase in synoptic moisture, combined with cold
air advection will increase the potential for lake effect snow.
Winds will back to more of a southwest flow Tuesday ahead of a
surface cold front. This will bring lake snows off Lake Erie more
towards metro Buffalo, and later Watertown off Lake Ontario. These
bands will be shifting, such that overall snow totals will be held
at bay, and overall we are not looking at anything significant
Tuesday, with just a few inches of snow possible, and maybe slightly
greater east of Lake Ontario on the Tug Hill where bands may linger
longer. Snows that do fall will likely end through the night as
drier air flows through the snow dendritic growth zone, limiting the
snow potential. Tuesday night an initial cold front will pass across
the region starting the flow of much cold air from the northwest.

Temperatures Monday will be slightly above mid December norms, with
highs ranging through the 30s. Tuesday will be slightly cooler than
Monday, with Tuesday night the colder of the two nights with a
colder airmass reaching the Eastern Great Lakes region.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/...
The southern 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 short lived 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 longwave trough
and eventually exit off the Canadian maritimes. As far as the day to
day details go...

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. Thursday may also become windy,
with a possible surface low forming on a lake aggregate surface
trough over the warmer Great Lakes, and building surface high over
the western Ohio Valley increasing the pressure gradient over the
eastern Great Lakes.

Thursday night may be the coldest night with this arctic airmass,
with many areas dropping down into the single digits for lows, with
sub-zero readings possible across the traditionally colder spots of
the Southern Tier and North Country. The wild card will be lake
effect snow/clouds with this limited the cooling overnight, possibly
remaining lows in the lower teens/around 10F for the night. A still
gusty wind will lower apparent temperatures to below zero across the
region.

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 synoptic snow anticipated. Given the
lack of significant surface development of any waves...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 /10Z SUNDAY THROUGH THURSDAY/...
Lake effect snow lingering along the immediate lakeshores will
lift northward through this morning. It now appears that these
bands will only clip the BUF/ART terminals, so the latest TAFs
reflect this slightly more optimistic trend. Otherwise, expect
VFR conditions this morning away from the lake snows outside of
the Southern Tier where lower clouds from the next system have
already started to move in.

Weak low pressure will bring a widespread, but generally light
snowfall to the region. Generally expect this to drop vsby to 1-2
miles as light snow gradually spreads from west to east this
afternoon. Snow may mix with sleet or rain late tonight, but for
the most part it should be all snow. Conditions will lower late
tonight as low level moisture increases as the low passes by.

Outlook...
Monday...Improving to MVFR with leftover light snow/rain/drizzle.
Tuesday...MVFR with a chance of snow showers.
Wednesday and Thursday...VFR/MVFR with a chance of snow
showers...with IFR east of the lakes in lake effect snow.

&&

.MARINE...
Winds associated with the meso-low on Lake Ontario are localized,
and probably not enough to maintain waves greater than 4 ft.
Meanwhile winds on Lake Erie nearshores have shifted to the SSW,
which will also cause waves to drop off quickly. As a result, all
small craft headlines were dropped.

Expect a brief break in the winds today, following by an increasing
southerly flow tonight. This will likely result in some more small
craft headlines ahead of low pressure, followed by stronger winds in
the cold air advection behind it. Winds may approach gale late
Monday, prompting a Gale Watch for Eastern Lake Ontario.

&&

.BUF WATCHES/WARNINGS/ADVISORIES...
NY...Winter Weather Advisory from 1 PM this afternoon to 7 AM EST
     Monday for NYZ001>004-010>014-019>021-085.
     Lake Effect Snow Advisory until 9 AM EST this morning for
     NYZ007.
     Winter Weather Advisory from 4 PM this afternoon to 10 AM EST
     Monday for NYZ005>008.
     Lake Effect Snow Warning until 9 AM EST this morning for
     NYZ006.
MARINE...Gale Watch from Monday afternoon through late Monday night for
         LOZ043>045-063>065.

&&

$$

SYNOPSIS...APFFEL
NEAR TERM...APFFEL
SHORT TERM...THOMAS
LONG TERM...RSH/THOMAS
AVIATION...APFFEL
MARINE...APFFEL


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