Area Forecast Discussion
Issued by NWS Buffalo, NY

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603
FXUS61 KBUF 062345
AFDBUF

Area Forecast Discussion
National Weather Service Buffalo NY
645 PM EST Tue Dec 6 2016

.SYNOPSIS...
A weak low pressure system will pass to our southeast overnight and
bring a wintry mix of precipitation. The mix will change to all wet
snow for a time across higher terrain with minor accumulations.
Colder air will then gradually deepen across the Lower Great Lakes
during the second half of the week with significant lake snows
becoming established east of both lakes Thursday through Saturday.

&&

.NEAR TERM /THROUGH WEDNESDAY NIGHT/...
Water vapor imagery showing a strong mid level baroclinic wave
crossing the region downstream of a strong closed low over the upper
midwest. This initial mid level wave is of southern stream origins
and has a strong push of moisture associated with it. This system
will bring a 3-5 hour period of precip to the region tonight, with a
variety of precip types depending on location.

A messy mix continues to overspread the area this evening. The lower
elevations have been a mix of rain and sleet. Precipitation should
tend to become mainly rain by mid to late evening across the lower
elevations of Western NY, with a few wet flakes possibly mixing in.
Across the interior Southern Tier, precip has been a mix of rain,
freezing rain, sleet, and wet snow so far. Increasing precip rates
should cool the column enough to change most of the precip to wet
snow for a few hours this evening across higher terrain, but prior
to this some minor ice accumulation may occur. Precip may change
back to spotty freezing rain or freezing drizzle overnight as precip
rates diminish, snow growth is lost in the cloud layer, and the mid
levels begin to warm again. With this in mind, we have issued a
winter weather advisory mainly for the periods of freezing rain
early and late in the event across Cattaraugus, Allegany, and
Wyoming counties. There may also be an inch or so of snow in this
area as well. Elsewhere, expect rain to mix with some wet snow and
produce little or no accumulation, with surface temperatures
remaining above freezing.

Interior portions of the North Country will stay colder, with mainly
snow from the Tug Hill into the western foothills of the
Adirondacks. This area may see an inch or two of snow overnight.

Also of note will be some strengthening southeasterly winds along I-
90 corridor between the PA line and southern Erie County, resulting
in gusty cross winds. Downslope winds off the Chautauqua ridge will
increase after sunset this evening, with wind gusts 40 to 45 mph
possible. It appears this will fall short of Wind Advisory criteria
at this time, and thus will hold off on any headlines. These
stronger gusts will decrease after midnight as the flow turns more
southwesterly by Wednesday morning.

Wednesday will offer a reprieve in the weather, just in time to make
any final preparations for the significant lake effect event taking
aim at the forecast area for Thursday into Saturday (more on that
below). Most of Wednesday will feature ample dry time, with perhaps
just a few light lake effect showers or sprinkles northeast of the
lakes. Temperatures will remain cool, with highs in the mid 30s at
elevation to near 40 across the lake plains.

By Wednesday night, the upper-level low will track from near Lake
Superior to just south of James Bay, will colder air spills into the
lower Great Lakes. Increasing synoptic moisture and lift, along with
850 mb temperatures falling to about -9C over the lower Great Lakes
by Thursday morning, will result in developing lake effect snow.
Initially, this will be on southwesterly flow (240 to 250 degrees)
early Thursday morning, which will bring some snow into Buffalo and
Watertown area for the morning commute. This may bring a quick 2 to
3 inches to these locations by around sunrise, before the snow bands
begin shifting southward after sunrise. Would not rule out some
rumbles of thunder with this initial lake effect band Thursday
morning. Our local research thundersnow nomogram shows the forecast
-10C level of 5kft and a forecast lake induced equilibrium level of
10kft supports likely thunder.

&&

.SHORT TERM /THURSDAY THROUGH SATURDAY NIGHT/...
...Significant lake effect snows expected east and southeast of
Lake Erie and Lake Ontario Thursday through at least Friday night...

Significant lake effect snow event setting up during this time
period, as deep upper level trough slowly makes its way across the
Great Lakes and the Northeast CONUS. The associated cyclonic flow
will pull progressively colder air across our region, with 850 mb
temperature dropping into the negative mid teens by late Friday and
Friday night. This will lead to the development of extreme over-lake
instability with lake induced CAPES up around 1000 J/KG and lake
equilibrium levels surging up to near 20K feet. Such strong
instability combined with the arrival of deeper moisture would
suggest the potential intense snowfall rates in excess of three
inches per hour at times.

Latest 12z guidance similar to that of continuity in the favored
orientation of the lake snows, a general 250-260 degree flow should
be in place at the start of Thursday morning, then gradually veer to
260-270 degrees through the course of Thursday, before turning more
northwest Thursday night, following the passage of a surface trough.
After that, a general west-northwesterly to northwesterly flow will
predominate both Friday and Friday night.

Off Lake Erie, the lake snows will initially set up across the
Buffalo Southtowns early Thursday, drifting to the south and
settling across Ski Country by late Thursday or early Thursday
night, with snowfall rates likely intensifying. Once the flow veers,
the lake snows will drift further to the south into the western
Southern Tier Friday into early Saturday.

Off Lake Ontario, the lake snows will initially set up across the
area between Watertown and the Tug Hill early Thursday, drifting to
the south and settling across the Tug Hill by late Thursday or early
Thursday night, with snowfall rates likely intensifying. Once the
flow veers, the lake snows will drift further to the south into the
Monroe-Oswego county corridor Friday into Saturday.

Confidence has increased to the point where we have converted the
lake effect snow watches to warnings east of both lakes and have
included the Monroe-Oswego county corridor in a lake effect snow
watch.

Outside of the main lake effect bands, the remainder of the region
can expect much more scattered snow showers at times between
Thursday and Saturday, with mainly dry weather otherwise prevailing.

With the arrival of progressively colder air, temperatures will drop
off to typical midwinter levels by the end of the week, with daytime
highs only reaching to between 25 and 30 on Friday, and nighttime
lows then dropping into the 10-20 range Friday night. Brisk winds
will plunge wind chill readings into the teens or single digits at
times from Thursday night into Saturday.

By the time we reach mid day Saturday, the lake snows should be
steadily weakening off both lakes as high pressure and drier air
build into the region, and as shear increases under the advancing
ridge. A general backing of the low level flow will probably still
send the remnants of these bands back north across the Buffalo and
Watertown areas Saturday afternoon and evening, before these die out
altogether Saturday night as the low level flow becomes too
southerly.

&&

.LONG TERM /SUNDAY THROUGH TUESDAY/...
On the heels of the extended lake effect snow event, 12z models
continue to indicate a synoptic low will bring another opportunity
for some wintry precipitation either late Sunday or Monday. There
is still quite a difference between the GFS and EC in timing and
tracking of the surface low. The GFS is faster with a track to the
north of New York State Sunday night which would bring warmer temps
and a better chance for mixed precip while the EC is slower with a
track that is just south of the western and central NY which would
favor cooler temps and likely all snow.

The depth of the low looks at this range to only be around 1000mb so
even if the GFS verifies better with its northerly track at this
point we do not see a threat of widespread high winds. The highest
confidence is in using a blend of both solutions with likely POPs
for Sunday night and chance POPs Sunday ahead of the system then
Monday through Tuesday to cover for any slower timing and wrap
around moisture. Temperatures were also blended at this range which
leads to p-type of snow during the overnight periods and some mixed
rain/snow during the day especially at lower elevations and closer
to the lakeshore. Temperature forecasts will likely change as future
model runs lock in on either the northerly or southerly track.

&&

.AVIATION /00Z WEDNESDAY THROUGH SUNDAY/...
A trough of low pressure advancing northward from Pennsylvania will
bring a mix of precipitation to the area tonight. Across lower
elevations most of the precip will fall in the form of rain, with a
few wet snowflakes mixing in at times. Across the higher terrain,
the rain will change to wet snow for a few hours before changing
back to a rain/snow mix and diminishing late tonight. Some pockets
of freezing rain are also likely at times across portions of the
Southern Tier, mainly east of KJHW. The steadier precip will produce
areas of MVFR VSBY tonight, with IFR where precip changes to all
snow across higher terrain.

Most of the precip will end late tonight with mainly dry conditions
for Wednesday, although a few widely scattered light lake effect
rain or wet snow showers are possible northeast of the lakes.

Widespread MVFR CIGS will develop this evening and continue into
early Wednesday morning, with IFR favored across higher terrain.
During the day Wednesday, the lower elevations of Western NY will
improve to VFR for a time, with MVFR and IFR lingering across higher
terrain. Lake effect clouds will develop later Wednesday afternoon
on a southwest flow, bringing a return to MVFR CIGS on the lake
plains of Western NY.

Outlook...

Thursday through Saturday...IFR in lake effect snow east and
southeast of the lakes. Occasional snow showers with more brief IFR
elsewhere.
Sunday...MVFR/IFR in widespread light snow.

&&

.MARINE...
This evening, southeasterly downslope winds will pick up off the
Chautauqua ridge along eastern Lake Erie. This will result in small
craft advisory worthy winds up to 25 knots, despite the bulk of the
wave activity being pushed into Canadian waters. Winds will turn to
the west after midnight, allowing waves to build back into US
waters.

Small craft conditions will then continue into the weekend on both
lakes Erie and Ontario as low pressure moves very slowly east across
Quebec, producing moderate west to northwest winds on the Lower
Great Lakes.

&&

.BUF WATCHES/WARNINGS/ADVISORIES...
NY...Lake Effect Snow Warning from 6 AM Thursday to 6 AM EST Friday
     for NYZ007-008.
     Lake Effect Snow Warning from 6 AM Thursday to 6 AM EST
     Saturday for NYZ006-012-019-020-085.
     Lake Effect Snow Watch from Thursday evening through late
     Friday night for NYZ003>005.
     Winter Weather Advisory until 4 AM EST Wednesday for NYZ012-
     020-021.
MARINE...Small Craft Advisory until 7 PM EST Friday for LEZ040-041.
         Small Craft Advisory from 7 AM to 7 PM EST Wednesday for
         LEZ020.
         Small Craft Advisory from 7 AM Wednesday to 7 AM EST
         Saturday for LOZ042>045.
         Small Craft Advisory from 10 AM to 7 PM EST Wednesday
         for SLZ022.

&&

$$

SYNOPSIS...CHURCH/HITCHCOCK
NEAR TERM...CHURCH/HITCHCOCK
SHORT TERM...TMA
LONG TERM...SMITH
AVIATION...HITCHCOCK
MARINE...CHURCH/HITCHCOCK



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