Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 062039
AFDBUF

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

.SYNOPSIS...
A weak low pressure system will pass to our southeast overnight,
this will lead to deteriorating conditions this afternoon and
evening a wintry mix of precipitation will move across our region
through tonight. 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/...
A messy mix of rain and snow will overspread the region this
afternoon and evening. Some sleet is being observed mixing with the
rain on the leading edge in Buffalo, and sleet will continue to mix
in through early evening as the precip shield spreads north. The
sleet will rapidly end by mid evening as the mid levels cool and
support a change to mainly wet snow. This will bring up to a few
inches of slushy wet snow to the higher terrain of the western
Southern Tier and Tug Hill, while the lower elevation lake plains
may see some flakes mixed in, but no snow accumulation.

Water vapor satellite imagery shows a shortwave trough over eastern
KY and TN tracking northeastward toward New England. Meanwhile
surface analysis shows a weak surface high over New England keeping
cooler surface temperatures in the 30s feeding into western NY on an
easterly wind. As this wave of low pressure tracks northeastward
this afternoon and evening, the large swath of precipitation along
the band of frontogensis, vorticity advection and warm air advection
will overspread the forecast area. BUFKIT thermal profiles from the
NAM and GFS reveal a very marginal setup for wet snowfall with a
deep nearly isothermal layer up through 10kft just below freezing.
Thus snow vs rain will be determined by both elevation and
precipitation intensity, both of which will help to get the boundary
layer temperatures just at or slightly below freezing. Where the
combination of elevation and precipitation intensity line up, as in
the western Southern Tier and the Tug Hill, expect a slushy 1 to 3
inches (2 to 4 inches on the highest hills). Otherwise, the lake
plains locations may see a brief change over to snow when
precipitation intensity increases (dynamical cooling), however, will
likely change back to rain as precipitation lightens up. This
combined with likely marginal surface temperatures stay just above
freezing (33 to 34) should prevent any snow accumulations across the
lake plains (including Buffalo, Rochester and Watertown). As far as
timing is concerned, the western Southern Tier can expect
precipitation to end just after midnight. The Genesee valley
starting between 5 and 6pm, and ending after midnight. The North
Country starting between 7 and 9pm and tapering off to light showers
by sunrise.

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/...
By the time we reach Saturday morning...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. This stated...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.

After that...our attention will turn to the next synoptic system...
which the medium range guidance suggests will cross our region
during the Sunday to Monday time frame. This system still looks to
bring us our next general chance of precipitation...which will probably
come in the form of a rain/snow mix again given that it should
ultimately draw somewhat warmer air back into our region.

With respect to temperatures...midwinter-like highs in the mid 20s
to lower 30s on Saturday will probably rebound some for both Sunday
and Monday given the aforementioned influx of somewhat warmer air...
with readings probably recovering into the upper 30s and lower 40s
by Monday.

&&

.AVIATION /21Z TUESDAY THROUGH SUNDAY/...
A trough of low pressure advancing northward from Pennsylvania will
bring a mix of precipitation to the area tonight, transitioning into
some lake effect snow on Wednesday. Expect ceilings to lower to MVFR
range by 00Z with IFR ceilings at KJHW as early as 21Z today.
Precipitation type will be rain or a rain/snow mix this evening
possibly mixed with some sleet briefly, changing to all snow across
higher terrain after 00Z with visibility reduced to 2SM or lower at
KJHW and 4SM or less at KBUF/KIAG/KROC. KART will experience mainly
rain but will be far enough to the north of the trough to maintain
VFR visibility.

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.

Outlook...

Thursday through Saturday...MVFR/IFR with lake effect snow.
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, with small craft conditions continuing into the weekend on
both lakes Erie and Ontario.

&&

.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.
MARINE...Small Craft Advisory from 7 PM this evening to 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
NEAR TERM...CHURCH/HITCHCOCK
SHORT TERM...TMA
LONG TERM...JJR
AVIATION...CHURCH/HITCHCOCK
MARINE...CHURCH



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