Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 060631
AFDBUF

Area Forecast Discussion
National Weather Service Buffalo NY
131 AM EST Tue Dec 6 2016

.SYNOPSIS...
High pressure will move across the region tonight with slowly
thinning clouds and a partial clearing. Weather conditions will
deteriorate Tuesday afternoon and Tuesday night when widespread
mixed rain and wet snow can be expected. Daily temperatures will
steadily decrease during the second half of the week when some
impactful lake snows will develop east of both lakes.

&&

.NEAR TERM /THROUGH TODAY/...
High pressure centered over Pennsylvania will supply our region
with fair weather overnight...although there will be a few
sprinkles or flurries over Oswego County as a result of dying lake
effect. The mercury will drop off into the upper 20s to lower 30s
for most areas.

After a brief respite from rain or snow...precipitation will return
later Tuesday afternoon as an extended trough of low pressure will
move across the region. Model consensus has slowed by a couple
hours...with a rain/snow mix expected to start across the Southern
Tier early in the afternoon...and to encompass most of Western New
York by Tuesday evening. Precipiation type will be strongly
elevation dependent with mostly rain over the lake plains and
mostly snow across the Southern Tier hilltops...with a mix of each
in the in- between zone. Accumulations of snow should be limited
to the higher hills of the Southern Tier. In Cattaraugus and
Allegany counties...up to 2 inches could accumulate by nightfall.

Highs on Tuesday should climb to the upper 30s to lower 40s in the
filtered and weak December sun.

&&

.SHORT TERM /TONIGHT THROUGH FRIDAY/...
Large precipitation shield will be across nearly the entire region
as we start the period, as weak surface wave lifts northeast from
the Ohio Valley. The precipitation will then quickly taper off from
west to east through the second half of Tuesday night, as primary
lift region pushes off to the east, followed by pronounced drying
at mid and upper levels. Marginal thermal profiles will result
in precipitation type being largely elevation dependent, with
mainly rain or a mix of rain/wet snow falling across the lower
elevations and a rain/snow mix or wet snow more likely across the
higher terrain, where perhaps an inch or two of slushy
accumulation will be possible. Overnight low temperatures Tuesday
night mostly in the lower to mid 30s.

Surface high pressure nosing in from the southwest will follow the
passage of the system for Wednesday. The ridge axis will then slide
to the southeast of the area by Wednesday night as upper level
troughing steadily deepens across the Great Lakes and Northeast,
resulting in 850 mb temperatures slowly but steadily cooling to
between -6C and -10C Wednesday night.

The cooling airmass will be cold enough to generate a lake
response downwind of the lakes by late in the day Wednesday and
Wednesday night. The backing low level winds initially will focus
on a 250-260 degree flow Wednesday, then on a 240-250 degree
flow Wednesday night. Lake parameters during this time are far
from outstanding with a rather low capping inversion and limited
synoptic- scale moisture. Profiles become more favorable as we
near Thursday morning. This will likely mean that what ever lake
response we do get will be rather disorganized. Thermal profiles
suggesting more of a mixed rain/snow precipiation type for
Wednesday transitioning to all snow over the course of Wednesday
night. Any accumulation likely limited to minor amounts on the
higher terrain east of the lakes.

...Significant lake snows developing Thursday through at least
Friday...

The main forecast concern throughout the remainder of the period
revolves around the development of significant lake effect snows
off both lakes.

Broad troughing is expected to be anchored over the Great Lakes
Thursday with broad cyclonic flow sending arctic air southward
across the Lower Great Lakes. 850 mb temperatures start out around
-10c Thursday morning cooling to near -14c by the end of the day
Friday. Over-lake instability during this time will be extreme
with Lake Induced Capes peaking around 1000 J/KG and Lake
Equilibrium Levels nearing 20K feet. Such a strong instability
signal would suggest the potential intense snowfall rates in
excess of three inches per hour at times.

Lastest model guidance trending a bit further to the north during
the day Thursday on a west southwest flow which would bring the
lake snows into the Buffalo Southtowns off Lake Erie and up to
Watertown or even farther to the north off Lake Ontario. A
prolonged period of west northwest flow develops behind the
passage of a trough late Thursday into Friday. This will likely be
when the more intense lake snows develop, focusing on the Western
Southern Tier off Lake Erie and from east of Rochester to the
southern Tug Hill off Lake Ontario.

At this stage of the forecast it seems nearly certain that heavy
lake snows will develop and our first extended look at winter so
far this season, however trying to pin down the exact placement of
the most significant threat areas remains at a lower confidence
level, due to subtle differences in wind direction. Will need to
wait for more run-to-run consistency to begin detailing the
highest threat locations.

&&

.LONG TERM /FRIDAY NIGHT THROUGH MONDAY/...
Significant lake effect snow event will likely continue Friday night
through Saturday under a general 290-300 west-northwest flow. 12z
GFS BUFKIT profiles show lake plumes will continue to feature ideal
snow production environment with moisture and lift aligning within
the snow growth region. Weak surface riding will lower lake induced
equilibrium levels to around 10kft through Saturday with some
weakening of lake bands then winds will begin to back Saturday night
ahead of a developing low over the central Plains or Midwest states.
These backing winds will quickly swing the lake bands northward
through Buffalo and Watertown before becoming sheared apart by
south to southeast flow by Sunday morning.

Sunday into Monday, 12z GFS/EC are showing differences in location,
timing and strength of the next potential storm system to pass near
New York. Overall there is agreement at least with the 12z runs that
this low center will pass just northwest of the lower Great Lakes
with an associated moisture plume and synoptic lift pointing to at
least a chance of precipitation Sunday into Monday with narrowing
of this to come with better model agreement.

As for temperatures...these will progressively lower through the end
of the week...with daytime highs dropping off to typical midwinter
levels in the mid 20s to lower 30s in time for both Friday and
Saturday...before perhaps rebounding a little by Sunday and Monday
as the aforementioned synoptic system draws slightly warmer air back
into our region. This would keep bring back a potential for a mix of
rain and/or snow during the daytime periods Sunday and Monday with
chance of snow Sunday night.

&&

.AVIATION /07Z TUESDAY THROUGH SATURDAY/...
High pressure centered just to our south will provide VFR conditions
across the bulk of our region...although some residual low level
moisture across the southern tier will keep some MVFR cigs in place
through about 10z.

Expect VFR conditions Tuesday morning as high pressure crosses
the region. Precipitation will then spread into the region late
in the day over the far western counties. At JHW...precipitation
may mix with snow with conditions quickly deteriorating to
MVFR.

Outlook...

Tuesday night...IFR to MVFR in mixed pcpn.
Wednesday...Improvement to VFR for most areas...although this trend
will be delayed east of Lake Ontario.
Thursday through Saturday...MVFR/IFR with lake effect snow.

&&

.MARINE...
Winds will continue to diminish as high pressure builds in. Small
craft headlines were extended a few hours based on current
conditions. These are set to expire late tonight as high pressure
builds in.

On Tuesday...the area of high pressure will exit across New England
while a complex frontal boundary will approach from the southwest.
While this will result in freshening southeast winds on Lakes Erie
and Ontario...speeds should remain below SCA criteria for the
nearshore waters and the associated building waves will be confined
to Canadian waters.

Looking further ahead...guidance suggests that there will be a
lesser threat for gale force winds for Thursday and Friday. While
strong westerlies can still be expected...sustained winds are now
more likely to remain below gale force.

&&

.BUF WATCHES/WARNINGS/ADVISORIES...
NY...None.
MARINE...Small Craft Advisory until 4 AM EST early this morning for
         LOZ043>045.

&&

$$

SYNOPSIS...WCH
NEAR TERM...APFFEL/RSH/WCH
SHORT TERM...TMA
LONG TERM...SMITH
AVIATION...APFFEL/RSH
MARINE...APFFEL/WCH



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