Area Forecast Discussion
Issued by NWS Buffalo, NY

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664
FXUS61 KBUF 051958
AFDBUF

Area Forecast Discussion
National Weather Service Buffalo NY
258 PM EST Mon Dec 5 2016

.SYNOPSIS...
High pressure will move across the region tonight with thinning
clouds and some clearing skies. Weather conditions will deteriorate
late Tuesday 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 TUESDAY/...
A trough of low pressure will continue to depart our region to the
east late in the day, as high pressure builds in from the west. The
upper ridging will be rather flat, initially, then amplify some on
Tuesday, helping the surface high to build a little more strength.
The amplification aloft will also bring drying which should erode
the layer of lower level moisture and cloud cover, likely later
tonight and early Tuesday based on BUFKIT forecast profiles.

Actual temperatures have been running a it warmer during the past
few night, than the forecast number, so I have bumped the overnight
temps up a bit above the best forecast guidance. Low temps should be
in the lower 30s for most of the region and in the id-upper 20s for
the colder spots. Highs on Tuesday should climb to the upper 30s to
lower 40s in the filtered and weak December sun.

After a brief respite from rain or snow, precipitation will return
later on Tuesday afternoon as an extended trough of low pressure
moves across the region. A rain/snow mix should start across the
Southern Tier around noon and spread north/northeast to encompass
most of Central and Western New York by the day`s end. Precipiation
type will be strongly elevation dependent with mostly rain over the
lake plains and mostly snow across the Southern Tier hilltops, and a
mix of each in the in-between zone. Accumulations of snow should be
limited to the higher hills of he Southern Tier, in Cattaraugus and
Allegany counties, and 2-3 inches limited by the wet character of
the snow and the short duration of the event.

&&

.SHORT TERM /TUESDAY NIGHT 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 tapering 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
focusing 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 SUNDAY/...
By later Friday night and/or Saturday...the lake snows should be
weakening as high pressure and drier air builds into the region
along with dramatically increasing amounts of shear...with these
likely to give way to a passing synoptic system and the chance of
a much more general light snowfall to close out the period
Saturday night and Sunday.

As for temperatures...these will progressively lower through the
end of the week...with daytime highs in the lower to mid 30s Thursday
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 as the aforementioned synoptic system draws
slightly warmer air back into our region.

&&

.AVIATION /20Z MONDAY THROUGH SATURDAY/...
VFR conditions across Western New York will hold through the
duration of the TAF cycle, and MVFR conditions to the south and east
of Lake Ontario will improve to VFR after 00Z, and last through
Tuesday afternoon. This is due to high pressure building in from the
west.

Outlook...

Tuesday...Mainly VFR.
Tuesday night and Wednesday...VFR deteriorating to MVFR with some
rain becoming likely.
Thursday through Saturday...MVFR/IFR with snow likely.

&&

.MARINE...
SMall Craft Advisories in place this evening will drop off from west
to east as high pressure builds across the Lower Great Lakes and
allows winds and waves to subside.

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 6 PM EST this evening for LEZ040-
         041.
         Small Craft Advisory until 10 PM EST this evening for
         LOZ043-044.
         Small Craft Advisory until 6 PM EST this evening for
         LOZ042.
         Small Craft Advisory until 1 AM EST Tuesday for LOZ045.

&&

$$

SYNOPSIS...WCH
NEAR TERM...WCH
SHORT TERM...TMA
LONG TERM...JJR
AVIATION...WCH
MARINE...WCH



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