Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 052358
AFDBUF

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

.SYNOPSIS...
High pressure will move across the region tonight with slowly thinning
clouds and a partial clearing. 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 to the east of
our region, 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.
Latest BUFKIT soundings for the NAM/RAP show a shallow layer of
low moisture which is forecast to gradually erode overnight. While
this is often overdone by model guidance, satellite trends support
this guidance with clouds expected to gradually erode this evening
and overnight. By late tonight, expect significant breaks in the
cloud cover with the high building in.

Actual temperatures have been running a bit warmer during the
past few nights than consensus forecasts, so the forecast has
warmer overnight temps than forecast guidance. Low temps should
be in the lower 30s for most of the region and in the mid-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 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 /00Z TUESDAY THROUGH SATURDAY/...
Lingering low level moisture will result in VFR/MVFR conditions
this evening and overnight. Cigs will be lowest across higher
terrain, and should lower slightly through the evening hours with
a lowering inversion. While low moisture in BUFKIT is often
overdone, satellite trends suggest clouds will last well into the
tonight period before finally scattering out late tonight. Timing
this is difficult, with a mix of MVFR/VFR conditions until winds
shift to the southeast and high clouds move in. There is a risk
of lower IFR cigs east of Lake Ontario (including ART) if skies
clear and then lower clouds re-develop shortly after. Confidence
in this is too low to include in the 00Z TAF but may be included
if skies do scatter out at ART.

Expect VFR conditions Tuesday morning as high pressure crosses
the region. Precipitation will spread into the region late
Tuesday, mainly as rain at the onset. At JHW, precipitation may
mix with snow late Tuesday afternoon with conditions lowering to
MVFR.

Outlook...

Tuesday night and Wednesday...VFR deteriorating to MVFR with rain
and snow.
Thursday through Saturday...MVFR/IFR with lake effect
snow.

&&

.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 10 PM EST this evening for LEZ040-
         041.
         Small Craft Advisory until 1 AM EST Tuesday for
         LOZ043>045.
         Small Craft Advisory until 10 PM EST this evening for
         LOZ042.

&&

$$

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



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