Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 041436

Area Forecast Discussion
National Weather Service Buffalo NY
936 AM EST Sun Dec 4 2016

Dry weather will be in place today, as a large area of high pressure
will drift across the Lower Great Lakes. A weak disturbance passing
over the forecast area tonight will then produce several hours of
widespread light snow. Another surface high will offer a short break
from the unsettled conditions Monday afternoon and night before our
weather will become quite active for the remainder of the week. This
will include a trend towards wintry weather after Wednesday when
significant lake snows will impact areas east of both lakes.


The axis of a large surface high will gradually pass over our region
during the course of today. This will assure us of dry weather,
although the cloud cover over the western counties may be stubborn
to break. This will be due to a wealth of low level moisture that
will be at least partially trapped beneath a pronounced subsidence
inversion at 5k ft.

First few visible images this morning showing partial clearing has
taken place for portions of the Genesee Valley and Finger lakes.
These areas will see more sun than far western and far eastern
areas, where low clouds will be stubborn to clear. As we progress
through the afternoon, mid and high level moisture will start to
advance over our forecast area ahead of the next weather maker.

This next feature will be a negatively tilted trough that will
advance from the Upper Great Lakes and Ohio Valley this afternoon to
our forecast area tonight. While this system will lack significant
baroclinicity and low level forcing, a divergent flow aloft will
combine with 100m hgt falls to provide plenty of lift to generate
some widespread snow. The snowfall will be generally be light,
but with liquid equivalent amounts of pcpn forecast to be in the
vcnty of 0.2 of an inch, snowfall amounts should range from an
inch or so across the lake plains and near the lakes to two inches
across the higher terrain.

In regards to temperatures, the mercury will climb into the upper
30s to lower 40s across the western counties this afternoon while
sites east of Lake Ontario will be a few degrees chillier.
Tonight, the mins will range from around freezing over the
western counties to the mid 20s over the North Country.


This period will open with a mid and upper level trough shearing
out as it finishes sliding northeastward across our region on Monday.
At the attendant surface trough/pseudo warm-frontal
boundary will finish crossing our area in a similar fashion...with
surface high pressure over the Ohio Valley then ridging into our
area in its wake.

A swath of mixed light rain and snow out ahead of the trough will
continue to taper off from southwest to northeast through the day...
with any additional snowfall accumulations on the order of an inch
or less and mainly confined to the North Country. Following its
passage...a lingering westerly flow will help to generate some
scattered light lake effect/orographically-induced rain and snow
showers east of the lakes...though these should be pretty short-
lived and inconsequential in nature as subsidence and drying increase
under the building surface ridge. Temperatures will be right around
average...with daytime highs ranging in the upper 30s to lower 40s.

Monday night the aforementioned ridge will crest across our region
during the first half of the night...before slowly drifting east
toward New England overnight. This feature will provide our region
with a period of fair and quiet weather along with some partial
clearing...which will allow overnight lows to bottom out between
the mid 20s and lower 30s.

After a dry start to the day Tuesday...a southern stream shortwave
and attendant weak surface low will quickly ripple northeastward
from the Tennessee and Lower Ohio Valleys and cross our region
between Tuesday night and Wednesday. Increasing moisture and warm
advective lift out ahead of this next system will begin working its
way across areas south of Lake Ontario as early as Tuesday afternoon...
before spreading across the remainder of the area Tuesday night. At
this point both available moisture and forcing look plentiful enough
for this system to generate another round of widespread light rain
and snow as it crosses our region...with the bulk of this falling
between later Tuesday afternoon and night...then quickly tapering
off from west to east during the day on Wednesday as the wave slides
off to our northeast. Meanwhile temperatures will remain around or
slightly above seasonal averages...with highs both Tuesday and
Wednesday ranging in the upper 30s to lower 40s...and lows Tuesday
night mostly in the lower to mid 30s.


This period will feature a major pattern change across our region...
as deep upper level troughing develops across eastern North America
and drags much colder Canadian air into our region...with 850 mb
temperatures consequently dropping off into the negative mid teens
by the end of the week. For our area...this will translate into
the arrival of considerably more wintry temperatures later on in
the well as the development of potentially significant
lake effect snows downwind of the Great Lakes.

As the trough digs toward our region early on in the
should spur the development of yet another wave of low pressure on
its eastern flank during the Wednesday night-Thursday time frame...
though the medium range guidance continues to exhibit considerable
differences with the timing and strength of this wave...with the
00Z/04 ECMWF remaining consistently deeper and slower with this
feature than the much weaker and faster GFS.

For our region...this wave will likely bring another round of
fairly widespread snow and rain showers as it passes through our
region later on in the week...with these remaining centered on
Thursday at this time following both a model consensus approach
and our existing continuity.

Following the passage of this wave...much colder Canadian air will
then flood across our region through the remainder of the week as
highlighted above. Coupled with what should be plentiful moisture...
this will set the stage for a round of potentially significant lake
snows Thursday night through Friday night...with these then likely
weakening at least some and shrinking up in coverage on Saturday as
high pressure and drier air ridge into our area from the Central
Great Lakes and Ohio Valley.

With respect to the location of the lake snows...current guidance
continues to suggest that the large-scale flow across our region
will generally be westerly to west-northwesterly once the colder
air arrives later on in the week...which would tend to direct the
most significant lake snows across the traditional snow belts east
and east-southeast of the lakes. The above stated...attempting to
pin down exact band locations this far in advance is generally a
fruitless enterprise as much can still change...with even small
variations in the low level wind field having a large influence
on lake band positions and consequently the areas of greatest
overall forecast concern. With this in will likely be
at least a few more days before the exact details of this next
event begin to become more clear.

As for temperatures...these will progressively lower through the
period...with daytime highs in the mid to upper 30s Thursday
dropping off to typical midwinter levels in the mid 20s to lower
30s in time for both Friday and Saturday.


A wealth of low level moisture trapped beneath a subsidence
inversion will keep strato-cu cigs of 2500-4000 ft across the
region. Partial clearing can then be expected...mainly east of
the Genesee Valley. As a result...the MVFR cigs across the
southern tier and portions of the IAG Frontier will improve to VFR
levels for the late morning...joining the rest of the forecast

Tonight...a disturbance over the Ohio valley will push northeast
across the Lower Great Lakes. This feature will produce widespread
light snow with conditions deteriorating to at least MVFR levels.
The snow should reduce vsbys to IFR levels for several hours after
about 03z.

Monday...MVFR with a chance of rain and snow showers.
Tuesday...Mainly VFR.
Tuesday night and Wednesday...VFR deteriorating to MVFR with some
rain becoming likely.
Thursday...MVFR/IFR with snow likely.


A large surface high will drift across the Lower Great Lakes
today...and this will maintain light winds and negligible waves.

Tonight...while a disturbance with widespread light snow will push
across the region...winds are not expected to notably freshen until
Monday when the gradient will temporarily tighten between the
exiting system and the next surface high. A short period of small
craft advisories may be needed...mainly over the eastern half of
Lake Ontario during the afternoon and early evening. Winds and waves
will subside with the arrival of another surface high overnight
Monday night.

While winds will freshen from the east on Tuesday...they will remain
below small craft advisory levels while the higher waves will be
confined to Canadian waters.

Looking further ahead...a deep storm forecast to track across James
Bay late in the week will likely generate gale force winds over the
Lower Great Lakes Thursday night and Friday.





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