Area Forecast Discussion
Issued by NWS Buffalo, NY

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000
FXUS61 KBUF 091101
AFDBUF

Area Forecast Discussion
National Weather Service Buffalo NY
601 AM EST Sat Dec 9 2017

.SYNOPSIS...
A bubble of high pressure is expected to bring fair and dry
weather today, before a passing weak low pressure system brings
a general light snowfall tonight and Sunday, along with areas
of heavier lake snows east of the lakes. Otherwise, temperatures
will remain at seasonable levels through the weekend. More
accumulating snow is expected Monday night through Wednesday as
another surge of arctic air arrives.

&&

.NEAR TERM /THROUGH TONIGHT/...
Some leftover flurries northeast of Lake Ontario into the Saint
Lawrence Valley early this morning. This activity will continue to
lift northward out of the region through this morning. The rest
of the day today, western and north central New York will be in
between systems as one area of low pressure tracks just offshore
of the mid Atlantic and New England coastlines, and a second
weak clipper low pushing eastward across the Upper Great Lakes.
The bubble of high pressure, subsidence and drier air over the
region will result in a dry and quiet conditions, along with at
least some partial sunshine. With some moderation in our
airmass, we can expect daytime highs to recover to the mid to
upper 30s in most locations, with a few of our normal warmer
locals in the Genesee Valley and Finger Lakes perhaps reaching
the 40 degree mark.

Area of ascent in advance of approaching digging shortwave/clipper
low will steadily expand eastward into the forecast area early
tonight. Leading band of light snow associated with trough axis/cold
front will work into the area around the 00z to 06z time frame.
Model soundings showing a rather dry wedge of air below 850 mb
and this will likely cut into snow rates to some extent on the
leading edge of this area of light snow, keeping most areas to
an inch or less of accumulation. Moisture quality will be better
east of the lakes with lake enhancement bringing higher amounts
of an inch or two additional. Lake effect parameters become
more favorable as we approach Sunday morning with better
moisture and shear profiles developing.

&&

.SHORT TERM /SUNDAY THROUGH TUESDAY NIGHT/...
There will be two distinct weather events of concern during this
period. The first will be a lake effect snow event that will could
potentially result in significant snow accumulations east of both
lakes.

A well established cyclonic flow of cold air will already be in
place across the Lower Great Lakes as we open this period Sunday
morning. Unlike the previous lake effect event though...this one
will have plenty of synoptic moisture to work with...including a
dendritic growth zone that will average 3k ft deep. There will also
be a higher cap of 10k ft over Lake Erie and nearly 15k ft over Lake
Ontario. These combined variables will support more efficient snow
making with snow rates averaging a half to one inch per hour...with
rates as high as two inches an hour.

In terms of placement of the bands...a fairly well aligned 280 flow
will initially direct the accumulating lake snows across the
Southern Tier and also on the south side of the Tug Hill plateau. As
a potent shortwave digs across Georgian Bay during the midday...
winds are forecast to back to about 260 on Lake Erie and possibly to
250 on Lake Ontario. This will lift the corresponding bands
northward through early afternoon. The moderately heavy band off
Lake Erie is expected to approach Buffalo...stalling in the vcnty of
the Southtowns before drifting back to the south during the mid and
late afternoon. Likewise...the band off Lake Ontario will move to
the north side of the Tug...although there is a possibility that it
could briefly make it to Watertown. While snowfall amounts are
currently forecast to range from 3 to 5 inches within these somewhat
mobile bands...higher amounts will be possible if they are less
transitory. Would not rule out some thunder in either of the
bands...but given the forecast heights of the -10c isotherm
(arnd 2.5k ft)...am not confident enough to include in the package.

An arctic frontal boundary will then settle south from the northern
shores of Lake Ontario Sunday night. While this boundary will likely
generate an inch or two of accumulation across the bulk of the
forecast area...an additional 4 to 6 inches of lake enhanced snow
could pile up to the east of both lakes. As the steering flow veers
more to the west immediately ahead of the approaching front Sunday
evening...the snow bands will push back to the south. This
movement...while slow at first...will be followed by an abrupt re-
organization to a multiple banded event in a northwest flow in the
wake of the arctic front. Since 24 hour snowfall could exceed nine
inches within the lake snow belts...a winter storm watch for lake
effect snow has been issued. The watch will be in effect for Sunday
and Sunday night.

In the wake of the arctic frontal passage...the lake snows will
quickly fall apart and dissipate early Monday morning as winds
become more northerly and synoptic moisture is stripped away from
the lakes. Interestingly...it will also be this northerly upslope
flow that will keep a fair amount of clouds in place...especially
over the western counties. It will be at this time that we will
transition to our second event of concern...that being a synoptic
driven storm that should bring healthy snow accumulations to the
entire forecast area.

A pair of robust shortwaves digging south across the Upper Great
Lakes on Monday will generate a broad area of low pressure that will
be centered over Lower Michigan by the end of the day. Warm
advection ahead of this system will lead to some light snow over our
region during the afternoon...especially over the far western
counties where an inch or so could accumulate.

Deterministic guidance packages are in fairly strong agreement that
the surface low will deepen and consolidate Monday night...while it
will track nearly due east across our western counties. Fairly
strong isentropic lift ahead of the surface low will initially be
the primary source of lift...then as the low tracks directly
overhead by daybreak...the snow may briefly taper off as lift will
be limited to height falls and upper level forcing. Regardless of
the source of lift...several inches of snow can be expected
regionwide by daybreak. Have raised pops to categorical...but
refrained from going to 100 to better collaborate with surrounding
offices.

During the course of Tuesday and Tuesday night...the still deepening
sfc low will track east across New England to the Canadian
maritimes. The pronounced deformation zone on the backside of the
exiting storm will cross our region in the process...and this is
when the snow could become rather heavy at times. This will
especially be the case for counties lining the south shore of Lake
Ontario...where lake enhanced snow could accumulate at a rate of one
to two inches per hour. Winter weather headlines will be likely for
this event. Given that there will be already be headlines in effect
for some of these counties for the lake effect event...will address
the problematic potential with the synoptic event in the Hazardous
Weather Outlook (HWO product).

&&

.LONG TERM /WEDNESDAY THROUGH FRIDAY/...
While temperatures will remain below normal during this period...
there will be a gradual day to day moderation in temperature. This
may be a clue to the weather further down the road...as the
impressive +PNA pattern is expected to break down by the weekend.
The collapse of the western ridge will allow Pacific based air to
flood the country...which in turn will lead to moderating conditions
across our region. This is outside the scope of this package
though...so lets focus on the last three days of the week.

The significant storm system from Tuesday will still be helping to
generate snow over our region on Wednesday...as leftover synoptic
moisture will combine with sub-arctic air (H85 arnd -20c) within a
cyclonic flow to generate lake lake effect snow showers. The
accumulating snows will be aimed at the typical snowbelts southeast
of both lakes...especially Wednesday and Wednesday night when
several more inches of accumulation will be possible. This will also
likely be the coldest 24 hour period of the young season...as
daytime temperatures away from the lakes will be in the teens while
Wednesday night lows will range form the minus single digits across
parts of the North Country to near 10 elsewhere.

While a shallow shortwave ridge will transit our region on
Thursday...will hold onto low chc pops for nuisance snow showers due
to weak warm advection. Temperatures will be as much as 10 degrees
higher on Thursday...as max temps will generally be in the mid 20s.

There is relative low confidence in the timing of a trough that will
move through our region Thursday night or Friday. While this should
produce more scattered snow showers...significant amounts of snow
are not expected.

&&

.AVIATION /11Z SATURDAY THROUGH WEDNESDAY/...
An elongated ridge of high pressure will lead to VFR conditions
expected through this evening. Ceilings will lower from west to east
through tonight as an area of low pressure moves into the area.
Areas of snow will develop ahead of the low with MVFR and
possible IFR conditions spreading from west to east by the end
of the TAF period.

Outlook...
Sunday through Monday...Areas of IFR in lake effect/lake
enhanced snow east and southeast of the lakes...otherwise MVFR/VFR.
Monday night and Tuesday...IFR in snow.
Wednesday...Areas of IFR in lake effect snow southeast of
lakes...otherwise MVFR/VFR.

&&

.MARINE...
Low pressure over western Quebec will make its way to Labrador
through today, while weak high pressure builds across New York State.
Winds have decreased enough to allow the small craft advisory to be
cancelled. This improvement in conditions will be fairly short-lived
however, as winds will freshen out of the northwest and waves will
build again following the passage of another weak surface low/cold
front tonight and Sunday morning. After that, a continued brisk
westerly to southwesterly flow is expected for the rest of
Sunday and Sunday night.

&&

.BUF WATCHES/WARNINGS/ADVISORIES...
NY...Winter Storm Watch from Sunday morning through late Sunday
     night for NYZ006>008-019-020-085.
MARINE...None.

&&

$$

SYNOPSIS...TMA
NEAR TERM...TMA
SHORT TERM...RSH
LONG TERM...RSH
AVIATION...TMA
MARINE...TMA



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