Area Forecast Discussion
Issued by NWS Buffalo, NY

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000
FXUS61 KBUF 192141
AFDBUF

Area Forecast Discussion
National Weather Service Buffalo NY
441 PM EST Sun Nov 19 2017

.SYNOPSIS...
Much colder air will establish increasingly organized bands of lake
effect snow southeast of both lakes where several inches of snow are
expected by Monday morning. High pressure will pass well to our
south on Monday, but lake snows will persist east of the lakes.

&&

.NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
The main lake effect event will get better organized tonight...

First for Lake Erie, disorganized lake effect snow bands will
become better organized through tonight as cold air continues to
deepen. Lake induced equilibrium levels will lift to 10k ft or
better. This, along with nocturnal cooling will favor higher and
more widespread accumulations. The multibanded nature from the
short lake fetch and weak/distant upstream connections should
limit accumulations, peaking in the 2 to 4 inch range overnight from
the lake, with the higher amounts being concentrated across the
higher terrain. Also, the core of the coldest air aloft is
expected to miss this lake and instead head over eastern Lake
Ontario. This plus any snow accumulations from the afternoon
should warrant the low end advisory.

Lake Ontario, similar to off Lake Erie, the cold air will deepen
enough by early evening to generate a fairly well organized
band of lake snow focused most likely on Wayne and northern
Cayuga counties. Lake equilibrium levels will lift to more than
10k ft with a fairly solid upstream connection becoming established
with Georgian Bay. While this will lead to increased snowfall
rates, the band is forecast to oscillate enough to keep the snow
from accumulating more than 3 to 5 inches in any one spot from
the northeast corner of Wayne County to south central Oswego
County. An advisory is out for far NE Wayne County while a
warning is in place for Northern Cayuga and (Western) Oswego
counties. The new HREF seems to highlight this well, although
the ensemble amounts appear to have a little bit of a high
bias. However, the location/placement from the HREF so far for
the early lake effect season has been on target.

&&

.SHORT TERM /6 AM MONDAY MORNING THROUGH WEDNESDAY NIGHT/...
The opening 24 hours of this period will feature surface-based
ridging over the Deep South sliding eastward and off the Atlantic
coastline...while low pressure over the Canadian Plains slides
east toward Hudson Bay. As a result...the cold westerly to west-
northwesterly flow that will be over our region at the start of
Monday morning will gradually back to southwesterly and give way
a strengthening warm air advection regime. Consequently...ongoing
lake snows downwind of the lakes will gradually lift northward
and steadily weaken during Monday...before falling apart altogether
Monday night.

Looking more specifically at the evolution of the lake effect...
off Lake Erie a general 270-280 flow at the start of Monday will
initially focus light to modest lake snows across the higher
terrain of the Southern Tier and Southern Erie/Wyoming counties
at daybreak. As the day progresses...the activity will steadily
weaken and lift northward as the low level flow steadily backs
and becomes increasingly sheared...and background synoptic scale
moisture/inversion heights crash. This should result in the lake
snows degenerating to scattered remnant mixed rain and snow showers
by Monday afternoon...and then to a few leftover sprinkles/flurries
across the Buffalo area late Monday. Expect additional accumulations
out of the Lake Erie activity on Monday to be on the order of
an inch or two...with these primarily focused across the ski
country.

Meanwhile off Lake Ontario...a general west-northwesterly flow and
an upstream connection to Georgian Bay will help to focus a fairly
cohesive band of moderate snow across areas southeast of the lake at
the start of Monday. As the morning progresses...this band will slowly
lift north toward the Tug Hill while beginning to weaken with the loss
of its upstream connection...though this may be at least partially
counteracted by the improving fetch across the long axis of the lake...
and increasing interaction with the Tug Hill. During the afternoon
the band will continue lift north to the Tug Hill proper and will
continue to gradually weaken with increasing shear and slowly falling
inversion heights. Additional accumulations out of the Lake Ontario
activity during Monday should range on the order of 3-6" in the
most persistent snows...with the bulk of these focused from
northeastern Wayne to Oswego and southern Lewis counties...
where the band will be most organized during its northward
migration.

Monday evening as 850/500 hPa ridge axis crosses the region lake
effect snow will subside...with up to an additional inch of snow
across the Tug Hill region. Backing winds will send lake snow east
of Lake Ontario northward, with snows ending across Watertown-Fort
Drum region.

Later Monday night and into Tuesday a period of dry weather is
expected as an axis of the surface high pressure crosses the region.
Skies will clear Monday night, but a southerly breeze will prevent
temperatures from rapidly dropping, with overnight lows in the lower
30s. Tuesday, these southerly winds will increase further, with
gusts up to 30-40 mph. Highest gusts will likely be across the hill
tops of SW NYS, where a 55 knot LLJ will occasionally produce gusts
30 to 35 mph, and also across the lake plain of WNY where downslope
flow, and under the strong LLJ may bring gusts upwards towards 40
mph. These winds and warming aloft will bring afternoon temperatures
into the 40s to low to mid 50s. A spot 60 cannot be ruled out across
the Genesee Valley. Warmest areas will be the Lake Plains and
Genesee Valley, except for SE of Lake Ontario where a melting
snowpack will slow warming.

Surface low pressure will cut across the Central Great Lakes
Tuesday, deepening to about 990 mb as it crosses James Bay Tuesday
night. A cold front front this surface low will be tracking across
the Central Great Lakes. Aloft a potent shortwave trough will be
crossing the central Great Lakes, pushing the cold front and its
precipitation eastward. Ahead of the cold front clouds will begin to
increase through Tuesday afternoon, and as these clouds increase
we`ll see some diminishment to the gusty winds.

Rain showers along the front will reach WNY Tuesday evening, and
continue to push inland across the region with the passage of the
cold front. The warm southerly flow ahead of the cold front will
start this precipitation off as plain rain. As temperatures cool
behind the front, rain will mix with and likely change to snow by
late Tuesday night. Not much snow accumulation at this time, with
perhaps an inch across higher elevations.

Wednesday limited lake effect snow is likely to develop behind the
cold front. Equilibrium levels are lower...in the 8-9K feet range
and moisture is also a bit lower than with this current lake snow
event. A northwest flow will bring snows again to the southeast of
the Eastern Great Lakes, with accumulations possible through the
daytime. As high pressure pushes towards the region Wednesday night,
falling inversion heights and drier air should end lake snows...with
a partly cloudy sky remaining. Temperatures Wednesday night will
drop back into the 20s...lower 20s inland, and upper 20s near the
lakes.

&&

.LONG TERM /THURSDAY THROUGH SUNDAY/...
Moving on into the extended portion of the forecast...the really good
news is that Thanksgiving Day itself should feature continued fair dry
weather as sprawling surface-based ridging remains in place a little to
the south of our area. Otherwise...high temperatures will again generally
range between 35 and 40 degrees.

Thursday night and Friday model solutions begin to diverge a little
with the GFS keeping us under the influence of high pressure and
therefore dry...while the ECMWF drags a weak cold front through
Northern New York Thursday night...before rebuilding high pressure
across our area during Friday. At worst...this would result in a
chance of snow showers across the North Country Thursday night...
with dry weather and somewhat below average temperatures otherwise
continuing.

After that...both guidance packages suggest that another low will
pass by to our north sometime during the Friday night-Saturday time
frame...while in the process dragging another cold front across our
region. This should translate into renewed chances for rain and snow
showers to close out the week...along with near to slightly below
normal temperatures.

&&

.AVIATION /22Z SUNDAY THROUGH FRIDAY/...
Cigs will average 1500-2500 feet for all of the TAF sites. Due
to the nature of the multi-banded lake effect, there will be
short burst of snow and temporary IFR conditions at times
overnight. This will be most noticeable over the Southern Tier
(KJHW/KOLE). Expect a coherent single band development SE of
Lake Ontario toward KFZY overnight.

Lake effect bands will weaken and drift north on Monday, with
IFR under the lake bands and VFR elsewhere. Expect mostly VFR
over all WNY and CNY by the end of the day.

Outlook...

Tuesday...Mainly VFR.
Wednesday...Rain and snow showers with a chance of MVFR/IFR.
Thursday...VFR, but MVFR in lake effect -SHSN E of the lakes.
Friday...Mainly VFR.

&&

.MARINE...
A tight pressure gradient will remain across the lower Great Lakes
through Monday. Small craft advisories will remain in place through
tonight.

Winds will subside somewhat Monday, as expansive high pressure
over the southern plains drifts to the east across the the
southern states. This will weaken the pressure gradient while
also establishing a warm advective pattern, which is less
conducive for low level mixing.

Fresh to strong southwesterlies found ahead of the next cold front
on Tuesday will keep rough conditions in place on Lake Erie.
Meanwhile offshore winds for the bulk of Lake Ontario will direct
the highest waves into canadian waters. The exception will be over
the northeast portion of the lake, north of Mexico Bay.

&&

.TIDES/COASTAL FLOODING...
Strong onshore winds will continue into tonight building
significant wave action along the Lake Ontario shoreline. The
combination of already higher lake levels and continued wave
action will result in increased shoreline erosion, especially
where the lakeshore is already unstable from erosion earlier in
the year.

&&

.BUF WATCHES/WARNINGS/ADVISORIES...
NY...Lakeshore Flood Warning until 1 AM EST Monday for NYZ001>007.
     Lake Effect Snow Warning until 6 PM EST Monday for NYZ005-006.
     Winter Weather Advisory until 10 AM EST Monday for NYZ004.
     Winter Weather Advisory until 1 PM EST Monday for NYZ012-019-
     020-085.
MARINE...Small Craft Advisory until 1 AM EST Monday for LEZ020.
         Small Craft Advisory until 6 PM EST Tuesday for LEZ040-041.
         Small Craft Advisory until 1 AM EST Monday for LOZ030.
         Small Craft Advisory until 6 PM EST Tuesday for LOZ042>045.

&&

$$

SYNOPSIS...RSH/TMA
NEAR TERM...RSH/TMA/ZAFF
SHORT TERM...JJR/THOMAS
LONG TERM...JJR
AVIATION...RSH/TMA/ZAFF
MARINE...RSH/TMA
TIDES/COASTAL FLOODING...TMA/JJR


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