Area Forecast Discussion
Issued by NWS Buffalo, NY

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000
FXUS61 KBUF 191153
AFDBUF

Area Forecast Discussion
National Weather Service Buffalo NY
653 AM EST Sun Nov 19 2017

.SYNOPSIS...
A strong storm exiting across the Saint Lawrence Valley will send a
blast of much colder air across our region today...and this will
allow our precipitation to change to a little snow. The colder air
will also 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 /THROUGH TONIGHT/...
Today will be a transitional day across the region...at least
through the morning hours...as a strong storm will push away from
our region via the St Lawrence Valley. The relatively mild and wet
weather that has been over the region for the past twenty four hours
will now give way to more typical late November weather...with
colder air encouraging the departing precipitation to end as a bit
of wet snow.

The colder air will also wake up Lakes Erie and Ontario...as
moderately strong cold advection will set up favorable conditions
for accumulating lake snows. Winter weather advisories will remain
in place southeast of Lake Erie with winter storm watches southeast
of Lake Ontario. The details...

First for Lake Erie...a general 290-300 flow will be in place
throughout the day with a cap below 10k ft. While this flow will
allow for some upstream priming off multiple lakes...the relatively
low cap and only marginally cold near surface conditions should
limit snowfall to a few inches...with the higher amounts coming over
terrain above 1500 ft. As we push through tonight though...the cold
air will deepen and the inversion will lift to 10k ft or better.
This...and the overnight timing...will favor higher and more
widespread accumulations. Nighttime accumulations will be in the
vcnty of 2 to 4 inches away from the lake...with the higher amounts
being concentrated across the higher terrain.

In regards to lake snows from Lake Ontario...as is usually the case
it will take several more hours for a noticeable lake response.
There will be a little more shear and slightly `warmer` profile over
the lake through early afternoon...but as we progress through the
afternoon into the evening hours...a fairly well organized band of
lake snow should become focused on Wayne and northern Cayuga
counties. As in the case of Lake Erie...the cap will be under 10k ft
so am not expecting overly generous amounts of snow for the daylight
hours. Amounts will likely range from a coating to an inch or so. As
we push through tonight...the cap will lift to more than 10k ft with
a fairly solid upstream connection becoming established with
Georgian Bay. While this will lead to greater 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.
Will maintain the winter storm watch for lake effect snow for this
area and allow additional guidance to help in doing additional fine
tuning. From this vantage point though...the most significant lake
snows in this area should occur from late tonight through Monday.

For the remainder of western New York and the North Country...today
will feature near to steady falling temperatures with cold blustery
winds. Gusts to 40 mph will be possible near the lake shores and
across higher terrain...and with temperatures generally in the
30s...we can expect wind chill values to be in the teens and
twenties.

&&

.SHORT TERM /MONDAY 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/early Monday evening - before
dying out altogether by late Monday evening. 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...before lifting north to the Watertown area Monday
night and dissipating due to further increases in shear and more
rapid crashing of the subsidence inversion. 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.
Only minor additional accumulations of under an inch are then expected
Monday night as the activity dissipates across the North Country.

Outside of the main lake effect areas...the Monday-Monday night time
frame will merely be quiet and dry...albeit with continued moderately
brisk winds as a decent pressure gradient will remain intact across
our region. With respect to temperatures...highs on Monday will range
from the mid to upper 30s over the higher terrain to the lower 40s
across the lake plains of far western New York...with mainly evening
lows in the 30s Monday night giving way to rising temperatures
overnight as warm air advection strengthens across our region.

As we move into Tuesday...the aforementioned surface low will slowly
drift east into North Central Quebec...while slowly dragging its
trailing cold front in our direction. At this juncture...it still
appears that the front will remain too far to our west to bring any
showers into our region during the daylight hours...so have removed
any mention of such from the forecast...with fairly plentiful sunshine
through midday/early afternoon only giving way to some increasing
clouds during the mid to late afternoon hours. With a tightening
pressure gradient in place out ahead of the cold front...we can also
expect another round of rather breezy to windy conditions to develop
during Tuesday...with the strongest winds found across far WNY which
will lie under an axis of stronger (40-50 knot) flow in the 925-850
mb layer. While the continued warm air advection regime will prevent
these stronger winds from totally mixing down to the surface...the
prevailing south-southwesterly low level flow may still be supportive
of some wind gusts approaching advisory levels across Niagara and
portions of Northern Erie Counties...as well as along the Lake Erie
shoreline. Otherwise it will be a rather mild day...with the breezy
to windy conditions...fairly plentiful sunshine...and 925 mb temps
of +4C to +7C leading to widespread high temperatures in the lower
to mid 50s.

Tuesday night the surface low will gradually lift toward the Ungava
Peninsula while dragging its trailing cold front across our region.
Low level convergence and moisture pooling along the boundary should
be just enough to produce a broken band of rain showers as it passes
through our region...with modest cold advection behind the front
possibly forcing a mix with/changeover to a little wet snow before
these quickly end from west to east. Otherwise...the cold advection
regime behind the front will result in low temperatures falling
back to between the upper 20s and mid 30s.

In the wake of the front...high pressure with colder and much drier
air will then build across the region Wednesday and Wednesday night.
While the airmass will be plenty cold enough to support a lake response
east and/or southeast of the lakes...its rather dry nature should limit
this to just some scattered light snow showers during Wednesday...with
these then likely falling apart altogether Wednesday night as the ridge
crests overhead. As such... conditions should be good for pre-Thanksgiving
travel across our area...with high temps in the mid to upper 30s Wednesday
followed by lows ranging through the 20s Wednesday night.

&&

.LONG TERM /THURSDAY THROUGH SATURDAY/...
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 /12Z SUNDAY THROUGH THURSDAY/...
IFR to MVFR cigs will be found across all of western and north
central New York this morning...as a strong storm exiting across the
St Lawrence Valley will gradually pull its low cloud cover and
associated mixed precipitation over the region.

As we progress through the day...cigs will average 1500-2500 feet
for all of the TAF sites. The only IFR conditions after 14z will be
found southeast of Lake Erie where lake snows will limit vsbys to 2
miles or less for sites like KJHW and possibly KELZ. There will also
be fairly strong winds over the western counties through the
day...with surface gusts as high as 40 knots near the lakes and
across elevated TAF sites.

For tonight...generally VFR conditions will be found at the major
TAF sites...however lake snows southeast of both lakes will produce
MVFR to IFR conditions for KJHW and sites east of KROC.

Outlook...

Monday...Lake effect SN, IFR SE of lakes, otherwise VFR/MVFR.
Tuesday...Mainly VFR. Wednesday...Rain and snow showers with a
chance of MVFR/IFR.
Thursday...VFR, but MVFR in lake effect snow showers E of the lakes.

&&

.MARINE...
A tight pressure gradient left in the wake of a strong storm exiting
across the St Lawrence Valley will maintain gale force winds across
the Lower Great Lakes into at least this afternoon...although the
bulk of the actual gales will come from the frequent gusts.
Meanwhile small craft advisories will stay in place for the BUF
Harbor and entire length of the Niagara River.

Winds will subside somewhat later today through tonight...as
expansive high pressure over the southern plains will drift 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. Gales over the nearshore waters
will be replaced by small craft advisories that will persist through
Monday.

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...
Developing very strong onshore winds will build significant wave
action along the Lake Ontario shoreline today and into tonight.
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.
     Winter Storm Watch from 4 PM EST this afternoon through Monday
     evening for NYZ005-006.
     Winter Weather Advisory until 1 PM EST Monday for NYZ019-020-
     085.
MARINE...Small Craft Advisory until 1 AM EST Monday for LEZ020.
         Gale Warning until 5 PM EST this afternoon for LEZ040-041.
         Small Craft Advisory until 1 AM EST Monday for LOZ030.
         Gale Warning until 8 PM EST this evening for LOZ042>045-
         062>065.

&&

$$

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



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