Area Forecast Discussion
Issued by NWS Buffalo, NY

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000
FXUS61 KBUF 181514
AFDBUF

Area Forecast Discussion
National Weather Service Buffalo NY
1014 AM EST Sat Nov 18 2017

.SYNOPSIS...
A strengthening storm system approaching from the mid western states
will generate occasional rain across our region this afternoon, then
a strong cold front will plow across the Lower Great Lakes to
produce mixed precipitation tonight. Gusty winds in the wake of the
powerful front will introduce a notably colder airmass into the area
for the second half of the weekend. This December like airmass will
generate accumulating snows southeast of both lakes Sunday into
Monday.

&&

.NEAR TERM /THROUGH TONIGHT/...
As promised for nearly a week now...it will be quite unsettled over
our region today as a complex storm system will approach our region
from the mid west. The system will be responsible for two distinct
rain events...some gusty winds...and eventually some accumulating
lake snows for sites southeast of both lakes.

Regional radars showing rain ending from west to east. This will
leave plenty of cloud cover into the early part of the afternoon
with little in the way of rain.

As we progress through the afternoon...the second round of rain
will take aim on the region as the deepening cyclone will make
its way to the northeast across Lake Erie. The impetus for this
round of rain will be even stronger jet induced lift brought
about in the left front entrance region of a 140kt H25 jet over
the Lower Ohio and Tennessee Valleys. In fact...the rain will
probably be heaviest with this particular batch for the late
afternoon and early evening. Rainfall amounts during the
daylight hours will range from roughly three quarters of an inch
over the Southern Tier to under a quarter inch over the North
Country. A southerly flow will factor into this...providing a
little orographic enhancement for the Southern Tier...and a
general 25 percent reduction from downsloping along a 25 mile
swath straddling the New York state Thruway between Buffalo and
the Montezuma wildlife refuge west of Syracuse. This same effect
will be found on opposing sides of the Tug Hill plateau.

Meanwhile...much milder air will pour northward across the region in
the wake of the initial `warm front`. This will allow temperatures to
climb to within a few degrees of 50 today across the western
counties...with readings likely stalling in the 40s for the Eastern
Lake Ontario region. The warmth will be immediately proceeded by
some gusty winds...mainly over the higher terrain (abv 1500 ft)
where winds could gust to 35 mph.

Tonight...the deep cyclone will move from southernmost Ontario and
the Niagara Frontier to the St Lawrence Valley. This forecast track
is too close to our forecast area to be a legitimate high wind
threat...although it will certainly become windy in its wake. The
strongest winds tonight will come after midnight...and will be
focused across the immediate lake shows and for the higher
elevations where winds could gust to 45 mph.

Along with the freshening winds overnight...strong H925-70
frontogentic forcing along and behind a powerful cold front will
lead to more precipitation. All of this will be in the form of rain
through midnight...then as colder air deepens in the wake of the
cold front...the rain will taper off as a mix of light rain and wet
snow. This change over will be a couple hours slower than earlier
forecasts...but there could still be enough snow for some of the
higher elevations to pick up a light coating by daybreak. This will
mainly be the case across the Southern Tier where the synoptic
driven precipitation will transition to lake effect by morning. Also
working against more significant accumulations will be temperatures
that are forecast to only drop to the low to mid 30s for the high
spots...and the mid to upper 30s across the lake plains.

&&

.SHORT TERM /SUNDAY THROUGH TUESDAY NIGHT/...
By the start of Sunday morning...deep low pressure will be
situated just north of the New York-Quebec border...with this
feature sliding further down the Saint Lawrence Valley during
the course of the day. Consequently...lingering synoptically-
driven rain across the eastern Finger Lakes and North Country
(with snow mixed in across the higher terrain) at the start of
the day will steadily wind down from west to east during Sunday
morning.

In the wake of the low...a cyclonic flow of much colder air will
overspread the region Sunday...then will remain in place through
Sunday night and Monday morning...before giving way to building
ridging and a corresponding warm advection regime Monday afternoon
and Monday night. With lake-deltaT values more than favorable for
a lake response and inversion heights looking to be at respectable
levels at points...this will set the stage for a round of lake
effect snows to the southeast of both lakes in a predominantly
west-northwest flow regime. The above said...synoptic-scale
moisture levels/inversion heights will also be a limiting factor
at points...as will a relative lack of consistent strong lift
within the prime dendritic snow growth zone. Putting all of this
together...it currently still appears that advisory-type snows
will be the most likely outcome within the favored lake effect
areas.

Looking more closely at the forecast details...off Lake Erie the
incoming colder airmass will already be cold enough to support
a lake response by the start of Sunday morning...with marginal
temperature profiles supporting a mix of rain and wet snow
initially...before steady cold air advection drives a changeover
to predominantly snow as the day progresses. While the lake effect
activity will be relatively weak/light in nature Sunday morning...
it should ramp up to moderate levels for Sunday afternoon and
evening as an synoptic-scale moisture improves...an upstream
connection develops to Lake Huron and inversion heights temporarily
climb to around 8-9 kft. After that time...light to occasionally
moderate lake snows will then continue through the balance of
Sunday night and into Monday morning on gradually backing low
level flow...before rapidly weakening Monday afternoon as inversion
heights crash and supporting background moisture diminishes. Expect
just some remnant flurries to possibly reach the Niagara Frontier
as the flow backs further to west-southwesterly early Monday
evening...before ending altogether by midnight.

Meanwhile off Lake Ontario...the developing lake effect activity
will likely remain considerably hampered (and thus lighter in nature)
through most if not all of Sunday by the more limited synoptic-scale
moisture that will be in place over that region for the latter two
thirds of the day...as well as somewhat lower inversion heights.
This will then change late Sunday and Sunday night as synoptic-scale
moisture improves...an upstream connection develops to Georgian Bay...
and inversion heights climb to more favorable levels...all of which
should allow for a more cohesive band of moderate snow to develop
east-southeast of the lake...where it will continue through Monday
morning. After that time...the band will gradually lift north toward
the Tug Hill while tending to weaken through the rest of Monday as
the upstream connection is lost and background moisture fades again...
before lifting further north to the Watertown area and falling apart
Monday night as the flow backs to southwesterly and inversion heights
rapidly crash.

Again...snowfall totals from this event look to primarily be in the
advisory range off both lakes...with current forecast projections
supporting the potential for a general 3-6" per 12 hours while the
lake snows are at their peak...which will not last much more than
that off either lake. Given this...have elected to hold off on
issuing any winter weather headlines for at least one more forecast
cycle. While some lower-end warning-criteria snows cannot be completely
ruled out southeast of Lake Ontario given the potential upstream
connection to Georgian Bay...such amounts remain rather questionable
at the present time.

Outside of the lingering synoptic-scale precipitation Sunday morning
and the aforementioned lake snows...the Sunday-Monday night period
will feature colder temperatures and some scattered snow showers
at times...with rather windy conditions on Sunday diminishing to
more moderate levels Sunday night and Monday.

Taking a quick gander at the last third of this period...Tuesday
still looks to be mainly dry and breezy to windy again out ahead
of an approaching cold front...while the strong warm advective
regime out ahead of the front also temporarily drives temperatures
back to above average levels in the upper 40s to mid 50s. The front
itself will then ease its way across our region Tuesday night...
with scattered rain showers mixing with/changing over to snow
as colder air returns in the wake of the boundary.

&&

.LONG TERM /WEDNESDAY THROUGH FRIDAY/...
On Wednesday a colder west-northwesterly flow will again be in place
across our region...however the colder airmass also appears to be
rather dry. This should tend to keep any lake response downwind of
the lakes fairly limited in scope...with a return to largely dry
weather then more likely for Wednesday night and Thanksgiving Day
as surface-based ridging builds across the region..

Another frontal boundary will likely dip southward from Canada
Friday...maintain chances for snow, especially across Lake Ontario
and points eastward which at this time will be closer to the frontal
boundary.

&&

.AVIATION /15Z SATURDAY THROUGH WEDNESDAY/...
A weak warm front pushing north across our region will keep or lower
cigs to MVFR. Meanwhile, cigs will be at IFR levels across the
bulk of the Southern Tier. These lowering cigs will be
accompanied by a little light rain.

As we progress through the afternoon into the early evening, a
strengthening storm system moving by just to our west will
spread another round of steady rain across the region. The rain
will help to further lower cigs to IFR levels in most areas for
tonight, with low IFR cigs anticipated for the Southern Tier
and portions of the Finger Lakes.

Outlook...

Sunday-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.

&&

.MARINE...
A deepening storm system over the mid western states will approach
the Lower Great Lakes today...and this will result in freshening
southerly winds. While winds will approach small craft advisory
criteria...the highest waves will be directed into Canadian waters.

The real problem during this forecast package will be the strong
winds that will follow the strengthening storm system as it exits to
our north and east late tonight and Sunday. Winds veering to the
west and northwest in the wake of the storm will increase to gale
force late tonight through Sunday. Gale warnings will remain in
place.

These very strong winds will only marginally subside Sunday night
and Monday...as expansive high pressure over the southern states
will make its way off the southeast coast.

&&

.TIDES/COASTAL FLOODING...
Developing very strong onshore winds will build significant wave
action from late tonight into Sunday night 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 Watch from late tonight through late Sunday
     night for NYZ001>007.
MARINE...Gale Warning from 10 PM this evening to 7 PM EST Sunday for
         LEZ040-041.
         Gale Warning from 1 AM to 10 PM EST Sunday for
         LOZ042>045-062>065.

&&

$$

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



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