Area Forecast Discussion
Issued by NWS Buffalo, NY

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000
FXUS61 KBUF 171810
AFDBUF

Area Forecast Discussion
National Weather Service Buffalo NY
110 PM EST Fri Nov 17 2017

.SYNOPSIS...
High pressure shifting across our region will continue to provide a
brief return of fair weather this afternoon into tonight. Saturday,
a strong storm system will track across the region with a round of
widespread rain and gusty winds. This will be followed Sunday by
colder temperatures and accumulating lake snows southeast of the
lakes.

&&

.NEAR TERM /THROUGH TONIGHT/...
High pressure is centered over western NY early this afternoon.
Subsidence brought on by the high pressure and dry air advection has
just about completely cleared out the low clouds south of the
eastern Great Lakes. Partly to mostly sunny skies will be in place
for most through this afternoon before mid and high clouds arrive
from just to our west ahead of a slowly organizing storm system over
the Central Plains. Temperatures have been slow to warm today with
cold advection but still feel most areas will see highs range from
the upper 30s to around 40 today with the removal of the cloud cover.
The higher terrain of the North Country will be unlikely to make it
out of the low to mid 30s.

Tonight, the high pressure will slide off to our east...while the
aforementioned area of low pressure tracks toward Chicago and
continues to slowly get better organized. In between these two
systems...much of our area can expect fair/dry weather to prevail
for much of the night under fairly widespread mid and high cloud
cover. Some returns across Michigan this afternoon are expected to
dissipate before reaching western NY as they shift into the back of
the departing high pressure. Late tonight, thickening/lowering
clouds and warming temps aloft bring rain to far western New York
between 4am and 6am. Have speed up the timing an hour or two based
on 12z guidance. Expect temps to cool this evening before southerly
flow yields steadily rising temps for the balance of the night.
Across far western New York...this will result in lows in the lower
to mid 30s giving way to readings in the upper 30s to lower 40s by
daybreak... while further east lows in the 20s will give way to
temps in the 30s by the start of Saturday morning.

Expect that surface temps will be above freezing for nearly all
locations before the rain arrives late tonight. There is just a
slight/low chance that across the interior North Country, away from
Lake Ontario, temps may still be at or just below freezing when the
leading edge of the rain arrives. This has led to a chance of
freezing rain for an hour or two. Confidence remains below 50%
however do to slight but still important in this case differences of
when the models bring in the rain.

&&

.SHORT TERM /SATURDAY THROUGH MONDAY NIGHT/...
Winter headlines may be needed by the end of this period...as a
soaking rain will transition to potentially significant snow
accumulations east of both lakes.

The system that will be responsible for a plethora of interesting
weather for our region during the coming days will be the remnants
of an impressive storm system over the Pacific Northwest. This once
anchored system has since opened up and is in the process of
releasing an abundance of mid level energy across the Inter-Mountain
West...and this will lead to lee side cyclogenesis over the
plains. The resulting disorganized surface low will consolidate
and push northeast to the Great Lakes as a `cutter storm`.
During the past several days though...guidance has been
suggesting that the storm will track further to the
east...across the LOWER Great Lakes. This is somewhat of an
anomalous track...but it should help to spare most (if not all)
of the forecast area from damaging winds. What we will not
escape from though will be a soaking rain...and eventually the
risk for significant lake snows east/southeast of both lakes. As
for the day to day details...

Putting it bluntly...this period will start off with simply lousy
weather. Widespread rain will overspread our region during the
course of Saturday morning...as the headline storm system over the
mid western states will consolidate and strengthen while advancing
to Lake Erie by the end of the day. In the process...it will push a
slow moving warm front up across our region. An anomalously strong
40-50kt low level jet impinging upon the gentle sloping frontal
boundary will provide some lift in the low levels while pumping rich
moisture into our region directly from the GOMEX. The majority of
the lift from this very dynamic system though will be driven from a
coupled H25 jet that will track directly across our region. The
strongest portion of this will be supplied by a 140kt jet over the
Lower Ohio Valley. This lift will interact with PWAT values in the
vcnty of one inch to produce rainfall amounts on Saturday that will
range from three quarters of an inch over the Southern Tier to a
third of an inch east of Lake Ontario. Some sites south of Buffalo
and Rochester could even experience higher amounts if the suggested
elevated instability prompts some embedded thunderstorms. While 00z
guidance is not as aggressive as previous runs with this
instability...will hold onto the chc pops from continuity for
thunder. The range in QPF will also be a function of some orographic
lift over southern sites and downslope `drying` along a 20 mile wide
swath centered roughly along the New York state Thruway from Buffalo
to Syracuse. Meanwhile despite the widespread nature of the rain...
relatively strong winds aloft within the warm advective pattern will
provide enough turbulent mixing so that temperatures will climb into
the 50s. The exception will be east of Lake Ontario in the vcnty of
the yet to pass warm frontal boundary.

A big turn in the weather can then be expected Saturday night. The
still strengthening cyclone will pass by to our north while slinging
a powerful cold front through the region. There will be a burst of
moderately heavy rain as the front plows through during the
evening...then as we progress through the night...the deepening cold
air will set the stage for some mixed lake effect precipitation east
of both lakes. There could even be a few inches of wet snow in the
vcnty of the Chautauqua ridge and Tug Hill plateau.

Normally with a deepening low passing to our west and north...there
would be a concern for strong winds in the wake of the cold front.
In this case though...guidance has been consistent for the past
several days in tracking the storm system further to the east. In a
few model packages...the system is forecast to track over or to our
southeast...which is very ATYPICAL. In any case...the risk for
problematic winds continues to diminish.

As we move into the second half of the weekend...it will feel more
like the Christmas season rather than the days leading up to
Thanksgiving. The combined efforts of the departing storm system
over the St Lawrence Valley and expansive high pressure over the
Lower Mississippi Valley will open the door for a blast of wintry
air over our region. H85 temps are forecast to drop to as low as
-12c...which will not only keep temperatures in the 30s on Sunday...
but will help to fuel moderate to strong convection over the still
relatively warm lakes. Therein lies the concern for Sunday into
Monday.

The base of a fairly deep longwave trough will be centered just to
the east of our forecast area Sunday and Sunday night. This will
keep a cyclonic flow of cold air over the lakes with little
variation in steering flow. While this would normally be favorable
for significant snowfall downwind of the lakes...there will not be
an abundance of synoptic moisture to help support the lake response.
The persistent 300 flow though will allow for some priming from
upstream lakes...so its not out of the question that a plowable
snowfall could take place. This would especially be the case over
Oswego County.

As for some specifics...
Off Lake Erie...a 320 flow early on Sunday will back to around
300 degrees during the midday and afternoon while the limiting cap
will rise to around 10k ft. This will be accompanied by a short
lived upstream connection to lake Huron...with the most favorable
lake snow conditions forecast to come between 18z Sunday and
06z Sunday night. Several inches of snow are anticipated...but
nothing significant. As the low level flow starts to back to 290
later Sunday night...the upstream connection will be lost and
snowfall rates will suffer accordingly. Only minimal snow
accumulations are then expected for the second half of Sunday
night and Monday when warming aloft will eventually bring an end
to the lake response.

Meanwhile off Lake Ontario...organized lake effect snow may take
several hours to get going on Sunday as the cold air will need time
to deepen. Initially...a 300 flow will direct a lake response
across Oswego County with snowfall rates of a half inch or so
per hour within the developing band. As we push into Sunday
night though...a backing of the winds aloft will establish an
upstream connection to Georgian Bay. This will encourage more
significant snowfall rates within a well established band that
will primarily stretch across northern Wayne and Cayuga Counties
to southern Oswego County. It will be from Sunday night into
the first half of Monday that will be the most likely time frame
when winter headlines for lake effect snow would be needed. As
we progress through he day Monday...winds will back a little
during the course of the day...breaking the upstream connection
to Georgian Bay while also allowing accumulating snows to fall
over a new area.

&&

.LONG TERM /TUESDAY THROUGH FRIDAY/...
Tuesday, a strong wave will dig into the north central Great
Lakes, encouraging deepening southwesterly flow across the region.
This will usher in slightly more mild temperatures, with daytime
highs in the mid to upper 40s. However, it may not feel too
much warmer, as the stiffening southwesterly winds 20 to 30 mph
keep apparent temperatures in the mid to upper 30s. Cloud cover
will also increase through the day ahead of the wave.

Model solutions diverge into Wednesday and Thanksgiving Day, with
differing opinions on the strength of the aforementioned shortwave,
and the attendant cold front passage. In the GFS/GEM camp, solutions
are more robust, with a stronger cold front, more active frontal
passage with rain/snow showers, and even some lake effect in the
cold air behind the front for Thanksgiving. However, the EC camp
suggests a weaker/more progressive wave with a less interesting
frontal passage and high pressure quickly building in its wake, with
a dry Thanksgiving. Unfortunately that leaves plenty of uncertainty
around the Wednesday travel day and holiday on Thursday, but
confidence is fairly high in a good travel day on Tuesday with mild
temperatures and dry conditions.

&&

.AVIATION /18Z FRIDAY THROUGH WEDNESDAY/...
High pressure over our region and associated subsidence and dry air
advection have scattered out the previous MVFR stratus leaving
behind VFR conditions. Mid and high clouds will then increase from
the west through this afternoon out ahead of a slowly organizing
storm system over the Central Plains states.

Tonight the elongated ridge of high pressure will slide off to our
east...while the aforementioned area of low pressure tracks toward
Chicago and continues to slowly get better organized. Expect VFR
conditions to continue for much of the night under widespread mid
and high cloud cover.

Rain is then expected to reach far western New York (KBUF/KIAG/KJHW)
by 9-10z with lowering cigs to MVFR/IFR by 12z Saturday. As winds
aloft increase overnight out ahead of the approaching low, LLWS is
also expected at all western TAFs (KBUF/KIAG/KJHW/KROC) 8-12z.

Outlook...
Saturday...Widespread rain and IFR/MVFR conditions developing from
west to east...with LLWS remaining possible.
Saturday night...Rain changing to snow...and becoming windy.
Sunday and Monday...Lake effect snow and attendant IFR southeast
of the lakes...otherwise VFR/MVFR with scattered snow showers.
Tuesday...Mainly VFR.

&&

.MARINE...
High pressure over Lake Ontario will continue a short period of sub-
advisory-level conditions this afternoon and evening.

As we move on into the weekend...a stronger storm system will track
northeastward across the Great Lakes...and may bring a period of
gale force winds to Lakes Erie and Ontario Saturday night and
Sunday. Accordingly...Gale Watches have been hoisted as outlined
below.

&&

.BUF WATCHES/WARNINGS/ADVISORIES...
NY...None.
MARINE...Gale Watch from Saturday evening through Sunday afternoon for
         LEZ040-041.
         Gale Watch from Saturday evening through Sunday
         afternoon for LOZ042>045-062>065.

&&

$$

SYNOPSIS...JJR/SMITH
NEAR TERM...JJR/SMITH
SHORT TERM...RSH
LONG TERM...CHURCH
AVIATION...JJR/SMITH
MARINE...JJR/SMITH


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