Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 221412
AFDBUF

Area Forecast Discussion
National Weather Service Buffalo NY
912 AM EST Wed Nov 22 2017

.SYNOPSIS...
A brisk northwest flow will bring a cooler airmass over the
Eastern Great Lakes region today with some scattered showers. Nuisance
lake effect snow and rain may develop late tonight and into
Thanksgiving. The next system is expected to affect the region
Saturday into Sunday.

&&

.NEAR TERM /THROUGH TONIGHT/...
A cold front is moving through eastern NY this morning, leaving
our region in a brisk and cold northwest flow. The back edge of
large scale precipitation is similarly moving into Central and
Eastern NY, with only light/scattered lake effect/upslope
precipitation remaining over the region.

Temperatures aloft at 850 hPa will drop to around - 9 to -10C
today. This will bring a much cooler day, with highs in the low
to mid 30s, and also create enough lake instability to generate
lake effect snow showers. Inversion heights will be low, around
5k feet, and the shallow structure of the bands will guarantee
little or no accumulation of snow SE of the lakes.

Lake effect snow will weaken further or end altogether tonight
as high pressure from the Midwest nudges eastward. This feature
will bring drier air in the lower levels, as well as increase
wind shear as winds back to more southwesterly. Later in the
night as the winds align, lake effect snow will likely
"intensify" off Lake Erie, with again light snow/flurries now
forming and carried inland across So. Erie county, and then up
towards Metro Buffalo on a southwest wind. Shallow inversion
heights and the influx of drier air will warrant just high
chance pops for now...as any accumulation will be minor.

Likewise off Lake Ontario, backing winds will begin to carry light
snow showers and flurries northward towards Watertown.

Lows tonight will drop back into the upper teens to mid 20s, several
degrees below normal.

&&

.SHORT TERM /THURSDAY THROUGH SATURDAY/...
We can look forward to moderating temperatures for much of this
period...as the mercury will climb back to well above normal as we
work past Thanksgiving. Our warm up will be the result of an
oscillating low amplitude flow...that with the mean H25 jet
straddling the Canadian border...will mean that our source region
will be increasingly Pacific in nature. This is not to say that
there won`t be weather to deal with though...especially as we head
into the end of the week.

As we open this time frame on Thursday...the axis of a large surface
based ridge will exit to the east across New England while a mid
level trough will pass by to our north. Without significant height
falls though...weak subsidence should persist across much of the
region. This will maintain generally fair weather for Thanksgiving
Day...however there will be some nuisance lake effect pcpn to deal
with northeast of both lakes.

Instability over the lakes on Thursday will be relatively weak with
a cap in place around 5-7 k ft...so the good news is that a
significant lake response is NOT in the cards. Also working against
any real snow accumulations will be the lack of moisture between 5-
10k ft. Since the snow dendritic growth zone will be found in an
alto-cu layer...this snow will fall through a dry layer and
experience a fair amount of sublimation/evaporation before making it
to the surface. It will not be until the column moistens up during
the passage of the mid level trough that the pcpn will really have a
decent chance of making it to the ground. At that point...temps will
be at or above freezing...so am not expecting more than a coating or
so of snow for sites generally northeast of the lakes.

As the passing mid level trough pushes a weak cold front to just
north of Lake Ontario Thursday evening...the steering flow over the
lakes will veer from southwest to west. This will initially allow
for some enhancement across the long fetch of Lake Ontario so that
an inch or so of accumulation could be found in the vcnty of the Tug
Hill. Keep in mind though that with a low cap of about 5k ft and a
shrinking dendritic growth zone...snowfall rates should not be an
issue. Even less impressive lake snow showers will be found east of
Lake Erie where the onset of warm advection will bring a complete
end to the lake response. Otherwise...the majority of the forecast
area will experience fair weather with seasonable temperatures.

On Friday...a flat shortwave ridge will push across the Lower Great
Lakes...while the large surface high will make its way off the Mid
Atlantic coast. A deepening southerly flow will then become
established between the high to our east and an approaching Pacific
based cold front over the plains. The ensuing warm advection will
boost H85 temps into the upper single digits C over the western
counties...and this will allow our max temps to climb well into the
40s (nr 50 Gen Valley) while we return to at least partial sunshine.

A deep storm system in the vcnty of James Bay will push the first of
two cold fronts to our doorstep by daybreak Saturday. While the
majority of Friday night will be rain free...there could be some
showers over the far western counties as we work our way through the
wee hours of Saturday morning. Since the southerly flow ahead of the
front will not allow temperatures to drop to critical winter
levels...the precipitation will be in the form of rain showers.

The Pacific front will cross our forecast area Saturday morning.
While some morning rain showers will likely accompany the passage of
this fast moving front...the midday and early afternoon could be
pcpn free. Have backed way off on pops for this 4-6 hour window...
but as we progress deeper into the afternoon...a digging longwave
trough over the Upper Great Lakes and continued cold advection over
our region will increase the potential for some mixed rain and snow
showers.

&&

.LONG TERM /SATURDAY NIGHT THROUGH TUESDAY/...
This period will feature two distinctly different weather regimes. A
relatively short stint of wintry weather with widespread snow will
give way to a pronounced day to day warming trend as we head through
the first half of the work week.

A longwave trough pushing from eastern Ontario to Quebec will be the
driving force for a second cold frontal passage Saturday night. This
will be a stronger frontal passage...one that will usher H85 temps
as low as -12c across Lake Ontario and the eastern Lake Ontario
region. While this is not as cold as earlier forecast...it is
certainly cold enough to establish accumulating lake snows southeast
of both lakes. The frontal passage itself should generate a coating
an inch of accumulation across the forecast area...with several
inches looking more plausible for the Chautauqua ridge and from
Wayne County east across the southern half of Oswego county. Have
raised pops to categorical for these areas. The stage will then be
set for what could be a challenging day for travel on Sunday.

Medium range ensembles remain in good agreement that a cold cyclonic
northerly flow will be in place over our region on Sunday...while
fairly deep synoptic moisture will persist. Given that H85 temps are
forecast to be in the vcnty of -12c and that there should be some
added lift from the northerly upslope flow...fairly steady snow
should be found southeast of both lakes...including along the bulk
of the New York State Thruway. While accumulations are not expected
to be significant in themselves...it could be cold enough for the
snow to be a little more greasy so that untreated roadways would be
more difficult to navigate. Stay tuned.

Sunday night and Monday...a broad ridge over the Mid West and Upper
Great Lakes will push across the Lower Great Lakes. The ensuing warm
advection and subsequent lowering subsidence inversion will bring an
end to the accumulating...problematic lake snows southeast of both
lakes...while fair weather will be found elsewhere. This will be
start of the aforementioned warm up. Temperatures that will struggle
to the freezing mark on Sunday will climb to near 40 over the
western counties (mid 30s North Country) on Monday.

The broad mid level ridge is then forecast to amplify over the
eastern third of the country on Tuesday. This will pump H85 temps to
near 10c...which would easily support afternoon highs in the 50s for
the majority of western New York.

&&

.AVIATION /14Z WEDNESDAY THROUGH SUNDAY/...

A prevailing NW flow will remain over the region into the
afternoon with a mix of MVFR and some IFR over higher terrain,
improving a little over time. There should be some clearing
skies and VFR flight conditions expanding across the region
into the night as high pressure moves toward the region. Some
weak localized lake effect plumes may form on a WSW flow
however.

Outlook...

Thursday...Mainly VFR, with a chance of localized MVFR in scattered
snow and rain showers northeast of the lakes.
Thursday night...A chance of snow showers/MVFR across the North
Country, otherwise mainly VFR.
Friday...Mainly VFR.
Saturday...VFR/MVFR with rain showers likely.
Sunday...Mainly MVFR, with chance of localized IFR in lake effect
snow showers southeast of the lakes.

&&

.MARINE...
Northwest winds will continue small craft advisories on the lakes
today. Winds and waves will diminish through the day as high
pressure nears the region from the Midwest. There is some
possibility that the SCA will need to be extended on Lake Ontario by
a few hours as waves may not diminish past 4 feet until closer to
mid evening hours.

A bit further down the road, it appears like that another round of
Small Craft Advisories will be needed again for Thursday afternoon
and night as the pressure gradient tightens around a cold front
passing by just to our north.

&&

.BUF WATCHES/WARNINGS/ADVISORIES...
NY...None.
MARINE...Small Craft Advisory until 1 PM EST this afternoon for LEZ040-
         041.
         Small Craft Advisory until 7 PM EST this evening for
         LOZ042>045.

&&

$$

SYNOPSIS...THOMAS/ZAFF
NEAR TERM...THOMAS/ZAFF
SHORT TERM...RSH
LONG TERM...RSH
AVIATION...THOMAS/ZAFF
MARINE...JM/JJR/THOMAS/ZAFF



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