Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 222105

Area Forecast Discussion
National Weather Service Buffalo NY
405 PM EST Wed Nov 22 2017

A large area of surface high pressure will move from the Midwest
eastward toward the mid Atlantic states during Thanksgiving. While
there may be some scattered rain and snow showers around east and
northeast Lake Erie and Lake Ontario, most of the holiday should be
dry across Western and Central New York. Dry weather will last
through Friday. A weak cold front will move across the region on
Saturday with some lake effect snow east and southeast of the lakes
on Sunday.


A low amplitude upper level longwave trough will be over the eastern
US through Thanksgiving.  While this will ensure temperatures aloft
will be low enough for some lake effect, a broad area of surface high
pressure will be moving from the Central US toward the Mid Atlantic.
The corresponding dryish airmass should keep lake effect processes
from doing much more than sputter at times. The best chance for some
semblance of a lake band will be late Thanksgiving morning into the
the evening as the flow becomes WSW over both lakes (and maximizing
fetch), with a resulting potential plume of flurries or light snow
showers moving N with time.

For Erie, this would start a plume over the Boston Hills or perhaps
a little south during the morning, moving northward across the
Buffalo Metro area and into the Northtowns by the evening.  For
Ontario, a band could form on a WNW flow over Wayne and then move
northward across the Eastern Lake Ontario during the day and toward
or into the entrance to the St Lawrence River by days end.

But, in both cases, dry air aloft will severely limit potential.  In
fact, the atmospheric profile in the worst case (but realistic from
current conditions) would barely get the airmass moist enough for
efficient dendrite growth.  In short, this means the potential for
weather related impacts during the holiday remain low, with mostly
passing flurries or non-accumulating snow showers east of the Lower
Great Lakes region, possibly even just non-consequential rain
showers, with dry weather outside of lake effect regions.


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. 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

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 through
Sunday. 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.


Surface and mid-level ridging will build across western and central
New York Sunday night and Monday. A lowering subsidence inversion
will bring an end to accumulating lake effect snow southeast of both
lakes during the evening hours leveling just chance/slight chance
POPs through the overnight. BUFKIT time-height cross sections
indicate that plentiful lift and low level moisture will be
available through the night even with lake induced equilibrium
levels crashing below 5kft. This could be a set up for some freezing
drizzle as surface temperatures will be below freezing and lingering
moisture will be below the dendritic snow growth zone. This is
something to watch closely over the next few days. Otherwise expect
dry weather with low clouds breaking for some sun on Monday. Temps
with the riding aloft will make a run into the low 40s across WNY
but likely linger in the 30s east of Lake Ontario.

The surface and mid-level ridge axis shift east of our area by
Tuesday with southerly flow picking up ahead of what the GFS/GEM
show as another cold frontal passage for Tuesday night/Wednesday.
The ECMWF however does not show a front crossing our region. With
these differences in mid have only brought in slight chance POPs for
Tuesday night with Tuesday expected to largely be dry. The global
models do show a strong 50 knot low level jet crossing our region
Tuesday which could lead to some gusty winds with diurnal mixing.
Temps Tuesday under Southerly flow may make a run toward the upper
40s to low 50s with clouds possibly increasing from the west if the
GFS is correct with the approach of a front. Wednesday continues a
spread in guidance with the GFS/GEM showing -10C 850mb temps behind
a cold front and the EC showing +2C to +6C in zonal flow. Have
leaned toward GFS/GEM with chance POPs while surface temps have a
high bust potential with such spread. Have leaned toward a blend of
guidance for now yielding mid to upper 40s for highs but cloud be
cooler especially if the cold frontal passage occurs.


VFR and some MVFR CIGS (higher terrain) will will last through the
night as low level moisture remains trapped under an 850mb level
inversion. Expect a little improvement overnight with less low cloud
cover with time, although some higher level clouds will move into
toward Thanksgiving morning.  Some lake effect plumes are expected
to form off both lakes during the day and migrate northward.
However, these bands will be rather weak with minimal precipitation
(rain and/or snow) reaching the ground as they move past KBUF/IAG
and KART during the afternoon.


Thursday night...Chance -SHSN/MVFR E of the lakes, otherwise VFR.
Saturday...VFR/MVFR with rain showers likely.
Sunday...Mainly MVFR, localized IFR in -SHSN SE of the lakes.


WNW flow will continue into tonight and weaken with time, then back
to WSW on Thanksgiving.  SCAs may be needed again by afternoon
together with scattered but light lake effect showers.  Relatively
quiet weather will then last through Friday with WSW or SW flow. The
next cold front moves through on Saturday with lake effect
developing on a NW flow by Sunday.


MARINE...Small Craft Advisory until 7 PM EST this evening for LEZ040-
         Small Craft Advisory until 7 PM EST this evening for



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