Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 230601
AFDBUF

Area Forecast Discussion
National Weather Service Buffalo NY
101 AM EST Thu Nov 23 2017

.SYNOPSIS...
A large area of surface high pressure will move from the Midwest
eastward toward the mid Atlantic states today. While there will 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.

&&

.NEAR TERM /THROUGH TODAY/...
A low amplitude upper level longwave trough will be over the eastern
US today.  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 dry airmass should keep lake effect processes from
doing much more than sputter at times.

Off Lake Erie, Scattered flurries have ended across the western
Southern Tier as of mid evening, as moisture became too shallow to
support any crystal growth. Late tonight and Thursday morning the
surface ridge axis will move east of the lake, allowing boundary
layer flow to back to the west and southwest and becoming better
aligned with the major axis of the lake. Expect a band of lake
effect snow to become a little better organized over the lake by
early morning, and mostly stay over the lake initially with weak
boundary layer flow.

By mid morning the flow will increase and allow the band to move
onshore, while at the same time moving northward towards metro
Buffalo as flow becomes southwest. The band of snow will quickly
cross Buffalo and end up in the Northtowns and Niagara County by
early afternoon, where it will mix with or change to rain. Cloud
microphysics will be interesting. The shallow lake effect cloud band
will be too warm for dentritic crystal growth, but there may be a
brief seeder/feeder mechanism in place as mid level synoptic
moisture crosses over the top of the lake effect band and seeds the
lake effect clouds from above. Accumulations will be very limited as
temperatures move above freezing at the surface as the band is
coming together and crossing Buffalo. Expect nothing more than a
light coating on the grass, which will promptly melt in the afternoon.

Off Lake Ontario, expect flurries and scattered snow showers to
continue through tonight from Wayne to Oswego counties with a spotty
dusting of accumulation. Farther west, expect just very light
flurries across Rochester and Orleans County. This band may try to
come together a little late tonight and early Thursday morning from
Sodus Bay to Oswego as convergence increases in an anticyclonic arc
at the southeast end of the lake following the passage of the ridge
axis. Again however, low inversion heights and poor dentritic snow
growth will keep any accumulation to a dusting. During the day, what
remains of the lake effect will push quickly north into Jefferson
County, with scattered rain and wet snow showers during the
afternoon.

Outside of lake effect areas, expect a good deal of cloud cover most
of the time tonight and Thursday, with just a few odd breaks of
partial clearing. Lows tonight will be in the 20s for most. Highs on
Thanksgiving Day will be in the upper 30s to around 40 on the lake
plains, and mid 30s higher terrain.

&&

.SHORT TERM /TONIGHT THROUGH SUNDAY/...
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
showers.

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.

&&

.LONG TERM /SUNDAY NIGHT THROUGH WEDNESDAY/...
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.

&&

.AVIATION /06Z THURSDAY THROUGH MONDAY/...
MVFR and low end VFR flight conditions in low ceilings will start
the 06Z TAFS, with these conditions persisting through much of the
day.

As winds become southwesterly today, there will remain enough lake
instability such that light lake effect snow may pass the KBUF/KART
airfields. Marginal temperatures may allow for some rain to mix into
the snow band.

High pressure with associated fair weather and mainly VFR flight
conditions will pass by just to our south later tonight. Scattered
lake effect flurries, and MVFR CIGS will linger across KART.  A
tightening pressure gradient between this high pressure and nearing
weak surface trough to the north will maintain a moderate
southwesterly flow through tonight.

Outlook...

Friday...VFR.
Saturday...VFR/MVFR with rain showers likely.
Sunday...Mainly MVFR, localized IFR in -SHSN SE of the lakes.
Monday...VFR.

&&

.MARINE...
WNW flow will continue into tonight and weaken with time, with Small
Craft Advisory conditions slowly ending from west to east overnight
on Lake Ontario. Winds will become southwest and increase again on
Thursday as fairly strong low pressure moves across northern Ontario
and Quebec. This will produce another round of Small Craft Advisory
conditions on both lakes starting late morning to early afternoon.
These moderate southwesterlies will last right through early
Saturday.

&&

.BUF WATCHES/WARNINGS/ADVISORIES...
NY...None.
MARINE...Small Craft Advisory until 4 AM EST early this morning for
         LOZ043>045.

&&

$$

SYNOPSIS...HITCHCOCK/THOMAS/ZAFF
NEAR TERM...HITCHCOCK/THOMAS/ZAFF
SHORT TERM...CHURCH/RSH
LONG TERM...SMITH
AVIATION...THOMAS
MARINE...HITCHCOCK/ZAFF


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