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000
FXUS61 KBUF 210231
AFDBUF

Area Forecast Discussion
National Weather Service Buffalo NY
1031 PM EDT Tue Apr 20 2021

.SYNOPSIS...
Low pressure will pass just south of the region late tonight through
Wednesday, producing widespread accumulating snow across the region.
The snow will likely have an impact on the Wednesday morning
commute, with slushy and slippery travel. The colder air will bring
some lake effect snow showers to areas southeast of Lake Erie and
Lake Ontario Wednesday night into Thursday morning. Dry and warmer
weather will return on Friday.

&&

.NEAR TERM /THROUGH WEDNESDAY NIGHT/...
A late season accumulating snow event is still on track through
Wednesday. A cold front will remain stalled just to the south of the
area across Pennsylvania. A mid level trough over the Upper Mid
West will dig and sharpen overnight...reaching the central Great
Lakes Wednesday. DPVA and flow adjustments downstream of the digging
trough will force a strong baroclinic wave to develop along the
stalled frontal zone over the Ohio Valley, with the resulting
surface low tracking from the Ohio Valley to southern New England by
Wednesday afternoon. A shield of widespread precipitation will be
forced by strong frontogenesis and differential temperature
advection to the north of the low track, with an added boost of
strong DPVA and right entrance region jet dynamics ahead of the
digging trough.

Light precipitation has already developed across the Niagara
Frontier and will spread northeast to the North Country by around
midnight. Precipitation will be slow to develop at first from
eastern Cattaraugus County ENE through the Finger Lakes to Oswego
County, with stronger frontogenesis focused north and west of there.
Widespread precipitation will hold off in these areas until late
tonight and Wednesday morning. Colder air will still be in the
process of filtering into the region on low level northerly flow
this evening, so this initial light precipitation may be a rain/snow
mix.

Later tonight through Wednesday morning frontogenesis will intensify
as the baroclinic wave reaches our longitude, resulting in a period
of moderate to heavy precipitation. The combination of ongoing cold
advection, cooling from melting processes, and wet bulbing of the
column will cool the atmosphere sufficiently to change all the
precipitation to snow. Forecast soundings continue to suggest there
may be a narrow zone of freezing rain/sleet near the rain/snow
transition line, and some of this may fall briefly across southern
Cattaraugus and much of Allegany County late tonight through early
Wednesday morning.

The ground is warm in the second half of April, so that will
initially inhibit snow accumulation. Snowfall rate is the key to
accumulation at this time of year, if it snows hard enough the snow
can overcome the warm surface and cover the ground. Once the ground
is covered, the warm ground becomes irrelevant with an insulating
layer of snow between the ground and new snow accumulation on top.
Given the strong forcing in this case, we expect the warm ground to
be overcome for a window from later tonight through Wednesday
morning when snowfall rates are greatest.

As far as accumulations go, expect 3-5" across much of Western NY
and the lower Genesee Valley, including Buffalo and Rochester. The
highest totals are likely to fall along the Chautauqua Ridge, Boston
Hills, and Wyoming County where around 6" is expected. The North
Country can also expect 3-5", with local amounts of up to 6" across
the higher terrain of the Tug Hill. Snowfall rates in these areas
may reach 1" per hour for several hours early Wednesday morning. 2-
4" is expected in Allegany County, along with the potential for a
little freezing rain. The lowest accumulations will likely be found
across Ontario, Wayne, N.Cayuga, Oswego counties where 2-3" is
expected. Forecast snowfall amounts in this area are lower as a
result of surface temperatures being slightly warmer, and the fact
that most of the snow will fall during daylight hours.

The widespread snow will taper off quickly from west to east
Wednesday afternoon, to be replaced by a few snow showers as cold
air pours into the eastern Great Lakes in the afternoon. Lake effect
and upslope snow showers will become more widespread Wednesday night
through Thursday morning as the pool of coldest air aloft crosses
the region. Northwest flow will direct most of this into the western
Southern Tier off Lake Erie, and from Orleans County eastward across
Rochester to Oswego County and down into the Finger Lakes off Lake
Ontario. Upslope flow will also keep more widespread snow showers
going across the higher terrain east of Lake Ontario. All of this
may produce localized additional accumulations of 1-2" Wednesday
night.

&&

.SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/...
A cold, northwest flow will continue Thursday with 850mb
temperatures between -10C and -8C. Lake effect snow showers will be
ongoing Thursday morning with the most concentrated areas southeast
of the Lakes. An organized band of lake effect may be present along
the Lake Ontario shoreline from Orleans and Monroe counties to the
Finger Lakes region Thursday morning. Snow showers will diminish
through the day and snowfall accumulations will be light, less than
an inch. There could be an isolated 1-2 inches across the higher
terrain but chance is low due to the late April sun angle making it
hard for snow to accumulate. Breezy conditions expected with
northwest winds 15 to 25 mph. High temperatures will only reach the
upper 30s to low 40s and with the winds, will feel like the low 30s.

Snow showers will come to an end by Thursday night as warm air
advection occurs across the region. Low temperatures will reach the
low 30s across the higher terrain to the mid to upper 30s near the
lake shores.

Dry weather expected Friday and Friday night as an upper level ridge
moves overhead. Warmer temperatures, however still below normal are
expected with highs in the low to mid 50s and low temperatures in
the low 30s across the higher terrain and mid to upper 30s towards
the lakes.

&&

.LONG TERM /SATURDAY THROUGH TUESDAY/...
Two jets, polar and subtropical will produce their own troughs
across the central United States Saturday. These two troughs and
associated surface lows will eventually cross the area late
Saturday afternoon through Sunday. The first of the two lows that
will impact the region will be the southern low. This low will
track northeast from the south into the Mid-Atlantic states
Saturday before exiting off the East Coast Sunday. Meanwhile the
low over the northern Mid-West will track northeast across
eastern Canada just on the tails of the southern low. Therefore,
expect increasing chances for showers late Saturday afternoon
through Sunday with the passages of these two systems. Shower
activity will decrease late Sunday afternoon as both features
exit to the east.

An omega block over the central United States will work its way east
as the troughs exit the Northeast. Therefore, upper level ridging,
supporting surface high pressure will push into the region beginning
Sunday night and remain overhead through Tuesday. While under the
influence of the surface high pressure, expect a couple of days of
dry weather to start off the week.

Otherwise, expect a range of 50s for the weekend with Sunday being
the coolest day due to the passage of the frontal boundaries. Warm
air advection associated with the upper level ridging will support a
warming trend for the start of the new work week. Highs Monday will
warm up into the mid to upper 50s, while Tuesday will be the warmest
of the period with a range of 60s, and possibly a few 70s.

&&

.AVIATION /03Z WEDNESDAY THROUGH SUNDAY/...
Low pressure will move from the Ohio Valley to southern New England
by Wednesday afternoon. A shield of widespread precipitation will
develop to the north of the low track. An initial rain/snow mix this
evening will quickly change to all snow overnight, with widespread
snow lasting through Wednesday morning before tapering off from west
to east Wednesday afternoon. The snow will produce widespread IFR
conditions later tonight through Wednesday. The snow may become
heavy for several hours early Wednesday morning, with VSBY possibly
nearing airfield minimums at the TAF sites. Expect improvement to
MVFR/VFR from west to east in the afternoon as the snow tapers off.

Outlook...

Wednesday night and Thursday...MVFR/local IFR with scattered
snow showers and some lake effect snow showers southeast of the
lakes. Windy.
Friday and Saturday...Mainly VFR.
Sunday...VFR/MVFR with a chance of showers.

&&

.MARINE...
Low pressure will move east across Pennsylvania late tonight night
to New England by Wednesday afternoon. Winds will become northeast
and increase tonight as this low approaches, then become northwest
and increase further Wednesday afternoon and Wednesday night as cold
air pours into the eastern Great Lakes behind the departing low. A
long period of Small Craft Advisory conditions is expected on Lakes
Erie and Ontario from Wednesday through Friday. The strongest winds
will occur on Thursday, when sustained winds may approach 30 knots
on Lake Ontario.

&&

.BUF WATCHES/WARNINGS/ADVISORIES...
NY...Winter Weather Advisory from 4 AM to 5 PM EDT Wednesday for
     NYZ007-008.
     Winter Weather Advisory from 2 AM to 2 PM EDT Wednesday for
     NYZ003-013-021.
     Winter Weather Advisory until 2 PM EDT Wednesday for NYZ001-
     002-010>012-019-020-085.
MARINE...None.

&&

$$

SYNOPSIS...Hitchcock
NEAR TERM...Hitchcock
SHORT TERM...HSK
LONG TERM...EAJ
AVIATION...Hitchcock
MARINE...Hitchcock