Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 200807
AFDBUF

Area Forecast Discussion
National Weather Service Buffalo NY
407 AM EDT Sun Aug 20 2017

.SYNOPSIS...
High pressure will move off the the mid-Atlantic coast today into
tonight. Return flow developing between this surface high and the
next approaching low pressure system will build heat and humidity
through Tuesday. A strong cold front will cross the region late
Tuesday, and may be accompanied by strong to severe thunderstorms.
The work week will end with a touch of fall in the air as a
sprawling Canadian high pressure builds in cooler and drier air.

&&

.NEAR TERM /THROUGH TONIGHT/...
GOES-16 advanced nighttime microphysics RGB early this morning shows
an area of mid-level moisture denoted by clouds containing ice
crystals exiting through the eastern Lake Ontario region. A few
isolated/scattered rain showers coincide with this deeper moisture,
but will taper off and exit the forecast area before sunrise.
Meanwhile, areas of valley can been seen covering portions of
western PA, and with low dew point spreads across the western
Southern Tier early this morning, expect this valley fog will expand
into the western Southern Tier. Temperatures just prior to sunrise
will range from the low to mid 50s in the interior valleys and the
upper 50s to low 60s across the lake plains.

Any patchy fog will dissipate after sunrise today, with diurnal
heating and increased mixing. A few to scattered diurnal cumulus
will develop inland from the lake shadows from Niagara Falls to
Rochester and southward into the upper Genesee Valley and Finger
Lakes. Otherwise, expect abundant sunshine elsewhere today as high
pressure and increase subsidence builds into the region, with very
seasonable temperatures in the mid 70s to low 80s.

Tonight, high pressure will slide off the mid-Atlantic coast with
light southerly return flow developing. Nocturnal decoupling of
winds in interior valleys, along with mainly clear skies, will
support another round of patchy Southern Tier valley fog. Otherwise,
temperatures tonight will run a touch warmer than last night, with
lows in the mid 50s in interior valleys to the low to mid 60s across
the lake plains.

&&

.SHORT TERM /MONDAY THROUGH TUESDAY NIGHT/...
On Monday surface high pressure will drift off the Mid Atlantic
coast and take up the classic late summer Bermuda high position,
pumping increasing heat and humidity into our region. The latest 00Z
NAM develops widely scattered convection across the higher terrain
of the western Southern Tier and western Finger Lakes during the
afternoon. Other guidance such as the GFS and ECMWF are less
aggressive with the moisture return, and warming mid level temps will
likely provide a cap to deep moist convection. With this in mind,
have kept the Monday forecast dry. Viewing conditions for the
partial solar eclipse should be good, with just scattered diurnal
cumulus and thin/high cirrus. 850mb temps will warm to around +17C
by afternoon, supporting highs in the mid to upper 80s across lower
elevations inland from the lakes.

Monday night a mid level trough will sharpen over the Upper Midwest,
with a gradual increase in moisture and ascent downstream bringing
thickening clouds and possibly a few scattered showers and
thunderstorms to our region overnight. A southerly breeze and
increasing clouds will keep temperatures very warm, with lows in the
lower 70s on the lake plains of Western NY, and mid to upper 60s
elsewhere.

The biggest period of interest over the next week will focus on
Tuesday and Tuesday night. A longwave trough will deepen over the
Great Lakes region, with an embedded strong shortwave approaching
our region late Tuesday, then crossing Tuesday night. An associated
strong cold front will move through the area Tuesday night. As is
almost always the case, the cold front will be preceded by a pre-
frontal trough Tuesday afternoon and evening, which will interact
with lake breeze boundaries and provide an initial focus for
convection Tuesday afternoon, followed by another round Tuesday
night with the actual cold front. It will again be warm and humid,
with highs in the mid 80s inland from the lakes. Southwest winds
will become quite gusty as the pressure gradient tightens,
especially northeast of Lake Erie where gusts to 40 mph are possible.

This system will have severe weather potential, with strong deep
layer shear developing across the warm sector and strong forcing
provided by the deepening mid level system. One potential negative
will be the warm mid level temperatures and poor lapse rates which
could temper instability to some extent, although cooling mid level
temps late in the day with the approach of the mid level trough may
mitigate this. The primary risk will be damaging winds given the
strong unidirectional flow regime. PWAT will be high and storms will
produce heavy downpours, but fast storm motion will likely prevent
much risk of flooding.

Tuesday night the cold front sweeps across the area with showers and
storms ending from west to east overnight.

&&

.LONG TERM /WEDNESDAY THROUGH SATURDAY/...
A seasonably deep longwave trough will become established for the
second half of the week over the Great Lakes and Northeast, bringing
a period of below normal temperatures to our region.

Wednesday and Thursday a northwest flow across the lakes and cold
air aloft will support a good deal of lake effect and upslope
clouds, and possibly even a few scattered showers at times. The
airmass is plenty cold enough for lake effect rain, but short
northwest fetch and a dry synoptic scale background will keep this
limited. Surface high pressure will begin to build into the area by
Friday and should bring an end to any scattered showers.
Temperatures will bottom out on Thursday and Friday when highs may
struggle to get out of the 60s even at lower elevations.

Next weekend high pressure will settle directly overhead. This
should provide a dry and sunny weekend with highs in the 70s and
cool nights with low humidity and light winds.

&&

.AVIATION /08Z SUNDAY THROUGH THURSDAY/...
Clearing skies overnight will lead to the development of valley fog
late tonight across the western Southern Tier with local IFR.

Any valley fog will quickly dissipate this morning, with mostly
clear skies and some scattered diurnal cumulus resulting in
widespread VFR conditions during the day.

More patchy Southern Tier Valley fog is expect tonight with
localized IFR, otherwise widespread VFR will prevail under mostly
clear skies.

Outlook...
Monday...VFR.
Tuesday...Mainly VFR, with a chance of showers and
thunderstorms in the afternoon.
Tuesday Night...MVFR/IFR, with showers and thunderstorms
likely.
Wednesday and Thursday...VFR/MVFR with a chance of
showers.

&&

.MARINE...
Northwest wind on the east end of Lake Ontario will continue to
subside this morning as a ridge of high pressure settles over the
lake today. Otherwise, a stiff lake breeze will develop on Lake Erie
this afternoon as the surface high slides off the mid-Atlantic
coast. Expect winds and waves today will remain below small craft
criteria level, although Lake Erie may be a bit choppy this
afternoon.

Lighter southerly flow Monday and Monday night will direct any waves
mainly toward Canadian waters. Then a strong cold front will cross
the region Tuesday which will eventually require small craft
advisory headlines that may linger through Wednesday or Wednesday
night. A few strong to possibly severe thunderstorms are also
possible Tuesday ahead of the cold front.

&&

.BUF WATCHES/WARNINGS/ADVISORIES...
NY...None.
MARINE...None.

&&

$$

SYNOPSIS...CHURCH
NEAR TERM...CHURCH
SHORT TERM...HITCHCOCK
LONG TERM...HITCHCOCK
AVIATION...CHURCH
MARINE...CHURCH


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