Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 280021
AFDBUF

Area Forecast Discussion
National Weather Service Buffalo NY
821 PM EDT SAT AUG 27 2016

.SYNOPSIS...
High pressure over the Saint Lawrence Valley this evening will drift
east overnight. This will allow a weak warm front to lift across our
region overnight...followed by a prefrontal trough and trailing weak
cool front Sunday afternoon and evening. In the process...our region
can expect a couple rounds of scattered to numerous showers and
thunderstorms between tonight and Sunday. The next round of fair
weather will then move in for Monday and Tuesday when high pressure
builds in from the Upper Great Lakes.

&&

.NEAR TERM /THROUGH SUNDAY/...
Regional surface analysis at 00z shows sprawling surface high
pressure centered over the Saint Lawrence Valley...and weak/broad
surface low pressure over the western Great Lakes. This latter
feature is a reflection of a several spokes of shortwave energy
that are in the process of lifting across the central Great
Lakes...while also generating some attendant clusters of showers
and embedded thunderstorms. The first of these is now weakening as
it lifts by to our northwest across nearby Southern Ontario...
while a second area of convection continues to make its way
northeastward across north-central Ohio.

Through the course of tonight...the above shortwave energy will
continue to lift northeastward around the periphery of an
anomalously strong Bermuda High...a track that will take it across
nearby Southern Ontario and Lake Ontario. At the same time...a
pseudo-warm frontal boundary (really a dew point front) over Ohio
will also lift northeastward across our region. As was pointed out
earlier this afternoon...this boundary separates the lingering
drier air over our region (dewpoints of around 60) from
considerably more humid air (dewpoints near 70) lying over the
Ohio Valley.

As this airmass makes its way into our forecast area later
tonight its leading edge will quickly destabilize our airmass...and
this in conjunction with the passing shortwave energy aloft will
help to trigger the development of some scattered showers and
thunderstorms...with the bulk of these likely to be found over the
far western counties in the vcnty of the mid level support and
corresponding 925-700mb frontogenesis. This scenario is identical
to what happened over our region just a few days ago.

By the start of Sunday morning...the last of the shortwave energy
will be in the process of crossing Lake Ontario...with any attendant
convection making its way across parts of the Finger Lakes and
Eastern Lake Ontario regions while ending across the far western
counties.

After several hours of subsidence in the wake of this feature...a
second round of convection is expected to develop on Sunday...in
conjunction with both renewed daytime heating of our warm and
newly humid-again airmass...and the arrival of a prefrontal trough
and developing lake breeze boundaries. Expect the bulk of this
convection to focus along and inland from these surface
boundaries...which should result in the most numerous activity
extending from the Southern Tier northeastward across the Finger
Lakes and into interior portions of the North Country... where
likely PoPs remain intact from continuity. Meanwhile...relatively
lower chances for convection will be found over and immediately
northeast of the lakes...with a secondary maxima extending from
the Niagara Peninsula eastward across Niagara and Orleans counties...
in the lake breeze convergence zone and in advance of the actual
surface cold front...which will not actually cross our region
until Sunday evening.

As was mentioned previously...the convection could become rather
strong... as SBCAPEs of 1500-2000 j/kg will be accompanied by
impressive 10 deg c/km low level lapse rates. This `fat CAPE` in
the presence of modest amounts (20-30 kts) of bulk shear should be
supportive of storms capable of producing some gusty winds and
possibly some small hail...as well as some heavy downpours given
precipitable water values pushing 2 inches. Given all this...have
added some enhanced wording to the forecast and HWO to better
reflect this potential.

Temperatures on Sunday will climb to between 85 and 90 with
increasingly uncomfortable levels of humidity...as surface
dewpoints will again reach into the mid and upper 60s.

&&

.SHORT TERM /SUNDAY NIGHT THROUGH WEDNESDAY/...
By Monday, a shortwave trough will track overhead and into New
England by the afternoon. Morning cloud cover will dissipate from
northwest to southeast as drier air and subsidence move into the
region through the afternoon. Temperatures will take a step back on
Monday, as 850 mb temperatures fall to +12 to +14C, thus highs will
be in the upper 70s to low 80s. Drier air will mix down in the
afternoon, allowing dew points to fall into the comfortable 50s.
High pressure will build across the region Monday night and Tuesday
leaving dry conditions and mostly clear skies. Monday night will
feature optimal radiational cooling with a dry airmass, and light
winds allowing temperatures to fall into the 50s for most locations,
except near the lake shores and urban locations. Tuesday will see a
warming trend as high pressure slights to slide east and the airmass
modifies with 850 mb temps to +17C, resulting in high temperatures
back into the mid 80s.

The upper level pattern across the CONUS starts to become more
interesting into Wednesday as a large Pacific trough craves out
across the western US which helps with downstream development of a
large ridge across the central plains. During the day Wednesday,
this will open the door for Canadian sourced airmass to dive from
the Hudson Bay region toward the northeast US. This airmass will be
on our doorstep by the second half of the day Wednesday. A warm,
humid and unstable airmass will be in place ahead of this front
across our region for the first half of Wednesday, and thus we
should see at least some scattered convection across the region
ahead of the frontal passage. However, with models trending quicker
on the arrival of the front, there may not be enough time
destabilize ahead of the frontal passage on Wednesday to develop
stronger thunderstorms across western NY.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH SATURDAY/...
An upper level trough axis will dig across Northern New England
through Friday. This will set up a persistent northwesterly flow
with cold air advection during this time. Model guidance is in
good agreement, with the 12Z GGEM/ECMWF/GFS all showing a similar
pattern. As such, model consensus will serve as a good starting
point, however some adjustments are still needed.

GFS BUFKIT shows moisture trapped beneath an inversion at about 5k
ft south of the lakes. Although deep layer mean RH largely misses
this, consensus 850 mb forecast to drop to +3C should support
some lake effect clouds during during the Wednesday through Friday
night period. Moisture is shallow and the flow is mostly
perpendicular to the lakes which will make lake effect rain
showers unlikely. However, because of this consensus guidance is
likely too warm for high temperatures on Thursday and Friday.
Daytime highs will be cooler than they have been for some time,
with many areas struggling to reach 70 on Friday.

For Friday night and Saturday, high pressure is forecast to build
across the region with dry and pleasant weather going into the
weekend. Good radiational cooling will result in chilly overnight
temperatures Friday night, followed by seasonable temperatures for
Saturday.

&&

.AVIATION /01Z SUNDAY THROUGH THURSDAY/...
High pressure over the St Lawrence Valley will provide the region
with fine flying weather through most of this evening...as dry VFR
conditions will be accompanied by light winds. As we push into the
overnight hours though...a mid level disturbance will pass by to
our north and bring the risk of some scattered showers and
thunderstorms to our region. Outside of any convection...cigs and
vsbys should remain at VFR levels.

On Sunday...a period of subsidence in the wake of tonight`s
disturbance will allow for a period of relatively dry weather
between the late morning and very early afternoon hours...before a
prefrontal trough pushes through the region and (in conjunction
with developing lake breeze boundaries) triggers a second round of
scattered to numerous showers and thunderstorms...which will be
most numerous from the Southern Tier northeastward into the Finger
Lakes. Expect the risk of localized reductions to MVFR/IFR within
any convection...which could also present the risk for some gusty
winds and heavy downpours. Otherwise...general VFR conditions
should prevail.

Outlook...
Sunday night...Mainly VFR with some leftover evening showers and
thunderstorms.
Monday and Tuesday...VFR.
Tuesday night and Wednesday...Mainly VFR with a chance of showers and
thunderstorms.
Thursday...VFR.

&&

.MARINE...
High pressure over the St Lawrence Valley this evening will move
east overnight...and this will allow the first of two frontal
boundaries to push across the region. While light winds and
negligible waves will remain in place...there will be an increasing
risk for thunderstorms as we progress through the night.

While winds will freshen somewhat across the Lower Great Lakes on
Sunday...the main concern will be the continued risk for thunderstorm
activity as a prefrontal trough and weak cool front will both cross
our region between Sunday afternoon and evening.

Fine weather for boating will return late Sunday night and Monday as
the next Canadian sfc high will build across the region.

&&

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

&&

$$

SYNOPSIS...JJR/RSH
NEAR TERM...JJR/RSH
SHORT TERM...CHURCH
LONG TERM...APFFEL
AVIATION...JJR/RSH
MARINE...JJR/RSH


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