Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 292301

Area Forecast Discussion
National Weather Service Buffalo NY
701 PM EDT FRI JUL 29 2016

Showers and possibly a few thunderstorms will be possible very late
tonight across the Southern Tier, and then through the day Saturday
south of Lake Ontario. More widespread activity is then expected on
Sunday as an area of low pressure crosses the region. The enhanced
cloud cover and persistent northerly flow will keep temperatures
near normal through the weekend before high pressure and
increasingly hot weather returns next week.


Water Vapor imagery this early evening displays a shortwave that is
slowly passing over the Corn Belt States. At the surface an area of
low pressure is reflected under this upper level low, with the
center of the low near the IA/IL border. High pressure over southern
Canada will bring an anti-cyclonic flow across our CWA for much of
the night, with the northerly to northeasterly flow bringing a
comfortable overnight period with dewpoints dropping down into the
mid to upper 50s, and air temperatures dropping down into the mid
50s across the North Country, to lower 60s over much of the region
south of Lake Ontario.

Showers and thunderstorms that formed this afternoon and early
evening have largely weakened and drifted south of the NYS/PA border
within this anti-cyclonic flow. This activity, that was heavy in
localized locations, has moisten the ground enough such that some
patchy fog will be possible overnight where prior storms passed.

A warm front stretching from this surface low to the Virginias will
slowly edge northward overnight. Aloft isentropic flow will begin to
increase clouds through the night. Clouds will likely remain over
far SW NYS this evening, and slowly push north and eastward towards
the SLV through the night.

As the warm front and surface low advance across OH late tonight and
early tomorrow morning a few showers could possibly reach the
western Southern Tier. This surface low will weaken as it tracks
from OH to PA, and tracking the surface dewpoint gradient, a good
marker for the warm front, we see much higher dewpoints reach south
of Lake Ontario, while over the Lake Ontario and points eastward a
still northeaster flow to the north of the warm front will maintain
a drier airmass. Thus will paint chance pops for showers and
possibly an isolated thunderstorm south of Lake Ontario, while
maintaining a dry day east of the lake. High temperatures tomorrow
will range from the mid 70s (under the clouds and showers to the
south) to the lower 80s (near Lake Ontario and across the Black
River Valley and SLV where a greater degree of sunshine will
be present).


A broad mid level trough over the central Great Lakes on Saturday
night will move to the Lake Ontario basin and Saint Lawrence Valley
on Sunday night, then to northern New England by Monday evening.
This will be reflected at the surface as an ill-defined region of
low pressure along and north of the Ohio Valley, moving eastward
across Pennsylvania with the track of the upper trough. This system
will bring some beneficial rain over the weekend, although the slow
moving and diffuse nature of large scale ascent will make timing and
placement of more organized rainfall difficult.

The first half of Saturday night our region will be between waves,
with many areas becoming mainly dry. Scattered showers will linger
across the Southern Tier where deeper moisture and weak low/mid
level deformation will remain. Overnight lows will range from the
upper 50s across higher terrain to the lower to mid 60s for the rest
of the forecast area. Easterly breezes will reduce or remove any
warming influences of the lakes Saturday night.

Late Saturday night and Sunday the mid level trough will slowly
cross the region, with  better differential PVA supporting more
robust large scale ascent. There continue to be some minor
differences in the details of this evolution amongst various
operational models, particularly GFS and ECMWF, but enough
confidence is there in the overall pattern to support increasing
POPS into the likely range in most areas Sunday. Some limited
instability will develop by afternoon, supporting scattered
thunderstorms as well. This system has potential to bring some
beneficial rain to the region, although the heavier amounts are
likely to remain unevenly distributed and tied to areas of
convection. The clouds and showers will keep temperatures down, with
upper 70s or near 80 at lower elevations and mid 70s higher terrain.

Sunday night the mid level trough will meander slowly east into
eastern NY and western New England, while the surface low moves to
eastern PA and southeast NY. More organized showers and a few
scattered thunderstorms will focus on the eastern Lake Ontario
region overnight, with showers slowly tapering off from west to east
across the remainder of the area as better forcing and moisture
shift eastward.

The passage of the upper trough axis on Monday will bing about one
last chance for precip from this pattern, limited mostly to Northern
and Central New York. Skies will clear through the day from west to
east and north to south, as the upper trough exits the region to
the east. High temperatures will be in the middle 70s to the lower
80s with more comfortable and less humid conditions. High pressure
building in from the west will bring drying and clearing over the
entire region with overnight low temperatures in the upper 50s to
the middle 60s.


Upper level ridging will build into the Great Lakes during this
period as the eastern Canadian trough breaks down, although that
appears likely to redevelop by the end of the period or just beyond.
Regardless, it does look like the large scale pattern is suggestive
of convective complexes developing within the northern periphery of
the growing heat/instability dome, which will be shifting
eastward with time. The growing ridge will more than likely keep
these complexes north of the region. Therefore, the region we
will have to wait until the ridge shifts far enough to the east to
allow a medium range consensus frontal boundary to approach by the
end of the period, for the next chance for rainfall.

Temperatures will steadily warm through the period as the ridge
builds and a southwesterly return flow becomes established.
Most high temperatures during the period should reach the mid to
upper 80s, with some potential for lower 90s if the warmer solutions


At 00Z VFR flight conditions were found, and these flight conditions
should prevail for the KBUF/KIAG/KROC and KART airfields through
this TAF cycle. For the KJHW airfield earlier showers and
thunderstorms that have rolled over the airfield have moistened the
ground, such that patchy fog will be possible later overnight. With
dewpoints expected to run a few degrees higher than model guidance
will place IFR VSBYS within the TAF to account for fog formation,
mainly after 06Z.

The northern edge of an area of showers will slowly lift northward
from near the western NYS/PA line around 12Z to near the NYS thruway
by late in the day. Activity will remain scattered through the day,
and will place just a VCSH in the KJHW TAF where greatest confidence
for nearby shower activity will be, as activity may remain just to
the south of the NYS Thruway through the day. It should remain dry
across the KART airfield.

Winds will remain generally light and from the northeast direction
through the TAF cycle.


Saturday Night through Monday...Mainly VFR with a chance of showers
and thunderstorms.
Tuesday and Wednesday...VFR.


Persistent northeasterly flow will maintain a light to moderate chop
on the lakes this weekend, though waves should remain below advisory
levels as the pressure gradient will remain relatively weak as a
weakening area of low pressure passes to the south of the lakes.
Otherwise, high pressure returns to the region next week, ensuring
generally tranquil conditions on the lakes.





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