Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 300512

Area Forecast Discussion
National Weather Service Buffalo NY
112 AM EDT SAT JUL 30 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 showing a well defined mid level shortwave
moving through southern lower Michigan early this morning. This
shortwave will approach western NY by around 12Z, with increasing
DPVA and associated moisture bringing an increase in cloud cover
from west to east through the remainder of the night. A few
scattered showers may develop towards daybreak across the western
Southern Tier.

On Saturday the 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


A mid level trough will cross the region late tonight and Saturday
morning and bring thickening and lowering mid level clouds to the
region. A few widely scattered showers will develop by around
daybreak across the western Southern Tier, then spread into the rest
of western NY by mid to late morning. By Saturday afternoon and
evening most of the remaining showers should be focused across the
Southern Tier. Expect mainly VFR to prevail through the day,
although local/brief MVFR may develop near any heavier showers.

Tonight a second, more significant mid level trough will move out of
the midwest states and approach western NY by early Sunday morning,
while a warm front lifts north out of PA and into western and
central NY. These two features will combine to produce more
widespread showers overnight, first across the Southern Tier after
03Z, then spreading north to the KBUF-KROC-KSYR corridor after 09Z.
Expect CIGS and VSBY to deteriorate to at least MVFR as the steadier
showers become established, with some potential for IFR as well.


Sunday...Areas of MVFR and local IFR with occasional showers and
scattered thunderstorms.
Monday...Areas of MVFR with showers likely east of Lake Ontario,
mainly VFR with widely scattered showers elsewhere.
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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