Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 301320

Area Forecast Discussion
National Weather Service Buffalo NY
920 AM EDT SAT JUL 30 2016

An upper level disturbance will produce a period of showers this
morning and midday across western NY, with showers and a few
isolated thunderstorms lasting through the afternoon and evening
across the Southern Tier. Another more potent mid level disturbance
and warm front will move into the area late tonight and Sunday,
bringing more widespread and beneficial showers and thunderstorms to
much of the region. Scattered showers will linger into Monday before
dry weather returns by Tuesday.


Water Vapor imagery showing a well defined mid level shortwave
moving east across Lake Erie and into Western New York this
morning. Divergence aloft is helping to enhance lift in the DPVA
regime ahead of this shortwave. Radar shows an area of
precipitation across Western New York, mainly south of I-90. There
are embedded convective elements in portions of the Western
Southern Tier with thunderstorms and locally heavy rainfall since
the driving synoptic feature is very slow moving.

Recent runs of the HRRR have now captured this feature, though it
appears to be a couple hours slow with its progression. Using a
blend of the HRRR and radar trends, this area should continue to
move slowly east, bringing some steady (and sometimes heavy)
rainfall to the Western Southern Tier through early afternoon.
Expect lighter showers north of this, with just scattered light
showers by the time it reaches the Lower Genesee Valley.

The scattered showers across the western Southern Tier will continue
into the afternoon near a shallow frontal zone aloft, and may
increase in coverage as diurnal instability develops and supports
more numerous showers and even a few scattered thunderstorms.
Farther north, expect a good amount of cloud cover to linger from
the Niagara Frontier to central NY to the north of the stalled
frontal zone. The North Country will escape all of this, with at
least partial sunshine near the Saint Lawrence River.

High temperatures will be close to mid summer seasonal norms in most
areas, with lower 80s at lower elevations and mid to upper 70s
across higher terrain. A northeast flow will keep the south shore of
Lake Ontario cooler.

This evening a few scattered showers and isolated thunderstorms may
continue across the western Southern Tier near the stalled frontal
zone aloft, while the remainder of the area stays dry. This will
change overnight as a more substantial mid level trough moves out of
the midwest states and approaches western NY. A period of deep layer
ascent will develop late tonight in strong DPVA and upper level
divergence ahead of the trough, and low level convergence,
isentropic upglide, and moisture pooling will increase as the
stalled frontal zone begins to lift northward as a warm front. The
strongest low level convergence will develop near the nose of a 35
knot low level jet from the Finger Lakes to central NY late tonight.
Quality moisture will also move into the area as a plume of both
Gulf and Atlantic moisture is captured by the system and advected

Given the quality of forcing and dynamics coming together, expect
showers to become more widespread again across the Southern Tier by
late evening or early overnight. These showers will then spread
northward into the Niagara Frontier, Genesee Valley, and central NY
late tonight as stronger forcing and moisture spread north and east.
With this in mind, have increased POPS into the high likely to
categorical range late tonight. The heaviest rain will likely focus
on the Finger Lakes and central NY late tonight near the nose of the
aforementioned low level jet. Elevated instability is marginal, but
should still support a few areas of embedded thunder in the areas of
heavier rain.


By Sunday morning, a broad trough will be in place across the Great
Lakes region, tracking toward Lake Ontario and the Saint Lawrence
valley Sunday night into Monday. 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.

An associated weakly defined surface low will track from near Lake
Erie Sunday morning to near the Saint Lawrence river by Monday
morning. Frontogensis ahead of the surface low along with enhanced
moisture advection and a developing low-level jet will bring a surge
of precipitation through the Finger Lakes and central NY into the
North Country during the day Sunday. Increasing instability with
diurnal heating and cooling aloft with the lowering heights will
increase thunderstorm potential across western NY to the Finger
Lakes region through the day Sunday, where shower and thunderstorm
coverage will be more scattered near the center of the surface low.
The best potential for more widespread rainfall is ahead of the
surface low across central NY and the North Country, while heavier
amounts are likely to remain unevenly distributed and tied to areas
of convection. The thick cloud cover and rain chances will help keep
daytime high temperatures limited to the mid to upper 70s. By Sunday
night, showers and thunderstorms will diminish across WNY as the
trough shifts eastward, but will remain likely east of Lake Ontario
and near the Saint Lawrence river. Overnight lows will fall into the
mid 60s with some upper 50s across the western Southern Tier were
some overnight clearing will help with radiational cooling.

By Monday, upper-level trough overhead with a weak trailing
shortwave will excite some isolated to scattered showers and
thunderstorms in the unstable northwesterly flow. The best moisture
will be pushed to the east by Monday, thus these showers and
thunderstorms will be mostly isolated to scattered in coverage and
mainly confined to the North Country and higher terrain of the
northern Finger Lakes and interior Southern Tier. 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 mid 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.


Tuesday through Friday, heat and humidity will build day-to-day as a
broad upper-level ridge builds eastward across the Great Lakes
region. This will be a dry stretch of weather with above normal
temperatures reaching the upper 80s to low 90s by Thursday and
Friday. Global models remain in consensus for a cold frontal passage
with showers and thunderstorms Friday night into Saturday.


A mid level trough will cross the region this morning and bring
thickening and lowering mid level clouds to the region. An area of
rain over Lake Erie will spread ENE into much of western NY through
mid morning, then diminish to scattered showers by midday. A few
embedded weak thunderstorms will also be found across the western
Southern Tier through mid to late morning. Expect mainly VFR
conditions even in the rain with unsaturated low levels, although
some brief MVFR is possible across the Southern Tier in the heavier
rain through 14Z.

By this afternoon and evening most of the remaining showers should
be focused across the Southern Tier, with enough afternoon
instability to support a few isolated thunderstorms as well. Expect
mainly VFR to prevail through the day, although local/brief MVFR may
develop near any heavier showers across the Southern Tier.

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 late tonight, with IFR developing across
the higher terrain of the Southern Tier and Finger Lakes.


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 northeast flow will last through the weekend and produce
choppy conditions just shy of Small Craft Advisory levels along the
south shore of Lake Ontario today and tonight. On Sunday low
pressure will move slowly east across PA, while high pressure will
build across Quebec. The pressure gradient between the two may allow
for northeast winds to increase further, with a potential for Small
Craft Advisory conditions along the south shore of Lake Ontario.
Winds will diminish later Sunday night as the gradient weakens.
Winds and waves will be lower through the weekend on Lake Erie.





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