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Area forecast discussion 
National Weather Service Buffalo New York
1055 am EDT Sat Jul 30 2016

Synopsis...
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.

&&

Near term /through tonight/...
there will be a divergent flow aloft ahead of a a shortwave
approaching from the upper Great Lakes region. Meanwhile there is
a subtle surface boundary across the western southern tier with
increasing boundary layer moisture. As this upper level wave moves
across this stalled boundary it will enhance showers, with
embedded thunderstorms and locally heavy rainfall possible. The
latest runs of the hrrr and now 12z NAM guidance do capture this
feature, with model guidance generally tracking this area of
precipitation slowly to the east through early afternoon. Guidance
which does not capture this feature has been discounted through
this evening.

This will bring a soaking, and sometimes heavy, rainfall to the
western southern tier with localized rainfall amounts over an
inch. Amounts will diminish rapidly to the north of this, with
little if any rain expected north of I-90 from Buffalo to
Rochester to Syracuse. Forecast highs were lowered a bit due to
the rain and cloud cover, however there may be some time late this
afternoon for temperatures to recover a bit in western areas since
the precipitation should exit into central New York by late
afternoon. Cloud cover should still be quite prevalent south of
Lake Ontario, with increasing sunshine to the north. Northeaster
winds south of Lake Ontario will be enhanced some by a developing
lake breeze this afternoon once temperatures rise.

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 New York. 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 New York 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 northward.

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 New York
late tonight as stronger forcing and moisture spread north and east.
With this in mind, have increased probability of precipitation into the high likely to
categorical range late tonight. The heaviest rain will likely focus
on The Finger lakes and central New York 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.

&&

Short term /Sunday through Monday night/...
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 New York 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 New York 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 New York 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.

&&

Long term /Tuesday through Friday/...
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.

&&

Aviation /15z Saturday through Wednesday/...
an area of showers with embedded thunderstorms across western New
York this morning will gradually move east today. By 15z this will
largely be to the south and east of our taf locations, with only
brief and/or localized impacts to flight conditions expected.
There is a small chance showers will develop behind this wave
across the western southern tier (jhw), but this chance appears to
be diminishing due to the stabilizing influence of the departing
rain showers.

Tonight a second, more significant mid level trough will move out of
the Midwest states and approach western New York by early Sunday morning,
while a warm front lifts north out of PA and into western and
central New York. 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 ceilings and visibility 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.

Outlook...

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.

&&

Marine...
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.

&&

Buf watches/warnings/advisories...
New York...none.
Marine...none.

&&

$$

Synopsis...Hitchcock
near term...apffel/Hitchcock
short term...church
long term...church
aviation...apffel/Hitchcock
marine...apffel/Hitchcock

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