Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 241729

Area Forecast Discussion
National Weather Service Buffalo NY
129 PM EDT SAT SEP 24 2016

An expansive area of high pressure will build across the region
tonight and Sunday with mainly clear skies and cool fall
temperatures. A cold front will then cross the region late Monday
afternoon and Monday night and bring a period of showers. Scattered
showers and even some lake effect rain will continue through the
middle of next week as a slow moving area of low pressure crosses the
Great Lakes.


Visible satellite imagery showing a rather extensive cumulus field
across the majority of the area early this afternoon, due to a
combination of leftover lake effect moisture and cold air aloft
promoting steep low level lapse rates. The shallow layer of moisture
will be effectively mixed out during the mid to late afternoon with
increasing amounts of sunshine. Temperatures will remain on the cool
side with highs in the mid 60s across most lower elevations, with
some mid to upper 50s across the higher terrain east of Lake

High pressure over Ontario and Quebec will continue to build
southward into the eastern Great Lakes tonight. Strong subsidence
and drying associated with the high will provide mainly clear skies
for most. High resolution guidance such as the NAM and Canadian GEM
support some limited lake effect clouds re-developing south of Lake
Ontario tonight with lake induced instability still present beneath
the steepening inversion near 5k feet. This would primarily target
the Genesee Valley and other counties along the south shore of Lake
Ontario from Wayne County westward. What remains of the lake effect
clouds will evolve into a tea kettle band and retreat to mainly just
offshore by daybreak Sunday as a land breeze circulation develop.

Light winds and clear skies for most will allow for good radiational
cooling tonight in this cool/dry airmass. Expect lows in the low to
mid 40s in most areas away from the immediate lakeshores. The cooler
areas of the interior Southern Tier will drop into the upper 30s,
and a few of the coolest sheltered valleys may drop into the mid
30s with isolated light frost. The larger river valleys will
likely stay in the upper 30s to around 40 with typical river valley
fog forming. Given the sparse coverage of frost potential will hold
off on a frost advisory for the western Southern Tier. It will be
colder east of Lake Ontario deeper into the cool airmass. Expect
mid 30s to be more widespread across Jefferson and Lewis counties,
with the coldest spots on the Tug Hill approaching 32. With this in
mind, continued the frost advisory for Lewis County and added
Jefferson County.

High pressure will remain in place on Sunday and provide plenty of
sunshine, with just a modest amount of diurnal cumulus developing
mainly across the higher terrain well inland from the lakes. The
airmass remains cool, with highs in the mid 60s across most of
western NY, and lower 60s across lower elevations east of Lake
Ontario and mid 50s higher terrain.


Sunday night...fair and dry weather will continue across our
region as the axis of the surface ridge slides eastward into New
England. The developing warm air advection regime on its backside
will allow some mid and high cloud cover to spill southeastward
across New York State... which will eventually help to render
skies partly cloudy for all but the North Country. The combination
of the increasing cloud cover and a developing southeasterly low
level return flow should result in a milder night for areas from
the Genesee Valley westward...where lows should range between the
mid 40s and lower 50s. Further east...a close facsimile of the
previous night`s low temperatures is expected given lesser amounts
of cloud cover and lighter winds...with readings again dipping
into the mid 30s across the North Country...where some patchy
frost will again be possible. Accordingly...have retained a
mention of such in both the ZFP and HWO.

Monday and Monday night...our weather will deteriorate again as the
aforementioned ridge gives way to a rather deep mid and upper level
trough digging southeastward across Ontario Province and the Great
Lakes...with this feature eventually morphing into a deep closed
upper low over Central Ontario by Tuesday morning. At the surface...
an attendant surface wave will slide along a similar path...and will
eventually swing its trailing cold front through our region. The
00z/24 guidance suite continues to universally speed up the timing
of this feature...and now brings it through our area between later
Monday afternoon and Monday night. With this in mind...have sped up
the frontal timing some 3-6 hours from our previous continuity...
while maintaining an attendant swath of likely PoPs given favorable
moisture pooling and a band of narrow but deep lift accompanying
the frontal passage. At this point instability looks limited enough
for the bulk of the precip to be in the form of plain rain showers...
though cannot rule out an isolated thunderstorm or two approaching or
reaching extreme far WNY later on Monday afternoon. Following the
passage of the front...strong dry slotting should build into the
region Monday night...while bringing a temporary return to mainly dry
weather for many areas.

With respect to temperatures Monday and Monday night...a strengthening
southerly downslope flow and the overall warm air advection regime out
in advance of the cold front will allow temperatures to rebound back
into the upper 60s across the North Country and to the lower to mid 70s
across far western New York. The warmest overall temps will be found
across the favored downslope flow regions along the Lake Erie shoreline
and across the lake plains from Buffalo over to Rochester...where we
have accordingly continued to aim some 5 degrees above current guidance.
Developing cool air advection in the wake of the cold front will then
send readings tumbling back into the upper 40s to mid 50s Monday night.

After that...the remainder of this period will then feature the large
closed upper low slowly meandering its way southward/southeastward
across the Great Lakes. The deep cool pool and region of deeper
wraparound moisture attendant to this feature will result in a return
to cooler temperatures along with a general opportunity for some
scattered showers...with some bands of more concentrated lake effect
rain showers also possible downwind of the lakes. Given the many model
permutations noted in both the evolution of the closed low and its
consequent influence on the orientation of the low level wind field
over the past few this point it remains difficult to pin
down exactly where the lake enhanced/driven showers might for
now have just indicated some very broad areas of mid range chance PoPs
to the lee of both Lakes Erie and Ontario. Temperature-wise...850 mb
temps of +4C to +6C should translate into daytime highs in the lower
to mid 60s on Tuesday...with lows in the mid 40s to lower 50s then
following for Tuesday night.


In the longer term portion of the forecast...the medium range models
remain in loose agreement on the closed upper low eventually exiting
our region and expansive surface-based ridging building into our area
in its wake...though some differences in the exact details of all this
still persist. This is most notably the case with the 00z/24 ECMWF...
which sharply broke continuity with its previous runs and the other
guidance packages and now takes the closed low due south rather than
due east Wednesday and Thursday...before slowly lifting this feature
back northward toward our area on Friday. Given the outlier nature of
this solution...feel it best to stick to our continuity of lingering
scattered showers Wednesday giving way to a return to fair and dry
weather Thursday and Friday...along with temperatures climbing back
to near or slightly above normal levels by the end of the work week.


Extensive diurnal cumulus with VFR CIGS across much of the region
early this afternoon will scatter out and dissipate in most areas
during the mid to late afternoon as the limited low level moisture
mixes out. This will leave mainly clear skies in most areas tonight.
Expect some limited lake effect clouds to re-develop south of Lake
Ontario near KROC with spotty MVFR CIGS. These should retreat to
mainly over the lake by 12Z Sunday. Expect some typical river valley
fog with local IFR across the western Southern Tier from about 06Z
to 13Z Sunday. Some patchy fog is also possible elsewhere especially
near any bodies of water inland from the Great Lakes.

On Sunday the fog will burn off by mid morning and leave VFR to
prevail with just a few scattered diurnal cumulus developing across
the higher terrain well inland from the lakes.


Monday...VFR...except for some patchy overnight/ early morning valley
fog with local IFR across the Southern Tier.
Monday Night...Areas of MVFR with showers likely.
Tuesday and Wednesday...VFR/MVFR with a chance of showers.
Thursday...Mainly VFR.


Winds and waves will continue to diminish this afternoon and evening
with choppy conditions along the south shore of Lake Ontario and the
Lake Erie shore southwest of Dunkirk slowly subsiding as the gradient
wind continues to diminish.

The large Canadian surface high will move by to our north tonight
and Sunday. This will produce gentle to moderate northerlies across
the Lower Great Lakes for the remainder of the weekend...with waters
being choppy at times along the south shore of Lake Ontario and also
southwest of Dunkirk.

Winds will veer to the southeast and freshen Sunday night and Monday
ahead of an approaching frontal system. Moderately strong winds will
then become southwest later Monday night and Tuesday behind a cold
front. This system may bring the next round of Small Craft Advisory
conditions by early next week.


NY...Frost Advisory from 2 AM to 9 AM EDT Sunday for NYZ007-008.



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