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000
FXUS61 KBUF 250540
AFDBUF

Area Forecast Discussion
National Weather Service Buffalo NY
140 AM EDT SUN SEP 25 2016

.SYNOPSIS...
A large area of high pressure centered over eastern Canada will push
east this afternoon and tonight...and this will guarantee another
sun filled day to end the first weekend of Fall. A strong cold front
will then plow across Western and North Central New York late Monday
and early Monday night...producing several hours of showers and
possibly some thunderstorms in the process. Colder air in the wake
of the front will generate some lake effect rain showers northeast
of both lakes on Tuesday.

&&

.NEAR TERM /THROUGH TODAY/...
High pressure centered over Ontario and Quebec will continue to
build southward into the eastern Great Lakes overnight. Strong
subsidence and drying associated with the high will provide mainly
starlit skies for most. Infrared satellite imagery is showing some
limited lake effect clouds re-developing along the southern shore of
Lake Ontario with lake induced instability still present beneath the
steepening inversion near 5k feet. This will primarily impact 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 overnight 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.

&&

.SHORT TERM /TONIGHT THROUGH TUESDAY NIGHT/...
Sunday night PV streaming southeastward over the Canadian Prairies
will combine with an upper level shortwave trough of low pressure
over the Northern Rockies to produce strong upper level height falls
over the Western Great Lakes, and deepening surface low near Lake
Superior. Downstream, a ridge of high pressure will maintain its
influence over the Eastern Great Lakes with a mainly clear sky
Sunday evening over our region. This clear sky and light winds will
allow for evening temperatures to quickly drop into the mid 40 to
lower 50s. High clouds will begin to stream over our region from the
upstream storm system, and this will slow the cooling some, as
overnight lows will eventually bottom out in the 40s, while the
North Country farther away from upstream clouds may drop down into
the mid 30s. Will continue to mention some patches of frost here.

Monday will start off dry over our region, but conditions will
quickly go downhill as the surface low advancing from the Western
Great Lakes to southwest of James Bay swings a cold front towards
our region. The model consensus continues to maintain a quicker
frontal passage than advertised 24 to 48 hours ago...with the
surface front reaching WNY by the late Monday afternoon hours. We
can already see the moisture building ahead of the cold front this
afternoon on water vapor imagery, with both Gulf of Mexico moisture,
and moisture from convection over Mexico streaming northward. As a
deep PV anomaly nearing 600 mb circulates around this upper level
shortwave over the Great Lakes Monday, it will deepen a surface low,
with a low level jet also increasing. The combination of upper level
height falls, low level convergence and lift along both the cold
front and low level jet through a deep saturated low level should
bring a line of showers across the region Monday afternoon and
evening. The earlier timing to the showers will allow for some
daytime instability, which may trigger a thunderstorm or two over
WNY. This instability will wane deeper into NYS, and thus will
include a chance for thunder Monday afternoon and evening from
roughly the Genesee River westward.

A dry slot will follow the cold front Monday night, leaving much of
the region relatively dry, with just a few scattered light showers.
Will maintain the low chance mention of precipitation through the
night. The partly to mostly cloudy skies will limit overnight
cooling, with most areas dropping back to the lower to mid 50s,
which is several degrees above normal.

Behind the cold front, there will be strong cold air advection, such
that temperatures at 850 hpa drop from around +12c Monday afternoon,
to near +2c by Tuesday morning. Several factors will come together
such that a plume of lake effect rain may develop before daybreak
Tuesday off Lake Erie, and then during the morning hours of Tuesday
east of Lake Ontario. Moisture associated with the upper level
trough will return over Lake Erie Monday night, and Lake Ontario
Tuesday as the upper level closed low slowly rotates eastward.
Meanwhile over the lake the colder airflow will steepen low level
lapse rates, and lake induced CAPE values will reach 1000 to 1500
J/KG. While the instability, and moisture are favorable for a band
of lake effect rain, there is still some uncertainty to the wind
field, and amount of shear. This is due primarily to the position
of the upper level low. The past few model runs have yet to resolve
how this closed low will evolve and move across the Great Lakes
region. Though the 12Z model suites are converging on an upper level
low position that favors a SW flow over the Eastern Great Lakes
region, we would like to see more model-to model run consistency
before going any higher than low likely Pops for lake effect rain on
Tuesday east of the Lakes.

Though the greatest probability for precipitation will be within
lake effect rain to the northeast of the eastern Great Lakes, we
will carry a low chance probability for showers across the entire
CWA, as daytime instability combined with steepening lapse rates may
contribute to scattered showers.

Instability wanes a bit Tuesday night over the lakes, but a still
potential southwest flow may continue lake effect rain showers. Will
continue with chance pops northeast of the lakes, while allowing
inland areas to likely dry out with the loss of daytime heating and
instability.

High Tuesday will remain in the low to mid 60s under the cool flow
aloft. Overnight lows will remain in the lower to mid 50s within any
lake effect activity, while inland areas drop back into the mid to
upper 40s.

&&

.LONG TERM /WEDNESDAY THROUGH SATURDAY/...
A closed 500 mb low is forecast to be near the Eastern Great Lakes
on Wednesday. Some lake enhanced rain showers should linger
Wednesday into Wednesday night, but with diminishing winds these
will trend closer to the lakes and become more localized.
Temperatures underneath the upper low will be seasonably cool on
Wednesday with highs in the 60s.

After this, there continues to be a disagreement among model
guidance. The 12Z GFS/GGEM open the closed low and move the
resulting trough slowly to the east. This would allow high pressure
to build into the region late in the week with dry weather and much
above normal temperatures. The 12Z ECMWF differs considerably by
breaking off a closed low and stalling it across the Appalachians.
This would result in much cooler conditions for our region, and
possibly some showers depending on where the closed low ends up.
Even if a closed low does break off it is uncertain where it will
stall and with the majority of guidance keeping our region dry there
is only a slight chance of showers during this timeframe. If model
guidance settles definitively on a dry pattern then highs on Friday
and Saturday can be raised several degrees.

&&

.AVIATION /06Z SUNDAY THROUGH THURSDAY/...
Expansive high pressure stretching from Hudson Bay to the Carolinas
will provide VFR conditions across our region through Sunday night.
The only possible exception will be across the Western Southern Tier
where there could be some valley fog between 08 and 13z to reduce
VSBYS to MVFR levels.

Outlook...

Monday...While VFR weather will prevail for most sites...showers and
possible thunderstorms will arrive in the far west after 18z to
produce localized MVFR conditions.
Monday Night...Areas of MVFR with showers likely.
Tuesday and Wednesday...VFR/MVFR with a chance of showers.
Thursday...Mainly VFR.

&&

.MARINE...
A large Canadian surface high will move by to our north for the
remainder of the overnight 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.

&&

.CLIMATE...
Saturday marked the end of the longest stretch of 70+ degree days
in Buffalo and Rochester since records began in the early 1870s.
The high temperature today topped out at 65 for Buffalo and 64 for
Rochester which marks 102 days since the last time (June 13th)
that the high temperature did not reach the 70 degree mark.
Following is a top ten list of longest stretches of 70+ degrees
days for Buffalo and Rochester.

Buffalo
Rank Run Length Ending date
1      102       2016-09-23
2       95       2005-09-22
3       94       2012-09-08
4       80       1959-09-10
5       78       2006-08-28
6       77       1878-09-10
7       75       1993-08-20
-       75       1987-08-23
9       73       1980-09-11
10      71       1952-08-21

Rochester
Rank Run Length Ending Date
1      102      2016-09-23
2      101      2002-09-26
3       88      2005-09-15
4       84      1959-09-10
5       83      1939-09-04
6       82      2011-09-04
7       81      1975-08-29
8       79      1881-09-12
9       78      2006-08-29
10      75      2012-09-08

&&

.BUF WATCHES/WARNINGS/ADVISORIES...
NY...Frost Advisory until 9 AM EDT this morning for NYZ007-008.
MARINE...None.

&&

$$

SYNOPSIS...RSH
NEAR TERM...HITCHCOCK/RSH
SHORT TERM...THOMAS
LONG TERM...APFFEL
AVIATION...RSH
MARINE...HITCHCOCK/RSH
CLIMATE...STAFF