Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 202315

Area Forecast Discussion
National Weather Service Buffalo NY
715 PM EDT Sun Aug 20 2017

High pressure will move off the the mid-Atlantic coast tonight.
Return flow developing between this surface high and the next
approaching low pressure system will build heat and humidity
on Tuesday. A strong cold front will cross the region late
Tuesday, and may be accompanied by strong to severe thunderstorms.
The work week will end with a touch of fall in the air as a
sprawling Canadian high pressure builds in cooler and drier air.


Skies this evening are clear, with some cirrus beginning to near the
region from the west.

Tonight, high pressure will slide off the mid-Atlantic coast with
light southerly return flow developing. Nocturnal decoupling of
winds in interior valleys, along with mainly clear skies, will
support another round of patchy Southern Tier valley fog. Some mid
and high level clouds may limit the coverage area compared to recent
nights. Otherwise, temperatures tonight will run a touch warmer than
last night, with lows in the mid 50s in interior valleys to the low
to mid 60s across the lake plains.

On Monday, surface high pressure will drift off the Mid Atlantic
coast with increasing heat and humidity in our region. This will
largely result in dry weather, however a majority of 12Z guidance
develops scattered convection across the western Southern Tier. The
culprit is a subtle shortwave embedded in a zonal 500 mb flow which
is across Illinois late this afternoon. Model guidance brings the
remnants of this wave eastward into our region on Monday, with
NAM/GFS/RGEM guidance all forecasting at least some measurable QPF
with this wave. Even so, areal coverage of this convection will be
sparse, only briefly impacting southern portions of the cwa.

For those interested in viewing the solar eclipse, mostly clear skies
are still expected across the lake plains where lake shadowing is
expected to minimize cloud cover. Partial cloud cover is expected
across the Southern Tier and upper Genesee Valley due to the
shortwave, however periodic breaks and movement of any showers will
likely provide an opportunity to view the eclipse even in these
areas. Forecast hedges 1-2 degrees below forecast consensus due to
the patchy cloud cover and the slight reduction in mid-day heating
due to the eclipse. Highs should average in the lower to mid 80s.


As we move into Monday night, Western and North-Central New York
will become firmly entrenched within the warm sector of an
organizing/deepening surface low over the upper Great Lakes. A solid
return flow of GOMEX air will make for a very warm and sticky night
across the forecast area, with temperatures in many locations
failing to fall below 70. Even the coolest spots across higher
terrain will only bottom out in the mid 60s.

The big story of the week remains the storms expected on Tuesday
ahead of an approaching strong upper level trough and accompanying
cold front that will cross the area Tuesday night. There remains
some disagreement between models on the degree of instability that
will exist when the pre-frontal trough arrives Tuesday afternoon,
with model soundings running anywhere 400 and 2000 j/kg by the
afternoon. Some of this difference is due to disagreements on
surface temperatures, likely tied to the model sky cover.
Regardless, models are in solid agreement that there will be an
abundant amount of vertical shear in play, as much as 40kt between 0-
6km, and this combined with plenty of synoptic forcing more than
warrants the Day 3 Slight Risk that SPC has placed across the entire
forecast area. Fairly unidirectional profiles suggest that damaging
wind gusts with QLCS segments will be the primary severe threat. In
addition, with PWATs exceeding 2 inches, torrential downpours are
likely to accompany storms as well, though storms should be dynamic
enough that flash flooding is not expected to be an issue at this

It will be a very warm day on Tuesday, with temperatures climbing
into the upper 80s from the Genesee valley eastward, where cloud
cover is expected to be less of an issue during the first half of
the day. Temperatures will "only" reach the low to mid 80s in
western New York, thanks to clouds and convection starting earlier
in the day. Temperatures will drop sharply Tuesday night with the
passage of the cold front with readings falling into the upper 50s
to lower 60s as much cooler and drier air sweeps across the lower
Great Lakes. While 850mb temps falling as low as +6C are certain to
elicit a lake response from late Tuesday night through Wednesday
night, the combination of a relatively short WNW fetch and very dry
air aloft should yield mainly cloudy skies, with only very low
chances of a shower or two. It will however feel almost fall-like on
Wednesday, with highs struggling to reach 70 across many areas, and
temperatures will dip into the 50s area-wide Wednesday night.


A seasonably deep longwave trough, on the order of 1 to 2 standard
deviations below normal will become established for the second half
of the week over the Great Lakes and Northeast, bringing a stretch
of below normal temperatures to our region.

Cyclonic flow across the lakes and associated cold air aloft,
along with some shortwave energy moving through will support a
good deal of lake effect and upslope clouds, and possibly even a
few scattered showers at times. The airmass is plenty cold
enough with 850 mb temperatures falling to near +6C for lake
effect rain, but short northwest fetch and relatively dry
synoptic scale background should keep this potential limited.

Surface high pressure will begin to build into the area by
Friday and should bring an end to any scattered lake showers.

Temperatures will be quite cool both Thursday and Friday when
highs may struggle to get out of the 60s even at lower
elevations. So a definite "taste of fall" feel for Thursday and

Next weekend high pressure will settle directly overhead. This
should provide a dry and sunny weekend with temperatures
improving back to seasonal normals with highs in the 70s with
with low humidity and light winds.


For the 00Z TAFS, VFR flight conditions are expected across the TAF
region throughout the duration, except for the So. Tier where some
valley fog is possible. Though dewpoints are higher tonight than
what they were last night at this time, some mid and high level
clouds late tonight may limit the extent of the fog formation. Will
continue low end MVFR vsbys for KJHW as there is still come
uncertainty for IFR VSBYS.

Several showers and thunderstorms are possible tomorrow afternoon,
mainly to the east of KJHW and south of KROC. For now will not place
any precipitation qualifier in the TAF as activity should remain
away from the airfields.

Monday night...VFR.
Tuesday...Mainly VFR in the morning, then showers and
thunderstorms in the afternoon and night with MVFR/IFR
conditions. Some storms may contain strong winds.
Wednesday and Thursday...VFR/MVFR with a chance of showers.


Lighter southerly flow Monday and Monday night will direct any waves
mainly toward Canadian waters. Then a strong cold front will cross
the region Tuesday which will eventually require small craft
advisory headlines that may linger through Wednesday or Wednesday
night. A few strong to possibly severe thunderstorms are also
possible Tuesday ahead of the cold front.





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