Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 192040

Area Forecast Discussion
National Weather Service Buffalo NY
440 PM EDT Sat Aug 19 2017

A weak upper level disturbance will cross the region with a few
widely scattered showers and thunderstorms through this evening.
High pressure will then bring a return to dry weather Sunday
and Monday with gradually warming temperatures. A strong cold
front will cross the region Tuesday, with widespread showers
and thunderstorms. This will be followed by much cooler and less
humid conditions for the end of next week.


A rather sharp mid/upper level trough axis will move across the
region through this evening. A moderate southwesterly flow ahead
of this has resulted in lake shadows northeast of the lakes this
afternoon, but the lake shadows should break down as a surface
trough pushes across the region this evening. Expect mainly
scattered showers and thunderstorms, which will be most
widespread across the Western Southern Tier which is closer to
the strongest DPVA and instability located to the south of the
region. Showers and storms will also develop across
Niagara/Orleans counties and these should weaken as they push
southward into the lake shadow. Afternoon temperatures will be
in the 70s.

A surface trough trailing the upper level trough will drop from
north to south across the region this evening. This will shift
winds to the northwest and may result in another round of
lake/upslope enhanced showers east of Lake Ontario. Farther
west, increasing subsidence and drying will bring clearing
skies. Expect fog to develop in river valleys across the
Southern Tier. Lows will range from the lower 60s on the lake
plains to the mid 50s across the interior Southern Tier and
Lewis County.

On Sunday, high pressure will ridge into the region with dry
weather and partly to mostly sunny skies. In the morning there
may be some lingering lake induced cloud cover, then in the
afternoon diurnal cumulus should develop inland of the lake
breeze. In each case, cloud coverage should be partial at most.
Otherwise, temperatures will be a bit warmer with highs ranging
from the mid 70s to lower 80s.


Sunday night and Monday will be characterized by generally fair
weather as upper level ridging transits the region. Speaking of
transits, the ridging should provide for good viewing conditions for
the upcoming solar eclipse - while we may see some diurnal fair-
weather cu developing inland of the lakes during the afternoon,
skies will remain mostly clear during the early afternoon hours on
Monday. Skies will be clearest northeast of the lakes, where the
prevailing southwesterly flow should generate a lake shadow.

The aforementioned southwesterly flow will be the product of broad
return flow around a prominent western extension of the Bermuda High
that will be solidly in place across the southeastern U.S.. While
temperatures Sunday night will be seasonable, with clear skies and
light winds allowing for radiational cooling that will in turn cause
temperatures to drop into the upper 50s in the coolest Southern
Tier/North Country spots, with low to mid 60s elsewhere, readings
will climb into the mid 80s on Monday as the return flow advects
increasingly warm and moist air into the region, with 850mb temps
climbing to +18C by Monday afternoon. The southwesterly flow and
associated downsloping will tack on an additional couple of degrees
across the Genesee valley and the Lake Ontario plain, where highs
will reach the upper 80s.

The warm temperatures will be accompanied by increasing mugginess as
well, as GOMEX moisture is drawn across the Mississippi and Ohio
valleys and into the Great Lakes. Dewpoints will surge into the mid
to upper 60s Monday night, resulting in lows that will fail to dip
below 70 across much of the Lake Erie and Lake Ontario plains, while
the rest of the forecast area will only dip into the mid 60s, making
for a sticky night.

The hot and muggy weather will persist into Tuesday. The warm and
moist airmass will become increasingly unstable on Tuesday, with the
approach of a deep upper level trough tracking across the Great
Lakes. This will likely drive showers and thunderstorms that will
develop across the forecast area Tuesday afternoon ahead of a strong
cold front that will be crossing the region Tuesday night. An
anomalously strong 40+ LLJ ahead of the trough will advect
additional moisture and significantly increase low-level shear as
well, and latest 12Z runs are a little faster with the timing of the
trough driving the convection, with lift arriving during peak
diurnal heating Tuesday afternoon. Given the very warm and moist
airmass preceding the system, and the strength of the synoptic
system and associated vertical shear, severe weather is looking
increasingly likely Tuesday afternoon into Tuesday evening, as noted
in the SPC Day 4 Outlook. After another day in the mid to upper 80s,
readings will fall off into the low to mid 60s in the wake of the
cold frontal passage.


A much cooler airmass will flow over the region this period, behind
a strong cold front from Tuesday.

Initially Wednesday a few lake effect rain showers will be possible
east of the lakes as cool 850 hPa temperatures of +7 to +8C flow
over the lakes. As the upper level trough passes Wednesday
additional scattered showers will be possible across the region. In
addition winds will be brisk Wednesday especially over the lakes and
along the shoreline where WNW winds upwards to 25 knots and
increasing wave heights will be likely.

Thursday we will still have anticyclonic flow aloft, and with the
core of the cold air passing over the lakes, additional lake effect
showers and clouds will be possible...though with a thinner moisture
layer...expect the lake response to be more clouds than showers.

Both Wednesday and Thursday will be cool with highs in the upper 60s
to lower 70s.

Sprawling high pressure will then reach our region for Friday and
Saturday with abundant sunshine and temperatures around 70
Friday increasing to the lower 70s for Saturday.


Scattered showers and thunderstorms will develop inland of lake
breezes this afternoon, with lake shadowing early this afternoon
gradually eroding late afternoon and early evening. Outside of
this, expect partly cloudy skies with VFR conditions. Showers
and a few thunderstorms may produce brief/local MVFR to IFR

The convection will end this evening, although a few upslope showers
may continue across the higher terrain east of Lake Ontario where
some MVFR CIGS may also develop. Valley fog will develop late
tonight across the western Southern Tier with local IFR. Otherwise
VFR will prevail for the rest of the region.

Any valley fog will quickly dissipate Sunday morning, with
mostly clear skies and VFR conditions during the day Sunday.


Sunday night and Monday...VFR except for late night and early
morning valley fog with local IFR.
Tuesday...Mainly VFR, with a chance of showers and
thunderstorms in the afternoon.
Tuesday Night...MVFR/IFR, with showers and thunderstorms
Wednesday and Thursday...VFR/MVFR with a chance of showers.


A moderate to strong WSW flow will continue this afternoon, then
will diminish and shift to the NW tonight. This will bring Small
Craft Advisory conditions to much of Lakes Erie and Ontario
through this evening. Winds will then diminish from west to east
tonight. There is a chance of waterspouts through this evening.
Although the 850mb to lake temperatures difference is marginal,
equilibrium levels will increase significantly with the arrival
of the upper level trough.

High pressure will bring a return to lighter winds Sunday and
Monday. Then a strong cold front will result in increasing winds
Tuesday which may require another round of small craft headlines
before and after the cold frontal passage. There also may be
strong thunderstorms along and ahead of the front.


NY...Beach Hazards Statement until 11 PM EDT this evening for
MARINE...Small Craft Advisory until 11 PM EDT this evening for LEZ020-
         Small Craft Advisory until 8 PM EDT this evening for
         Small Craft Advisory until 5 AM EDT Sunday for LOZ044-045.



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