Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 191432

Area Forecast Discussion
National Weather Service Buffalo NY
1032 AM EDT Sat Aug 19 2017

A weak upper level disturbance will cross the region today with a
few widely scattered showers and thunderstorms. High pressure will
then bring a return to dry weather Sunday through Monday.
Temperatures will be briefly cooler today before becoming very warm
and humid again early next week. A cold front will bring another
round of showers and thunderstorms Tuesday, followed by much cooler
and less humid for the end of next week.


A weak low level trough will continue to cross the region through
this morning, bringing areas of cloud cover to much of the region.
The combination of convergence along the weak boundary, cooling air
aloft, and lake induced instability will support an area of lake
enhanced rain showers east of Lake Ontario. This should quickly
break apart by midday as diurnal mixing disrupts the delicate
lake induced convergence zone.

During the afternoon a fairly sharp mid level trough will approach
the region with increasing DPVA. Radar shows line of showers
developing along the leading edge of this across Eastern Ohio,
with these likely to expand in coverage but largely stay just
south of the NY/PA border. Otherwise, lake breeze boundaries
will play the primary role, with scattered showers and
thunderstorms developing from the western Southern Tier into the
western Finger Lakes and southern Tug Hill region this
afternoon and evening. Showers and thunderstorms may also
develop across Niagara and Orleans counties where the breeze off
Lake Erie will encounter a northwesterly wind shift in the
afternoon. Otherwise, stable lake shadows will develop northeast
of the lakes this afternoon and prevent any convection from the
rest of the Niagara Frontier to Rochester and across Jefferson
county. This shadowing will also provide more sunshine.

A brief push of cooler air aloft will keep highs in the mid to upper
70s across lower elevations and lower 70s on the hills. It will
become breezy once again northeast of Lake Erie with gusts in the 20-
25 mph range.

The mid level trough will cross the region this evening. A few
showers may linger into this evening, elsewhere the trough will
just bring a period of greater cloud cover. Overnight a few
upslope showers will linger east of Lake Ontario along with more
cloud cover. Farther west, increasing subsidence and drying
will bring clearing skies. Expect lows to range from the lower
60s on the lake plains to the mid 50s across the interior
Southern Tier and Lewis County.


A sprawling surface high will move through upper Ohio Valley Sunday
then off the mid-Atlantic coast Monday. A dynamic trough will dive
through the Great Lakes Tuesday sweeping a potent cold front across
the area Tuesday night.

Sunday and Monday will be characterized by building warmth and
humidity as return flow develops between the surface high moving off
the mid-Atlantic and the impinging trough over the upper Great
Lakes. Temperatures will be near seasonal normals on Sunday with 850
mb temps around +12 to +14C, while increasing southwesterly flow
Monday and especially Tuesday ahead of the cold front will push 850
mb temperatures to around +18 to +19C. The combination of the warmer
airmass and downslope flow across the Genesee Valley and northern
Finger Lakes will result in upper 80s readings both day, with a
potential for a few sites to see the 90 degree mark. Northeast of
Lakes Erie and Ontario, a stiff lake breeze will keep temperatures
slightly cooler, in the low to mid 80s. Dry weather and ample
sunshine will prevail with subsidence under the ridging aloft on
Sunday and Monday. This will provide ideal viewing conditions for
the solar eclipse on Monday afternoon. Monday night will be quite
warm and muggy ahead of the approaching front, with low temperatures
not likely to fall out of the 70s across the lake plains and dew
points in the upper 60s to near 70.

By Tuesday afternoon and into the overnight hours, shower and
thunderstorm chances will increase as the trough approaches and
crosses the region. A fairly dynamic shortwave rounding the base of
the trough will help provide forcing for ascent and also increasing
the wind shear Tuesday late afternoon and early evening. Current
model depictions of thermodynamic profiles suggest fairly tall,
skinny CAPE. However, given the strongly forced nature of the trough
passage with ample shear certainly cannot rule out a severe weather
threat of mainly damaging wind gusts. Severe weather potential is
still uncertain, and may change based on the timing of the front.


A cooler airmass will work its way across the northeast in the wake
of the cold frontal passage Tuesday night, bringing a fall feel to
the second half of the work week. 850 mb temperatures will fall to
around +6C for Thursday and Friday with drier northerly flow as a
sprawling Canadian surface high builds over the Great Lakes. The
result will be daytime high temperatures in the mid 70s Wednesday,
but in the upper 60s to low 70s for Thursday and Friday. Initially
some lingering moisture and upslope flow Wednesday will support some
isolated to widely scattered showers, especially over the higher
terrain where northerly upslope flow can help with orographic
ascent. Progressively drier air will build in, along with a lowering
subsidence inversion, with the surface high for Thursday and Friday.
Thus, despite the cooler temperatures aloft normally sufficient for
a lake response, don`t expect we`ll see much other than some
stubborn lake enhanced cloudiness south of the lower Great Lakes.


Areas of clouds will continue through early afternoon with a
mix of VFR and MVFR CIGS. Scattered showers and a few
thunderstorms will develop this afternoon with VFR prevailing in
most areas, although any thunderstorm may produce brief/local
MVFR to IFR conditions.

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.


Sunday 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 likely.
Wednesday...VFR/MVFR with a chance of showers.


Based on latest guidance and the development of a convergence
band on Lake Erie have added a chance of waterspouts to the
forecast late this afternoon into tonight. Although the
850mb to lake temperatures difference is marginal, equilibrium
levels will increase significantly with the arrival of the upper
level trough.

Low pressure will move slowly east across Quebec today and bring
another round of moderate to strong WSW winds to the eastern Great
Lakes. 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. High pressure will bring a
return to lighter winds Sunday and Monday.


NY...Beach Hazards Statement through this evening for NYZ006-007-
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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