Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 161831

Area Forecast Discussion
National Weather Service Buffalo NY
231 PM EDT Sun Jul 16 2017

A weak trough of low pressure will cross the region tonight and
Monday with some showers and thunderstorms. High pressure will
bring a return to mainly dry weather Tuesday and Wednesday before
chances for showers and thunderstorms return late in the week as
summer heat and humidity builds.


Early this afternoon, an upper level trough axis extends from
the Ontario/Quebec border southwestward to Eastern Michigan.
Showers and thunderstorms have developed just ahead of this
feature across Southern Ontario, from PVA and aided by diurnal
instability. This is fairly well resolved by model guidance,
with the leading edge of this trough axis providing the synoptic
scale lift to generate showers and storms in our area tonight.

Until then, regional radar imagery shows a few scattered
showers and thunderstorms which have developed along or just
inland of lake breeze boundaries. These appear to be aided by a
subtle mid-level wave which is moving across central New York
early this afternoon and should continue to move eastward. This
wave was not well handled by any available model guidance.
Meanwhile, visible satellite imagery shows lake shadows
developing northeast of Lake Erie and Lake Ontario with upstream
convection initially stalling or riding along these boundaries.
This should result in mainly dry weather across Western New
York late this afternoon, outside of sparse showers and
thunderstorms in a few spots.

Chances for precipitation will gradually increase from west to
east tonight as the trough axis gradually moves closer. There
will be ample instability for some showers and thunderstorms,
even without diurnal heating, but areal coverage should be
mainly scattered in nature with slow storm motion.

One thing to watch for tonight is the potential for slow moving
showers and thunderstorms to develop along a residual lake
boundary near the east shores of Lake Erie. Some mesoscale
guidance shows this happening with localized rainfall amounts in
excess of an inch possible. Precipitable water values will be
around 1.5 inches. This suggests a small risk for flash flooding
considering antecedent conditions, but amounts from even the
high resolution guidance are too marginal to support a Flash
Flood Watch. This will be added to the HWO, in addition to the
flood concerns for Monday.


By Monday morning an upper level low will be meandering slowly
eastward just north of Lake Ontario, eventually reaching the Saint
Lawrence valley Monday night. Scattered showers and thunderstorms
just ahead of and under the trough axis will work their way across
the region from over the lower Great Lakes in the morning to central
and eastern NY by the evening.

Initially Monday morning, land breeze convergence over the
relatively warmer lakes under the trough axis will help organized
showers and thunderstorms over the lake and the nearby locations
northeast of the lakes. As the trough meanders slowly eastward and
diurnal heating occurs over land, the lake breeze will develop,
drying out shower/thunderstorm chances for locations east northeast
of the lakes, while helping to organized a broken line of
thunderstorms inland, which will track off into central NY by late
afternoon. A few scattered thunderstorms will likely linger across
Niagara, Orleans and Monroe Counties along the Lake Erie / Lake
Ontario convergence zone. Overall shear will marginal, on the order
of 20 knots, despite the decent CAPE values near 2000J/kg. Thus any
severe weather threat would be limited to any pulse convection
likely interacting with a lake breeze boundary, which will be
overall fairly isolated.

Monday night into Tuesday the through will continue to slowly move
eastward to near Montreal. Given the height rises aloft and moisture
plume exiting into New England, this will overall lead to a dry
forecast Monday night through Tuesday night. There will only be a
low chance for showers and a rumble of thunder across the North
Country which may remain under the influence of the cyclonic flow of
the exiting trough. Temperatures Monday through Tuesday will be
seasonable, but warming slightly into Tuesday with the height rises
in the wake of the trough. Highs will be around 80 Monday and in the
low to mid 80s Tuesday. Nighttime lows will mainly be in the 60s,
expect for mid 50s in the interior western Southern Tier where light
winds and clearing skies will lead to better radiational cooling and
valley fog.

By Wednesday, increasing southwesterly flow and warm advection ahead
of an approaching shortwave will push high temperatures solidly into
the mid 80s with some upper 80s readings in the downslope prone
Genesee Valley and northern Finger Lakes. Wednesday should remain
mostly dry with increasing sky cover late in the day ahead of the
approaching shortwave in the northwesterly flow aloft.


The overall pattern for the second half of the week and into the
weekend will be dominated by a broad ridge over the center of the
country with northwesterly flow aloft from the Canadian high plains
across the Great Lakes to New England. The result will be mainly
seasonal temperatures for the forecast area as we remain aligned
along the temperature gradient between the warmer air under the
ridge and the cooler air under the trough to the north. Also due to
our position along the northern periphery of the ridge, we can
expect showers and thunderstorms at times as various shortwaves run
the ridge. The first chance for showers or thunderstorms will come
Wednesday night as a dampening wave approaches the region, however
with the weakening expect fairly isolated to widely scattered
coverage. The better chance thunderstorms, perhaps severe and with
heavy rainfall chances will come with a better wave either Thursday
or Thursday night. Due to the timing uncertainty have maintained 50
PoPs both Thursday and Thursday night. Surface high builds in for
later Friday into Saturday bringing a return to fair weather.


Mid level clouds will continue to gradually increase today from
northwest to southeast across the area with the approach of a
mid level trough. While clouds will increase, it appears any
showers and thunderstorms will hold off until this evening when
deeper moisture and better ascent arrive as the mid level trough

From late evening through the overnight expect widely scattered
showers and a few thunderstorms to develop, with coverage possibly a
little greater over and northeast of Lakes Erie and Ontario where
warm lake waters will aid in maintaining greater instability through
the night. A few of these showers and storms will contain
local/brief MVFR to IFR, otherwise expect VFR to continue in most
areas tonight. Some fog will again form across the Southern Tier
valleys with local IFR.


Monday night...VFR/MVFR. A chance of showers and thunderstorms.
Tuesday and Wednesday...Mainly VFR.
Thursday...VFR/MVFR. Showers and thunderstorms likely.


Winds will remain light through the first half of the week as
the pressure gradient remains weak across the eastern Great


The KBUF WSR-88D has experienced a major equipment failure and
will remain out of service for an extended period of time,
likely most if not all of next week. A national support team
is scheduled to arrive Sunday night and will begin work Monday
morning on the extensive repair project. A more definitive time
table for return to service will be available once the national
team arrives.





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