Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 131721

Area Forecast Discussion
National Weather Service Buffalo NY
121 PM EDT Sun Aug 13 2017

High pressure will settle across the region tonight with mainly
clear skies. A weak ridge of high pressure will remain in place
through the first half of the week. This will provide dry weather
most of the time, although a few widely scattered showers and
thunderstorms cannot be ruled out during the afternoon and evening
on Monday and Tuesday. Low pressure moving into the Great Lakes will
then bring a better chance of more numerous showers and
thunderstorms on Thursday.


Satellite imagery showing an abundance of diurnal cumulus this
afternoon inland from the lakes. This will continue to bring a mix
of clouds and sun for inland areas through early evening, while
stable lake shadows provide more complete sunshine along the
lakeshores. A local convergence zone forced by lake breeze
boundaries interacting with the synoptic scale NW flow will force
cumulus to become congested along an arc from Grand Island through
the Buffalo Metro Area to eastern Erie and Wyoming counties.

The diurnal cumulus will dissipate this evening with the loss of
daytime heating. Surface high pressure will drift east across NY/PA
tonight and bring mainly clear skies for most areas, with a modest
increase in high/thin cirrus from west to east late tonight. There
may also be an increase in lower clouds late tonight across the
higher terrain of the western Southern Tier as SSE return flow
develops. Good radiational cooling conditions will allow lows to
drop into the upper 50s to around 60 on the lake plains, with low to
mid 50s in the cooler Southern Tier valleys and Lewis County. Expect
typical Southern Tier valley fog late tonight and early Monday
morning. Patchy fog may also develop near other inland bodies of
water, such as the Seneca/Oswego Rivers, Black River Valley, and
near some of the lakes of the North Country.

On Monday high pressure will drift off the New England coast, with
weak southerly return flow developing in the low levels. This will
allow a plume of somewhat more moist and unstable air to advect
northward out of PA and into Western NY and the western Finger Lakes
during the afternoon. A weak/subtle mid level trough will move
through the Upper Midwest tonight and reach our region Monday
afternoon, providing some weak large scale ascent. A weak low level
inverted trough will also develop, and interact with mesoscale lake
breeze and terrain forced boundaries to increase low level
convergence during the afternoon and evening. The aforementioned
forcing and moisture, while not impressive, may be enough to support
a few widely scattered showers and thunderstorms during the
afternoon and evening across Western NY, Genesee Valley, and western
Finger Lakes. Expect highs within a few degrees of 80 across lower
elevations, with mid 70s across higher terrain and along the
immediate lakeshores as lake breezes develop.


By Tuesday, a shortwave trough currently back over Manitoba will
pivot across the forecast area on the southern periphery of a large
trough near the Hudson Bay. The region will be embedded in westerly
to west-northwesterly flow as the plume of moisture advection and
DPVA moves through on Tuesday. Instability will be fairly modest,
and any convection that develops will be placed inland from the lake
breeze boundaries. Given the west-northwesterly flow direction, this
best potential for showers and thunderstorms would be the Niagara
Peninsula, southeastward between Buffalo and Rochester to the
western Finger Lakes. Otherwise, outside of any scattered
showers/thunderstorms, and within the lake shadows, expect a mainly
dry with seasonable temperatures.

Weak ridging will build across the region on Wednesday in the wake
of this shortwave passage. The result will be fair weather with
continued seasonable temperatures as 850 mb temps remain around
+14C. Nighttime lows will be near normal as well each night, Monday
night through Wednesday night, mid 50s to low 60s. The warmest night
will be Wednesday night as warm advection ensues ahead of the next
approaching weather system, with upper 50s to mid 60s and increasing
cloud cover.

A low pressure system will then cross the region sometime Thursday
into early Friday, with timing differences between the various
global models. This will bring out next best chance for widespread
showers or thunderstorms across the region. Have increased PoPs to
low end likely on Thursday, keeping in mind timing will certainly
need to be fine tuned as the day approaches.


Cool air advection behind the trough passage with weak shortwaves
make it difficult to rule out a few showers Friday and Friday night.
Another more significant shortwave is possible Saturday with a
chance of thunderstorms. Overall temperatures will remain near
seasonal values, trending slightly cooler for the end of the week
into next weekend.


Extensive diurnal cumulus inland from the Great Lakes will continue
through early evening with bases in the 4-5K feet range. The diurnal
cumulus will dissipate this evening with the loss of daytime
heating, leaving behind mainly clear skies with just a modest
increase in high/thin cirrus late tonight. Valley fog will develop
across the western Southern Tier late tonight and early Monday
morning with local IFR, and some of this will likely impact KJHW.
Some patchy fog may also bring spotty IFR elsewhere, especially near
rivers and lakes inland from the Great Lakes.

On Monday a modest increase in moisture and instability will allow
diurnal cumulus to build again from late morning into the afternoon
across Western NY and the western Finger Lakes. A few widely
scattered showers and thunderstorms may develop during the afternoon
and evening. VFR will prevail, but any heavier shower may produce
brief/local MVFR to IFR conditions.

Tuesday...Local IFR in southern Tier valley fog early. Otherwise VFR
with isolated showers and thunderstorms during the afternoon and
Wednesday...Local IFR in southern Tier valley fog early, otherwise
Thursday...Areas of MVFR with showers and thunderstorms likely.
Friday...Mainly VFR with a chance of showers.


Surface high pressure will build into the eastern Great Lakes region
tonight before drifting off the New England coast Monday, although a
weak ridge will remain in place back into the eastern Great Lakes
through Wednesday. This will bring an extended period of light winds
and flat wave action with local lake breezes each afternoon
producing weak onshore flow.





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