Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 200640

Area Forecast Discussion
National Weather Service Buffalo NY
240 AM EDT Sat May 20 2017

High pressure building south across the area will bring cooler
and drier air to start the weekend. Showers with a chance of
thunderstorms are then expected Sunday as low pressure lifts
north from the mid-Mississippi valley across the eastern Great


Mid-level clouds continue to stream across the area, as a weak upper-
level vortmax passes across eastern Ontario into the St. Lawrence
valley. Light precipitation falling out of the mid-level deck is
making it to the ground as a few sprinkles. These sprinkles will
continue to press on to the east overnight, giving way to dry but
cloudy skies for most areas along and south of the Thruway this
morning. Clearing skies across the North Country will press
southwest towards Rochester during the day, as mid-level cloud
associated with the passing shortwave moves off to the east, while
across western New York and the Southern Tier, clouds will linger
through the day, as warm advection aloft increases in advance of a
low pressure system moving across the Missouri valley.

Regarding temperatures, persistent cloud cover and northeasterly low
level flow will depress temperatures across Western NY, with lows in
the low 60s, with upper 50s along the Lake Ontario shore. Sunshine
across the North Country will allow temperatures to rise into the
mid 60s, in spite of cooler 850 mb temperatures in place.


During the first 48 hours of this upper level low over the
High Plains will drift northeastward to the Ontario-Quebec border...
with an associated modest surface reflection over the Central Mississippi
Valley also lifting northeastward and becoming co-located with the upper
low. In the process...the surface low will push one or more attendant
warm frontal segments across our area late Saturday night and Sunday...
followed by its trailing cold front later Sunday night and Monday

After a mainly dry start to Saturday evening...the above developments
will result in slowly increasing shower chances across areas south
of Lake Ontario through the rest of Saturday night...with showers then
becoming more widespread across the entire region Sunday and Sunday
evening...when a few embedded thunderstorms will also become possible
due to the development of some weak elevated instability. The showers
will then gradually taper off from west to east later Sunday night and
Monday with the passage of the trailing cold front...resulting in the
return of mainly dry conditions to most areas by early to mid afternoon.

The deep southwesterly flow out ahead of this system will help to pump
plentiful GOMEX-based moisture into our region during the Sunday-Sunday
night time frame...resulting in PWATs climbing to as high as one and
two-thirds of an inch. Coupled with the slow movement of the system...
this will lead to another soaking rainfall for our region...with a current
multi-model/WPC consensus suggesting the potential for as much as a half
inch to an inch of rain. Otherwise...any severe threat from this system
looks to be rather minimal given both the weak and elevated nature of
the instability.

In the wake of this system...the arrival of weak surface-based ridging
and associated drier air/subsidence should then allow for a dry Monday
night. Mainly dry weather should then continue through at least a good
portion of Tuesday...though a few showers cannot be totally ruled out
across western sections as the next system approaches later in the day.

With respect to temperatures...lows Saturday night should range from
the mid 40s across the North Country to the upper 40s and lower 50s
across the rest of the region. Highs on Sunday are more problematic...
as 850 mb temps will surge to between +10C and +15C at the same time
cloud and shower coverage will be on a marked west-east increase. For
now...have elected to stick with continuity and aim a bit above guidance
with highs mostly in the mid 70s...though such readings could be a bit
optimistic if the clouds and showers arrive quickly enough. After a mild
Sunday night with lows mainly in the lower to mid 50s...modest cool air
advection in the wake of the cold front will then suppress highs back
into the mid to upper 60s on Monday...before modest warm air advection
out ahead of the next system allows readings to climb back into the
lower to mid 70s on Tuesday.


Unsettled weather will return to our region during this
additional shortwave energy dropping southeastward from the Canadian
Prairies leads to re-amplification of upper level troughing over the
Great Lakes and Northeast...with another complex surface low extending
from the Central Great Lakes to the Carolinas gradually consolidating
and lifting northeastward to Quebec Province. This will result in
fairly widespread showers returning to our region for Wednesday and
Thursday...then slowly diminishing Thursday night and Friday as the
low departs into Quebec. Meanwhile near to slightly above normal
temperatures Tuesday night and Wednesday will slip back to a little
below average for the remainder of the period...with highs only
expected to reach to around 60 Thursday...and to the lower to mid
60s on Friday.


Much of the forecast area remains under VFR cigs, as mid-level
clouds continue to cross the region, courtesy of a passing upper
level shortwave. VFR cigs will persist across much of the forecast
area today - even as mid-level clouds associated with the wave move
off to the east, they will be replaced with increasing clouds from
the southwest due to increasing warm advection aloft ahead of an
approaching warm front. The one exception will be across the North
Country, where skies will remain clear through much of the period.


Sunday through early Monday morning...MVFR. Showers likely with a
chance of thunderstorms.
Later Monday and Tuesday...Mainly VFR.
Wednesday...VFR/MVFR with showers likely.


High pressure will build south across the lower Great Lakes
tonight and Saturday. Northeast winds tonight and Saturday
will make for choppy conditions on the west half of Lake
Ontario...and particularly for the nearshore waters west of
Hamlin Beach and for the Lower Niagara River...although criteria
is not expected to reach small craft advisory levels.





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