Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 271846

Area Forecast Discussion
National Weather Service Buffalo NY
246 PM EDT Sat May 27 2017

High pressure crossing the region will keep conditions quiet and dry
across the forecast area through Sunday morning. Shower and
thunderstorm chances will increase from west to east Sunday
afternoon through Sunday night, as a low pressure system crosses the
area. This initial round of precipitation will exit the region
Monday. Thereafter, expect mild temperatures with afternoon showers,
as an upper level trough will linger across the region through much
of the coming week.


As expected, widespread stratocu has developed across the forecast
area this afternoon, with most areas inland from the immediate
lakeshores showing cloudy to mostly cloudy skies. This cloud cover
will dissipate tonight, as upper level ridging moves overhead and
diurnal heating disappears. However, given lingering low level
moisture, should expected clearing materialize, we can expect to see
areas of fog develop late tonight. Look for temperatures to fall
into the lower 50s overnight, with upper 40s in the typically cooler
North Country.

Generally tranquil conditions will prevail through the first half of
the day on Sunday, as the upper level ridge axis crests across the
forecast area before moving off to the east. Once again, with
lingering low-level moisture, widespread diurnal cu/stratocu can be
expected in the afternoon. In addition, there will be a potential
for showers, and possibly even a few thunderstorms across western NY
in the afternoon, as a shortwave trough approaches the area in place
of the departing ridge. Increasing south-southeast flow and
strengthening warm advection ahead of the approaching low pressure
system will boost 850mb temps to +12C, yielding highs in the mid 70s.


Showers and thunderstorms will become more numerous Sunday night as
first a warm front, and then a cold front and upper level shortwave
bear down upon the region. Water Vapor imagery this Saturday
afternoon displays a shortwave advancing across CO/NE and this
feature will reach WNY by late Sunday night. A surface low pressure
will track to the west and north of WNY Sunday night with a warm
front lifting across the region Sunday night, followed quickly by a
cold front reaching WNY late.

Initially showers Sunday night will be associated with a warm front
lifting northward across the Eastern Great Lakes region. Modestest
instability for overnight will remain across far WNY, where a few
rumbles of thunder are possible. Lower instability to east will
reduce thunder chances.

Later and ahead of a surface cold front, limited prefrontal
instability coupled with upper level height falls will generate
showers and perhaps some thunder. This unsettled weather will cross
the region Sunday night and into Monday. The cold front will slice
across WNY Monday morning, and lingering showers and thunderstorms
will exit our region through the morning hours across WNY, and
afternoon hours across the Eastern Lake Ontario region. Dry weather
and weak high pressure will then expand from west to east across WNY
and NCNY.

Monday night and Tuesday another upper level low will settle
southward across the Great Lakes region. Several impulses rotating
through this feature will bring chances for showers and perhaps
thunder. The first of these impulses is expected to reach WNY Monday
night, with a chance for showers and thunderstorms before the upper
level impulse dampens out within the mean flow.

Tuesday another impulse, combined with the upper level low drawing
closer will bring increasing chances for showers and thunderstorms.
The bulk of this activity will likely form on lake breeze boundaries
through the afternoon and evening hours. MLCAPE values of 500 to 750
J/KG and PWAT values around three quarters of an inch will provide
the instability and moisture that when interacting with the lift
along lake breeze boundaries, convection is likely to form. A
morning inversion will likely keep this convection at bay, but by
noontime convection should be able to overcome the decreasing
capping inversion. Moisture is not overly impressive, and activity
may be just scattered in nature. Northeast of the lakes, a southwest
stabilizing wind flow will promote sunshine and likely a dry day for
areas such as metro Buffalo, and north of Watertown. Temperatures
Monday and Tuesday will be within a few degrees of normal. Sunday
night, with the late arrival of the cold front, will remain mild
with lows in the low to mid 60s, while behind the cold front Tuesday
night overnight lows will drop back into the 50s.


Moving on into the extended portion of the forecast...the massive upper
level low will only slowly meander its way across the Great Lakes and
southeastern Canada through the middle portions of next week...with its
associated pool of cooler air slowly filtering across our region.
This will result in highs pulling back into the mid to upper 60s for
both Wednesday and Thursday...while nighttime lows gradually settle
back to between 45 and 50.

With respect to precipitation potential...numerous shortwave impulses
rotating around the upper low will result in frequent opportunities
for showers across our region...with some embedded thunderstorms
also possible Wednesday with diurnal heating/destabilization of our
slowly cooling airmass. While timing such waves remains a difficult
exercise this far in general precipitation chances
should tend to peak each afternoon across areas away from the lakes
owing to diurnal heating...before diminishing each night with
nocturnal stabilization of the boundary layer.

Late in the period...the upper level low will begin to lift out into
the Canadian Maritimes...while allowing surface high pressure to build
eastward into our region. This should finally bring a return to drier
weather to close out the work week...though temperatures will probably
remain a bit below normal with highs remaining confined to the mid
and upper 60s on Friday.


Widespread VFR cigs dominate the forecast area this afternoon, as
lingering low-level moisture has led to the development of
widespread stratocu. Expect clearing to take place tonight, with
upper level ridging moving overhead and the loss of diurnal heating.
However, enough low-level enough moisture should linger across the
region that, should expected clearing occur, fog development will be
possible across much of the forecast area.

Morning fog will dissipate by around 12-13Z, leaving widespread VFR
conditions in place through the rest of the forecast period. Diurnal
VFR cloud cover is expected to re-develop in the afternoon once
again, with additional clouds moving overhead western NY late in the
period, as a low pressure system approaches the area.


Sunday night...Areas of MVFR with widespread showers and
Monday...Mainly VFR...Showers and thunderstorms exiting the east.
Tuesday through Thursday...Mainly VFR with scattered afternoon showers
and thunderstorms.


Weak high pressure will cross the lower Great Lakes through tonight.
This will maintain a weak surface pressure gradient that will keep
light winds in place along with negligible waves.

As a warm front approaches on Sunday...east to southeast winds will
freshen a bit...but will remain well below small craft advisory
levels. The only ill effects will be choppy conditions on the
western end of Lake Ontario...and particularly near the mouth of the
Niagara River, where the winds will oppose the outflow from the
river. The main concern though will come from thunderstorm activity
that could develop during the afternoon. This threat will be
greatest over Lake Erie.

Showers and thunderstorms will become widespread over the Lower
Great Lakes Sunday night...gradually spreading across the St
Lawrence Valley late Sunday night and Monday. Meanwhile...winds will
continue to freshen Sunday night as they veer to the south in the
wake of the passing of a pair of frontal boundaries...including the
aforementioned warm front.

For Memorial Day...the showers and thunderstorms will push east...
clearing Lake Erie by mid morning and eventually clearing Lake
Ontario by mid afternoon. Moderate southwesterlies will be found on
both lakes...but winds and waves should remain below small craft
advisory levels.





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