Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 261818

Area Forecast Discussion
National Weather Service Buffalo NY
218 PM EDT Fri May 26 2017

Low pressure will move slowly away from the region and into the
Canadian Maritimes, but will continue to support scattered showers,
drizzle, and fog tonight through Saturday morning. Conditions will
slowly improve later Saturday. High pressure will then bring dry and
much warmer weather Sunday before a cold front crosses the area
Sunday night with a good chance of showers and thunderstorms.


Radar imagery showing scattered light showers and a few pockets of
drizzle across much of the region this afternoon, with all of the
activity very light. Expect this to continue through early evening
as the area remains under a broad/moist cyclonic circulation from
low pressure moving through New England. Expect a relative maximum
of scattered showers across the eastern Lake Ontario region near a
weak low/mid level deformation zone, and also from the Niagara
Frontier to the interior Southern Tier and western Finger Lakes near
a channelled mid level vorticity maxima and low level convergence

Synoptically, the low over New England will move into the Canadian
Maritimes through Saturday as a weak area of high pressure moves
into the Lower Great Lakes. A small but potent mid level vorticity
maxima and associated weak surface low will move through the upper
Ohio Valley tonight, and reach PA by Saturday morning. The GFS
continues to be an outlier with a farther north position of this
feature, and we continued to follow the farther south NAM and ECMWF
based guidance. With this in mind, most of the associated showers
will remain south of the area Saturday morning, except perhaps right
near the PA state line.

The scattered showers will slowly diminish this evening with the
loss of weak diurnal instability and the breakdown of the supporting
bands of convergence. Expect plenty of cloud cover to remain into
the evening with abundant low level moisture in place. Low stratus
will likely expand and lower even further from late evening through
the overnight. BUFKIT soundings show a wealth of deepening low level
moisture, and a strengthening inversion overnight due to nocturnal
cooling and weak mid level subsidence. The combination of increasing
moisture and a developing inversion will support widespread low
clouds, and also areas of fog and drizzle overnight. The extensive
cloud cover will keep temperatures from dropping much despite the
cool airmass, with lows in the lower 50s in most areas and upper 40s
across the interior Southern Tier and eastern Lake Ontario region.

Expect the fog, drizzle, and low clouds to last through Saturday
morning in most areas. This will then give way to slow and steady
improvement during the afternoon as the low moves into the Canadian
Maritimes and the broad/moist cyclonic circulation slowly fades
away. Expect the fog to dissipate by mid to late morning, with the
drizzle ending by midday. Clouds will be stubborn to clear across
the higher terrain well inland, but developing stable lake shadows
should aid in increasing amounts of sunshine by late afternoon on
the lake plains. Temperatures will remain on the cool side, with mid
to upper 60s for most.


Good news for those with outdoor plans this weekend, there continues
to be a slowing trend with the arrival of precipitation on Sunday.
Not only will this slowing trend keep much of the day Sunday dry
(until late afternoon/early evening), but will also allow
temperatures to close in on the 80 degree mark for the downslope
locations across the lake plains and Genesee Valley.

Looking at the large scale details, the upper-level low seen on WV
imagery over southeastern Alberta today will slowly meander eastward
to the upper Great Lakes during the day Sunday. Ahead of this low, a
warmer and more moist airmass will push northward across the
forecast area. 850 mb temperatures will surge to +11 to +14C, which
when combined with southeasterly flow ahead of the deepening surface
low, will bring downslope warming across the Lake Plains and Genesee
Valley. High temperatures will be solidly in the 70s Sunday, with a
few readings near the 80 degree mark in the downslope regions. Much
of the day should be dry before precipitation arrives from southwest
to northeast later Sunday afternoon and into Sunday evening. An axis
of instability will arrive with the better forcing, and so there
should be some embedded thunder in the area of showers. Overall,
timing of arrival during the overnight should limit the instability
and thus keep the storm strength in check. Temperatures will remain
fairly mild Sunday night with the warm advection and southerly
downslope flow ahead of the cold front, despite the areas of showers
and thunderstorm moving through. Lows will only bottom out in the
upper 50s to low 60s.

By Monday, the stacked low pressure system will be centered over
northern Lake Superior. Slightly cooler, yet still seasonable, air
will filter into the region behind the initial cold front, but the
occluded nature of the low will keep the colder air held up back
across the upper Great Lakes. 850 mb temps around +10 will keep
highs in the low to mid 70s. The increasing pressure gradient across
the lower Great lakes with the surface low over northern Lake
Superior will develop a synoptically aided southwesterly lake breeze
across the Niagara Frontier on Monday. Winds will peak in the
afternoon there with gusts of 35 to 40 mph. Additionally, the lake
breeze will act to keep the Niagara Frontier dry through the day,
while locations on either side of the lakes (southern Ontario and
the western Southern Tier to southern Tug Hill) will see showers and
thunderstorms develop during the afternoon with the steeper lapse
rates aloft under the cyclonic flow. The stout lake breeze will act
as a focus for any convection that develops. Would not rule out a
couple stronger pulse storms that get to near severe criteria given
the wind shear and limited instability, although most of the storms
should be garden variety. Continued breezy conditions overnight with
only minor cold advection (nearly neutral) will keep overnight lows
Monday night in the 50s.


The large 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 in the upper 60s to lower 70s
Tuesday pulling back into the mid to upper 60s for both Wednesday and
Thursday...while nighttime lows gradually settle back to the upper 40s
and lower 50s.

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 each day with diurnal heating/destabilization of our slowly
cooling airmass. This said...conditions will also be far from a complete
washout...with periods of drier weather likely found in between the
passing impulses.


Low pressure will move slowly east out of New England and into the
Canadian Maritimes through Saturday, but the broad circulation will
still hold influence across our area through the TAF period. A
wealth of low level moisture will remain in place with widespread
low stratus through the first half of Saturday. CIGS will be mainly
MVFR at lower elevations with areas of IFR higher terrain into early
this evening. By late evening and overnight low level moisture
deepens and the inversion strengthens, which should allow for
widespread IFR CIGS to develop. Low level winds will also be light,
which will likely allow for areas of fog, drizzle, and IFR VSBY to
develop as well. Conditions may approach airfield minimums at times
overnight into Saturday morning.

Expect slow improvement by late Saturday morning with fog and
drizzle dissipating and VSBY improving to MVFR/VFR by 18Z. CIGS will
remain IFR through much of the morning, with improvement to MVFR in
the afternoon.

Sunday night...Areas of MVFR with showers and thunderstorms likely.
Monday through Tuesday...MVFR/VFR with a chance of showers.


Weak high pressure will slowly build east across the Lower Great
Lakes over the next few days. Winds will remain light through the
weekend with winds generally 12 knots or less and waves less than 2
feet through Sunday.

Low pressure will then move into northwest Ontario province Sunday
night and stall Monday through Wednesday. This will generate a
little more in the way of wind, with southwest winds increasing on
Lake Erie and Ontario through the period and generating some modest
wave action at the northeast end of both lakes.





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