Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 270257

Area Forecast Discussion
National Weather Service Buffalo NY
1057 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 areas of drizzle
and fog overnight 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 late this evening shows earlier scattered showers with
diurnal influences have diminished to areas of of drizzle under the
overcast skies across much of western and central NY. This is
associated with moisture wrapping around a stacked low near southern
Nova Scotia and upslope flow off the lakes.

The low will continue to move into the Canadian Maritimes through
Saturday as a weak area of high pressure moves into the Lower Great
Lakes. Upstream a small but potent mid level vorticity maxima and
associated MCV will move through the upper Ohio Valley overnight,
and reach PA by Saturday morning. This is showing up on regional
radars near 11pm over southwest Ohio. We continue to follow the
NAM/ECMWF/HRRR/GEM based guidance which has the best handle on the
current location and track of this system. With this in mind, most
of the associated showers with this system will remain south of our
forecast area Saturday morning, except perhaps right near the PA
state line.

Plenty of cloud cover will remain through the night with abundant
low level moisture in place. BUFKIT soundings show a wealth of low
level moisture, and a strengthening inversion overnight due to
nocturnal cooling and weak mid level subsidence. The combination of
increasing moisture and an 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.


A large upper level low will slowly move from the upper Great Lakes
to southeastern Canada during the middle portion of next week, with
its associated pool of cooler air filtering across our region. As a
result, temperatures will average slightly below normal during the
period, with daytime highs will be in the mid to upper 60s in most
areas, with lower 70s across the Genesee Valley and Finger
Lakes regions. Lows will average in the upper 40s to lower 50s.

Numerous shortwaves will be embedded in the cyclonic flow around the
upper level low. Although long range model guidance is in good
agreement with the upper low, timing these waves is difficult in
this timeframe. This will result in a prolonged period of mainly
chance PoPs, which will be greatest in the afternoon and early
evening hours due to diurnal heating, then diminish overnight. A
moderate southwesterly flow will likely result in lake shadowing
which will diminish chances downwind of the lakes.

Model consensus builds high pressure across the region on Friday,
which should finally bring dry weather to the region.


Stacked low pressure will move slowly east across the Canadian
Maritimes through Saturday, but the broad circulation will still
hold influence across our area overnight and into Saturday. Low
clouds and areas of drizzle will continue to be possible across all
TAF sites. Through the 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 to MVFR VSBY as well. Conditions
may approach airfield minimums at times overnight into Saturday

Expect slow improvement by late Saturday morning with fog and
drizzle dissipating and VSBY improving to MVFR/VFR around 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 Wednesday...MVFR/VFR with scattered to numerous


Low pressure near southern Nova Scotia late this evening will exit
tonight and Saturday with weak high pressure then slowly building
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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