Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 250635

Area Forecast Discussion
National Weather Service Buffalo NY
235 AM EDT Tue Jul 25 2017

Low pressure over central New York will move east of our region and
dissipate on Tuesday. This will allow any leftover spotty light rain
or drizzle to come to an end...with skies then slowly clearing out
between late Tuesday afternoon and Wednesday morning as high pressure
settles across New York State. The high will then provide our area
with a nice day on Wednesday...before a cold front crosses the area
on Thursday along with another round of showers and thunderstorms.
Following the frontal passage...high pressure will then build across
the area for the end of the week and the upcoming weekend...along
with a return to dry and comfortable weather.


As of 06Z...surface low pressure is now situated near Watertown...
with its trailing cold front having cleared the Southern Tier.
While some showers still persist across the Saint Lawrence Valley
out ahead of the low...the remainder of the region is now enveloped
in the moist northwesterly/northerly upslope flow on the backside
of the low...which is producing widespread low clouds and some
spotty very light rain or drizzle. This will continue to be the
story for the rest of the night as the low slowly meanders its
way across the North Country...while ongoing cool air advection
helps drive overnight lows down to between 55 and 60 degrees.

During the day Tuesday, it will actually feel quite fall-like for
this area, with northerly flow and trapped low-level moisture under
a sharping inversion leading to overcast skies lasting much of the
day. The cool pool of air aloft will keep daytime highs below
normal, with highs only in the 60s to near 70. Thick low overcast in
the morning will likely lift some and break into a stratocumulus
deck that will linger through the afternoon. This process will also
allow for any morning fog/drizzle to dissipate as we get into the
late morning.


Expect dry and quiet weather Tuesday night through Wednesday as
surface high pressure and low amplitude mid-level ridging shifts
across western and central New York. Added some Southern Tier river
valley fog Tuesday night with the high cresting overhead. Expect
partly to mostly sunny skies on Wednesday. Expecting a cool night
Tuesday night with lows in the mid 50s inland to around 60 at the
lakeshores. Wednesday, southerly flow develops helping to boost
temperatures back to late July levels with highs forecast in the
upper 70s to low 80s except low 70s across the Tug Hill.

Most of Wednesday night should still remain dry ahead of an
approaching cold front. Have gone with a low chance of some showers
across the Niagara Peninsula and SLV Thursday morning either from a
narrow leading warm sector or well ahead of the cold front.
Otherwise, mugginess will return overnight as the lower Great Lakes
comes under strengthening warm and moist southerly flow. Temps may
actually rise late in the night. Expect lows to bottom out only in
the mid to upper 60s with 70 possible for lows near the lakeshores.

The next likely period of showers and thunderstorms will then arrive
Thursday with the passage of a cold front. There remains some timing
differences among the 12z guidance with some showing a morning
frontal passage and others later in the afternoon. This leaves
uncertainly in more exact timing so have held on to continuity which
better aligns with the 12Z ECMWF and brings the highers POPs across
our region in the afternoon hours. Instability at this range looks
weak but with better insolation ahead of the front we could be
looking at a risk for some strong/severe storms as 0-6km shear is
forecast to run about 40kts. SPC has not included a severe risk area
at this time. 850mb temps are forecast to rise to +13C to +14C which
will translate into highs in the upper 70s to low 80s. Combined with
dewpoints rising to the mid/upper 60s will make for a very
uncomfortable mid summer day across the region.

Showers and thunderstorms will taper off from northwest to southeast
with the passage of the front Thursday night. Northerly flow behind
the front will likely hold in cloudiness and possibly a few
scattered showers in wrap around moisture ahead of the trailing mid-
level trough axis. Dewpoints holding in the 60s will keep mild
overnight temps only bottoming out in the low 60s.


The axis of the upper level trough that is expected to bring the
showers and thunderstorms on Thursday/Thursday night will be
centered to the east of the forecast area by the time we get into
Friday, and the bulk of the precipitation should be clear of the
area. However, the 12Z ECMWF and GEM are both slower with the
progression of the trough than the GFS, and under this scenario
precipitation wrapping around a surface low over New England may
linger across the eastern half of the forecast area during the day
on Friday. Even if precipitation associated with the low stays clear
of the area, it is worth noting that 850mb temps falling to around
+9C in addition to lingering cyclonic flow aloft have the potential
to generate a few lake-effect showers south of Lake Ontario Friday
into Friday night. The precip potential may be limited by a dearth
of moisture aloft, as well as a short northerly fetch across the
lake. For the time being, will keep a low chance of pops in the
forecast pending better agreement on timing of the upper level
trough progression between models. As would be expected in this
scenario, temperatures will be a tad on the cool side for late July,
with highs in the lower 70s, with upper 50s to lower 60s Friday

Even if lake effect showers do not pan out Thursday into Thursday
night, we can expect lake-enhanced stratocu to hang around south of
Lake Ontario during this period. Clouds should diminish for the
weekend as the upper level trough continues to pull away from the
region and a col develops overhead in the upper levels, with broad
surface ridging in the lower levels. This should make for a pleasant
weekend across western and north-central New York, as high pressure
persists at least through Sunday and light north-northeasterly flow
helps keep dewpoints down in the upper 50s to around 60, with
seasonable highs in the upper 70s.


Low pressure near KART will slowly drift eastward and dissipate through
the course of Tuesday...with high pressure and drier air over the Upper
Great Lakes gradually building into the region between Tuesday and Tuesday
night. On the backside of the low...a moist northerly upslope flow will
maintain widespread IFR to MVFR stratus and areas of spotty light rain
and/or drizzle into Tuesday morning...before the combination of diurnal
influences and dry air advection leads to slowly improving conditions from
late Tuesday morning on through Tuesday night. This will first result in
the lower stratus evolving into an MVFR stratocu field and the dissipation
of any spotty light rain and drizzle...followed by gradual improvement to
VFR conditions...with the latter beginning first across far western and
northern New York late Tuesday afternoon...then gradually spreading
southeastward through Tuesday night. At this point it appears that the
Finger Lakes and Upper Genesee Valley will be the last to see conditions
improve back to VFR...with some patches of IFR/MVFR possibly hanging on
into Wednesday morning in these areas.

Later Tuesday night...the combination of at least some partial clearing...
light winds...and a cooler airmass will probably allow some patchy
valley fog and attendant IFR conditions to develop across the Southern

Wednesday...Patchy IFR/MVFR across the Southern Tier/Upper Genesee Valley/
Finger Lakes dissipating during the morning...otherwise VFR.
Thursday...MVFR/VFR with showers and thunderstorms becoming likely.
Friday and Saturday...Mainly VFR.


With winds and waves having both fallen below advisory levels on Lake
Erie...have dropped the Small Craft Advisory that had previously been
in effect.

Elsewhere...a period of brisk northerly to northeasterly winds will
continue through Tuesday morning on Lake Ontario...before diminishing
Tuesday afternoon as low pressure drifts east of our area and weakens.
This will lead to advisory-level winds and waves along the entire
south shore of Lake Ontario through the majority of Tuesday...before
building high pressure brings improving conditions late Tuesday
afternoon and Tuesday night.

Looking further out into the week...the aforementioned area of high
pressure will provide our region with fine boating conditions on
Wednesday...before a cold front crosses the area Thursday along
with another round of showers and thunderstorms. In its wake...
high pressure and fair dry weather will then return for Friday
and the upcoming weekend.


A period of stronger northerly winds is expected to set up across
Lake Ontario late tonight and Tuesday morning as low pressure departs
off to our southeast. With the wind direction now expected to be
a bit more northerly than previously forecast, especially
across the eastern end of the lake, have expanded the Lake Shore
Flood warning to cover the entire southern shoreline of Lake
Ontario. Conditions will worsen first across the western end of
the lake overnight with a more northeasterly wind direction
expected. Into Tuesday morning, impacts will expand eastward to
include northern Cayuga and western Oswego counties as winds
take a more northerly direction.


NY...Lakeshore Flood Warning from 6 AM this morning to 4 PM EDT
     this afternoon for NYZ004>006.
     Beach Hazards Statement through this afternoon for NYZ004>006.
     Lakeshore Flood Warning until 4 PM EDT this afternoon for
     Beach Hazards Statement through this evening for NYZ001>003.
MARINE...Small Craft Advisory until 8 PM EDT this evening for LOZ042-
         Small Craft Advisory from 6 AM this morning to 4 PM EDT
         this afternoon for LOZ044.



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