Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 250748
AFDBUF

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

.SYNOPSIS...
Low pressure over central New York will move east of our region and
dissipate today. This will allow any leftover spotty light rain or
drizzle to come to an end...with skies then slowly clearing out
between late this 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 build across the
area for the end of the week and the upcoming weekend...along with
a return to dry and comfortable weather.

&&

.NEAR TERM /THROUGH TONIGHT/...
During this period...low pressure near Watertown will slowly drift
eastward and dissipate today...while high pressure and drier air
over the Upper Great Lakes gradually builds into the region through
tonight.

On the backside of the slowly departing low...a moist northerly
upslope flow will maintain widespread low clouds and areas of spotty
light rain and/or drizzle into this morning...before the combination
of diurnal influences and dry air advection leads to slowly improving
conditions from late this morning on through tonight. This will first
result in the lower stratus lifting into a stratocumulus field with
any spotty light rain and drizzle dissipating today...followed by
gradually clearing skies late this afternoon and tonight...with the
clearing trend beginning first across far western and northern New
York late this afternoon...then gradually spreading southeastward
across the rest of the region through tonight. At this point it
appears that the Finger Lakes and Upper Genesee Valley will be the
last to experience the clearing...with some areas of lower clouds
possibly hanging on into Wednesday morning in these areas per the
latest guidance. Late tonight...the combination of at least some
partial clearing...light winds...and a cooler airmass will probably
also allow some patchy valley fog to develop across the Southern Tier.

Otherwise...this period will be much cooler and more comfortable
than recent days...as a cooler and much drier airmass continues to
overspread our region. With plenty of cloud cover in place...highs
today will only be in the mid to upper 60s for the most part...with
just far northwestern portions of the Niagara Frontier seeing highs
reach or break the 70 degree mark as some partial sunshine returns
late this afternoon. Lows tonight will then range from the upper 40s
in the interior valleys of the Southern Tier to the lower to mid 50s
elsewhere...which will be quite a change from the rather warm and
muggy nights we have experienced of late.

&&

.SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/...
Low level anticyclone will build east-southeast through the Great
Lakes region under building heights Wednesday. No real sensible
weather concerns as we start the period Wednesday, other than some
possible patchy Southern Tier river valley fog early Wednesday with
a stable/dry airmass in place. Developing warm air advection along
with ample sunshine will allow afternoon temperatures to recover
into the mid to upper 70s, perhaps a bit warmer for interior
sections.

Wednesday night will feature warmer and more humid weather as a
southwest flow becomes established behind the exiting area of high
pressure and in advance of an approaching cold front. Expect
overnight lows to bottom out only in the mid to upper 60s perhaps
only 70 near the lakeshores. Could see the leading edge of some
scattered shower activity reach the Niagara Peninsula and Saint
Lawrence Valley late in the night as a veering 30-40 knot low level
jet leads to increasing moisture advection ahead of the front.

The cold front will slowly sink southward into western and north
central New York Thursday as the upper trough exits east into Quebec
and it becomes parallel to upper flow. Small model differences with
timing of the front still exist, making convective forecast for
Thursday and Thursday night a bit challenging. Instability looks
marginal but if front is slowed to a more afternoon timing with
better insolation ahead of the front we could be looking at the
potenial for some stronger storms and locally heavy rainfall. High
temperatures should reach into highs in the upper 70s to lower 80s.
Combined with dewpoints rising to the mid/upper 60s will make for a
very uncomfortable mid summer day.

Showers and thunderstorms should start taper off from northwest to
southeast depending on the frontal timing 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 temperatures only bottoming out in the low 60s.

&&

.LONG TERM /FRIDAY THROUGH MONDAY/...
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
night.

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.

&&

.AVIATION /08Z TUESDAY THROUGH SATURDAY/...
Low pressure near KART will slowly drift eastward and dissipate
through the course of today...while high pressure and drier air over
the Upper Great Lakes gradually builds into the region through tonight.
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 this morning...before the combination of
diurnal influences and dry air advection leads to slowly improving
conditions from late this morning on through tonight. 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 this afternoon...
then gradually spreading southeastward through tonight. 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 tonight...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
Tier.

Outlook...
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.

&&

.MARINE...
A period of brisk northerly to northeasterly winds will continue
through this morning on Lake Ontario...before diminishing this
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 today...before
building high pressure brings improving conditions late this
afternoon and tonight.

Looking further out into the week...the aforementioned area of high
pressure will then 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.

&&

.TIDES/COASTAL FLOODING...
Behind slowly departing low pressure...a brisk northerly to north-
northeasterly low level flow will be in place across Lake Ontario
this morning...which will lead to lakeshore flooding concerns from
Niagara County eastward to about Northern Cayuga county. Lakeshore
Flood Warnings will thus remain in effect for these areas...though
we have dropped the warning for Oswego County as it now appears that
waves will not get quite high enough to produce flooding concerns
there.

As this afternoon progresses...both the winds and wave action will
diminish...which should allow any flooding and erosion to subside
by late afternoon.

&&

.BUF WATCHES/WARNINGS/ADVISORIES...
NY...Lakeshore Flood Warning until 5 PM EDT this afternoon for
     NYZ004-005.
     Beach Hazards Statement through this afternoon for NYZ001>005.
     Lakeshore Flood Warning until 5 PM EDT this afternoon for
     NYZ001>003.
MARINE...Small Craft Advisory until 5 PM EDT this afternoon for LOZ042-
         043.
         Small Craft Advisory until 5 PM EDT this afternoon for
         LOZ044.

&&

$$

SYNOPSIS...JJR
NEAR TERM...JJR
SHORT TERM...TMA
LONG TERM...WOOD
AVIATION...JJR
MARINE...JJR
TIDES/COASTAL FLOODING...JJR



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