Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 211039
AFDBUF

Area Forecast Discussion
National Weather Service Buffalo NY
639 AM EDT SUN AUG 21 2016

.SYNOPSIS...
A strong cold front will cross the region today along with some
showers and thunderstorms...some of which could produce locally
heavy rainfall. Following the front...a much cooler and drier
Canadian airmass will build across the region through Monday. This
much cooler airmass will not only trigger some scattered lake
effect showers east and southeast of the lakes tonight and early
Monday...but will also result in daytime temperatures on Monday
that will be the lowest we have experienced since mid June. After
that...high pressure will provide our region with fair dry weather
and a steady day to day warming trend through midweek.

&&

.NEAR TERM /THROUGH TONIGHT/...
During the course of today...moderately strong low pressure south of
James Bay will bodily lift northeastward into Northern Quebec...and
will push its trailing strong cold front eastward across our region
in the process. As of this writing...this boundary was draped across
Southern Ontario and central Lake Erie...and still appears poised to
cross far western New York this morning...the Finger Lakes by midday...
and the North Country by early to mid afternoon.

As expected...the shower and thunderstorm activity ahead of and
along the cold front has become considerably more numerous since
earlier this morning...thanks to both increasing forcing from the
front and height falls/DCVA aloft acting upon our very warm and
moist airmass. This general trend should continue through this
morning...warranting continued categorical PoPs.

As has been mentioned previously...the subtropically-sourced airmass
preceding the front (PWATS of 2 to 2.25 inches) in combination with the
increasing large-scale forcing will result in a continued potential for
some of the storms to produce locally heavy rainfall. At this point...
general rainfall amounts look to range from about a third to a half an
inch over far Western New York to as much as an inch or so across the
North Country...where the best overall combination of lift and moisture
still looks to be in play. At this point...expect little in the way of
potential for strong to severe storms...with the timing of the front
and its attendant cloud cover/convection likely helping to preclude the
development of much more than some weak instability...and the bulk of
any noteworthy shear looking to remain largely confined to the rear of
the boundary.

Following the passage of the front...a much cooler and drier Canadian
airmass will then flood across our region through the balance of today
and tonight. In conjunction with strong large-scale subsidence...this
should initially lead to a return to generally dry weather and at least
some partial sunshine from west to east this afternoon...with conditions
also turning rather breezy to windy across far Western New York as the
pressure gradient tightens in advance of an approaching secondary trough...
and the incoming cooler air airmass aids in the more efficient downward
transport of higher momentum air from aloft.

Very late this afternoon and early this evening...the aforementioned
secondary trough will cross our region...in conjunction with an associated
mid level shortwave and brief uptick in background moisture. In conjunction
with increasing lake-driven instability...this feature should generate a
round of scattered showers and possible thunderstorms over and downwind
of Lake Erie as it passes...with much more isolated/widely scattered
activity found over/near Lake Ontario where the overall moisture field
will generally be less favorable.

In the wake of this feature...some limited lake-effect showers will
then continue southeast of Lakes Erie and Ontario through the balance
of the night as 850 mb temps continue to cool to between +6C and +8C...
with the shortening fetch...relatively dry nature of the airmass and
a relatively low capping inversion helping to keep these from becoming
anything more than widely scattered to scattered in nature. Outside of
these...the remainder of the region should remain dry.

With respect to temperatures...the passage of the frontal clouds/
convection and developing cool air advection regime in the wake of the
boundary should result in highs largely remaining confined to the
75-80 degree range today...with lows tonight then falling back into
the mid and upper 50s as the incoming cooler/drier airmass deepens
across our region.

&&

.SHORT TERM /MONDAY THROUGH WEDNESDAY NIGHT/...
This period will begin with the coolest airmass in months moving
across the region...but as we progress through the first half of the
week...temperatures will quickly rebound to the sultry levels that
we have become accustomed to. The day to day warming trend will be
accompanied by mainly dry weather...with only localized lake effect
rain showers Monday morning spoiling an otherwise completely dry
period. The details...

In the wake of Sundays cold frontal passage...a short lived period
of much cooler air will sweep across our forecast area on Monday.
H85 temps will bottom out in the single digits...ranging from 8c
along the PA border to about 5c across the Thousand Islands. This
will not only produce the coolest day in over two months for our
forecast area...but will enable some lake effect clouds and nuisance
rain showers to be generated for areas southeast of Lakes Erie and
Ontario.

Other than the fact that we will actually have our first semblance
of a lake response of the infant lake effect season...this event
will not be worth getting excited about. The overall synoptic
airmass will not be especially moist...and with a somewhat sheared
environment in place and a relatively short fetch coming into play
over both lakes...the lake derived instability will not have a lot
of support. Lake induced CAPES are forecast to be in the vcnty of
500 j/kg...and while this moderate amount of instability will
encourage the development of lake effect clouds...rain showers will
be limited by both the dry synoptic environment and also a
strengthening subsidence inversion. A weak inversion is forecast to
be in place at 12z on Monday...but as we work through the morning...
this cap will significantly strengthen as expansive high pressure
over the Ohio Valley will build northward across the Lower Great
Lakes. Given the time of year...any nominal lake response will also
have to contend with strong diurnal effects.

Boiling all of this down...Monday will feature some lake induced
clouds southeast of both lakes in the morning...then sunshine will
dominate the midday and afternoon across all of the region. Max
temps will be within a few degrees of 70.

As the surface high drifts directly across our forecast area Monday
night...we can be assured of a very cool night under clear starlit
skies and light winds. Temperatures will drop into the 50s for the
majority of the forecast area...with some 40s likely across much of
the Western Southern Tier and possibly for the Black River Valley
and points east. This scenario will undoubtably support fairly
widespread valley fog across the Southern Tier as well.

It will be beautiful across the region on Tuesday. The surface will
gradually drift off the Mid Atlantic Coast...while the parent mid
level ridge will amplify over the Great Lakes region. This will
promote sun filled skies...and with H85 temps climbing to around
14c...we can expect our max temps to be in the upper 70s to lower
80s. The warming will be accompanied by dew points that will remain
in the 50s...so it should still feel quite comfortable.

After another star filled night Tuesday night...we can look forward
to sunny warm day on Wednesday. Broad ridging will be in place
across the eastern half of the country...while high pressure will
extend back across our region from the offshore waters of Nova
Scotia.

While fair weather will remain in place over our region Wednesday
night...a deepening southerly flow ahead of an approaching cold
front will support a very mild night. Temperatures will struggle to
fall into the mid 60s for most areas. Just as a point of
reference...normal mins for this time of year are in the mid to
upper 50s.

&&

.LONG TERM /THURSDAY THROUGH SATURDAY/...
Unlike the previous period when day to day mercury readings were
trending higher...the suppression of the sub tropical ridge over the
Southeastern states will lead to a normalization of temperatures as
we head into the weekend.

A strong shortwave crossing the province of Ontario on Thursday will
start to knock down the northern fringes of the Bermuda High...while
an associated cold front will become better organized over the Upper
Great Lakes. Very warm and increasingly humid air will stream
northeast and pool across the Lower Great Lakes in advance of this
sfc feature...and this will once again help to destabilize
conditions over our forecast area. While high and mid level clouds
will likely thicken over the region within the increasingly sultry
airmass...H85 temps in the upper teens c will encourage the mercury
to climb into the mid and upper 80s f. The bulk of the day will be
rain free...but scattered convection can be expected during the peak
heating of the day...especially over the far western counties.

As the aforementioned shortwave crosses to the north of the Great
Lakes Thursday night...it will push the first two frontal boundaries
across our forecast area. The first front...a glorified pre frontal
trough...will encounter a plume of sub tropical moisture over our
region with PWAT values in the vcnty of 2 inches. This will likely
result in a much greater coverage of showers and thunderstorms with
an elevated risk for torrential downpours. Will thus hold onto the
likely pops that we have in continuity.

The deep moisture plume will be pushed east and swept off the coast
Friday and Friday evening...and this will allow for improving
conditions over our forecast area. Scattered showers and
thunderstorms Friday will taper off from the northwest during the
course the process of the mid level drying. While the second frontal
boundary...the true cold front...is slated to push across the Lower
Great Lakes Friday night...no additional pcpn is expected. The front
will be moisture starved and hgts will already be rising...so this
should be a dry fropa.

On Saturday...a large area of high pressure will cross the Great
Lakes region. This should produce a fine day over our region with
H85 temps in the vcnty of 10c offering comfortable temperatures
within a couple degrees of 80f.

&&

.AVIATION /12Z SUNDAY THROUGH THURSDAY/...
The main feature of interest during the TAF period will be a strong
cold front that will cross the region from west to east between
roughly 12z and 18-19z today. General VFR conditions preceding
the front will give way to some showers and thunderstorms and a
possible period of MVFR as the front approaches and passes through
our region...with conditions then returning to VFR in the wake of
the departing boundary. Following the frontal passage...winds will
turn rather breezy out of the west-southwest across far western
New York...where sustained winds of near 20 knots and gusts to
near 30 knots will be likely this afternoon.

Tonight...a trailing secondary trough will cross the region during
the evening...with the combination of this feature and increasingly
cool air aloft helping to trigger some clouds and scattered showers
southeast of Lakes Erie and Ontario...and possibly a couple of early
evening thunderstorms over and immediately downwind of lake Erie.
While there will be the potential for some limited areas of MVFR
within the lake-driven clouds and showers...by and large conditions
should be VFR.

Outlook...
Monday...Mainly VFR with a chance of showers southeast of the lakes
early.
Tuesday and Wednesday...VFR.
Thursday...Mainly VFR with a chance of showers and thunderstorms.

&&

.MARINE...
A strong cold front will cross the Lower Great Lakes between this
morning and early this afternoon. Some showers and thunderstorms
will mark its passage...a few of which could produce brief/localized
areas of higher winds and waves.

With respect to the larger-scale gradient winds...brisk southerlies
out in advance of the front this morning will give way to freshening
west-southwesterlies following its passage...which will then veer to
northwesterly and slowly diminish tonight and Monday. With all of this
in mind...small craft advisories are in affect as outlined below.

In the wake of the cold front...very cool air will pour across the
region from this afternoon through Monday. This will result in enough
lake induced instability and a deep enough convective cloud depth to
promote the development of waterspouts...with the greatest risk of
these coming between late this afternoon and Monday morning.

&&

.BUF WATCHES/WARNINGS/ADVISORIES...
NY...Beach Hazards Statement through this evening for NYZ007.
     Beach Hazards Statement through this evening for NYZ010-019-
     085.
     Beach Hazards Statement from 1 PM EDT this afternoon through
     this evening for NYZ001-002.
MARINE...Small Craft Advisory until 5 AM EDT Monday for LEZ040-041.
         Small Craft Advisory from noon today to 8 PM EDT this
         evening for LEZ020.
         Small Craft Advisory until 11 AM EDT Monday for
         LOZ042>044.
         Small Craft Advisory until 11 AM EDT Monday for LOZ045.

&&

$$

SYNOPSIS...JJR
NEAR TERM...JJR
SHORT TERM...RSH
LONG TERM...RSH
AVIATION...JJR
MARINE...JJR/RSH



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