Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 211056

Area Forecast Discussion
National Weather Service Buffalo NY
656 AM EDT Fri Jul 21 2017

High pressure will remain over the area through tonight. A frontal
boundary will then set up in the vicinity of the lower Great Lakes
this weekend, potentially bringing wet and unsettled weather.
Temperatures will remain near average through the end of the week
and this weekend.


Patchy early morning fog/stratus will burn off by mid day leaving
plenty of sunshine today as high pressure continues to drift east
across the lower Great Lakes. Most high temperatures in the upper
70s to lower 80s. High pressure will pass overhead tonight with
continued dry conditions and seasonable temperatures.


During this period...a compact but potent mid level shortwave trough
will dig southeastward from south-central Canada...while gradually
evolving into a broader upper level trough as it pushes across the
Great Lakes and Ontario/Quebec provinces Sunday and Monday. In the
process...this feature will force a broad surface low initially over
the Upper Mississippi Valley to slide east-southeastward along a
quasi-stationary frontal boundary draped across the Great Lakes
and Northeast...with the surface wave itself meandering its way
across New York State and eventually to the mid-Atlantic coastline
between Sunday and Monday. As a result of all this...we can expect
another extended round of unsettled weather to affect our region
during this period...along with at least some potential for stronger
storms capable of producing heavy rain and/or some severe risk at

Digging into the forecast details...on Saturday the surface low will
make its way eastward into Lower Michigan...with the above mentioned
frontal boundary extending its way eastward along or near the NY/PA
border...and one or more (possibly convectively-driven) shortwaves
rippling eastward along this boundary. Moisture and forcing pooling
along and north of the front (and in the vicinity of these waves)
should support one or more clusters of convection developing and
then riding eastward along the boundary through the day...though
as is typically the case the available guidance continues to exhibit
considerable variability in the exact track and timing of these
features. This stated...a rough model consensus would suggest one
such wave pushing eastward into areas south of Lake Ontario during
the afternoon...which should generally lead to an increasing potential
for precipitation from west to east.

Given a continued dearth of instability seen in the guidance with
the 00Z/21 forecast cycle...would expect that the majority of this
initial activity would be in the form of showers...with just a chance
for some embedded thunder found west of the Genesee Valley. This said...
there is a chance that somewhat better instability could potentially
make its way into the Southern Tier during the afternoon...with this
strongly dependent upon both the amount of heating realized and the
exact position of the surface front. Were this to occur...0-6 km
bulk shear values of 40-50 knots would be more than sufficient to
support a risk for some severe storms with strong to locally damaging
wind gusts between the mid afternoon and early evening hours...for
which SPC has a marginal to slight risk of severe storms in place.
With PWATs also climbing to near 2 inches...any stronger convection
could also present a risk for locally heavy rainfall if any training
of cells manages to develop.

This initial wave should cross western New York between Saturday
afternoon and evening...with subsidence and some drying in its wake
then likely leading to at least some reduction in convective potential
from northwest to southeast during the second half of Saturday night.
This stated...some additional scattered showers and/or storms will
still be possible overnight with a warm and humid airmass still in
place...the frontal boundary remaining draped across our region...and
the main surface wave pushing into Southern Ontario and thus into closer
proximity to our area.

On Sunday the surface wave will make its way eastward into western New
York...and will provide a further increase in both lift and moisture
as it approaches...with some increase in instability also potentially
coming into play over time. For the bulk of our region...all of this
should result in convection becoming more likely again over time...with
the areal coverage of this likely reaching a maximum during the afternoon
hours in concert with peak diurnal heating. With PWATs ranging from
1.5-1.75 inches...any stronger convection will again be capable of
producing locally heavy rainfall provided areas of training cells are
realized...with favorable shear profiles also supportive of at least
another conditional risk for some stronger to severe storms...with this
highly dependent on the amount of instability that actually develops.
With the above in mind...will continue to advertise the possibility of
both heavy rain and some gusty winds/hail in the HWO.

As we move through Sunday night and Monday...the broad surface low
will slowly meander its way to the mid-Atlantic/New England coastlines...
while its parent broad upper level trough digs in overhead. As a result...
we can expect scattered to numerous showers and storms to persist across
our region...though the potential for heavy rain and stronger to severe
storms should both tend to diminish as drier air works in aloft...and
instability levels generally lower. At this appears that
the greatest overall precipitation potential will lie across eastern
portions of the area...which will lie in closest proximity to the slowly
departing surface wave.


Any leftover showers from this weekend`s system should finally come to
an end between Monday night and Tuesday as high pressure and cooler/drier
air builds into our area...with fair and dry weather then following for
Wednesday as the surface high settles directly overhead. Toward the end
of the period...the high will slide off to our east and allow a weak cold
front to push into the region from the Upper Great Lakes...thereby
allowing for lower-end chances for showers to return by Thursday.

Regarding temperatures...the aforementioned arrival of cooler and drier
air will translate into temperatures and humidity levels falling off
through Tuesday...which should feature daytime highs generally in the
mid 70s and comfortable dewpoints in the mid to upper 50s. Both will then
rebound some through the rest of the period as the axis of the ridge slides
to our east and a southwesterly return flow develops on its backside...
resulting in temps climbing back to around 80 degrees and dewpoints
recovering to around 60 or so.


The combination of light winds, clear skies, and high dewpoints has
lead to restrictions in fog/stratus early this morning. Once
the fog/stratus burns off by mid morning, expect widespread VFR
conditions to persist throughout the rest of the day, as high
pressure continues to slide east across the forecast area.

Saturday and Sunday...VFR/MVFR with some showers and thunderstorms.
Monday...Mainly VFR but with a chance of showers.


High pressure over the upper Great Lakes will expand across the
lower Great Lakes today and tonight. This will result in fine
conditons for recreational boating. A frontal boundary is then
expected to stall near the lower Great Lakes this weekend with
chances of showers and storms becoming a threat, but winds/waves are
expected to remain on the light side.





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