Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 221847

Area Forecast Discussion
National Weather Service Buffalo NY
247 PM EDT Sat Jul 22 2017

A warm and humid airmass residing along a stalled frontal boundary
over the Ohio Valley will continue to bring chances for showers and
thunderstorms through Monday. A few thunderstorms may become strong
to severe Sunday and Sunday night across Western New York. Surface
high pressure will build into the region Tuesday, with slightly
lower humidity and mostly sunny skies.


This afternoon two frontal boundaries lie across the region. A
stalled frontal boundary stretched across the Ohio Valley and
towards Lake Erie will separate a very warm and humid airmass to our that will bring numerous showers and thunderstorms
across OH/PA. A few of these showers and possible thunderstorms may
sneak across the state border this afternoon and evening.

Another frontal boundary remains to our north, near the country
border, and though not as unstable and humid to the south of this
boundary...a few light showers will remain possible, mainly across
the Lake Ontario region.

By tonight the main wave of showers and thunderstorms to our south
will be passing to the east, with the remainder of the evening hours
and overnight hours likely to be dry across the region. Mostly
cloudy skies tonight and a muggy airmass will be accompanied by near
normal the 60s.

Tomorrow a potent shortwave across the Western Great Lakes will
deepen a surface low across the central Great Lakes. This surface
low will push the stalled frontal boundary to our south a bit
farther northward, allowing for our region to tap into a more
unstable airmass of 1000 to 1500 J/KG. The combination of increased
instability, and 0-6 km bulk shear magnitude increasing to around 40
knots will support thunderstorms...of which some may increase in
strength to strong to severe. Primary concerns will be damaging
winds from the storms, though CAPE profiles suggest that hail will
also be possible.


A mid-level trough axis over Lake Huron will slowly pivot across the
eastern Great Lakes Sunday night through Monday driving a surface
low and associated low-level frontal zone/moisture axis across
western and central New York. This will all come together to produce
a fairly widespread band or bands of showers and thunderstorms
Sunday night into Monday from the Genesee Valley east into central
NY as the low shifts east before redeveloping off the coast of Cape
Cod later Monday. There remains some concern for potential hazardous
weather with these storms. Pwats ahead of the low are forecast to
rise to around 1.5 inches or near the 90th percentile of pwat
climatology which may bring a risk of heavy downpours and flooding
especially if storms train along any surface boundaries. There is
also a lingering risk for severe winds during the evening hours with
any organized storms Sunday evening where GFS forecasts Bulk shear
of 40-50kts. Veering vertical wind profiles in forecast soundings
may even bring about another isolated tornado risk. The most
favorable CAPE/surface based instability will be Monday afternoon
across central NY. SPC has included a Slight/Marginal risk area for
Sunday with Marginal risk area on Monday in our forecast area. These
potential hazards will also continue to be highlighted in the
Hazardous Weather Outlook.

By Monday night the consensus of the models shows the low positioned
off the New England coast with high pressure in the process of
building back across the Great Lakes. There will also be a
developing cool northerly flow on the backside of the coastal low.
POPs continue to show a trend of showers tapering off from west to
east overnight leaving dry weather across WNY.

Tuesday and Tuesday night will feature dry and quiet weather as high
pressure settles over western New York. Skies will feature
increasing clearing from west to east as the moisture axis exits off
the coast with the low.

Temperatures will run cooler Monday and Tuesday topping out below
normal late July levels. Highs run generally reach into the lower to
mid 70s in the cool northerly flow on the backside of the passing
low along with 850mb temps dipping to 10-12C in the passing trough.
Overnight lows will bottom out in the 60s Sunday night then slip to
50s Monday and Tuesday nights with cool and dry Northerly flow.


High pressure lingering across New England should promote another
day of mainly dry and quiet weather on Wednesday...before another
pronounced shortwave trough and associated weak cool front cross the
region during the Wednesday night-Thursday time frame...and bring
the potential for another round of scattered showers and storms.
Following the passage of this system...surface-based ridging and
drier air then looks to build back into our region for Thursday
night and Friday...offering a return to quieter and mainly dry
weather to close out the period.

As for temperatures and humidity levels...these will both rebound
to near-normal midsummer levels on Wednesday as the southwesterly
return flow between the departing high and approaching cool front
advects warmer air and increasing amounts of moisture back into
our region. After that...little change in either is then expected
for the rest of the the cool front will usher in little
to no change in airmass.


An area of showers and thunderstorms will cross eastward...mainly to
the south of the state line, though a few showers will be possible
across the KJHW terminal this afternoon. To the north of this
convection...light showers/virga will remain across Lake Ontario.

Flight conditions should remain VFR through this afternoon and
overnight...except for KJHW where some low stratus within the
IFR/MVFR range may form. Precipitation at the airfields will be
minimal tonight and into Sunday morning.

On Sunday a warm and humid airmass will begin to push farther
northward into Western New York, bringing additional chances for
showers and thunderstorms, mainly through the afternoon hours.

Sunday afternoon...VFR/MVFR with some showers and thunderstorms.
Monday...Mainly VFR but with a chance of showers.
Tuesday and Wednesday...VFR.
Wednesday night and Thursday...Mainly VFR but with a chance of
showers and thunderstorms.


A stalled frontal boundary just to the south of the Eastern Great
Lakes will continue to bring chances for showers and thunderstorms
across the lakes. An easterly wind will increase to 15 to 20 knots
on Lake Ontario Sunday, though the strongest winds should remain out
over the open waters of the lakes. These winds will bring waves on
the western shores of Lake Ontario to near 4 feet on Lake Ontario
late Sunday afternoon and through the evening hours.


As low pressure approaches our region Sunday and Sunday evening...
a low-level easterly to east-northeasterly flow will briefly
strengthen across Lake Ontario with this potentially
strengthening enough to generate some lakeshore flooding
concerns along the south shore of the lake from roughly Niagara
county east to Monroe County. Latest guidance keeps the
strength of the winds just below a critical threshold to drive
waves high enough for the issuance of a Lakeshore Flood Watch at
this point but will continue to be closely monitored.





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