Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 230256

Area Forecast Discussion
National Weather Service Buffalo NY
1056 PM EDT Sat Jul 22 2017

A warm and humid airmass 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.


A diffuse wave over Michigan will track across the region this
evening along a stalled frontal boundary producing some spotty light
rain across the region. Behind this weak wave overnight we will see
mainly dry conditions with perhaps a rogue shower or two in the
vicinity of the stalled boundary.

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.


Upper level ridging will crest across the region on Wednesday,
providing for one more fair weather day for western and north-
central New York, with seasonable temperatures in the upper 70s to
lower 80s. After a couple of cooler nights earlier in the week,
mugginess will return Wednesday night as surface high pressure moves
off the New England coast, opening up the lower Great Lakes to
warm and moist southerly flow. This will be reflected in lows
that will run in the mid to upper 60s. Along with this increase
in humidity, we will see a chance of showers and thunderstorms
towards daybreak, as a prominent upper level trough approaches
the area.

Model consensus continues to indicate the passage of the
aforementioned upper level trough on Thursday. This should result in
widespread showers and thunderstorms Thursday into Thursday evening
as a cold front sags south across the area. A secondary reinforcing
shortwave may end up aiding in the development of diurnal showers
Friday afternoon, particularly across the southern half of the
forecast area, closer to the hung up frontal boundary. nominally
cooler air filtering in behind the front may yield a milder day
Friday, with highs in the mid to upper 70s.


Expect widespread VFR conditions overnight with some mid and high
level cloud cover and some spotty light rain shower. JHW and the
western Southern Tier will be the exception, with IFR/LIFR stratus
developing as a low-level moist airmass advects into the region
overnight. Fairly light southerly winds may allow the low stratus
there to develop downward into fog overnight.

On Sunday scattered to numerous showers and thunderstorms will
develop over western NY and spread eastward across the Finger Lakes
and into central NY by Sunday night. Expect VFR conditions outside
of thunderstorms, with localized MVFR/IFR in 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.


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. Scattered thunderstorms are
also expected over area waterways Sunday which could produce small
hail and locally higher winds and waves.


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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