Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 212057

Area Forecast Discussion
National Weather Service Buffalo NY
457 PM EDT THU JUL 21 2016

High pressure will remain off the mid-Atlantic coast through Friday,
allowing heat and humidity to build through the weekend. Humidity
will markedly increase overnight and will promote a few scattered
thunderstorms across the region early Friday morning. A few storms
may produce isolated gusty winds. In the wake of the early round of
storms Friday, the remainder of the weekend will be sultry and
mainly dry.


High pressure drifting off the Mid Atlantic coast will continue to
promote dry and hot conditions into this evening. Extensive cirrus
debris clouds from upstream convection will increase and thicken
from west to east, with sunshine becoming more filtered behind the
increasing high clouds with time.

The weather turns both more interesting and more uncertain for the
overnight hours and into Friday morning. Most convective allowing
models have been struggling with depicting the ongoing MCS over
Michigan, but largely anticipate at least the northern portion
dissipating before approaching WNY. However, this gives little
confidence in how the hi-res models evolve the eventual overnight
convection for our forecast area. Looking at the big picture, a
shortwave across the upper Great Lakes, with strong warm air and
moisture advection ahead of it, will approach the region overnight.
This will allow dew points to surge through the 60s to near 70
overnight, with temperatures overnight remaining largely in the 70s
(not a comfortable sleeping night). This destabilizing airmass
across the region overnight will likely support the redevelopment of
convection upstream across Ontario near the weak mid-level front and
shortwave forcing. The exactly placement and timing of this
redevelop remains uncertain as it will depend somewhat on the
influence of the initial ongoing MCS. Overall convection will evolve
from northwest to southeast across the forecast area, but the amount
of coverage is largely uncertain. Numerous scenarios are possible,
from just widely scattered thunderstorms, to a solid line of
potentially strong thunderstorms moving through. The deterministic
forecast follows closer to the middle-ground, which depicts the
potential for a cluster of thunderstorms moving south-
southeastward toward NW PA, but clipping across western NY in the
early morning hours (roughly 4am to 8am). Meanwhile another
cluster of storms may evolve eastward across the Saint Lawrence
valley and across the North Country during the same early morning
time frame. SPC maintains the forecast area under a marginal risk
for severe weather, which is reasonable given the poor diurnal
timing, but increasing instability and wind shear. The main threat
will be a few isolated to scattered strong to damaging wind gusts

After the initial round of thunderstorms in the morning, most of
Friday should feature plenty of dry time but with a hot and very
humid airmass in place. In the wake of the shortwave, there should
be enough subsidence and mid-level warming to keep afternoon
convective chances very low. While can`t rule out a few isolated to
scattered thunderstorms inland from the lake breeze boundaries, it
should be a largely dry day. Dew points will maintain in the upper
60s to low 70s, and combined with high temperatures in the upper 80s
to low 90s will push heat indicies into the 90s, with some upper 90s
across the Genesee Valley and northern Finger Lakes region.


Lingering convection from Friday afternoon should taper off during
the evening hours with the loss of diurnal heating. Otherwise, it
will be another sultry night across Western and North-Central New
York as the region will continue to stew in a humid summertime
airmass thanks to Gulf moisture that will be in place across much of
the Eastern U.S. Dewpoints in the mid-60s will ensure that
temperatures will struggle to fall below 70 along the lake plains,
with mid-60s only across the highest elevations of the Southern Tier
and North Country.

As we move into the weekend, it should be dry across much of the
forecast area as upper level ridging upstream across the central
Great Lakes will maintain a stout mid-level cap and abundant dry air
aloft across the region that will discourage any convection. The
lone exception will be across the Saint Lawrence valley, which will
be exposed to shortwave energy rolling down the front of said ridge,
resulting in a potential for showers and thunderstorms that will
persist into Saturday evening. It will be another hot day across the
forecast area, with 850mb temps in the 18-20C range yielding highs
in the mid 80s to lower-90s. Drier air sagging south across the
forecast area should yield a slightly cooler night with lows in the
60s, and perhaps even a few 50s in the North Country.

It should remain dry across much of the forecast area on Sunday as
the weakening upper level ridge works its way over the Lower Great
Lakes. The next chance for showers and thunderstorms will not be
until later Sunday night as an upper level trough approaches from
the West. The aforementioned cooler/drier air sagging south out of
Canada will keep temperatures in the low to mid 80s, however
increasing southerly flow ahead of the approaching trough will begin
to advect more humid air back into Western New York Sunday night.
Lows will rebound back into the upper 60s to lower 70s, with lower o
mid 60s across the eastern half of the forecast area.


An upper-level shortwave trough is forecast by 12z guidance to cross
the Great Lakes on Monday. This trough will drive a surface cold
front across our forecast area with renewed chances of showers and
thunderstorms. Southwest winds ahead of the front will increase
humidity levels with dewpoints again pushing toward the 70 degree
mark. The combination of heat and humidity with frontal timing
working with the diurnal heating cycle could lead to an unstable
environment for storms to work with. GFS forecast SBCAPE of 1000-
2000 j/kg and 0-6km bulk shear increasing to 30-35 kts may even
support the risk for a few strong to severe storms if these levels
or higher can be reached. High temperatures are expected to reach
into the mid to upper 80s ahead of the front with 850mb temperatures
forecast to build toward +20C.

The front will push east Monday evening with chances of showers and
thunderstorms tapering off west to east. Temperatures will remain on
the mild side overnight as humidity will be slow to scour out behind
the front. Modified continental polar air building in with high
pressure behind the cold front should lead to a string of mainly dry
days Tuesday through Thursday with more comfortable temperatures
running closer to but still a touch above 30-year normal levels.
Humidity levels will also be a touch more comfortable but still
noticeable with dewpoints hovering around 60. High temps should run
in the upper 70s to low 80s with overnight lows ranging through the
60s. Have included a slight chance for showers on Thursday as some
700mb divergence and increasing low level moisture may
trigger a shower.


VFR flight conditions will prevail through through this evening.
Winds will become south to southwesterly today, with some gusts to
around 15 knots across the lake plains.

Showers and thunderstorms will drop southeastward within a northwest
flow overnight tonight. Much of this activity will arrive after
midnight, with the most likely time period for activity being
between 06Z and 12Z. A few of these storms could contain gusty to
strong winds. Flight conditions may drop to MVFR/brief IFR within
such activity. Most of the storms will move east of the area by
midday Friday, with just a few more isolated to widely scattered
storms possible late in the day along lake breeze boundaries.


Saturday and Sunday...VFR.
Sunday night and Monday... Mainly VFR with a chance of showers and
thunderstorms and associated brief MVFR/IFR.
Tuesday... VFR.


A weak frontal boundary will move across the region overnight, with
a chance for early Friday morning showers and thunderstorms on both
Lakes Erie and Ontario. Winds will increase ahead of the front later
tonight and Friday, especially on Lake Erie where sustained winds
will increase to around 20 knots. This will produce a period of
small craft advisory conditions from late tonight through Friday.

Otherwise, high pressure will build across the region through the
weekend and should mostly promote fine boating conditions. Lingering
west to northwest winds and some chop on the lakes early Saturday
should give way to lighter winds and calmer waters through Sunday.

The next front, with associated showers and thunderstorms and
increased winds, will move through early next week.


MARINE...Small Craft Advisory from 2 AM to 8 PM EDT Friday for LEZ020-



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