Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 221503

Area Forecast Discussion
National Weather Service Buffalo NY
1103 AM EDT Thu Jun 22 2017

Surface high pressure will slide off the East Coast today...while
allowing a warm front to press into our region from the Ohio Valley.
The warm front will bring a general chance of showers and thunderstorms
as it lifts across the area this afternoon and tonight...while also
ushering in a much warmer and more humid airmass. Showers and storms
will then become more widespread across our area on Friday as tropical
moisture surges across our area in advance of an approaching cold
front...which will then cross New York State late Friday and Friday
night. In the wake of the front...cooler and somewhat unsettled
weather will follow for this weekend as broad upper level troughing
digs across the Great Lakes.


As of 15Z...regional surface analysis shows surface high pressure
draped across the spine of the Appalachians...with an elongated warm
frontal zone extending all the way from low pressure over Manitoba to
central Lake Erie and northwestern Pennsylvania. While skies are still
mostly sunny across the bulk of our region...mid and high clouds are
beginning to increase across extreme far western New York...out in
advance of a weakening complex of showers and embedded thunderstorms
working its way eastward across the central Great Lakes and Southern

Through the rest of the day...the surface warm front will continue to
make its way northeastward into our region...with the boundary reaching
the lower Genesee Valley and western Finger Lakes by late this afternoon
or early this evening. This will result in a continued increase in cloud
cover from west to east...with the risk for some showers and a few
thunderstorms also spreading across the area in a similar manner this
afternoon as the remnants of the upstream convection work into our region.
The above stated...these should tend to be much more scattered by the
time they the much drier and more stable airmass currently
across our region should take a decided toll on the already weakening
MCS as it presses further eastward.

Following the warm frontal passage...much warmer and more humid air
will overspread far western New York this afternoon. With some heating...
this could result in the development of enough instability to pop a
few additional scattered showers and storms late this afternoon/early
this evening across the Southern Tier. Should these actually materialize
(a factor that remains in considerable question as of this writing)...
available instability and shear profiles could be sufficient for a couple
of storms capable of producing locally gusty winds in this latter area...
something that we`ll need to keep an eye out for as we push through the
afternoon and early evening hours.

Temperature-wise...continued steady warm air advection across our region
will push 850 mb temps up to between +13C and +17C by late this afternoon...
which should translate into high temps ranging from the mid and upper 70s
across the North Country to the lower to mid 80s across much of the area
south of Lake Ontario. Humidity levels will also be on the increase as
well...particularly across far western New York where surface dewpoints
will climb into the lower to mid 60s by late this afternoon.

Tonight the surface warm front will continue to make steady northward
progress...with this feature largely clearing our area by daybreak on
Friday. While the bulk of any associated convection should tend to
focus along and northward of the advancing front (i.e., across Lake
Ontario...the North Country...and Saint Lawrence Valley)...some additional
scattered convection will still be possible across the rest of the area
given the continued advection of an increasingly warm and moisture-rich
airmass across our region...for which some chance PoPs remain in play.
The increasingly tropical nature of the incoming airmass will also
result in a rather uncomfortable night for sleeping...with surface
dewpoints surging well into the mid and upper 60s...and overnight lows
failing to drop below the upper 60s (higher terrain) to lower 70s
(lake plains).


Very warm and humid conditons will be in place across western and
central NY Friday ahead of an approaching cold front. Expect
building instability and an increase of synoptic scale forcing will
bring an increasing threat for heavy showers and thunderstorms
through the afternoon and evening hours. Southwest flow off the
lakes may try to limit convection across the Niagara Frontier and
near Watertown, with the greatest coverage likely from the western
Southern Tier into the Finger Lakes and southern Tug Hill region.
Dewpoints 65-70 degrees will combine with temperatures climbing into
the upper 70s to mid 80s will make for very steamy conditions during
the day away from any cooling lake breezes or thunderstorm outflows.

Concerning severe potential, SBCAPE forecast 1000-3000 j/kg (highest
east of lake breezes) and 30-35 kts of 0-6km Bulk shear could
support a risk for strong to severe storms. SPC has included NY
within a Marginal Risk area for severe storms so have added wording
for gusty winds to higher thunderstorm POPs. Heavy rain is also a
concern as GFS forecast PWAT reaches 1.75-2 inches just ahead of the
front supporting heavy downpours with a few storms. WPC has also
included much of NY within a Marginal Risk area for Excessive
Rainfall. Have added some enhanced wording to higher POPs for heavy
rain within thunderstorms.

Friday night the cold front will sweep east across the area, with
showers and thunderstorms ending from northwest to southeast by or
shortly after midnight. Cold air advection behind the front will
bring in a cooler and drier airmass with partial clearing developing
from northwest to southeast overnight. The drier airmass will allow
temperatures to fall into the lower 60s on the lake plains and upper
50s across the higher terrain.

Saturday, a broad longwave trough will shift over the Great Lakes
brings cooler air aloft over western and central NY. This cool air
aloft will steepen low and mid level lapse rates to support diurnal
cumulus and a chance for some showers and possibly weak
thunderstorms well inland from developing lake shadows. A stiff
southwest flow off Lake Erie will converge with westerly flow over
Lake Ontario supporting the development of a lake breeze convergence
zone from Niagara County east along the south shore of Lake Ontario
to Oswego County. This band of convergence combined with the steep
lapse rates may support a few scattered showers and isolated
thunderstorms Saturday afternoon. Continued cold air advection will
support more comfortable temperatures with highs in the low to mid
70s in most locations and some breezy winds. Any enhanced lake
breeze convection Saturday evening will end shortly after sunset
leaving partial clearing overnight. Expect the even drier airmass to
allow cool low temperatures in the mid 50s on the lake plains and
around 50 across the interior Southern Tier and Lewis County.


The longwave trough over the Great Lakes is forecast to remain in
place into at least next Tuesday and possibly even next Wednesday as
progression of the upper level flow pattern is stalled by a strong
high pressure area over the North Atlantic. This pattern will
support several days of cooler temperatures will comfortable humidly
levels with daytime highs perhaps topping out within a few degrees
of the 70 degree mark. The trough axis is starting to look like it
may cross western and central NY on Monday which supports higher
end chance range POPs. Otherwise, cool air aloft within the trough
will support a day-to-day threat of showers and possibly low topped
thunderstorms mainly inland of lake breezes. These showers and
thunderstorms will be further focused by any shortwaves rotating
around the mean trough. If this environment can be coupled with a
stout shortwave, would not rule out some small-hail producing storms
given the lower freezing level heights and steep lapse rates aloft,
especially if storms focus along a lake breeze boundary.


Mid/high clouds will increase across the region through the rest of the a warm front lifts north into the area. While a few scattered
showers and thunderstorms will be possible with the arrival of the warm
front...much of the afternoon should feature mainly dry weather and VFR

Showers may linger along the front across Lake Ontario and the North
Country overnight, as the front stalls over Ontario province. Overall
though, conditions should remain VFR, with occasional period of MVFR
conditions in -shra/-tsra. Increasing advection of deeper moisture into
the area late tonight may drive cigs down to MVFR across higher terrain
of the Southern Tier, however. In addition, a strengthening low level jet
aloft may result in LLWS later tonight, particularly across far western
New York.

Friday...VFR/MVFR with widespread showers and thunderstorms.
Friday Night...VFR/MVFR with rain showers and a chance of thunderstorms.
Saturday and Sunday...Mainly VFR with a chance of showers and thunderstorms.
Monday...Mainly VFR with a chance of showers.


High pressure will keep conditions tranquil on the lakes for much of
the day, with the exception of southwesterly lake breezes to around
15kts on the northeastern ends of the lakes this afternoon.
Southerly winds will freshen tonight as a warm front crosses the
region and low pressure develops over the central Great Lakes. This
will largely keep wave action confined to Canadian waters however,
and the only serious threat for small craft advisory conditions will
not be until Friday night, when a cold front crossing the region
will bring cooler temperatures and fresh westerly flow that will
raise waves above 4-5 feet on the eastern end of Lake Ontario. The
fresh westerly flow and subsequent SCA conditions will likely
continue through the weekend, thanks to persistent upper level
troughing in place across the region.





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