Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 170824

Area Forecast Discussion
National Weather Service Buffalo NY
424 AM EDT Thu Aug 17 2017

A warm front will approach the area today with clouds thickening
from west to east. Showers and scattered thunderstorms will arrive
in Western NY by late afternoon or early evening, and then
overspread the rest of the area tonight as the warm front moves
across the region. A few more scattered showers and thunderstorms
will develop Friday and Saturday along with plenty of rain free
time. High pressure will then bring a return to dry weather for
Sunday and Monday.


High pressure will drift east across New England this morning, with
the onset of mid/upper level warm advection bringing a west to east
increase in high cirrus level cloudiness. Southern Tier river valley
fog will continue through about 13Z before burning off.

An occluding surface low will track from Wisconsin this morning to
eastern Lake Superior by late tonight, with a seasonably deep mid
level trough taking a similar track. A warm front downstream of the
low will approach Western NY by late this afternoon before crossing
the region from southwest to northeast tonight. The warm front will
bring a period of isentropic upglide and moisture convergence, aided
by a 35-40 knot low level jet. A few weak, convectively augmented
mid level shortwaves will provide some additional forcing for
ascent. Strong moisture advection will boost PWAT to around 2 inches

The quality of forcing and moisture coming together continue to
support a high likelihood of showers along the warm frontal boundary.
These showers will reach Western NY during the late afternoon and
early evening, Genesee Valley by early to mid evening, then spread
into the eastern Lake Ontario region through the early overnight.
Some limited elevated instability will develop along the warm front
and support a few weak, embedded thunderstorms. The high PWAT
environment will support heavy downpours with any scattered storms
that develop, but weak instability and progressive nature of the
pattern should prevent any flooding issues despite the high PWAT.

Temperatures will warm into the lower 80s in most locations today
prior to the arrival of thicker clouds and showers. Ongoing warm
advection and an increasing southerly breeze will keep temperatures
very mild tonight, with lows in the lower 70s on the lake plains of
Western NY, and mid to upper 60s elsewhere. Dewpoints will surge
into the upper 60s to around 70 overnight, bringing a muggy feel to
the air.


On Friday, a low pressure system will track northwest of the forecast
area with several weak boundaries and shortwaves crossing the
region. A prefrontal trough and weak shortwave will cross western NY
in the morning, ill-timed for much significant convection early in
the day. This will result in some showers with a few rumbles of
thunder over WNY. However, as the trough tracks east of the Genesee
Valley by midday and the afternoon, thunderstorms will become
increasingly likely with diurnal heating coming into play. About 35
knots of 0-6km shear will accompany the roughly 1500J/kg of CAPE,
which could result in a few stronger to possibly severe
thunderstorms developing. However, the weak forcing will likely
yield fairly isolated coverage of any stronger thunderstorms. Storms
will also be exiting the forecast area by the time the better
instability develops, resulting in perhaps only a stronger storm or
two developing east of Rochester before exiting into Binghamton`s
forecast area. SPC matches this train of thought with the latest Day
2 SWO keeping the Marginal threat for severe weather east of the
Genesee Valley into central and eastern NY.

However, western NY won`t be completely out of the game for
thunderstorm potential on Friday. With diurnal heating after the
initial leading wave, lake breeze boundaries will develop southeast
of Lake Erie and Ontario. As a secondary frontal boundary interacts
with the lake breezes, expect some scattered showers and
thunderstorms to develop from near Jamestown to Batavia, and track
off to the east through the mid-late afternoon. A few additional
storms may also develop southeast of Lake Ontario from Wayne County
to Oswego County tracking off into central NY. This activity will
likely be broken in coverage and will not pose much of a severe
threat as the best shear and instability axis will be well to our
east by then. That said, can never rule out a pulse severe storm or
two when interacting with the lake breeze boundaries. High
temperatures on Friday will be near normal for this time of year, in
the upper 70s to low 80s.

By Friday night into Saturday, cold air advection in the wake of the
cold frontal passage will drop 850 mb temperatures to around
+10/+11C. With lake temperatures near 22C, this is marginally enough
for a lake response, however BUFKIT profiles suggest a fairly stout
inversion near 800 mb. Forecast models insist on generating some QPF
off the lakes overnight into Saturday morning, however given the low
inversion height it is more likely that this will manifest as some
low cloud cover. Have included just a low chance PoP east of the
lakes. Also considered the potential for waterspouts on the lakes
Saturday morning, however with the low inversion height and marginal
lake-850mb temperatures there was just not enough confidence to
include the chance in the forecast. By Saturday afternoon any
lingering lake response clouds or showers will diminish with diurnal
heating coupled with weak warm advection ahead of another trough
approaching the lower Great Lakes. This trough axis will cross the
region Saturday afternoon and evening, bring some scattered showers.
Showers will die out Saturday evening with the trough axis pushing
east and the loss of diurnal heating.

High pressure will build through the Ohio Valley Sunday and Sunday
night brining drier and warm air into the region. Temperatures will
be coolest Saturday, with highs in the mid 70s to upper 70s, while
upper 70s to about 80 degree readings return on Sunday. Lows will be
in the upper 50s to lower 60s.


Monday into the first half of Tuesday will bring fair weather with
building temperatures and humidity into mid week. A surface high
will move through Pennsylvania on Monday before sliding off the mid-
Atlantic coast on Tuesday. Return flow developing in the lee of the
surface high passage will result in 850 mb temperatures climbing to
+17/+18C by Tuesday afternoon, with dew points pushing the mid to
upper 60s. This will result in mid 80s readings Monday afternoon,
and mid to upper 80s Tuesday afternoon. The warmest temperatures
will be in the downslope areas of the Genesee Valley and northern
Finger Lakes, with the fairly moist soil conditions likely helping
keep any areas from reaching the 90 degree mark. With plenty of
attendant dry air/subsidence also in place...this will allow for
mostly sunny/mainly clear skies...thereby resulting in near ideal
sky conditions for viewing of Monday`s solar eclipse.

As we progress deeper into the new work week...our weather still
looks to turn more unsettled again for later Tuesday and especially
Tuesday night...when the medium range guidance suggests that the
next upper level trough and associated cold front will move across
our region. Have continued to advertise broadbrush chance PoPs to
cover the passage of these features...with our area then drying out
from northwest to southeast on Wednesday following the frontal
passage. Following the passage of the cold front...cooler and less
humid air will then spread back into our region on Wednesday...when
daytime highs will retreat back into the mid and upper 70s.


Valley fog will continue through about 13Z across the western
Southern Tier with local IFR. Otherwise VFR will prevail elsewhere
with a gradual increase in thin/high cirrus.

A warm front will approach the region today with increasing and
lowering clouds from west to east. Showers will reach Western NY by
late afternoon or early evening, then spread east across the rest of
the area tonight as the warm front crosses the region. A few
scattered thunderstorms will also be embedded in the showers as
marginal instability develops. CIGS/VSBY will remain VFR most of the
time through the first half of tonight, although any heavier showers
or thunderstorms will contain brief/local MVFR to IFR conditions.
Late tonight and Friday morning MVFR CIGS will become more
widespread as the low levels saturate, with some areas of IFR
developing across higher terrain.


Friday...MVFR/IFR CIGS improving to mainly VFR with scattered
showers and thunderstorms.
Saturday...Mainly VFR with a chance of showers.
Sunday and Monday...VFR except for late night and early morning
valley fog with local IFR.


East winds will increase today ahead of a warm front over the Upper
Midwest. The strongest winds will be found on the west half of Lake
Ontario where choppy conditions will develop, although the greatest
wave action will be found in Canadian waters. Winds will become
southerly tonight as the warm front moves northeast of the area,
again directing the greater wave action into Canadian waters.

Low pressure will then move slowly from the western Great Lakes
Friday to southern Quebec by Saturday. This will bring an increase
in winds to the eastern Great Lakes, with Small Craft advisory
conditions probable on Friday for Lake Erie, and Saturday for Lake
Ontario. Winds and waves will decrease by Sunday as high pressure
builds into the Ohio Valley and eastern Great Lakes.





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