Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 171924

Area Forecast Discussion
National Weather Service Buffalo NY
324 PM EDT Thu Aug 17 2017

Showers and thunderstorms associated with a well-organized low
pressure system over Upper Michigan will cross Western and North-
Central New York this afternoon and tonight, with more scattered
activity possible Friday afternoon as the low center slowly passes
to our north. An upper level disturbance will bring another chance
for showers Saturday before high pressure dries things out for the
latter half of the weekend into Monday. Temperatures will run near
to slightly above-normal through the weekend, with warmer
temperatures in the mid 80s possible next week.


WV imagery this afternoon shows an unusually well-organized (for
this time of year) low pressure system spinning near the western end
of Lake Superior. A warm front associated with this low is currently
moving northeast across the lower Great Lakes. MRMS shows a broad
area of showers tracking across southern Ontario, with activity
beginning to develop across the Niagara Frontier. A more organized
band of showers and thunderstorms has been developing across
Western Ohio and the western end of Lake Erie in association with a
low-level jet feeding into the low , and this band may end up being
more of a factor for our forecast area this evening.

SPC has a slight risk area associated with this developing line of
showers across northern Ohio and NW PA, extending up to the
Chautauqua county line. Latest mesoanalysis continues to indicate
little in the way of CAPE, as widespread warm-frontal associated
cloud cover this afternoon has hindered destabilization and therefore
the severe weather threat for our area. Nonetheless, in spite of
minimal instability attm, the advection of warm/moist air will
increase markedly late this afternoon and early this evening into
the forecast area as the aforementioned low-level jet axis continues
to track east, advecting 70+ dewpoint air into our area, helping to
fuel showers and a few embedded thunderstorms. While the deep low-
level moisture will fuel periods of heavy rain, the line should be
progressive enough in nature to negate any flash flooding threat.
Regarding temperatures, it will be a very muggy August night, as the
dewpoints in the upper 60s to low 70s will support similar lows.

The LLJ axis and associated line will cross the forecast area
tonight, with the bulk of the precipitation exiting the eastern
edge of the forecast area Friday morning. However, more scattered
showers and thunderstorms are likely Friday afternoon, as the region
will remain under the influence of troughing from the surface
through the upper levels, as the large, occluded low slowly tracks
to our northwest from the UP of Michigan into western Quebec.
However, with the LLJ and associated moisture passing to our east,
it will not be quite as muggy, with highs in the upper 70s, to lower
80s along the Lake Ontario plain, wheres downsloping southwesterly
flow will bump temps up a couple of degrees.


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.


VFR conditions continue to prevail this afternoon, though mid-level
clouds have overspread much of the forecast area, with a few showers
spreading into WNY as a warm front moves across the area. Conditions
should initially remain VFR with developing -shra this afternoon,
with brief periods of mvfr/ifr possible in heavier -shra/tsra. As
the atmosphere becomes more saturated in the wake of
afternoon/evening convection, we may see the development of MVFR
cigs, with IFR possible across the higher terrain of the Southern
Tier, after 06Z. In addition, LLWS will become a concern after 03Z,
as a LLJ strengthens over the area.

Conditions will improve to VFR
after 12Z, as low level moisture mixes out and slightly drier air
moves into the region. However, with troughing lingering across the
region, expect more scattered -shra/-tsra to develop Fri afternoon,
especially inland from the lakes.


Friday night...Mainly VFR with scattered showers and thunderstorms
tapering off in the evening.
Saturday...Mainly VFR with a chance of showers.
Sunday and Monday...VFR except for late night and early morning
valley fog with local IFR.
Tuesday...Mainly VFR, with a chance of showers and thunderstorms late.


East winds will increase this afternoon 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

Low pressure will then move slowly from the western Great Lakes
Friday to southern Quebec by Saturday. This will bring an increase
in southwesterly winds to the eastern Great Lakes, with Small Craft
advisory conditions probable by Friday afternoon 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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