Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 171938

Area Forecast Discussion
National Weather Service Buffalo NY
338 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.


By Friday night, a cold front will have passed through the region,
bringing in cold air advection behind it enough to drop 850mb
temperatures to around 10c. Combined with lake temperatures around
22c, this could produce showers overnight but will most likely
result in low clouds because of a low cap and very dry air above

On Saturday, a secondary trough will bring another frontal boundary.
This combined with diurnal heating may produce showers inland during
the afternoon. Saturday will have the coolest temperatures of the
weekend, with highs in the mid 70s and lows in the low 60s.

A broad area of high pressure will build over the region Sunday,
accompanied by warm and dry air, bringing us back to seasonal
temperatures (near 80). High pressure will continue into Monday,
where temperatures will begin to climb back up to the low to mid
80s. Monday night, the ridge will still be over the Eastern United
States, with low temperatures near normal in the 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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