Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 181554

Area Forecast Discussion
National Weather Service Buffalo NY
1154 AM EDT Fri Aug 18 2017

A cold front will move across the area this afternoon and produce a
few more widely scattered showers and thunderstorms mainly southeast
of the lakes. Another weak front will cross the area Saturday with
widely scattered showers and thunderstorms, especially across the
Southern Tier. High pressure will then bring a return to dry weather
Sunday and Monday.


Regional radar this morning shows the last of the overnight
precipitation has pulled well away to the east, while regional
visible satellite imagery shows a well-developed cu field, inland of
the prominent lake shadows northeast of the lakes. The very moist
airmass, featuring dewpoints in the upper 60s to low 70s is
supporting this cloud field. Given all of the moisture in place,
expect plenty of clouds to hang around outside of lake shadow areas,
which will be maintained thanks to a breezy southwesterly flow ahead
of an approaching cold front on the southeast flank of a well-
developed low centered just north of the Soo. This breezy southwest
flow, combined with the warm and muggy airmass that is in place will
make for an especially warm day across the Lake Ontario plains and
the Genesee valley, where highs will climb into the mid 80s, while
readings will be more mild downwind of the lakes, where temperatures
will top out in the upper 70s to low 80s.

The warm and muggy airmass, combined with troughing aloft will
provide a favorable environment for the development of scattered
afternoon showers and perhaps a few thunderstorms, along lake breeze
boundaries, which should be pushed well inland by the relatively
strong flow aloft. Most likely locations for convection in our area
will run from the Southern Tier across the Finger Lakes into the
southern Tug Hill/western Adirondacks this afternoon, while areas
to the west will remain well within the stable lake shadow and
should see a sunny and dry, albeit breezy afternoon, as brisk winds
aloft mix down to the surface, with gusts of 25-35mph expected.

The diurnally driven widely scattered convection will mostly end by
mid evening as the boundary layer begins to cool and stabilize, and
the weak synoptic scale cold front moves east of the area. This will
leave mainly dry weather overnight, although cooling aloft
interacting with the warm lake waters may allow a few scattered
showers to develop late tonight over and east of Lakes Erie and
Ontario. Expect lows in the mid 60s on the lake plains, and around
60 across the interior Southern Tier and Tug Hill region. The
airmass will slowly become less muggy overnight as weak cool/dry
advection continues.


On Saturday, a progressive amplified trough will swing across the
forecast area. Weak warm advection ahead of the trough passage will
bring some modest instability back into the western Southern Tier
during the morning with increasing wind shear. A few stronger
thunderstorms may develop with the trough passage in the western
Southern Tier, with storms possibly being aided by the developing
Lake Erie lake breeze boundary. SPC has included this area in a
Marginal risk for severe weather. Otherwise, more widely scattered
showers and rumbles of thunder are possible Saturday afternoon and
early evening for the rest of the forecast area with the trough
passage. With slightly cooler air aloft under the trough and some
cloud cover, expect high temperatures will run just below normal in
the mid 70s. Showers and any thunder will diminish overnight in the
wake of the trough axis passage and loss of diurnal heating. Lows
Saturday night will be a bit cooler than previous nights, in the mid
50s to low 60s.

A broad surface high will build in across the upper Ohio Valley
Sunday bringing moderating temperatures and drier air. This will
result in high temperatures near normal values. As the surface high
slides off the mid-Atlantic coast Monday, return flow developing
will push high temperatures into the low to mid 80s. Skies will
remain mostly clear Sunday into Monday, which should provide prime
viewing conditions for the eclipse on Monday. Low temperatures
Sunday night and Monday will follow the same warming trend as the
high temperatures, with lows Monday night well into the 60s with
building humidity.


This period will be dominated by a steadily amplifying longwave
trough over eastern Canada and the Lower Great Lakes. This will lead
to notably lower day to day temperatures...although at the onset of
the time frame...our temperatures will be well above normal. As for
the day to day details...

Our region will find itself between systems on high
pressure anchored off the coast and a cold front approaching from
the Upper Great Lakes will support a very warm and increasingly
humid environment. Temperatures should easily climb into the mid to
upper 80s across the lake plains and in the valleys. This airmass
will also be increasingly unstable...and while the vast majority of
the day will be rain free...some showers and thunderstorms will be
possible later in the day. This will mainly be the case over the
western counties where lake breeze interactions and/or the presence
of a pre-frontal trough could focus convection.

A cold front will then push southeast across our forecast area
Tuesday night and early Wednesday, likely bringing widespread
showers and/or thunderstorms across the forecast area.

Expansive high pressure over the Upper Mississippi Valley Wednesday
afternoon will gradually settle to the southeast across the Lower
Great Lakes and Ohio Valley by Thursday. This will allow for
significant improvement in our weather with fair weather
returning...along with a pronounced turn to cooler and more
comfortable (lower dew points) conditions. Temperatures by Thursday
will average at least 5 deg f below normal.


Stratus has largely mixed out this morning, leaving most locations
VFR, though a very moist boundary layer airmass featuring sfc
dewpoints in the upper 60s/low 70s is keeping cloud bases lower,
resulting in MVFR cigs hanging on at KIAG/KJHW/KART. These cigs
should lift with increasing mixing this morning, and the lake shadow
will also become more prominent as we move into the afternoon hours,
resulting in clearing skies at KIAG/KART.

Scattered -shra/-tsra can be expected well inland of the lakes this
afternoon, courtesy of a moist low level airmass and troughing
aloft, along with an approaching cold front. This diurnally driven
convection is not expected to affect any TAF terminals, and will be
roughly along a line from KOLE-KFZY eastwards. Convection should
diminish after sunset. While the airmass in the region will begin to
dry out somewhat overnight, dewpoints will remain the 60s, and it is
likely that we will see MVFR cigs return again tonight, with IFR
across the Southern Tier, as cloud bases lower once again.


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.


Low pressure will move from just east of Lake Superior today to west
central Quebec by Saturday, with a trailing cold front crossing the
eastern Great Lakes this afternoon. Southwest winds will become
quite strong on Lake Erie today as they funnel up the length of the
lake, with solid Small Craft Advisory conditions this afternoon and
evening. The southwest winds will be lighter at the east end of Lake
Ontario. West winds will slowly increase on Lake Ontario overnight
into Saturday, with Small Craft Advisory conditions probable on at
least the east half of the lake for Saturday, while winds remain a
little lighter on Erie.

High pressure will then bring a return to light winds Sunday and
Monday on both lakes.


NY...Beach Hazards Statement until 11 PM EDT this evening for
MARINE...Small Craft Advisory until 2 AM EDT Saturday for LEZ040-041.
         Small Craft Advisory until 11 PM EDT this evening for



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