Area Forecast Discussion
Issued by NWS Buffalo, NY

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946
FXUS61 KBUF 242100
AFDBUF

Area Forecast Discussion
National Weather Service Buffalo NY
500 PM EDT Sat Jun 24 2017

.SYNOPSIS...
Temperatures will run cooler than average this weekend and into
early next week, as upper level troughing moves across the
region. The cooler temperatures will be accompanied by showers at
times, as a series of upper level disturbances moving through the
trough cross the area. The trough will move east by Wednesday,
with warmer temperatures expected by mid-week.

&&

.NEAR TERM /THROUGH SUNDAY/...
Regional radars showing a narrow line of convection focused
along the lake breeze convergence between Niagara Falls/
Rochester and Buffalo/Batavia. A shortwave entering the Lower
Great Lakes will enhance this convection, then eventually cause
it to shift to the east as the shortwave moves across the
region. These trends are captured by mesoscale guidance, but
were blended to match latest radar trends with a narrow line of
likely PoPs along the Niagara/Erie county border which shifts
east toward Rochester but weakens with the loss of daytime
heating. Locally heavy rainfall amounts can be expected in this
line, with localized rainfall amounts in excess of an inch
possible.

Any convective activity will come to an end during the evening
hours, with the loss of diurnal heating, leaving quiet and cool
night across the area. The cooler temperatures aloft, combined
with diminished cloud cover will translate to cooler readings
than we have seen at night of late, with lows dipping into the
upper 50s along the lake plains, with mid 50s across the higher
elevations inland.

On Sunday, a broad mid-level longwave trough will extend from the
Northern Plains across the Great Lakes and into the Northeast
region. Latest guidance continuing to support the idea of a sharp
mid level shortwave and embedded vigorous vorticity max moving
through the base of the longwave trough and crossing western and
central New York during the day. Synoptic scale lift from this
shortwave and low-level forcing from a surface trough and 30+ knot
low level jet will contribute to scattered showers and isolated
thunderstorms. The most widespread coverage will be during the
afternoon corresponding to peak heating and closer to the passage of
the surface trough.

Gusty southwest winds 25-30kts downwind of the lakes should provide
a lake shadow to keep shower/storm coverage mainly inland.
Temperatures will top out a few degrees below normal due to the
influence of the longwave trough and associated pool of cool air
aloft. Highs are forecast to top out within a few degrees of 70.
Dewpoints only around 50 will yield very comfortable humidity
levels.

&&

.SHORT TERM /SUNDAY NIGHT THROUGH TUESDAY NIGHT/...
Through the course of this period a long wave trough will pass
across the Great Lakes region, reaching its greatest depth Monday
night as a strong shortwave passes through. Within this trough
temperatures at 850 hPa will bottom out around +5 to +6C, and at 500
hPa temperatures of -20C within this trough will be around 4SD below
normal for this time of year centered across the Ohio Valley.

Sunday night a swath of deeper moisture will settle southward across
Lake Ontario and later Lake Ontario. As temperatures at 925/850 hPa
drop to around +10/+6C lake instability will increase over a +22C
(Lake Erie) and +19C (Lake Ontario) lake surface. While rain showers
will be possible across the lake plains, greatest chances will be
across the higher terrain of SW NYS and the Tug Hill to SE of Lake
Ontario region where orographic uplift on a west to northwesterly
wind in addition to lake induced CAPE values of 1500 to 2000 J/KG
should generate rare late June lake enhanced precipitation. Any lake
enhanced rain will become more scattered during the late morning
hours during increased daytime mixing. The approach of an upper
level shortwave later Monday will again increase the coverage area
of showers and thunderstorms over the region. The deep 500 hPa
trough and its associated cool pool will encourage some thunderstorm
development, and with low freezing levels and WBZ heights around 6-
8K feet some graupel or small hail will be possible. However the
skinny CAPE profiles suggest that larger hail will be unlikely.

Showers and thunderstorms will diminish Monday night with the
passage of the shortwave within the base of the long wave trough.
There may linger a few lake enhanced rain showers deeper into the
night, especially across the North Country which will lie under the
cooler air aloft.

Tuesday we will remain within the influences of the upper level
trough, through now behind the robust shortwave, and slowly rising
heights aloft the rain shower coverage/intensity should be less than
Monday. The still cool pool aloft will continue the chances for a
few thunderstorms. By Tuesday night the upper level trough will be
pushing away from our region with any linger rain showers largely
ending by the late evening hours. A few lake effect rain showers may
develop off Lake Ontario as low level convergence increases on a
westerly flow. However we will also begin to warm a degree or two at
850 hPa, so lake instability will not be as great as previous
nights.

Temperatures will be well below normal Monday and Tuesday with both
days only pushing mid to upper 60s for highs. Overnight lows will
drop back into the upper 40s to mid 50s closer to the lakes.

&&

.LONG TERM /WEDNESDAY THROUGH SATURDAY/...
While humidity will increase to uncomfortable levels during this
period...temperatures will not stray very far from normal. This will
be due to an eastward displacement of the sub tropical ridge that
during the summer typically supplies our region with very warm
and sultry weather. The heart of this ridge will be centered well
off the coast of the Carolinas between 60-70W longitude rather than
70-80W...so this will allow a very broad low amplitude trough to
become established over the center of the country. The overall
result will be to keep the true heat out of area...although we
will have our fair share of humid conditions.

Unfortunately...the position of the low amplitude trough to our west
will also mean that the core of the sub tropical jet will undulate
back and forth across the Great Lakes region. This will keep active
weather over our forecast area...as several significant waves/sfc
lows will track east along a frontal boundary that will essentially
mimic the position of the overlying jet. Given the time of year...
this will enhance/focus diurnally enhanced convection. As for the
day to day details...

Wednesday should be the `nicest` weather day of the period...as a
shortwave ridge will work its way across the Lower Great Lakes.
While there could be some disorganized lake effect rain showers east
of Lake Ontario to start the day...any lake effect will quickly be
broken apart by strong diurnal mixing. Otherwise...we can expect at
least partial sunshine with a comfortable afternoon as temperatures
in the 70s will be accompanied by relatively low humidity.

Conditions will start to deteriorate Wednesday night as a warm front
will extend east across our region from the Upper Great Lakes. While
the strongest isentropic lift along and head of this boundary should
be to our north and west...there will be the potential for some late
night showers and storms.

While the warm front will push north of our region during the day
Thursday...we will become firmly entrenched within the more unstable
warm sector of the parent system. This will allow any boundaries to
touch off some showers and thunderstorms...which appears to be a
favored solution by many of the ensembles. Will thus raise pops to
likely. Otherwise it will be a warmer and more humid day with
temperatures climbing into the lower 80s for the bulk of western New
York...while readings will be in the 70s east of Lake Ontario and
across the higher elevations. The increased humidity will be more
noticeable as dew points will be some 10 deg higher...reaching into
the low to mid 60s.

As low pressure moves east and weakens across the st Lawrence Valley
Thursday night...its trailing cold front will settle south and
essentially stall over our forecast area. This will keep unsettled
conditions in place with fairly frequent showers and thunderstorms
expected into the start of the weekend. In fact...with dew points
forecast to climb into the upper 60s for most areas on Friday...
storms will likely contain heavier than normal downpours resulting
in an elevated risk for flash flooding. Stay tuned.

&&

.AVIATION /21Z SATURDAY THROUGH THURSDAY/...
A line of showers and thunderstorms extends from IAG-ROC with
brief periods of IFR conditions and gusty winds possible in
tstms. Convection should avoid other TAF locations, with VFR
conditions expected outside of these storms through the
remainder of today. Convection should rapidly diminish this
evening, with the loss of diurnal heating, giving way to more
VFR conditions overnight and into Sunday morning.

Outlook...
Sunday afternoon through Tuesday...Mainly VFR with scattered to
numerous showers and thunderstorms...mainly in the afternoons.
Wednesday...Mainly VFR.
Thursday...VFR/MVFR with a chance of showers and thunderstorms.

&&

.MARINE...
Southwesterly winds have increased on Lake Erie, requiring a
brief period of small craft headlines through this evening.
Winds will diminish rapidly with the passage of a disturbance
around sunset.

Waves will rise higher on Sunday, particularly on the eastern
half of Lake Erie, as winds strengthen further as a strong upper
level disturbance moves through. Small craft advisory
conditions will develop on the eastern end of Lake Erie by
Sunday afternoon and perhaps on the eastern end of Lake Ontario
by Sunday night.

After a brief respite Monday morning, another passing upper level
disturbance will likely bring another period of small craft advisory
conditions to Lake Erie Monday night.

&&

.BUF WATCHES/WARNINGS/ADVISORIES...
NY...Beach Hazards Statement until 10 PM EDT this evening for
     NYZ019-085.
MARINE...Small Craft Advisory until 10 PM EDT this evening for LEZ040-
         041.
         Small Craft Advisory from noon Sunday to 2 AM EDT Monday
         for LEZ040-041.

&&

$$

SYNOPSIS...TMA
NEAR TERM...APFFEL/TMA
SHORT TERM...THOMAS
LONG TERM...RSH
AVIATION...APFFEL/TMA
MARINE...APFFEL/TMA



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