Area Forecast Discussion
Issued by NWS Buffalo, NY

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447
FXUS61 KBUF 242003
AFDBUF

Area Forecast Discussion
National Weather Service Buffalo NY
403 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 MONDAY 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 /TUESDAY THROUGH SATURDAY/...
Latest runs of the medium range guidance indicate that the axis of
the longwave trough and associated vort max will be in the process
of shifting east of the forecast area on Tuesday. The cool air aloft
within the trough (again 850mb T around 6C) will support mainly a
diurnal threat for showers and possibly some thunderstorms. Similar
to Monday, taller shower/storm cells in this cool environment may
bring some small hail with a sub-10kft WBZ height. Again with
lingering cool air aloft, high temps will top out again in the 60s.

Surface high pressure centered over the southern Appalachians will
ridge north across the region Tuesday night and Wednesday allowing
for dry weather with mainly clear to partly cloudy skies. Temps will
still run a degree or two below normal.

Thursday and Friday, the high is forecast to shift off the Mid-
Atlantic coast with southerly winds bringing a return of increasing
moisture and warmth across western and central NY. Highs look to
push back above normal with readings in the upper 70s to low 80s. A
cool front looks to be the main focus for shower and thunderstorm
activity as both the GFS and EC models indicate it may become
stalled/stationary somewhere over the forecast area.

&&

.AVIATION /20Z SATURDAY THROUGH THURSDAY/...
VFR conditions will prevail throughout the day, though diurnal
showers and a few isolated thunderstorms may develop in the
afternoon, largely along lakebreeze boundaries along the KIAG/KROC
corridor. Convection should rapidly diminish this evening, with the
loss of diurnal heating, giving way to more VFR conditions
overnight.

Outlook...
Sunday 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...
Westerly flow developing in the wake of a cold front crossing the
area will generate a moderate chop across the eastern ends of the
lakes, with waves to 3 feet. 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...None.
MARINE...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...SMITH
AVIATION...TMA
MARINE...TMA



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