Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 270632
AFDBUF

Area Forecast Discussion
National Weather Service Buffalo NY
232 AM EDT TUE SEP 27 2016

.SYNOPSIS...
Cooler air following in the wake of a cold front will generate some
lake effect rain showers northeast of the lakes on Tuesday. After a
mainly dry day Wednesday...a slow moving storm system will then bring
unsettled weather to our region for the rest of the week and next
weekend.

&&

.NEAR TERM /THROUGH TUESDAY NIGHT/...
06z regional surface analysis shows the cold front and any
remnant scattered light showers now in the process of clearing the
North Country...with these likely clearing our area altogether by
07z. In the wake of this boundary...strong dry slotting
overspreading the region will result in mainly dry weather and
mainly clear to partly cloudy skies for most areas...except
downwind of Lake Erie where some lake effect rain showers will
continue to slowly develop.

So far...the aforementioned lake effect activity has been
sputtering along a low-level convergence axis over eastern Lake
Erie over the past couple of hours...with one small cluster of
showers already having pushed across Erie and Genesee
counties...and another now trying to become better organized out
over the lake. Like its predecessor...this next area is also now
producing some limited in-cloud lightning while out over the warm
lake waters.

For the rest of the night...this activity should continue to
become a bit better organized/more numerous as background synoptic
moisture increases a bit...while lifting northward from areas just
a little south of Buffalo to the Northtowns and southern Niagara
county as a general west-southwesterly flow backs to southwesterly
and becomes increasingly sheared. With this in mind...have
continued to indicate PoPs climbing into the high chance/low
likely range overnight in the above mentioned areas...and have
also bumped up thunder probabilities into the chance range given
the expected trend toward better organization and current upstream
obs of some lightning out over the lake.

As for temperatures...these will drop back through the 50s
overnight... with some 40s anticipated for the Southern Tier.

During Tuesday and Tuesday night...a large closed upper low will
slowly meander its way south-southeastward across the Upper Great
Lakes. While we will be firmly under of the influence of its
accompanying cool pool (850 mb temps of +4C to +8C)...the region
of the best/deepest wraparound moisture associated with the low
will remain well to our west over the Upper and Central
Lakes...with just an attendant spoke of this sliding across far
Western New York and adjoining portions of Lake Erie/Lake Ontario
during Tuesday. Thus for most locations...Tuesday and Tuesday
night should just be mainly dry and cooler...with daytime highs
ranging through the 60s on Tuesday...and nighttime lows falling
into the mid 40s to lower 50s Tuesday night. The two exceptions to
this will be over and downwind of Lakes Erie and Ontario...where
the cool airmass aloft and warm lakes will team up to generate
some lake effect rain showers.

Overall...the best setup still looks to be found over and
downwind of Lake Erie Tuesday morning...where a somewhat sheared
southwesterly low level flow and the aforementioned spoke of
deeper moisture should be supportive of a broken band of showers
and possibly a few embedded thunderstorms. At this point...do
not feel that a really cohesive rain band will form given the
combination of the aforementioned shear and increasing late
September diurnal influences...both of which should act to keep
the activity more cellular/showery in nature.

With respect to location...the slowly backing/sheared
southwesterly flow should direct the bulk of the rain showers
across areas from roughly the city of Buffalo northward into
Niagara and Orleans counties...with the axis of the most
persistent activity most likely to lie across far Northern Erie
and Niagara counties. This position is a bit further north than
our existing continuity...and is in line with both 00z model
trends and the propensity for early season events to be oriented a
bit further north than the guidance would suggest...as the models
typically underdo the influence of lake-driven diabatic processes
on the low level wind field... which in turn results in a slightly
more veered low level flow in the models than in the real
atmosphere.

Once we reach Tuesday afternoon...the combination of diminishing
background moisture...a continued slow but steady backing of the
low level flow...and a further increase in shear will result in
the showers weakening and becoming much more scattered while
lifting almost entirely into Niagara County...with these then
shifting west of the area and falling apart altogether Tuesday
night as our airmass becomes too dry and the low level flow too
southerly to support lake effect precipitation.

Meanwhile off Lake Ontario...the setup appears considerably less
impressive as the background airmass will be drier...and the low
level flow will likely be a bit more sheared and south-southwesterly
overall...which will also translate into a shorter fetch. This
should result in much more scattered showers primarily affecting
Jefferson County and the uppermost Saint Lawrence Valley on Tuesday...
with this activity then also shifting further northwestward and
falling apart altogether Tuesday night as the airmass dries out
and the low level flow backs to south-southeasterly.

&&

.SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/...
Wednesday is shaping up to be a relatively quiet and mainly dry
period for Western and North- Central New York. A large upper
level low will be continuing its gradual descent across the
central Great Lakes, while drier air wrapping across the
southeastern flank of this low will continue to remain overhead
much of the lower Great Lakes. This, along with a synoptically
unfavorable position in the left entrance region of an exiting
upper-level jet streak will keep things mainly dry. Temperatures
will rebound into the 70s along the lake plains on Wednesday,
aided by strengthening downsloping southeasterly flow, with 60s
across the higher elevations.

While this southeasterly flow should initially keep things dry
for much of the day on Wednesday, the main upper level low is
currently progged to drop south towards the Ohio valley, while a
secondary low off the Mid- Atlantic is shunted off to the
northeast. This will open up much of Pennsylvania and Western New
York to a long southeasterly fetch of Atlantic moisture that will
isentropically lifted along the northeastern flank of the main low
over Ohio, in turn generating an increasing number of showers
across much of the forecast area as we move from Wednesday evening
into Thursday. As a side note, the strengthening east-northeast
flow in the lower levels on the northern edge of the surface flow
will likely produce a considerable swath of lake- enhanced rains
just to our north across the Greater Toronto Area Wednesday night
into Thursday morning.

The increasing cloud cover and influx of maritime air into the
region will produce more mild temperatures Wednesday night, with
lows in the upper 40s to lower 50s, while the widespread clouds will
keep readings in the low to mid 60s Thursday. Showers will likely
persist through the end of the week as the upper low stalls out over
the region. The persistent mild air will keep lows in the 50s
Thursday night.

&&

.LONG TERM /FRIDAY THROUGH MONDAY/...
This period will be dominated by a large cut-off low which is
generally forecast by medium-range global models to meander from the
Ohio Valley north and east across the eastern Great Lakes then
shifting off the coast of New England as an open trough by early
next week. Among the 12z runs the ECMWF remains the slower of the
model solutions. The cut-off low will draw a broad slug of Atlantic
moisture westward across the tri-state region to the eastern Great
Lakes and upper Ohio Valley where it will interact with multiple
vorticity maxes circulating around the low. Each of these vort maxes
should force a swath of rain showers shifting from east to west
across the region. 12z NAEFS standard anomalies of PWAT for the
Friday into the weekend timeframe are running between 1.5 and 2
deviations above normal which means moisture will be plentiful for
numerous periods of rain showers. WPC 48-hour QPF guidance for 00z
Friday into 00z is showing a large swath of 1-2+ inches of rainfall.

Forecast-wise this requires an extended period of chance range POPs
and cloudy to mostly cloudy skies running Friday through Sunday only
tapering to slight chance by Monday when the low is forecast to be
shifting east of New England. Highest confidence is on Friday across
the Southern Tier closest to the center of the cut-off low. As model
agreement becomes better aligned as this event gets closer and
confidence increase expect that POPs will be increased further with
finer detail in timing of swathes of showers. The high level of
Atlantic moisture streaming across the region should ensure mild
seasonable temperatures with highs in the mid to upper 60s and lows
only slipping back into the 50s.

&&

.AVIATION /07Z TUESDAY THROUGH SATURDAY/...
Through Tuesday...mainly dry VFR conditions and scattered cloud
cover will prevail across the bulk of the region...with the
exception of areas northeast of Lakes Erie and Ontario where
some lake effect rain showers are expected. The most numerous
activity will be found off Lake Erie Tuesday morning...where
the northward-migrating showers could produce some MVFR conditions
at KBUF/KIAG before weakening and becoming more scattered Tuesday
afternoon. Off Lake Ontario...lighter and much more scattered
showers will be found from roughly KART northward...with these
much less likely to produce reductions to below VFR.

Tuesday night...VFR conditions will prevail...with any lingering
lake effect rain showers north-northeast of the lakes shifting
west of our region and falling apart as the low level flow turns
more southerly...and our airmass dries out even further.

Outlook...
Wednesday...VFR...with just a chance of showers late near KJHW.
Wednesday night through Saturday...VFR/MVFR with periodic rounds
of scattered to numerous showers.

&&

.MARINE...
Colder air will move in behind the cold front which will help
winds aloft mix to the lake surface. A stronger pressure gradient
following the front and the colder air will produce gusty winds
overnight and Tuesday, resulting in Small Craft Advisories along
the Lake Erie nearshores, and along the eastern end of Lake
Ontario. The Lake Ontario conditions will improve as winds become
southeasterly late Tuesday, but wind will remain rather strong
from the south to southwest along the Lake Erie shores. Although
conditions will not be ideal, there is a potential for some
waterspouts along a convergence band which is expected to develop
on Lake Erie late tonight and Tuesday.

&&

.BUF WATCHES/WARNINGS/ADVISORIES...
NY...None.
MARINE...Small Craft Advisory until 5 AM EDT Wednesday for LEZ040-041.
         Small Craft Advisory until 5 PM EDT this afternoon
         for LOZ045.

&&

$$

SYNOPSIS...JJR
NEAR TERM...JJR
SHORT TERM...WOOD
LONG TERM...JJR
AVIATION...JJR
MARINE...APFFEL/WCH


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