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Area forecast discussion 
National Weather Service Buffalo New York
1040 PM EDT Monday Sep 26 2016

Synopsis...
a cold front will exit east of our region overnight and the
colder air in its wake will generate some lake effect rain showers
northeast of the lakes on Tuesday morning. A slow moving storm
system to our south could then keep somewhat unsettled weather
over our region through much of the rest of the week.

&&

Near term /through Tuesday/...
a cold front which is across western New York late this evening
will continue to push to the east overnight. At 10 PM, showers
associated with this boundary moving into the Eastern Lake Ontario
region, with scattered showers just behind this main area along
the frontal boundary itself. All of this should slide east of the
cwa around or shortly after midnight. Otherwise, infrared satellite
shows drier air just behind the front which should provide a
period of partly to mostly clear skies for a period overnight.

Lake effect rain showers will develop late tonight as 850mb
temps drop to around 5c. This will result in moderately unstable
conditions over the lakes. This will be accompanied by a weakly
capped, low sheared environment that will still have some synoptic
moisture to further support the heat and moisture transfers off
the Warm Lake waters. We often see a dry environment that can not
be moistened enough by lake processes, but this case will not be
one of them.

The most pronounced lake response will be northeast of Lake Erie
where a near maximum fetch will encourage the development of a band
of lake effect rain that will extend across the Buffalo Metro area
after 09z. This band should drift north to the northtowns and
towards the falls by daybreak. The placement of this band is a
little further north than most of the mesoscale guidance packages,
taking into account the thermally induced low level backing of the
flow which is often discounted by guidance. Off Lake Ontario...the
240 degree flow will result in a shorter fetch and less impressive
organization. The associated showers will be directed across
northern Jefferson County to the Thousand Islands region.

Temperatures tonight will drop back through the 50s, with some 40s
anticipated for the southern tier.

&&

Short term /Tuesday night through Thursday night/...
Tuesday night into Wednesday morning 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 dry Tuesday
night. The cool and relatively dry air aloft should also allow
temperatures to fall into the 40s Tuesday night.

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 European model (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 precipitable water 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 quantitative precipitation forecast 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 /02z Tuesday through Saturday/...
a cold front near roc at 02z will push east late this evening.
Showers associated with this boundary and moisture pooled along
the boundary will result in some MVFR conditions in both ceilings and
visibility in heavier showers. Meanwhile conditions will improve rather
quickly behind the boundary as drier air builds in.

Expect mainly VFR conditions late tonight and Tuesday, with the
exception of some lake effect rain showers will will produce areas
of MVFR conditions. These will primarily impact buf/iag Tuesday
morning, with the band lifting northward during this Time. Lake
effect rain showers will weaken in the afternoon. Elsewhere,
expect mainly scattered clouds with VFR conditions.

Outlook...

Tuesday night...VFR.
Wednesday through Saturday...VFR/MVFR with a chance of 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...
New York...none.
Marine...Small Craft Advisory until 5 am EDT Wednesday for lez040-041.
Small Craft Advisory until 5 PM EDT Tuesday for
loz045.

&&

$$

Synopsis...rsh/wch
near term...apffel/rsh/wch
short term...wood
long term...jjr
aviation...apffel/wch
marine...apffel/wch

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