Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 261056
AFDBUF

Area Forecast Discussion
National Weather Service Buffalo NY
656 AM EDT MON SEP 26 2016

.SYNOPSIS...
The large high pressure system that produced a beautiful weekend
across our region will exit across New England today...while a
strong cold front will approach from the west. This front will
generate several hours of showers across our region this afternoon
into tonight...with colder air in its wake supporting some lake
effect rain showers northeast of the lakes on Tuesday. 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 TONIGHT/...
The beautiful early Fall weather of the past couple of days will now
give way to deteriorating conditions...as our controlling Canadian
surface high will give way to a fairly strong cold front. This front
was advancing from Lower Michigan early this morning...and by this
afternoon...it will be making its way across Lake Erie and southern
Ontario.

AMSU derived products this morning depict a plume of deep moisture
with PWAT values estimated at 1.75 inches streaming to the north
along and ahead of the front. This swath of anomalously rich
moisture(200% of normal) will gradually work across western New York
during the course of the midday and afternoon...while deep lift
through 25k ft will generate widespread showers and possible
thunderstorms.

The forcing associated with the front will come from a variety of
sources. To start with...large scale lift will be initiated by a
robust shortwave that will carve out a deep closed low over the
Upper Great Lakes. This will result in 200m hgt falls over our
region while establishing a strongly divergent upper level flow.
More focused lift will then come from a co-located cyclonically
curved H25 100 knot jet and strong H925-70 frontogentic forcing.
This will all interact with the abundant supply of moisture to
produce a 3-5 hour window of widespread showers. There will also be
the chance for some embedded thunderstorms with the main line of
pcpn...but this potential should be limited to the western counties
where some marginal diurnally assisted instability will be in place
into the early evening hours. The very dynamic nature of the system
will be the main impetus for any true convection.

In terms of timing...showers over the far western counties can be
expected anytime after lunchtime with the pcpn pushing east to the
Genesee Valley and Western Finger Lakes by late afternoon/early
evening. The strong southerly gradient ahead of the katabatic styled
cold front will favor a slower eastward propagation than most
frontal passages. The rain will then reach the Eastern Lake Ontario
region after nightfall (23z).

For tonight...the cold front will steadily push across the remainder
of our forecast area during the first of the night. The showers...
which could be moderate to heavy at times...will end fairly abruptly
from the west as the relatively narrow band of deeply forced lift
will rapidly give way to subsidence and notably drier air in the
wake of the front. The showers will end across the far western
counties by 02z...the Finger Lakes region by about 05z and finally
the Eastern Lake Ontario region by about 08z.

Rainfall from this significant frontal passage is forecast to range
from about a half inch in most areas to under a third of a inch in
the downslope areas of the Lower Genesee Valley and Lake Ontario
lake plains (Orleans Co to Nrn cayuga Co). Amounts could approach
three quarters of an inch across the Srn Tier.

While a significant dry slot in the wake of the front will promote
at least partial clearing overnight...there will be more weather to
contend with. Mother Nature will shift gears from synoptic to
mesoscale as H85 temps falling back to around 5c will establish
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 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.

As for temperatures...today will be warmer than the past few days as
a deepening southerly flow ahead of the cold front will advect
milder air into the region from the Ohio Valley. H85 temps that were
in the single digits C over the weekend will climb to around 14c
ahead of the front. This will support readings in the 70s for most
areas during the midday and afternoon. Temperatures tonight will
drop back through the 50s...with some 40s anticipated for the Srn
Tier.

&&

.SHORT TERM /TUESDAY THROUGH WEDNESDAY NIGHT/...
During the first 24 hours of this period...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 even 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 than was suggested
Sunday afternoon. Have thus reverted the forecast back to more
showery wording...while maintaining a period of categorical PoPs.

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. With this latter factor in mind... would in fact not
at all be surprised to see the rain showers wind up even a little
further north than depicted in the latest forecast...and primarily
across Niagara County (as is presently suggested by the typically
more reliable regional GEM).

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.

Moving on into the latter half of this period...it still appears
as if the closed upper low will continue to meander south-
southeastward into the Ohio Valley Wednesday and Wednesday night.
For our region...an initial southerly flow of drier air on the
eastern flank of this system should result in a mainly dry and
slightly warmer day for most of Wednesday...before increasing
moisture and lift attendant to the slowly encroaching low leads to
a renewed potential for some scattered showers late Wednesday
afternoon across far Southwestern New York...and Wednesday night
across the remainder of the area. Temperature-wise...850 mb
readings should recover to between +8C/+9C on Wednesday and allow
daytime highs to reach into the upper 60s to lower 70s in most
places...with lows in the upper 40s to mid 50s then following for
Wednesday night.

&&

.LONG TERM /THURSDAY THROUGH SUNDAY/...
While some variance persists amongst the medium range models for
later on this week and next weekend...the overall trend over the
last 48 hours has been toward a decidedly wetter overall scenario...
and the 00z/26 guidance suite is no exception with the GFS/GEM/ECMWF
all showing the deep cutoff low slowly meandering across the Ohio
Valley and Great Lakes through Saturday...and the ECMWF even more
pessimistic in keeping this feature around our area right through
Sunday.

Such a pattern would place our region on the northeastern flank
of this system for much of this period...where a persistent
easterly to southeasterly flow would act to feed plentiful
Atlantic moisture into our region... resulting in predominantly
cloudy skies along with periodic rounds of showers. While this
would be good news in that it would help bring some additional
much-needed rainfall to our drought-stricken region...it would
also likely put a bit of a damper on any outdoor activities...
particularly during the Thursday-Saturday time frame.

Taking the continued trend toward a more pessimistic scenario
into consideration in tandem with the lingering uncertainty in the
exact position of the cutoff low...have elected to raise PoPs a
bit more through the chance range throughout this period...with
the highest probabilities generally found across the Southern Tier
and interior portions of the Finger Lakes. Unfortunately...this
change does result in the extension of shower chances into Sunday
as well...out of respect for the slower ECMWF.

With respect to temperatures...the flow of mild Atlantic-based
air into our region should result in readings averaging out right
around or slightly above normal. More specifically...daytime highs
should largely be in the mid to upper 60s...with nighttime lows
ranging between the upper 40s and mid 50s.

&&

.AVIATION /12Z MONDAY THROUGH FRIDAY/...
High pressure stretching from Quebec to the New England will keep
fair VFR conditions in place across our region through the midday
hours.

A cold front will then approach from the west...with deteriorating
conditions in widespread showers and possible thunderstorms during
the afternoon and evening. Cigs will drop to MVFR levels after 20z
across the western counties...with a brief period of IFR cigs
possible between 22 and 01z. MVFR conditions are not anticipated for
KART and KGTB until after 00z.

Tonight...cigs will quickly rebound to VFR levels as skies will at
least partially clear in the wake of the front. This improvement can
be expected over the western counties after about 02z...and for the
Eastern Lake Ontario region after 08z.

Outlook...

Tuesday and Tuesday Night...Mainly VFR. Areas of MVFR/IFR in lake
effect rain showers northeast of Lake Erie.
Wednesday through Friday...VFR/MVFR with a chance of showers.

&&

.MARINE...
A cold front approaching from Michigan will promote freshening
winds across the Lower Great Lakes today...but given the south to
southeast flow...the highest waves will be found in Canadian waters.
Nevertheless...offshore winds should become strong enough to warrant
a small craft advisory for the nearshore waters of Lake Erie and
also for the far eastern end of Lake Ontario for the afternoon.

Along with the higher wind speeds...showers will become widespread
ahead and along the front this afternoon into tonight...with some
thunderstorms possible as well.

Winds in the wake of the front will veer to the southwest tonight.
This will allow waves to build across Lake Erie and also over the
far eastern end of Lake Ontario...so the aforementioned small craft
advisory will remain in place through the night.

Moderate to strong southwesterlies will remain in place across Lake
Erie and much of Lake Ontario through Tuesday as a stalled storm
system will be anchored over the Upper Great Lakes.

&&

.BUF WATCHES/WARNINGS/ADVISORIES...
NY...Frost Advisory until 9 AM EDT this morning for NYZ007-008.
MARINE...Small Craft Advisory from 2 PM this afternoon to 5 AM EDT
         Wednesday for LEZ040-041.
         Small Craft Advisory from 2 PM this afternoon to 5 PM
         EDT Tuesday for LOZ045.

&&

$$

SYNOPSIS...RSH
NEAR TERM...RSH
SHORT TERM...JJR
LONG TERM...JJR
AVIATION...RSH
MARINE...RSH


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