Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 290218

Area Forecast Discussion
National Weather Service Buffalo NY
1018 PM EDT WED SEP 28 2016

A large upper level low centered over the Midwest will spread
showers across Western New York starting late tonight. The low will
remain nearly stationary through Saturday before drifting into
Southern Ontario Sunday. As a result, expect mild but unsettled
weather to persist through the balance of the week and the upcoming


Radar imagery showing a few widely scattered showers developing from
the Buffalo area east into the western Finger Lakes. These showers
appear to be developing along a subtle east/west oriented boundary
where convergence is locally maximized between southeast flow across
the Southern Tier, and ENE flow along the south shore of Lake
Ontario. This boundary will remain in place or lift slowly north
through the early overnight, and continue to support a few scattered
showers. The Southern Tier should remain largely dry through 2AM
before a more significant push of moisture over the eastern half of
PA begins to approach.

Overnight a vertically stacked low will continue to drift southward
across the Ohio Valley, with increasing southeast low/mid level flow
capturing a plume of Atlantic moisture and advecting it through PA
and into western NY. An associated low level jet will provide
increasing low level moisture transport and convergence very late
tonight, bringing increasing coverage of showers. Expect showers to
become steadier first across the Southern Tier after 3AM, then
spread into portions of the Genesee Valley and Niagara Frontier
toward daybreak Thursday.

Shower coverage will increase further as we move into Thursday, when
the upper low drifts into the Ohio Valley. This will have the effect
of directing a fairly uniform southeasterly low level jet across PA
and central/western NY that will in turn tap into Atlantic moisture
and advect it across our region. Greatest pops and precip amounts
will be found across the Southern Tier northwards towards the
Niagara Frontier, closest to the low level jet axis and deepest
moisture. Conversely, the North Country will remain dry throughout
the period given its distance from the aforementioned low level jet
and the fact that it will be subject to a drier northeasterly
downslope flow into the low.

Regarding temperatures, expect readings tonight to reflect the
increasing cloud cover, with lows in the 50s. The influx of fairly
uniform maritime air into the region Thursday combined with the
widespread cloud cover and precipitation will keep temperatures on
the cooler side of average, with highs in the low to mid 60s on the
lake plains of Western NY. Thicker, low cloud cover will keep highs
in the mid to upper 50s across the higher terrain of the Southern
Tier. The North Country will be much warmer with no rain, and
thinner clouds. Expect highs in the lower 70s from Watertown to the
Thousand Islands region.


It will be quite unsettled across much of the region into the
upcoming an anomalously deep closed low in the vicinity
of the Ohio Valley will dominate the period. The stacked storm
system will be unusual at this time of year in both its relative
`strength` as well as its longevity. This will be the main feature
within a mid latitude block...and one that could technically be
defined as a Rex block given that positive hgt anomalies of equal
strength will be found to its north across eastern Canada. A clue to
the persistence of this block can be seen back across the North
Pacific...where a lack of a significant west to east H25 jet signals
the near term lack of an upstream `kicker`. More on this item in the
long term section.

This scenario will establish a prolonged fetch of rich Atlantic
moisture back across the Mid Atlantic Region and coastal states to
our region. PWAT values will increase to 1.5 inches at the start of
the period and remain there for into the start of the upcoming
weekend. It is also interesting to note that the unusually strong
east to southeast flow that will be bearing this moisture will be as
much as 5 STD above typical late Sept levels...territory that
translates to once in a 30 year return interval. While this could
prove problematic in terms of heavy rain for parts of the Mid
Atlantic region...the whole scenario should prove to be beneficial
for our forecast area.

Digging into the details, synoptically this setup favors periods of
showers with the broad divergent flow aloft and cyclonic low and mid
level flow. The periods with more widespread shower coverage and
beneficial rainfall amounts will be tied to individual shortwaves,
low-level jets and moisture advection. Picking up Thursday night, a
plume of moisture advection will shift westward into NE Ohio and NW
Pennsylvania which will shift the focus for the most widespread and
persistent showers into that region. This will allow for break in
precipitation across the region Thursday outside of a few isolated
to scattered showers remaining possible toward the southwest corner
of the state.

By Friday continued deep easterly moisture advection from the
Atlantic will fuel some diurnally driven showers especially across
western NY and the Finger Lakes, with decreasing chances toward the
North Country. Any subtle shortwaves in the cyclonic flow will prove
sufficient to spark off scattered showers through the day. By Friday
night, weakening moisture advection and low-level jet coupled with
loss of diurnal heating should see any shower coverage become more
isolated to scattered across the region.

On Saturday as the main cut-off low finally starts to shift ever so
slowly northward again, this will result in a rejuvenation of
moisture advection into the region with some vorticity advection.
The result should be another period of more widespread showers
across the region as the better forcing move northward. By Saturday
night this forcing and moisture advection will shift north of the
forecast area but with the forecast area in the warm sector in
cyclonic flow will maintain a chance of showers.

This whole stretch will not be a wash out and will feature some dry
periods, but overall we will pick up some beneficial rain for the
area. Through the whole forecast period rainfall amounts may total
anywhere from around a quarter inch in the North Country to up to 2
inches in the Southern Tier.


The cut-off upper level low is expected to begin tracking from
Michigan eastward across Lake Ontario through Sunday. This track
will continue the threat for scattered showers across western and
north-central NY as lift produced by the low continues to interact
with Atlantic moisture wrapping into and around the circulation. On
Monday the low is forecast by medium range models to transition to an
open wave while bring picked up by a strong northern stream trough
diving across Quebec. The axis of this trough and open wave will
cross our forecast area from northwest to southeast along with
residual moisture to continue chances of scattered showers to start
the new work week. During this 48-hour period we are not expecting
any significant rainfall accumulations due to the scattered nature
of the showers. WPC QPF averages and additional quarter to half inch
of rainfall on top of what falls later this week into Saturday.

A return to drier weather is then expected by Monday night through
Wednesday as the remnants of the upper low shift off the coast of
southern New England and an amplified 500mb ridge builds over the
eastern States all in response to a deep trough pushing across the
western states. Subsidence beneath this ridge will help scatter out
much of the lingering moisture leading to partly to mainly sunny
skies. Temperatures through the entire long term period will top out
in the mid 60s to low 70s during the day and upper 40s to low 50s
each night.


Clouds will increase from south to north across Western NY and the
Finger Lakes through 06Z, with nothing more than a few widely
scattered showers across western NY and the western Finger Lakes.
CIGS will remain VFR through 06Z. Late tonight and Thursday morning
deeper moisture will arrive, with more widespread showers moving
into the area from south to north. Expect CIGS to deteriorate to
MVFR across lower elevations by mid to late morning Thursday as the
low levels saturate, with IFR developing across higher terrain of
the western Southern Tier. The lower CIGS will remain through
Thursday evening across the higher terrain of the western Southern
Tier, while the lower elevations of the lake plains improve to VFR.

The eastern Lake Ontario region will avoid all of the rain, with a
consistent feed of dry air draining into the area on northeast low
level flow. This will maintain VFR conditions through the period.


Thursday Night through Friday...Areas of MVFR with periods of
Saturday through Monday...VFR/MVFR with scattered showers.


Moderate easterly flow will continue tonight before becoming even
stronger on Thursday as the pressure gradient tightens between low
pressure over the Ohio Valley and high pressure over Quebec. This
will produce higher end Small Craft Advisory conditions on the west
half of Lake Ontario tonight through the end of the week.

On Lake Erie, the east to southeast wind direction will keep waves
confined to Canadian waters and areas west of NY waters. Choppy
conditions will remain on the lakes through Saturday, before the low
finally begins to shift position into Southern Ontario and weakens
further by Sunday and Monday.


MARINE...Small Craft Advisory until 2 AM EDT Saturday for LOZ042-043.



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