Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 281846

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

An large upper level low centered over the Midwest will spread
showers across Western and North-Central New York starting 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 weekend.


The bulk of the forecast area continues to see sunshine this
afternoon as it remains under the influence of dry air and
subsidence on the outer fringes of a large, stacked upper low
currently centered over the Midwest states. Some high clouds on the
edge of this system continue to move across far western New York and
cu is building across the hills of western PA and far southwestern
NY, indicative of increasing low level moisture across the area.
While there is still a chance for an isolated shower across far SW
NY this afternoon and early this evening, the bulk of the forecast
area will not likely see showers develop and move in until after
midnight, when the deeper moisture currently visible over central
Ohio wraps northeastward into NY.

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 lower 60s. The one
exception will be across the North Country, which once again should
remain far enough displaced from the precip and under the
of downsloping to climb into the upper 60s, except for the cooler
top of the Tug Hill Plateau.


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.

Now that we know that the bulk of our region will have plenty of
moisture to work with...where is the lift going to come from to
generate the advertised rainfall? A divergent flow aloft on the
northeast side of the stacked low and orographic lift in the low
levels will be the most persistent sources for upward forcing...
while the steadier enhanced periods of rain will be supported by
strong low level jets and shortwaves circulating around the
periphery of the parent low. Time to get into some details.

There will be widespread showers found across the Southern Tier on strong lift produced from a 45 kt low level jet
traversing Pennsylvania will extend north across the border.
Additional lift will be supplied by a weakly coupled h25 jet. While
the corresponding thermodynamics are not impressive for
thunderstorms...this will be a strongly forced event with just
enough elevated CAPE to justify current TRWs that are in continuity.
Will continue the trend from continuity by further raising cat pops
to near 100 for that area...with pops tapering off to likely for the
corridor extending from BUF and ROC to SYR...and just low chc pops
for the North Country. In fact...areas north of the Tug may escape
with a relatively decent day...disregarding the increased cloud
cover. Basin average rainfall Thursday is forecast to range from a
half to three quarters of an inch across the Srn Tier to under a
third of an inch along the south shore of Lake Ontario and likely
no measurable for the North Country. It will be notably cooler
regionwide with afternoon temps in the low to mid 60s. Its possible
that some of the rainier higher terrain near the Pennsylvania border
could remain in the 50s.

As the strong low level jet pushes away from the Southern Tier
Thursday night...the showers across all of western New York will
diminish in intensity and coverage. Will remove any mention of
thunder as the dynamic forcing will be removed. This will leave chc
pops in most areas for the overnight...while likely pops will be
found across the Southern Tier. Again...mainly dry uneventful
weather can be anticipated across the North Country. It will be
relatively mild for late September very weak warm
advection of a saturated maritime airmass will prevent the mercury
from falling out of the 50s.

While the axis of deepest Atlantic moisture is forecast to still be
over our region on Friday...latest guidance is not indicating any
significant focusing mechanism for steady shower activity. Given the
proximity of the closed low though...experience suggests that there
would be at least one shortwave rotating back up and across the Srn
Tier. This should justify keeping likely pops across the Srn Tier
while chc pops will be found most elsewhere. The mercury on Friday
will once again top out in the a small diurnal temperature
range will persist across the region.

There is loose agreement between the various guidance packages that
the persistent upper low will start to push north from the Ohio
Valley Friday night and Saturday. The resulting hgt falls over our
region will couple with the continued cyclonic flow and any embedded
shortwaves to support continuity of likely pops over the western
counties...while chc pops will remain in place for the Eastern Lake
Ontario Region. It could very easily be a rainy day to start the
weekend...but the good news is that this would likely be the
beginning of the end for the unsettled weather over our region.


There will be a clear trend towards improving weather during this the persistent closed low that had been parked over the
Ohio Valley will slowly push northeast across the Lower Great Lakes
and exit across the St Lawrence Valley.

The impetus for the return to a progressive flow will be +100kt H25
latitudinal jet over the northern Pacific that will feed into and
deepen a west coast trough. This trough will amplify a downstream
ridge that will initially be blocked by the aforementioned closed
low. In time though...the ridge will build enough to lift out the
low and send it packing across the St Lawrence Valley. The end
result will be a significant building of the hgt field across the
Great Lakes region with sfc high pressure finally regaining control.

In terms of day to day weather across our forecast area...
Scattered showers will persist on Sunday as the core of the upper
low will pass by just to our west. While a cyclonic flow will keep
just the chance for showers over the region into Monday...increased
subsidence that will accompany the significant height rises will
lead to fair weather for Monday night and Tuesday. Temperatures all
three days are expected to range from 65 to 70 while overnight lows
will be mainly in the 50s.


VFR conditions continue to be found across the forecast area this
afternoon, with scattered cu being found across the hills of the
North Country and Southern Tier. High clouds moving into Western NY
will thicken and lower through the night as a large area of low
pressure spinning over the Midwest pushes showers into the forecast

While widespread showers are expected to spread north into the
forecast area from around 06Z onwards, brisk easterly flow of 10-
15kts should produce enough downsloping to keep KROC/KIAG/KBUF VFR
through the forecast period. However across the higher elevations of
the Southern Tier, cigs will drop into MVFR/IFR range late tonight
and into Thursday as Atlantic moisture spreads into the region and
deepens on the aforementioned easterly flow. Conditions are expected
to remain VFR across the North Country throughout the TAF period, as
the area will be under the influence of drier northeasterly flow and
should remain outside of the influence of the low pressure system.


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


Easterly flow is beginning to freshen on the lakes this afternoon as
a large area of low pressure continues to spin across the Midwest.
This flow will continue to strengthen to around 20kts tonight and
will persist though Friday as the low remains nearly stationary. A
small craft advisory has been issued for Lake Ontario to account
for the waves that will build on the southern/eastern half of the
lakes in response to this flow, while on Lake Erie, winds will keep
waves confined to Canadian waters. Choppy conditions will remain on
the lakes through Saturday, before the low finally begins to shift
position into Southern Ontario.

There will also be chances for waterspouts this period as
increasing lake induced equilibrium levels, combined with the cool
flow aloft over the warm lakes creates an environment that favors
waterspouts. Greatest chances for waterspouts associated with this
upper level closed low will be tonight through Thursday.


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



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