Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 281901

Area Forecast Discussion
National Weather Service Buffalo NY
301 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.

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.


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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