Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 202325
AFDBUF

Area Forecast Discussion
National Weather Service Buffalo NY
725 PM EDT Fri Oct 20 2017

.SYNOPSIS...
High pressure will remain anchored along the Mid-Atlantic coastline
this weekend with dry weather and well above normal temperatures
continuing. The pattern will then become more unsettled early next
week with a slow moving cold front bringing rain, followed by cooler
temperatures.

&&

.NEAR TERM /THROUGH SATURDAY/...
Simply put...expansive ridging at all levels will continue to
dominate our weather through this period.

There will be a weak gradient flow tonight with good (but not quite
ideal) radiational cooling. Winds will go calm in the sheltered
Southern Tier valleys where temperatures will be the coolest and
some patchy valley fog may form overnight - though expect lesser
coverage than last night with a notably drier low level environment
in place. Lows will range from around 40 in the Southern Tier
valleys to the upper 40s to around 50 across the lake plains where
weak downsloping will help to limit cooling a bit.

On Saturday, persistent ridging will allow for an outstanding start
to the weekend, with plenty of sunshine and temperatures warming to
a solid 15 to 20 degrees above normal - or more specifically to
the mid and upper 70s. Enjoy!

&&

.SHORT TERM /SATURDAY NIGHT THROUGH MONDAY NIGHT/...
A significant pattern change will take place during this period...so
our pleasant indian summer weather will GRADUALLY give way to wet...
and EVENTUALLY...much cooler conditions. The `cooler` portion of
that pattern change is not anticipated until the long term portion
of this forecast package (after Mon night).

The first of two Pacific shortwaves came ashore in the Pacific
Northwest this morning...and this feature will push a progressive
trough across the intermountain region during the first half of the
weekend. This will sharpen a downstream ridge over the eastern
portion of the country...resulting in beautiful weather that will
persist through the remainder of the weekend and for most areas...
through Monday.

While absolutely gorgeous weather will be experienced across our
forecast area...cyclogenesis in the lee of the Rockies Sunday and
Sunday night will temporarily lead to a split in the westerlies. The
resulting southern stream storm system will hold back the inverted
sfc trough to its north...effectively retarding the eastward
progressive of the lead cold front and its associated shower
activity. As a result... have backed off by some 6-12 hours with the
onset of the showers by not bringing showers into the far west until
late Monday afternoon. Meanwhile...the second Pacific shortwave will
be digging southeast across the Canadian/northern U.S. Plains.

As we progress through Monday night...the second shortwave will
significantly deepen the longwave trough over the nations mid
section. This will lead to a re-phasing of the brief split in the
pattern...with the southern stream storm effectively being `picked
up` and directed northwards towards our forecast area. As one would
expect...the deterministic guidance packages differ on the speed and
track of the energy being ejected from the southern stream
disturbance...but there is general agreement that showers will
become more numerous over our forecast area during the course of
Monday night...with steady rain expected by Tuesday morning over the
western counties. Given that a stalled baroclinic zone will be in
place somewhere between the Niagara Peninsula/Lake Erie and the
Genesee Valley...and that that a plume of moisture rich air within a
strong southerly flow will likely be in place...significant rainfall
will be a possibility (far wrn counties) as we move into the long
term portion of the forecast.

&&

.LONG TERM /TUESDAY THROUGH FRIDAY/...
It will be quite unsettled for the first half of this period...as a
vigorous mid level disturbance digging into a deep longwave trough
will develop a closed low in the vicinity of the Upper Ohio Valley/
Lower Great Lakes. This will keep a feed of rich sub tropical
moisture flowing northwards across our region for Tuesday...while a
strong surface low will pass either directly over our region or just
to our west. In either case...strong jet dynamics aloft will combine
with frontogenetic forcing in the low levels to lift the copious
moisture into moderate to heavy rain. Precipitation amounts should
exceed an inch in many areas for Tuesday...and pending the track of
the aforementioned surface low...amounts could exceed an inch and a
half. While this would certainly promote noticeable rises on smaller
tributaries...the dry antecedent conditions leading into this should
prevent widespread hydro issues. Would not rule out localized
problems though from possible training of any embedded convection.

As we push through Tuesday night...a dry slot should work across our
region within the occluded system. This will taper off the steady
rain to scattered showers from west to east...possibly completely
ending for a time over the western counties.

While the filling upper low will start to work its way across the
Lower Great Lakes on Wednesday...surface based ridging and a lack of
moisture in the mid levels will limit the extent of any showers
activity. The showers that we do develop should be most prevalent
over the far western counties where deeper moisture will be
available to interact with increasing lake instability. The lake
effect rain showers could become more widespread east of BOTH lakes
Wednesday night when the most unstable conditions and sufficient
synoptic moisture will be in place over the lakes. It will also be
much chillier...as H85 temps around zero c will only support
Wednesday afternoon temps that will range from the lower 50s in the
west to the mid to upper 50s east of the Genesee Valley. While this
will indeed by much cooler than the preceding week...these
temperatures will actually be close to where we should be for late
October.

Ridging is forecast to become established over the western counties
on Thursday...but the operational ECMWF is still suggesting that the
longwave trough will close off again...this time in the vcnty of
eastern Pennsylvania. This scenario could enable synoptic moisture
to back in across the eastern Lake Ontario for Thursday and Thursday
night before ridging finally takes hold of the entire region on
Friday.

&&

.AVIATION /00Z SATURDAY THROUGH WEDNESDAY/...
High pressure will largely maintain unlimited VFR conditions
through the period. The one exception to this will be across the
Southern Tier...where localized MVFR/IFR in valley fog will be
possible overnight and Saturday morning.

Outlook...
Saturday night through Sunday...VFR except for local IFR in
late night/early morning river valley fog.
Monday...VFR/MVFR with a chance of showers.
Monday night and Tuesday...MVFR/IFR with showers.
Wednesday...VFR/MVFR with a chance of showers.

&&

.MARINE...
High pressure ridged across the Lower Great Lakes will drift to the
mid-Atlantic coastline by Sunday. Consequently, light to modest
winds and relatively minimal waves will continue through the
weekend. A frontal system is then expected to push though the area
early next week, with strengthening winds and cold advection in its
wake eventually bringing an increasing potential for marine
headlines by either Tuesday or Tuesday night.

&&

.BUF WATCHES/WARNINGS/ADVISORIES...
NY...None.
MARINE...None.

&&

$$

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


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