Area Forecast Discussion
Issued by NWS Buffalo, NY

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135
FXUS61 KBUF 051506
AFDBUF

Area Forecast Discussion
National Weather Service Buffalo NY
1006 AM EST Mon Dec 5 2016

.SYNOPSIS...
High pressure over the Ohio Valley this morning will drift to the
Lower Great Lakes this afternoon and evening. This will allow fair
weather to gradually return to the region and persist through at
least the first half of Tuesday. Conditions will once again
deteriorate later Tuesday and Tuesday night when widespread mixed
rain and wet snow can be expected. Daily temperatures will steadily
decrease during the second half of the week when some impactful lake
snows will develop east of both lakes.

&&

.NEAR TERM /THROUGH TONIGHT/...
Synoptically induce precipitation across Eastern Lake Ontario
counties moving east this morning with a band of lake effect snow
forming behind. Will maintain the Winter Weather Advisory for
Jefferson and Lewis counties but drop Oswego as the lake snow will
move farther to the north. Expect an inch or so with a mix of rain
and snow over the lower elevations of the Upper Saint Lawrence
Valley, across Watertown and points north to the river, then another
couple inches of snow on the Tug Hill Plateau.

As we work through tonight...surface high pressure area will build
across our forecast area. This will provide the region with a fair
and uneventful night of weather...although there will be a few hours
of nuisance light rain and wet snow showers southeast of Lake
Ontario.

Temperatures across the forecast area will climb into the upper 30s
to lower 40s this afternoon...then are forecast to settle into the
upper 20s to lower 30s tonight.

&&

.SHORT TERM /TUESDAY THROUGH WEDNESDAY NIGHT/...
During the first half of this period...high pressure initially
draped across our region at the start of Tuesday will withdraw
northeastward into the Canadian Maritimes...while a southern
stream shortwave over the Tennessee Valley races across the Mid
Atlantic States and out to sea. This wave will be accompanied
by a weakening surface trough as it pushes northeastward...with
this latter feature crossing our region between later Tuesday
afternoon and Tuesday night.

While the surface reflection will be in a weakening phase as
it passes through western and north central New York...it will
still be preceded by a surge of deep moisture and decent warm
advective lift. This will be enough to generate another quick
round of widespread light precipitation across our area...with
this mainly coming between mid to late Tuesday afternoon and the
first half of Tuesday night...then quickly tapering off from west
to east through the balance of the night. Marginal temperature
profiles for rain versus snow will result in ptype with this
system being heavily elevation-dependent...with mainly rain
falling across the lower elevations and a rain/snow mix or wet
snow predominating across the higher terrain...where an inch
or two of slushy accumulation will be possible by the time
everything is said and done Tuesday night. Meanwhile...surface
temperatures Tuesday and Tuesday night will average out a little
above normal for early December...with highs in the upper 30s
and lower 40s Tuesday followed by lows mostly in the lower to
mid 30s Tuesday night.

Following the passage of this weak system Tuesday night...
surface-based ridging and drier air over the Ohio Valley will
nose northeastward across New York State during Wednesday...
with the axis of this ridge then sliding eastward to the Mid
Atlantic and New England coastlines Wednesday night. At the
same time...upper level troughing will be steadily deepening
across the Great Lakes and Northeast...resulting in 850 mb
temperatures slowly but steadily cooling to between -6C and
-10C by daybreak Thursday.

The cooling airmass be cold enough to generate a lake response
downwind of the lakes Wednesday and Wednesday night...with this
initially focusing on a 250-260 degree flow Wednesday into
Wednesday evening...before shifting southward during the balance
of Wednesday night as the low level flow veers closer to 270
degrees. This stated...any such response will also be greatly
limited through most of this time frame...as both a low (3-5 kft)
capping inversion and rather limited synoptic-scale moisture will
be in place through much of Wednesday night...before starting to
improve late in the night. With this in mind...do not expect
whatever lake effect activity that forms to actually do all that
much Wednesday and Wednesday night...with any activity on Wednesday
probably also coming in the form of a rain/snow shower mix given
both its limited extent and thermal profiles that will more than
likely be only marginally cold for rain versus snow. It will really
not be until later Wednesday night that lake parameters will begin
to improve and the activity will start to become a little better
organized...by which time it should be slipping southward into
the more traditional snow belts east of the lakes.

With all of the above in mind...have kept PoPs for any lake effect
areas confined to the chance range through most of Wednesday night...
with the remainder of the region more than likely to see largely
dry conditions prevailing. As for temps...we can expect one more
day of near to slightly above average highs in the upper 30s and
lower 40s Wednesday...before readings drop off into the 25-30 range
Wednesday night as our airmass continues to slowly but steadily cool.

&&

.LONG TERM /THURSDAY THROUGH SUNDAY/...
As we have been highlighting in this space for some time now...this
period will feature a major pattern change across our region...as
upper level troughing deepens across eastern North America and drags
much colder Canadian air into our area...with 850 mb temperatures
consequently dropping off into the negative mid teens by Friday and
Saturday. For our area...this will translate into the arrival of
considerably more wintry temperatures...as well as the development
of potentially significant lake effect snows downwind of the Great
Lakes.

Early on in this period...one notable change that has taken place
since this time yesterday is that all guidance has significantly
backed off on the idea of another surface wave developing and
tracking near or over our region during Thursday...with the 00z
ECMWF now also joining the other main guidance packages in this
respect. For our region...the absence of this wave will allow our
airmass to continue to steadily grow colder right through Thursday
and Friday...which in conjunction with significantly improving
background moisture should allow for more significant lake snows
to affect portions of the region from Thursday through Friday...
and potentially into Friday night.

As was the case yesterday...current guidance continues to suggest
that a general westerly to northwesterly flow will be in place
through this time frame...which would tend to direct the bulk of
the lake snows across the ski country east and southeast of Lake
Erie...and from the Tug Hill region to areas southeast of Lake
Ontario. This stated...plenty enough inherent uncertainty in the
exact orientation of the low level wind field still exists at this
time to preclude pinning down the precise locations to be most
affected...potential snowfall amounts...and consequently the
issuance of any watches.

By later Friday night and/or Saturday...the lake snows should be
weakening as high pressure and drier air builds into the region
along with dramatically increasing amounts of shear...with these
likely to give way to a passing synoptic system and the chance
of a much more general light snowfall to close out the period
Saturday night and Sunday.

As for temperatures...these will progressively lower through the
end of the week...with daytime highs in the lower to mid 30s Thursday
dropping off to typical midwinter levels in the mid 20s to lower
30s in time for both Friday and Saturday...before perhaps rebounding
a little by Sunday as the aforementioned synoptic system draws
slightly warmer air back into our region.

&&

.AVIATION /15Z MONDAY THROUGH FRIDAY/...
The worst conditions through daybreak will be found east of Lake
Ontario...otherwise slow improvement can be expected during the
course of the day.

The widespread area of light to occasionally moderate snow that
produced IFR to LIFR vsbys over the far western counties last night
is in the process of pushing northeast across the St Lawrence
Valley. While the steady snow will push away...widespread MVFR cigs
will remain. Much of the southern tier will keep IFR cigs until the
midday hours.

As we push through the afternoon...high pressure will advance to the
western counties from the Upper Ohio Valley. While this will bring
an end to some nuisance lake effect pcpn over the southern tier...
the low clouds will be stubborn to lift to VFR levels. Much of the
moisture will be trapped beneath a staunch subsidence inversion...so
it is possible that the MVFR cigs will hold through the afternoon.

For tonight...mainly VFR conditions can be expected as high pressure
will push over the region. The only area of concern will be between
KROC and KSYR where some MVFR conditions with nuisance lake effect
may persist during the first portion of the night.

Outlook...

Tuesday...Mainly VFR.
Tuesday night and Wednesday...VFR deteriorating to MVFR with some
rain becoming likely.
Thursday and Friday...MVFR/IFR with snow likely.

&&

.MARINE...
A tightening surface pressure gradient between an exiting trough and
high pressure advancing from the Upper Ohio Valley will help to
generate fresh to strong westerlies over both Lakes Erie and Ontario
today. Will thus keep small craft advisories in place for all of the
NY nearshore waters.

Tonight...high pressure will build across the Lower Great Lakes and
allow winds and waves to subside. The only small craft advisories
will be over the eastern two thirds of Lake Ontario where waves will
take a few more hours to drop off.

On Tuesday...the area of high pressure will exit across New England
while a complex frontal boundary will approach from the southwest.
While this will result in freshening southeast winds on Lakes Erie
and Ontario...speeds should remain below SCA criteria for the
nearshore waters and the associated building waves will be confined
to Canadian waters.

Looking further ahead...new guidance suggests that there will be a
lower threat for gale force winds for Thursday and Friday. While
strong westerlies can still be expected...sustained winds are now
more likely to remain below gale force.

&&

.BUF WATCHES/WARNINGS/ADVISORIES...
NY...Winter Weather Advisory until 1 PM EST this afternoon for
     NYZ007-008.
MARINE...Small Craft Advisory until 6 PM EST this evening for LEZ040-
         041.
         Small Craft Advisory until 10 PM EST this evening for
         LOZ043-044.
         Small Craft Advisory until 6 PM EST this evening for
         LOZ042.
         Small Craft Advisory until 10 PM EST this evening for
         LOZ045.

&&

$$

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



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