Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 061142

Area Forecast Discussion
National Weather Service Buffalo NY
642 AM EST Tue Dec 6 2016

High pressure over eastern New York will exit across New England
during the course of the day...while another complex frontal system
will approach us from the Ohio Valley. This will lead to
deteriorating conditions this afternoon and evening as another round
of mixed precipitation will move across our region through tonight.
Colder air will then gradually deepen across the Lower Great Lakes
during the second half of the week with impactful lake snows
gradually becoming established east of both lakes in the process.


High pressure centered over eastern New York this morning will
provide all of our forecast area with fair weather through at least
lunchtime. Mid and upper level cloud cover will then increase from
the southwest during the course of the midday and afternoon as
another complex frontal system will approach our region from the Ohio
Valley. While benign conditions will persist through the remainder
of the day across the Eastern Lake Ontario region...a 50kt low level
jet will quickly push the leading edge of a mixed precipitation
shield across the western counties.

This precipitation will race north across the southern tier by mid
afternoon. Initially...thermal profiles from both the GFS and NAM
suggest that the pcpn will be in the form of rain or wet snow...
but with dynamic cooling (evaporation cooling and cooling due to
melting)...this pcpn will change to a couple hours of just snow.
The pcpn could remain in the form of snow across the elevated
terrain...but expect at least a mix in the valleys. Further north
across the lake plains...the pcpn will arrive very late in the
afternoon/towards dinner time. The lowest few thousand feet of the
boundary layer should be warmer than that of the Srn am
expecting just mixed pcpn...if not mainly rain. An inch or two of
snow could accumulate by nightfall across the Southern Tier...again
mainly across the higher terrain.

The wavy frontal boundary will advance to the north across our
forecast area tonight with widespread mixed precipitation for all.
Again...the character of the pcpn will largely depend on elevation
as a slushy inch or two of accumulation will be possible across the
higher terrain of the Srn Tier and also across the Tug. As the mid
and upper level moisture gets stripped away in the wake of the front
over the western counties...the steadier pcpn will give way to some
light rain/drizzle and fog.

In regard to will be relatively least
across the lake plains. The mercury will climb to the mid 40s across
the lake plains west of Syracuse and in the Genesee Valley...while
readings will top out in the upper 30s to near 40 across the
Southern Tier and Eastern Lake Ontario region. Tonight...mins will
generally range from the low to mid 30s.


...Significant lake effect snows expected east and southeast of
Lakes Erie and Ontario Thursday through Friday night...

During the first 24 hours of this period...surface-based ridging
and drier air over the Ohio Valley will quickly 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.

While the cooling airmass will certainly be cold enough to generate
a lake response downwind of the lakes Wednesday and Wednesday night...
this will be very meager both through the day Wednesday and a sizable
chunk of Wednesday night owing to the presence of a very low (3-5
kft) capping moisture...and some directional shear.
Thus...even with the low level flow varying between 260 and 240
degrees and consequently aligned near or along the major axes of
both lakes...any lake response will likely not manage to do all
that much through at least the first half to two thirds of Wednesday
night...with the activity also likely coming in the form of a rain/
snow shower mix during Wednesday given both its limited extent and
marginal temperature profiles.

Later Wednesday night...both moisture levels and capping inversion
heights will begin to rise as the flow starts to veer a little again...
which should allow the lake effect to begin to get somewhat better
organized by daybreak Thursday. This should result in snow becoming
more likely off both lakes...with developing lake bands likely aligning
themselves across the immediate Buffalo southtowns off Lake Erie and
between the northern reaches of the Tug Hill and the Watertown area
east of Lake Ontario...where some accumulations of an inch or so will
be possible by Thursday morning.

After that things will get much more interesting through the remainder
of this deep upper level troughing slowly makes its way
across the Great Lakes and Northeastern States. The broad cyclonic flow
associated with this trough will pull progressively colder Canadian air
across our region...with 850 mb temps falling off into the negative mid
teens by later Friday and Friday night. This will lead to the development
of strong instability over the lakes...with lake equilibrium levels
surging to 18-20 kft. Coupled with the arrival of much deeper moisture...
this will set the stage for a much stronger lake that
will likely feature intense snowfall rates at times.

With respect to the placement of the lake snows...a general 250-260
degree flow should be in place at the start of Thursday morning...
then will gradually veer to 260-270 degrees through the course of
Thursday...before turning more northwesterly Thursday night following
the passage of a surface trough. After that time...a general west-
northwesterly to northwesterly flow will predominate both Friday
and Friday night.

As a result of the above...lake snows initially situated across the
Buffalo Southtowns off Lake Erie and between Watertown and the Tug
Hill will slowly drift south and become increasingly better organized
through the day on Thursday...with this activity settling across the
Ski Country east of Lake Erie and the Tug Hill east of Lake Ontario
by late Thursday or early Thursday which time it should
be packing a considerably more substantial punch. The passage of
the aforementioned trough Thursday night will then help to further
enhance the activity and send it further southward into the Southern
Tier off Lake Erie and into the Monroe-Oswego county corridor off
Lake Ontario...where it should then linger through the end of the
short term period.

Given increasing forecast confidence in both a forecast environment
conducive for significant lake effect snows and in the placement of
the main lake plumes...we have reached the point where it is necessary
to begin issuing lake effect watches. At this point we will start
with those areas that look to be affected first and where confidence
in significant accumulations is highest...which are Southern Erie...
Wyoming...Chautauqua...and Cattaraugus counties off Lake Erie...and
Oswego...Jefferson...and Lewis counties off Lake Ontario. For these
areas...lake effect snow watches have been hoisted as outlined in the
watch/warning/advisory section below. While we have held off elsewhere
for is likely that additional watches will also ultimately
become necessary for at least portions of the area southeast of Lake

Outside of the main lake effect bands...the remainder of the region
can expect much more scattered snow showers at times between Thursday
and Friday night...with mainly dry weather otherwise prevailing. With
the arrival of progressively colder air...temps will drop off to typical
midwinter levels by the end of the week...with daytime highs only reaching
to between 25 and 30 on Friday...and nighttime lows then dropping into
the 10-20 range Friday night.


By the time we reach Saturday morning...the lake snows should be
steadily weakening off both lakes as high pressure and drier air
build into the region...and as shear increases under the advancing
ridge. This stated...a general backing of the low level flow will
probably still send the remnants of these bands back north across
the Buffalo and Watertown areas Saturday afternoon and evening...
before these die out altogether Saturday night as the low level flow
becomes too southerly.

After that...our attention will turn to the next synoptic system...
which the medium range guidance suggests will cross our region
during the Sunday to Monday time frame. This system still looks to
bring us our next general chance of precipitation...which will probably
come in the form of a rain/snow mix again given that it should
ultimately draw somewhat warmer air back into our region.

With respect to temperatures...midwinter-like highs in the mid 20s
to lower 30s on Saturday will probably rebound some for both Sunday
and Monday given the aforementioned influx of somewhat warmer air...
with readings probably recovering into the upper 30s and lower 40s
by Monday.


High pressure will provide VFR conditions over the region through at
least midday...then mid and high level clouds will advance and
thicken across the western counties during the midday and afternoon.
Mixed precipitation associated with the thickening cloud cover will
start across the Southern Tier later this afternoon with conditions
rapidly deteriorating to IFR to MVFR levels.

As the mixed pcpn spread northeast across the remainder of western
and north central New York early tonight...VFR conditions will
quickly drop to at least MVFR. MVFR to IFR conditions will be likely
after midnight regionwide.


Wednesday...Improvement to VFR for most areas...although this trend
will be delayed east of Lake Ontario.
Thursday through Saturday...MVFR/IFR with lake effect snow.


High pressure over eastern New York this morning will exit across
New England during the course of the day. While this will result in
light to gentle easterlies across the Lower Great Lakes to start the
day...a tightening pressure gradient ahead of an approaching complex
frontal boundary will encourage winds to freshen during the midday
and afternoon. This should not be a major concern the
east to southeast flow will confine the highest waves to Canadian

Tonight...the wavy frontal boundary will push north across the Lower
Great Lakes. Veering winds on both lakes will be accompanied by a
further increase in speeds...which could lead to small craft
advisory conditions for the Lake Erie nearshore waters by daybreak.

On Wednesday...strong west to southwest winds will produce small
craft advisories across all of the NY nearshore waters...particularly
on Lake Erie and for sites north of Mexico Bay on Lake Ontario.
These small craft advisories could very well persist through at
least Thursday...particularly for the areas outlined above.


NY...Lake Effect Snow Watch from Thursday morning through late
     Thursday night for NYZ007-008.
     Lake Effect Snow Watch from Thursday morning through late
     Friday night for NYZ006-012-019-020-085.



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