Area Forecast Discussion
Issued by NWS Buffalo, NY

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000
FXUS61 KBUF 061532
AFDBUF

Area Forecast Discussion
National Weather Service Buffalo NY
1032 AM EST Tue Dec 6 2016

.SYNOPSIS...
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.

&&

.NEAR TERM /THROUGH TONIGHT/...
A messy mix of rain and snow will overspread the region this
afternoon and evening. This will bring up to a few inches of slushy
wet snow to the higher terrain of the western Southern Tier and Tug
Hill, while the lower elevation lake plains may see some flakes
mixed in, but no snow accumulation.

Water vapor satellite imagery shows a shortwave trough over eastern
KY and TN tracking northeastward toward New England. Meanwhile
surface analysis shows a weak surface high over New England keeping
cooler surface temperatures in the 30s feeding into western NY on an
easterly wind. As this wave of low pressure tracks northeastward
this afternoon and evening, the large swath of precipitation along
the band of frontogensis, vorticity advection and warm air advection
will overspread the forecast area. BUFKIT thermal profiles from the
NAM and GFS reveal a very marginal setup for wet snowfall with a
deep nearly isothermal layer up through 10kft just below freezing.
Thus snow vs rain will be determined by both elevation and
precipitation intensity, both of which will help to get the boundary
layer temperatures just at or slightly below freezing. Where the
combination of elevation and precipitation intensity line up, as in
the western Southern Tier and the Tug Hill, expect a slushy 1 to 3
inches (2 to 4 inches on the highest hills). Otherwise, the lake
plains locations may see a brief change over to snow when
precipitation intensity increases (dynamical cooling), however, will
likely change back to rain as precipitation lightens up. This
combined with likely marginal surface temperatures stay just above
freezing (33 to 34) should prevent any snow accumulations across the
lake plains (including Buffalo, Rochester and Watertown). As far as
timing is concerned, the western Southern Tier can expect
precipitation starting between 2 and 4 this afternoon, and ending
just after midnight. The Genesee valley starting between 5 and 6pm,
and ending after midnight. The North Country starting between 7 and
9pm and tapering off to light showers by sunrise.

Also of note will be some strengthening southeasterly winds along I-
90 corridor between the PA line and southern Erie County, resulting
in gusty cross winds. Downslope winds off the Chautauqua ridge will
increase after sunset this evening, with wind gusts 40 to 45 mph
possible. It appears this will fall short of Wind Advisory criteria
at this time, and thus will hold off on any headlines. These
stronger gusts will decrease after midnight as the flow turns more
southwesterly by Wednesday morning.

&&

.SHORT TERM /WEDNESDAY THROUGH FRIDAY NIGHT/...
...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 inversion...limited 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 period...as 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 response...one 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 evening...by 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 now...it is likely that additional watches will also ultimately
become necessary for at least portions of the area southeast of Lake
Ontario.

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.

&&

.LONG TERM /SATURDAY THROUGH MONDAY/...
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.

&&

.AVIATION /16Z TUESDAY THROUGH SATURDAY/...
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 or 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.

Outlook...

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.

&&

.MARINE...
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 though...as the
east to southeast flow will confine the highest waves to Canadian
waters.

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.

&&

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

&&

$$

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


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