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Area forecast discussion
National Weather Service Buffalo New York
1258 am EST Fri Dec 15 2017

Synopsis...
low pressure will track into Michigan today and then to New England
Saturday. This will result in periods of lake effect snow during and
below normal temperatures. High pressure will return to the region
Saturday night and Sunday with fair weather.

&&

Near term /through today/...
cold conditions continue across western/central New York tonight with
temperatures dropping quickly outside of the lake effect bands.
Light snow has been ongoing east of Erie/Ontario as high pressure
moves south of the region. Winds will begin to back overnight and
lake snows will shift northward as low pressure moves into the
northern Great Lakes.

Model consensus develops a convergent land breeze on both Lake Erie
and Lake Ontario late tonight. This will result in more organized
lake effect snows which may be enhanced by upstream moisture from
Lake Huron. Model quantitative precipitation forecast is minimal, with the NAM forecasting hardly
any measurable precipitation.

Expect the steadiest snows to be focused near Oswego County
where 3 to 6 inches is forecast. Off Lake Erie, expect 2 to 4
inches in Chautauqua and southern Erie counties. In each case,
the greatest amounts will be near the immediate shoreline due to
the light winds. This is a difficult forecast since band
intensity may vary given subtle differences in moisture and snow
ratio.

On Friday, winds will shift to the south-southwest as the clipper low moves
closer. This will result in a brief burst of snow from Buffalo
to Niagara Falls with 1-2 inches of snow possible Friday morning near
the Lakeshore. A similar story for the Watertown area off Lake
Ontario. After this, bands will mainly impact Canada most of
Friday afternoon. The exception is across western portions of
Niagara County where a few inches of snow is likely depending on
the exact location of the lake effect band. Otherwise, expect
mainly dry weather outside of the lake effect bands.

Otherwise, temperatures will continue to be below normal tonight
with lows ranging from the single digits to mid teens. Highs on
Friday will 'warm' slightly into the 20s.

&&

Short term /tonight through Sunday/...
Friday evening will become very active again as another clipper
passes by just north of Lake Ontario. The clipper will do little by
itself, with just some light snow and less than an inch of
accumulation outside of lake effect/lake enhanced areas.

More importantly, this clipper will produce a much more favorable
environment for heavy lake effect snow. The increased moisture,
synoptic scale ascent, and fresh batch of cold air aloft will raise
lake induced equilibrium levels to over 15k feet off both lakes near
the time of the cold frontal passage. Deep moisture and a deep
dendritic Crystal growth zone will support favorable cloud
microphysics. Convergence will Max out as the cold front crosses
each lake, and the frontal convergence combines with pre-existing
convergence along bands of lake effect snow already over the lakes.
These scenarios often produce a burst of very heavy snow with
snowfall rates of over 2 inches per hour along with thunder and
lightning, although they typically move quickly across the area as
wind direction changes quickly with the cold frontal passage.

Off Lake Erie...

Late Friday afternoon a band of lake effect snow will be found
across the Niagara peninsula, extending across Grand Island and into
Niagara County on south-southwest flow. As the front crosses the area, it will
merge with the lake effect band and intensify it further, with the
band moving fairly quickly south across the Buffalo Metro area. This
may occur during the latter half of the Friday evening commute, and
produce very poor travel conditions for an hour or two. By late
evening the band of snow will move all the way into the western
southern tier. It will initially still be strong when it first
arrives in the southern tier, with snowfall rates then backing off
overnight but still producing steady accumulations. The lake effect
snow will remain across the western southern tier and possibly
southern Erie and Wyoming counties through Saturday morning, before
increasing shear with the approach of a warm front disrupts the band.

As far as accumulations go, expect 3-6 inches across portions of
northern Erie and Niagara counties, with much of that falling in
just a 2 hour period during the early evening. Farther south, there
will be an initial quick burst of heavy snow when the bands,
followed by more persistent lake effect snow overnight into Saturday
morning. This may produce totals of 6-10 inches in persistent bands,
although there is some uncertainty on whether bands will be able to
remain over one location long enough to reach warning criteria. With
this in mind, we went with a Winter Storm Watch for lake effect snow
east of Lake Erie for Friday evening through early Saturday
afternoon.

Off Lake Ontario...

Late Friday afternoon a band of lake effect snow will be located on
the Canadian side of the border near Kingston, Ontario in south-southwest flow.
The front will arrive by late evening and merge with the lake effect
band. This will carry an intensifying band of lake effect snow south
and east across Jefferson County. This band will then further
intensify as it moves into the Tug Hill region, with some of the
mesoscale model guidance suggesting a short period of very high
snowfall rates (3-4 inches per hour) for the Tug Hill during the wee
hours of the morning Saturday. Given a very high cap and a
relatively deep mixed phase layer, have added the likelihood of
thunder and lightning with this activity. This will especially be
the case on the Tug Hill where higher terrain aids in the ability
for cloud to ground strikes. On Saturday morning the band will move
a little farther south into northern or central Oswego County and
begin to weaken, with further weakening by midday as shear increases.

As far as accumulations go, for central and northern Jefferson
County accumulations will be limited by the fast band motion, with 2-
4 inches for Watertown and points north and east, although this may
mostly fall in 2 hours. The band, and associated upslope flow will
likely last longer across the Tug Hill plateau. The longer residence
time combined with potentially very high snowfall rates may produce
8-12 inches accumulation in persistent bands, centered on the Tug
Hill. With this in mind, we went with a Winter Storm Watch for lake
effect snow east of Lake Ontario for Friday evening through early
Saturday afternoon.

Once the lake effect snow diminishes Saturday, the 12z model
guidance has made some significant changes for Saturday. A trend
that was first captured by the 06z GFS and 00z European model (ecmwf) is now shown by
most model and ensemble guidance. The latest models show warm
advection increasing rapidly across the central and eastern Great
Lakes Saturday as a weak wave moves through the Midwest. This warm
advection will spread abundant cloud cover, and possibly a streak of
light snow across western and central New York during the day Saturday
into Saturday evening. Given the big change from previous model
guidance, just introduced a chance of snow showers for now, but if
this trend continues, probability of precipitation will need to be increased.

Saturday night and Sunday the warm front will continue to creep
northeast across the region, with any remaining light snow ending as
forcing along the frontal zone weakens. Sunday should remain dry,
with the start of a modest warm up as temperatures rise into the mid
30s for the lower elevations of western New York.

&&

Long term /Sunday night through Thursday/...
for the start of next week, model solutions point to a
deamplification of the pattern, as upstream blocking degrades. This
will allow for a more progressive flow of Pacific influenced air
across the Great Lakes region. Expect a warming trend from Sunday
into Monday and Tuesday as upper-level ridging builds across the
eastern Great Lakes. The thermal ridge axis transits the forecast
area on Tuesday as the surface high slips to our east and next wave
moves into the upper Great Lakes, which will push 850 mb
temperatures briefly above the freezing mark Tuesday. Thus high
temperatures slowly improve from the 20/30s on Sunday, to the 30s to
near 40 on Monday, and right around 40 on Tuesday. Precipitation
wise, a southern stream clipper along the warm front may produce
some scattered showers Sunday night. The nearby retreating warm
front boundary combined with weak wave passages in the progressive
flow will then keep some scattered light shower chances in the
forecast through Monday and Monday night. A stronger weather system
will then plow a cold front across the region by Tuesday night,
bringing both showers ahead of the front and lake effect snows and
much cooler air behind the front for mid-week.

&&

Aviation /06z Friday through Tuesday/...
Lake effect snow showers will consolidate into bands east of the
lakes tonight...but this will only impact the kjhw taf site. Winds
will then shift to the southwest later tonight with lake snows
lifting northward to buf/iag/Art by Friday morning. This will result
in a few hours of IFR or lower conditions.

South-southwest winds will push lake snows mainly north and west of all taf
locations Friday afternoon, with the exception of iag which
will be close to these snow bands.

Outlook...

Friday night...VFR with IFR in lake effect snow. Expect a few hours
of heavy snow at buf/iag/Art Friday evening, then lake effect should
move to the south.
Saturday...mainly VFR. Lake effect snow showers mainly away from
taf sites.
Saturday night and Sunday...VFR.
Sunday night through Tuesday...VFR/MVFR with a chance of snow
showers.

&&

Marine...
Small Craft Advisory conditions will remain in place for the Lake
Ontario nearshore waters through the first half of tonight...then a
weakening surface pressure gradient will allow winds and waves to
subside.

While relatively light winds will be in place late tonight and early
Friday across the lower Great Lakes...an approaching area of low
pressure will encourage winds to freshen Friday afternoon and
evening. The increase in winds will be accompanied by renewed lake
snows...which will be heavy on Lake Erie and on the east end of Lake
Ontario.

&&

Buf watches/warnings/advisories...
New York...Lake effect snow warning from 10 PM this evening to 10 am EST
Saturday for nyz007.
Winter Weather Advisory until 7 am EST this morning for nyz006.
Lake effect snow warning from 10 PM this evening to 1 PM EST
Saturday for nyz006-008.
Winter Weather Advisory from 11 am this morning to 10 PM EST
this evening for nyz001.
Winter Storm Watch from this evening through Saturday
afternoon for nyz012-019-020-085.
Winter Weather Advisory from 4 PM this afternoon to 1 am EST
Saturday for nyz010.
Marine...Small Craft Advisory until 7 am EST this morning for
loz043>045.
Small Craft Advisory until 1 am EST early this morning for
loz042.

&&

$$

Synopsis...apffel
near term...apffel/hsk/rsh

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