Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 171943

Area Forecast Discussion
National Weather Service Buffalo NY
243 PM EST Sun Dec 17 2017

High pressure will move east of the region tonight. Temperatures
will warm above freezing Monday as a warm front lifts across the
region, bringing a chance for rain and snow showers into Tuesday.
A cold front will sweeps across the lower Great Lakes Tuesday night,
bringing a return to more wintry conditions by Wednesday.


Surface high pressure will move off to our east tonight, while the
warm frontal boundary across the Ohio Valley continues to slowly
advance northeast into the forecast area. A weak mid level wave will
approaching the forecast area from the upper Great Lakes late
tonight and interact with this boundary. There maybe enough lift to
bring some light snow late tonight across the northern half of the
forecast area, particularly across the North Country. The
combination of cloud cover and weak warm air advection will bring a
non-diurnal temperature trend overnight, with readings climbing into
the lower to mid 30s across western New York, and into the lower to
mid 20s in the North Country.

The warm front will get hung up across the forecast area on Monday
and may even work back to the southwest as a backdoor cold front
late in the day. Expect spotty precipitation throughout the day with
the best chances for any light snow accumulations across the North
Country in the morning. Boundary layer temperatures will warm enough
for some rain elsewhere. Highs will be near 40 degrees across
western New York to the mid 309s across the North Country.


Monday night a warm frontal boundary draped from Lake Ontario to
the North Country at the start of the night will continue to slowly
push northeastward and out of our area...though our region will
still remain under the influence of broad warm air advection out
ahead of a modest mid level trough/attendant surface low pushing
into the Upper Great Lakes. This will keep plenty of cloud cover in
place along with a continued potential for some light precip...
though with available forcing looking to be rather weak and diffuse
this should be rather spotty...with the best precip chances confined
to the North Country which will lie closest to the departing warm
front. As has been the case for the past two days...simple pattern
recognition...statistical guidance....and model thermal profiles all
suggest that the bulk of whatever precip there is will be in the
form of plain rain...thanks to relatively mild temps in both the
boundary layer and in the main cloud bearing layer...which will
remain too warm to support the formation of ice crystals. About the
only real chance for snow will lie across the North Country...where
temps may be initially cold enough to support more of a rain/snow mix
before slow but steady warm advection forces a changeover to all rain
overnight. Given the steady warm air advection regime...low temps will
occur fairly early in the evening and will range from the lower to mid
30s east of Lake Ontario to the mid and upper 30s elsewhere...before
yielding to steady or slowly rising temperatures overnight.

On Tuesday the aforementioned mid level trough/surface low will make
their way further eastward...and will eventually push their attendant
cold front across our area during the afternoon hours. With only limited
to modest forcing available...expect any precip out ahead of and along
the front to remain largely scattered. The antecedent mild airmass in
place out ahead of the front will result in milder temperatures and a
ptype of all rain...with highs reaching into the lower to mid 40s
areawide...and much of the Genesee Valley/Finger Lakes region even
making a run into the upper 40s. Otherwise the tightening pressure
gradient out ahead of the front will result in an increasingly breezy
to windy day...with the short term guidance packages having trended
stronger with the low level wind field compared to yesterday. As of
this point it now appears that most areas should see wind gusts to
30-35 mph...with the strongest overall winds found downwind of Lake
Erie...where the combination of funnelling effects and a stronger flow
aloft should result in gusts to the 40-45 mph range. The 12Z NAM (which
has a notably stronger low-level wind field and brings the stronger
winds closest to the surface) would even suggest the potential for
some lower-end advisory-range gusts within this latter area...though
with a general warm air advection regime in place and other guidance
remaining notably weaker with the low-level wind field...confidence
in this is not yet high enough for inclusion in the HWO. Nonetheless...
this possibility will be something for us to keep a close eye on over
the next day or so.

Following the frontal passage...a westerly to west-northwesterly flow
of colder air will overspread our region Tuesday night and Wednesday.
Given current lake surface temperatures of around +4C...this airmass
will eventually become cold enough to support a lake response by late
Tuesday evening off Lake Erie...and during the second half of Tuesday
night off Lake Ontario.

Digging a bit further into the forecast Lake Erie the
overall lake response continues to appear rather muted by a general
lack of deep background moisture through well as rather
low inversion heights ranging between only 3 and 5 thousand feet. This
should keep any activity fairly scattered and light in nature...with
any lake snows likely to end altogether Wednesday evening as high
pressure and drier air build into the region.

Meanwhile off Lake Ontario...conditions still seem to be at least
somewhat more favorable with somewhat deeper moisture and higher
inversion heights becoming available for a time between later Tuesday
night and early Wednesday...though compared to yesterday this period
of more favorable conditions also appears to be shorter and accompanied
by lower inversion heights than what had been previously projected.
Given this...conditions still appear to be less than ideal for an
overly significant lake response...and thus unworthy of anything higher
than some likely PoPs and modest snowfall accumulations at this vantage
point...with the bulk of these confined to between later Tuesday night
and Wednesday morning. After that time...the lake snows should steadily
wind down Wednesday afternoon and night thanks to the arrival of the
aforementioned ridging and drier air.

Outside of the lake effect areas...the Tuesday night-Wednesday night
time frame should be largely dry...with all areas seeing a return to
colder temperatures for midweek. Speaking more specifically...lows on
Tuesday night will settle back to between the mid 20s and lower 30s
in the wake of the cold front. Highs on Wednesday will then struggle
to get much above the lower to mid 30s...with readings south of Lake
Ontario then largely falling back to the teens and lower 20s Wednesday
night...while the North Country sees readings settle back to around
10 above.

Moving on into the last 24 hours of this period...on Thursday the
surface ridge will slide eastward across our region...and will bring
our region a dry and quiet day...along with high temps ranging from
the lower to mid 20s across the North Country to the lower to mid
30s south of Lake Ontario. The high will then slide eastward into
New England Thursday night...while a strengthening low pressure
system develops into the central Mississippi Valley. Developing
warm air advection out ahead of this system and its advancing warm
front will spread increasing amounts of cloud cover back across our
region...with perhaps a little spotty light wintry precipitation
developing overnight...though a consensus of the medium range
guidance would keep this largely confined to areas from Lake Ontario
and the North Country northward...where the airmass would more
than likely remain cold enough for primarily snow. Otherwise...
low temps will range from the teens east of Lake Ontario to the
20s elsewhere...with readings likely tending to rise in the warm
advection regime overnight.


The main focus for weather late week into next weekend will be on
what 00z models are showing as a strengthening low pressure system
crossing the Great Lakes just west of New York. Deep southerly flow
and moisture forced on the nose of a leading low level jet would
likely bring back warmer temps and perhaps some mixed wintry
precipitation changing to plain rain. Likely POPs continue in the
forecast for Friday with the global models in good agreement in
tracking this low. While precip is most likely to start as some snow
before switching to plain rain, 00z GFS temp profiles show a warm
nose may override sub-freezing surface temps bringing a threat of
perhaps freezing rain Friday morning until surface temps warm above
freezing. Have included chance for snow/fzra where warm nose temps
exceeded +1C. Not expecting any significant damaging winds with this
system at this time as the low level jet will remain decoupled above
the surface in warm advection. The storm center will past west and
north of New York where an associated cold front should shift east
across our forecast area Saturday. Cold air should then return
behind the front. Highs should reach into the 40s Friday with rain
then dip back down into the 30s behind the front Saturday. Overnight
temps coldest Thursday morning will become more mild by Saturday
morning ahead of the cold front.

Looking further down the road...yesterdays medium to long range
guidance is looking very interesting for the period from Christmas
to New Years. For a few model runs now...this longer range guidance
has been hinting at a return to notably colder weather for the Great
Lakes region. A closed low over Siberia is forecast to break down in
the coming days with a chunk of its energy being ejected out across
the Kamchatka Peninsula and northern reaches of the Pacific ocean.
Eventually...this very robust bundle of mid level energy is forecast
to help carve out a full latitude trough over the heart of North
America. While highly anomalous ridging off the West coast would
extend from 20N all the way to the Pole...a large portion 0f the
polar vortex would settle south to northern Ontario. This newly
phased pattern would not only include a cross polar flow...but more
importantly would allow H85 temps of <-30c within the vortex to make
their way across the northern plains.

While this long range forecast would place the coldest air of this
outbreak over the plains states...the air would eventually make its
way to the Ohio Valley. Climatologically...this is exactly where it
should be to support significant southwest flow lake effect for our
region. This would also come at a time (between Christmas and New
Years) when impactful southwest flow lake storms seem to be most
common. In a nutshell...the week or so from Christmas to New Years
should feature colder than normal weather with the hint for
substantial lake snows. Stay tuned.


Low-level moisture associated with a slow-moving frontal boundary
over northern OH/PA remains trapped underneath a subsidence
inversion associated with surface/mid-level ridging moving across
the region this afternoon is producing IFR/LIFR conditions in the
Southern Tier, including KJHW. Conditions should gradually improve
to at least to MVFR by mid afternoon, however IFR conditions will
likely return tonight, as the frontal boundary and moisture will
remain in the area. Elsewhere, expect current VFR cigs to fall into
MVFR territory after 18-00Z as the frontal boundary and associated
moisture gradually lifts northeastward across the forecast area.


Monday...IFR/MVFR with a chance for rain and snow showers.
Tuesday...IFR/MVFR with rain likely.
Wednesday...IFR in lake effect snow likely SE of the Lakes, MVFR/VFR
with a chance of snow elsewhere. Thursday...Mainly VFR.
Friday...VFR/MVFR with a chance for rain or snow showers.


Generally light northeasterly winds will give way to freshening
southwest winds on Lake Erie tonight as a warm front slowly moves
northeast into the lower Great Lakes and a weak clipper approaches
the region. Winds will flirt with small craft advisory criteria for
a brief time tonight as the disturbance passes by, but should remain
just below advisory levels.

Southwesterlies will strengthen further Monday night however, as a
deep area of low pressure centered well to our north crosses Hudson
Bay. THe resulting tightening of the pressure gradient across our
region sending winds into small craft advisory territory across many
zones Monday night into Tuesday, with winds staying brisk as they
veer to the northwest with a passing cold front Tuesday night.





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