Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 171018

Area Forecast Discussion
National Weather Service Buffalo NY
518 AM EST Sun Dec 17 2017

High pressure will cross the region tonight and Sunday, with
dry weather for the second half of the weekend. Temperatures
will warm above freezing Monday as a warm front lifts across the
region, bringing a chance for rain and snow showers into
Tuesday before a cold front sweeps across the lower Great Lakes
Tuesday night, bringing a return to more wintry conditions by


Surface analysis this morning shows high pressure centered south of
James Bay along the Ontario/Quebec border, with a ridge extending
south into New York. While this has kept skies mostly clear across
the North Country overnight, allowing temperatures to plunge below
zero in some locales, a band of lower clouds extending from Michigan
across southern Ontario and western New York has kept the mercury
from dropping much more than a couple of degrees overnight. This
band of clouds is associated with a stalled out frontal boundary
draped across the region from northern IL to north-central PA/NJ.

The aforementioned frontal boundary will gradually creep
northeastward as a warm front today in response to backing flow
associated with an upper level disturbance ejecting out of the
southern Plains. This will in turn push the band of low clouds
farther northeast across the forecast area, ensuring a rather gloomy
day for most areas south of Lake Ontario, though large scale
subsidence associated with broad ridging across the region will keep
conditions dry. The cloudy skies will keep temperatures on the cool
side and have backed off highs for today by a few degrees, going
with mid 20s for much of the Thruway corridor, though areas near the
PA border may climb just above freezing by late in the day as the
warm front edges towards the border. Meanwhile, while it will remain
sunny for most of the day across the North Country, the area will
remain far enough back into the cold air that highs will only top
out in the upper teens.

Surface high pressure will move off to our east tonight, while the
warm frontal boundary continues to edge northeast into the forecast
area. A weak clipper approaching the forecast area from the upper
Great Lakes late tonight will interact with this boundary, perhaps
producing enough lift to generate some light snow by daybreak across
the northern half of the forecast area, particularly across the
North Country, but activity looks to be poorly organized at this
time. The combination of increased cloud cover and the warm front
will actually cause temperatures to warm as we move through the
night, with readings climbing into the low 30s across western NY,
and into the low to mid 20s in the North Country.


Monday and Tuesday will bring a short warm up to western and central
NY after experiencing more than 10 days with below normal
temperatures. The above normal temps (reaching into the U30s/L40s
Monday and Tuesday) will come in the wake of a warm front and ahead
of a quasi-zonal flow at 500mb. A leading shallow shortwave trough
passing over the Great Lakes on Monday is forecast to be followed by
a broad but deepening trough on Tuesday and Tuesday night. Each of
these synoptic features will be accompanied by modified Pacific
moisture resulting in continued chance POPs for rain and snow
showers at times although plenty of drytime should also be expected.
An exception will be found for the eastern Lake Ontario and Niagara
Frontier regions where models indicate better forcing and moisture
will combine for likely POPs. Tuesday night or early Wednesday
morning, a cold front is forecast to cross our region supported by
the digging mid-level trough. These features will drive higher POPs
for lake-enhanced rain showers mixing with and changing to lake
effect snow showers. As cold air deepens overnight, temps will dip
back into the 20s overnight.

Lake effect snows are still expected to develop Tuesday night
through Wednesday behind the cold front. Northwest cold air
advection is forecast to drop 850mb temps to around -12/-14C. This
flow would impact locations south/southeast of the lakes with lake
effect snow with the best organized band most likely for Lake
Ontario. Outside of the lake effect, chance/slight chance POPs
remain in between the lake effect areas to cover for stray snow
showers in weaker lake streamers south of the lakes. Have lingered
chance POPs for snow showers south of the lakes into Wednesday. Then
expect some winding down, first on Lake Erie then on Lake Ontario,
as surface-based ridging and drier air build north into the region.
Still too early for forecast snowfall with this potential mid-week
event but lake induced equilibrium levels per GFS BUFKIT rise over
10kft east of Lake Ontario which may bring a few hours of
significant snow accum Wednesday morning. Temperatures will have
returned to a little below average with highs mainly in the upper
20s to lower 30s on Wednesday followed by a cold night with lows
ranging from the near single digits above freezing to lower 20s
Wednesday night.


Dry weather is still expected Thursday with a narrow progressive
surface high shifting over the eastern Great Lakes. 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 Thursday night and 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 in the 30s Thursday 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.


High pressure will move across the region over the next 24
hours, keeping conditions dry. However, low level moisture
associated with a warm front will make its way northeast across
the forecast area. IFR conditions associated with this front are
in place across the Southern Tier, with MVFR cigs to the
northeast across KBUF/KIAG/KROC. This front will gradually slide
northeast during the period, spreading cigs towards KART by the
end of the period.


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 should push winds into SCA 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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