Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 181813

Area Forecast Discussion
National Weather Service Buffalo NY
113 PM EST Mon Dec 18 2017

A mix of wet snow and some freezing rain for the North Country will
mostly change to rain by late this evening, with rain and drizzle
elsewhere through tonight. It will become windy on Tuesday,
especially northeast of the lakes with a few more rain showers or
some drizzle. The mild temperatures will last through Tuesday before
a cold front brings temperatures back to average for the middle of
the week. Another surge of warmer air will arrive for Friday and


Radar imagery showing the most widespread precip over and east of
Lake Ontario. Surface obs show a variety of precip east of Lake
Ontario with snow in most areas east of Interstate 81, while areas
closer to Lake Ontario are changing to rain. There has been a few
brief periods of freezing rain during the transition, including in
Watertown. For most areas the freezing rain will be brief. The one
exception will be near the Saint Lawrence River, where northeast
flow is locked in for the valley and draining low level cold air
into the Thousand Islands region. In this region, the snow will
change over to freezing rain later this afternoon, and likely
continue through late evening until increasing southwest flow scours
out the low level cold air. With this in mind, a Winter Weather
Advisory remains in place for Jefferson County through this evening,
with the main concern being the Thousand Islands region.

Otherwise for the rest of the area, temperatures are well above
freezing and will remain that way through tonight. Most of the
forcing for ascent associated with isentropic ascent will remain
from Lake Ontario northward, but abundant low level moisture will
continue to produce areas of light rain and drizzle this afternoon
through tonight. Despite the low level moisture and increasing
dewpoints over snowpack, it will likely become too windy by later
tonight for any fog concerns.

On Tuesday low pressure will pass across northern Ontario and
Quebec, with a trailing cold front crossing our region late in the
day. Most of the forcing will remain well north of the Canadian
border, but abundant low level moisture will continue to support
drizzle and a few scattered showers at times. The rain will begin to
change  back to snow late in the day across the higher terrain east
of Lake Ontario as cold air begins to move back into the region in
the wake of the cold front.

It will become quite windy on Tuesday as the strong low passes
through northern Ontario and Quebec, tightening the pressure
gradient across the eastern Great Lakes. A 45-50 knot low level jet
will cross the area. There is an inversion in place at about 3-4K
feet, but it appears there is enough wind below the inversion to
support some modest mixing of stronger momentum to the surface
despite the marginal lapse rates. Funnelling down Lakes Erie and
Ontario often adds 10 knots onto gusts for the Niagara Frontier and
near Watertown compared to surrounding areas, and this appears to be
the case for Tuesday as well. With this in mind, we have issued a
wind advisory in the typical areas northeast of Lakes Erie and
Ontario where wind gusts may reach 50 mph. Elsewhere expect gusts of
around 40 mph on Tuesday.

Temperatures will remain mild, with lows in the mid to upper 30s
tonight in most areas, and highs in the low to mid 40s on Tuesday.
The combination of somewhat higher dewpoints, mild temperatures, and
wind will allow significant melting of the snowpack, but with very
little rain, there are no flood concerns.


Lake effect and more scattered upslope flow snows are still
forecast to develop Tuesday night into Wednesday as 850mb temps
cool behind the cold front. Northwest cold air advection is
forecast to drop 850mb temps to around -12C Wednesday morning
during the time where lake snows will be most organized. Better
background moisture and higher equilibrium levels (to 15kft)
will allow the better organized lake effect snows to occur
east/southeast of Lake Ontario. Less favorable parameters
southeast of Lake Erie keep POPs capped in the chance range.
Have lingered likely POPs for Lake Ontario and chance POPs for
Lake Erie into Wednesday before winding down Wednesday night as
surface-based ridging lowers inversion heights and drier air
shifts into the region. Forecast snow totals for this event will
be more a nuisance than a shovelable snow. Have only gone with
an inch or less southeast of Erie and 2-4 inches across Oswego
and southern Lewis counties east of Lake Ontario. 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 single digits above freezing to
teens Wednesday night.

A progressive surface high shifting over the eastern Great Lakes
Thursday supports the dry forecast. North and West of NY, models
show a warm front shifting into southwest Ontario. Look for mainly
cloudy skies with temps on the cool side topping out in the 20s
(eastern Lake Ontario region) to low 30s (WNY). The main focus will
be on the strengthening low pressure system to our west crossing the
Great Lakes. Deep southerly warm advection and moisture forced on
the nose of a leading low level jet will bring back warmer temps but
the bigger concern would be for perhaps some mixed wintry
precipitation late Thursday night before changing to plain rain
Friday. 00z GFS temp profiles show a warm nose may still override
sub-freezing surface temps at the onset of precipitation bringing
the threat of freezing rain until surface temps warm above freezing.
Have included chance for -fzra where warm nose temps exceeded +1C
and surface temps are still less than 33.


Near 1000mb surface low is forecast to cut just north and west of
western NY on Friday. Chance of mixed precipitation/freezing rain
lingers where a warm nose shifts across our region Friday morning.
Expect chances for plain rain by Friday afternoon as surface temps
warm above freezing. Likely POPs have been pushed to Friday night
with a slower timing for the arrival of a cold front extending south
from the low. The storm center will past just north and west of New
York where an associated cold front should shift east across our
forecast area Saturday. Cold air should then return behind the front
with rain changing back to chance of rain/snow later Saturday then
chances for snow showers through Sunday/Christmas Eve beneath cool
cyclonic flow aloft. High temps should reach into the 40s Friday
with rain then dip back down into the 30s behind the front for the
weekend. Overnight temps will be coldest Friday and Sunday mornings
with more mild readings Saturday morning ahead of the cold front.

Winds may again become gusty late Friday in the wake of the
cold front associated with low pressure passing to our
northwest. A 50kt low level jet will accompany the front with
the strength of the jet in the wake of the low still unclear. If
a jet lingers as the 00z GFS shows then mixing in cold air
advection may support advisory gusts.

Looking ahead to the period between Christmas and New Years...the
long range guidance continues to suggest that the coldest airmass of
the season to this point will dive south into the heart of the
country. Energy ejecting out of a closed storm system 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 temperatures
of <-30c within the vortex to make their way across the northern

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.


For most areas, light rain and some drizzle will continue at times
through tonight with abundant low level moisture in place and a weak
system passing by just north of Lake Ontario. Across the North
Country, snow will gradually change to rain this afternoon and
evening in most areas. Some freezing rain will fall through mid
afternoon at KART until the temperature rises above freezing. The
freezing rain will last into the evening for areas near the Saint
Lawrence River.

With the abundant low level moisture, widespread MVFR to IFR CIGS
will continue this afternoon and tonight. On Tuesday, expect more
MVFR CIGS across lower elevations and IFR CIGS primarily across
higher terrain. VSBY will be MVFR to IFR in the light rain and
drizzle, and also in the snow for the North Country.

It will become quite windy on Tuesday, especially northeast of the
lakes where gusts may reach 40 knots.


Tuesday Night...Mainly VFR except localized IFR in lake effect SHSN
east of Lake Ontario.
Wednesday...IFR in lake effect snow likely SE
of the Lakes, MVFR/VFR with a chance of snow elsewhere.
Thursday...Mainly VFR.
Friday...MVFR with a chance of rain showers, possibly starting as
a brief period of snow or freezing rain.
Saturday...MVFR with rain likely.


Moderate southwest winds will continue this afternoon and tonight as
the pressure gradient tightens across the eastern Great Lakes. A
fairly strong low will move across northern Ontario and Quebec on
Tuesday, and allow southwest winds to increase to at least 30 knots
on Lakes Erie and Ontario. There is some potential for a low end
gale for a few hours, but the bulk of the time should remain just
below gale criteria. With this in mind, have held with high end
Small Craft Advisories for now. It will remain windy through
Wednesday before high pressure brings lessening winds later
Wednesday night and Thursday.


NY...Winter Weather Advisory until 10 PM EST this evening for
     Wind Advisory from 10 AM to 7 PM EST Tuesday for NYZ007.
     Wind Advisory from 6 AM to 7 PM EST Tuesday for NYZ001>003-
MARINE...Small Craft Advisory until 10 AM EST Wednesday for LEZ040-041.
         Small Craft Advisory until 7 PM EST Tuesday for LEZ020.
         Small Craft Advisory from 7 AM Tuesday to 7 PM EST
         Wednesday for LOZ043>045.
         Small Craft Advisory from 7 AM Tuesday to 1 PM EST
         Wednesday for LOZ042.



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