Area Forecast Discussion
Issued by NWS Buffalo, NY

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017
FXUS61 KBUF 070857
AFDBUF

Area Forecast Discussion
National Weather Service Buffalo NY
357 AM EST Wed Dec 7 2016

.SYNOPSIS...
A cold front will sweep across the region this morning, bringing a
mix of light rain and drizzle across Western New York, while a
little accumulating snow is possible across the higher terrain east
of Lake Ontario. Much colder air will filter in behind this cold
front such that scattered light lake effect precipitation today and
this evening east of the lakes will increase in intensity, while
transitioning over to snow. Bands of lake effect snow, initially on
a southwest wind tonight will drop southward later tonight and
continue to produce heavy snow through Saturday.

&&

.NEAR TERM /THROUGH TONIGHT/...
Water Vapor imagery this morning displays an upper level low
spiraling just to the west of Lake Superior. At the surface an area
of low pressure is found beneath this upper level low, with a cold
front now reaching WNY. Regional radars, and surface observations
display a mix of wet snow and rain across the northern Finger Lakes
and Eastern Lake Ontario region, with primarily just light rain
and/or drizzle across WNY.

For this morning, much drier air in the mid levels will strip
moisture from the snow dentritic growth zone such that any falling
precipitation through the day will be either drizzle, or light rain
across WNY, with this precipitation type spreading eastward across
the Eastern Lake Ontario region by mid-morning. Surface observations
are generally above freezing this early morning, with only areas
that are subfreezing east of Lake Ontario where light snow is
falling. Expect in areas where precipitation falls today, that
surface temperatures will be just above freezing. That said, since
just a few degrees change at the surface could bring freezing
drizzle/rain this will be closely monitored through the morning,
especially east of Lake Ontario.

Temperatures aloft will be falling through the day with cold air
advection, and this will bring limited chances for lake effect
precipitation. While we`ll be unstable for bands of lake effect to
form, the lack of moisture aloft will likely limit precipitation
chances until this evening when additional moisture from the Ohio
Valley arrives, and the snow dendritic growth zone lowers into a
more saturated layer. Thus bulk of the late morning and afternoon
hours outside of the weak lake effect response near the lakes will
be dry, mostly cloudy and cool.

Tonight lake induced equilibrium levels will begin to rise, upwards
towards 10K feet east of both lakes by late tonight. With moisture
again returning to favorable snow growth layer, and winds initially
aligning on a southwest flow, we should see a band of lake effect
snow become better organized, around the late evening hours east of
Lake Erie, and around midnight east of Lake Ontario. While there may
be a little rain mixed in at first, and near the lake shores, we
should see a transition to all snow as the band of snow matures
through the second half of the night.

For Lake Erie, a southwest wind will bring the onset of snow across
Buffalo/perhaps northwards initially. Then as the upper level low
pushes eastward to the north of Lake Ontario, we`ll see winds begin
to veer, pushing the lake effect snow southward across the
southtowns late tonight. While the best lake effect parameters come
during the day Thursday and into Friday, we could still see snowfall
rates up to an inch per hour within the Lake Erie band by dawn
tomorrow. This could lead to a messy commute tomorrow morning,
mainly for the areas along and just to the south of the city of
Buffalo and the airport.

For Lake Ontario, a southwest wind will bring the onset of snow
across Watertown, and perhaps extending as far north as near the
Saint Lawrence River. As the upper level low spirals across southern
Canada winds will begin to veer, dropping the band of snow to near
the northern Tug by dawn tomorrow morning. This band of snow will
still be in its infancy late tonight, and snowfall rates will not
start to push an inch per hour until the last few hours of the
period.

In total just a few inches of snow will be possible northeast of the
lakes tonight.  For both lakes would not rule out some rumbles of
thunder with this initial lake effect band late tonight. Our local
research thundersnow nomogram shows the forecast -10C level of 5kft
and a forecast lake induced equilibrium level of 10kft supports
likely thunder.

&&

.SHORT TERM /THURSDAY THROUGH SATURDAY NIGHT/...
...Significant lake effect snows to impact areas east and southeast
of Lake Erie and Lake Ontario into the weekend...

An impressive stacked storm system...seen in WV imagery churning
east across Lake Superior early this morning...will be the driving
force behind our upcoming major lake snow event. This deep system
will track due east across the heart of Ontario this afternoon
before crossing into Quebec`s air-space late tonight and Thursday. As
this slow moving feature drifts across southern Quebec to the
Canadian maritimes by the weekend...it will circulate H85 temps as
low as -16c across the wide open Lower Great Lakes. This will
establish a moderate to extremely unstable thermal profile below 10k
ft with SBCAPEs climbing to near 1000 j/kg and lapse rates below 5k
ft easily exceeding 10 deg c/km. Meanwhile equilibrium levels will
surge to around 20k ft...more than enough to support deep mesoscale
driven convection. Microphysically...the dendritic snow growth
region will average 3-4k ft deep...which is typical for lake effect
snows within a steady...cold advective regime. This thermal profile
will encourage the growth of large dendrites that should accumulate
at a rate of at least 2 to 3 inches per hour in the heart of well
organized bands.

Local research shows that the synoptic placement and track of the
stacked storm system coincides extremely well with analogs for
significant lake snows east of both lakes. Additional studies
strongly suggest that this event should become electrified...at
least during the initial 24-36 hours. Given the high inversion hgt
and placement of the -10c isotherm...there should be sufficient room
to separate charges generated by the mixed phase cloud. Lightning in
lake snows are typically within 20 miles or so of the source of
instability (the lakes)...so have tried to incorporate this into the
forecast. Have even placed a bullseye for thundersnow in the vcnty
of the large wind farm on the Tug Hill where cloud to ground
strikes are more common.

This impactful lake snow event will begin later tonight and
intensify Thursday and Thursday night as instability increases under
a rising cap. A 250 flow over Lake Erie will initially direct lake
snows across the immediate southern portion of the BUF metro area
and extend to southern Genesee County. Several inches of snow are
likely for this area by mid morning Thursday...so a lake snow
advisory may be issued as confidence increases with the placement.
The flow will steadily veer to 270 degrees during the course of
Thursday. This will allow the plume of moderate to heavy lake snow
to settle from the Southtowns to the heart of the southern tier
where snowfall rates will increase to 2 to 3 inches per hour...
partially due to the enhanced orographic lift. Meanwhile off Lake
Ontario...a 240 flow will support lake snows over Jefferson and the
northern half of Lewis County late tonight and Thursday morning. As
in the case of the activity off Lk Erie...winds will veer to 270
during the afternoon. The plume of moderate to heavy lake snow will
push south across the Tug Hill in the process. Again...snowfall
rates will increase during this southward push...largely due to
increased fetch and also from orographic lift from the Tug.

As a surface trough moves south across the Lower Great Lakes
Thursday night...the steering flow below 10k ft will veer to the
northwest. While this will keep the focus of the Lake Erie snows
over the Western Southern Tier (particularly Chautauqua and Catt
Co`s)...it will continue to push the Lake Ontario snows further
south...off the Tug. This will place Oswego County in the
cross-hairs of Mother Natures snow machine. In the process...
overnight snowfall amounts are forecast to average a foot along and
within 20 miles east of the Chautauqua Ridge...with a foot and half
of snow expected in the vcnty of the Tug Hill Plateau.

Friday and Friday night...a fairly persistent 300-310 flow will keep
widespread accumulating lake snows in place southeast of both lakes.
Another foot of snow could easily accumulate during this 24 hour
period...again...focusing on the Chautauqua Ridge and Oswego county.
Amounts should reach at least advisory levels for Wayne and Northern
Cayuga Counties...so the current lake snow watches for those areas
will likely be updated to either advisories or warnings later today.

By the time we reach mid day Saturday...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. 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.

&&

.LONG TERM /SUNDAY THROUGH TUESDAY/...
While the major lake snows of the previous days will not extend into
this period...it will remain quite wintry over our forecast area.
This scenario will be supported by a longwave pattern that will
feature a deep trough over the heart of North America that will keep
a cyclonic flow of seasonably cold air over all of the Great Lakes
region. This pattern will be highlighted by a large chunk of the
Polar Vortex that will break from its anchored position near the
Pole and dive southward across central Canada during the weekend and
early next week. The robust mid level gyre will not only keep a
relatively cold pattern in place by maintaining the deep trough...
but it will help to drag a new surge of Arctic air south towards the
Lower 48. We may have to contend this airmass as we advance outside
of this forecast period.

In the meantime...a developing storm system over the center of the
country will take aim on our region for later Sunday and Sunday
night. The medium range ensembles...while exhibiting a great deal of
spread within their solutions...are in fair agreement that
widespread snow will make its way across our forecast area. While
accumulations should be anticipated with this event...confidence is
not high for significant amounts. It could be enough though to make
for a messy Monday morning commute.

Model solutions continue to diverge on Monday...as scenarios range
from the passing of an organized storm to our south...to a cold
northwesterly flow that would support some nuisance lake effect.
After collaboration with surrounding offices...have decided to broad
brush with high chc pops for now as we should have at least some
snow showers over the region.

While a cold cyclonic flow will be in place across the Lower Great
Lakes on Tuesday...there is medium confidence that weak low level
ridging should support mainly fair weather. That being said...some
nuisance lake snows will be possible east and northeast of each
lake.

Looking further out through much of the remainder of the week...
temperatures should tumble to below normal levels across our region.
A pool of <-30c H925 air over the Canadian prairies on Tuesday will
be directed across southern Ontario and Quebec on Wednesday and
Thursday as the aforementioned arctic gyre will sweep through the
base of the longwave trough and eventually become positioned over
the Labrador Sea. Downstream ridging over the Norwegian Sea may keep
this general pattern in place for the bulk of the week. This would
favor at least some of the very cold air being left over our
region...where H925 temps would average in the minus teens c and H85
temps would be in the vcnty of -20 to -25c.

&&

.AVIATION /09Z WEDNESDAY THROUGH SUNDAY/...
For the 06Z TAFs a cold front will cross the TAF region through
the next 6 to 9 hours, with a brief period of IFR cigs likely
along and just behind the front. Ahead of the front lingering
light rain and snow will remain near KART and the North Country,
while patches of drizzle will linger across WNY.

After this front passes, and narrow wedge of drier air should return
most TAF sites to VFR flight conditions for a time today.

As cold air deepens across the Eastern Great Lakes behind the cold
front, lake effect snow will develop. This activity is expected to
develop in earnest after 00Z, and near the KBUF and KART airfields.
Rain may initially mix in these bands of precipitation, especially
near KBUF, but soon will transition to all snow with IFR and lower
flight conditions. The band of snow off Lake Erie may extend as
inland as KROC late tonight, while remaining just south of KIAG,
and north of KJHW through late tonight.

Outlook...

Thursday through Saturday...IFR in lake effect snow east and
southeast of the lakes. Occasional snow showers with more brief IFR
elsewhere.
Sunday...MVFR/IFR in widespread light snow.

&&

.MARINE...
A cold front will cross Lake Ontario this morning, with a westerly
flow increasing across the lakes behind the front. This will result
in small craft advisory worthy winds up to 25 knots on the lakes
today, with gusty winds also across portions of the major rivers.

Small craft conditions will then continue into the weekend on both
lakes Erie and Ontario as low pressure moves very slowly east across
Quebec, producing moderate west to northwest winds on the Lower
Great Lakes.

&&

.BUF WATCHES/WARNINGS/ADVISORIES...
NY...Lake Effect Snow Warning from 6 AM Thursday to 6 AM EST Friday
     for NYZ007-008.
     Lake Effect Snow Warning from 6 AM Thursday to 6 AM EST
     Saturday for NYZ006-012-019-020-085.
     Lake Effect Snow Watch from Thursday evening through late
     Friday night for NYZ003>005.
MARINE...Small Craft Advisory until 7 PM EST Friday for LEZ040-041.
         Small Craft Advisory from 7 AM this morning to 7 PM EST
         this evening for LEZ020.
         Small Craft Advisory from 7 AM this morning to 7 AM
         EST Saturday for LOZ042>045.
         Small Craft Advisory from 10 AM this morning to 7 PM
         EST this evening for SLZ022.

&&

$$

SYNOPSIS...THOMAS
NEAR TERM...THOMAS
SHORT TERM...RSH
LONG TERM...RSH
AVIATION...THOMAS
MARINE...THOMAS



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