Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 100906

Area Forecast Discussion
National Weather Service Buffalo NY
406 AM EST Sun Dec 10 2017

Lake effect snow will continue east of Lakes Erie and Ontario today
and tonight with areas of locally heavy snowfall...before high
pressure brings a brief break from the snow on Monday. Low pressure
will then cross the Great Lakes and New England Monday night through
Tuesday night while bringing a widespread accumulating snowfall to
our region.


As of early this morning...a weak surface low is situated over far
southeastern Ontario Province...with a weak trough/surface cold
front extending back across the Lower Great Lakes. While this system
itself will produce little more than some scattered light snow
showers as it crosses our region early this morning...areas of
lake effect/lake enhanced snowfall will continue east and southeast
of the lakes...thanks to the increase in background moisture
provided by this system.

Off Lake Erie...a broad and disorganized area of light snow showers
continues across the higher terrain of Southern Erie and Wyoming
counties and the western Southern Tier...thanks to a combination
of lake effect and orographic processes given the sheared west-
northwesterly flow that is in place at the lower levels. Expect
this area to largely remain in place through the early to mid
morning hours...while also remaining on the light side thanks to the
continued sheared west-northwesterly flow...and a brief incursion of
somewhat drier air following the passage of the aforementioned
surface boundary.

Meanwhile off Lake initial broad area of light snow
across the North Country is waning and giving way to a somewhat
more organized band of lake snows across Oswego and far northern
portions of Cayuga response to somewhat better alignment
of the low level flow across Lake Ontario. This better organized
state should be relatively short-lived the passing
surface boundary will induce veering of the low level wind field and
a temporary increase in shear...which should lead to this activity
shifting southward and spreading out/weakening through the early to
mid morning hours...similar to what has already been seen with its
Lake Erie counterpart.

After that time...the lake effect activity off both lakes is
expected to shift back northward and re-intensify through the
remainder of the another weak clipper low passes by across
Southern Ontario and Quebec...and forces the low level flow to back
to the west-southwest and become better aligned again...while also
providing another shot of more favorable background moisture. With
inversion heights also climbing back into the 10-12 kft range and a
fairly deep dendritic crystal growth region present in the cloud-
bearing layer below the inversion...the lake effect activity off
both lakes will also likely become fairly heavy for a time between
this afternoon and evening.

Compared to yesterday...the low level flow out ahead of this second
clipper appears a bit more backed (west-southwesterly) in most
guidance packages... particularly in the 00z NAM and GEM...both of
which have steadily trended in this direction since early yesterday
afternoon. While the overall difference in wind direction is not all
that should still be sufficient to bring the cores of the
lake bands a bit further north than previously expected. Off Lake
Erie...this should translate into the lake effect making it into the
immediate Buffalo southtowns/central and eastern Buffalo suburbs/far
southern Genesee County for a time later this afternoon and early
this evening...while the Lake Ontario activity now appears as if it
will make it as far north as the Watertown area during this same
time frame.

The forecast implications of this trend are these
will act to spread out the heavier snowfall a little bit more than
previously expected. While this will likely result in somewhat lower
total accumulations in the current Lake Effect Snow warning areas...
it should also translate into a period of at least advisory-worthy
snowfall this afternoon and early evening across portions of
Northern Erie and Genesee Counties...for which a Winter Weather
Advisory has thus been issued. Given snowfall rates that should
reach 1-2 inches per should be stated that it`s not at all
out of the question that warning-criteria amounts could also be
realized if the band were to remain in place over one area long

As we progress through the evening and early overnight hours...the
aforementioned clipper system will quickly push its attendant cold
front across our region...with the low level flow veering to
northwesterly and becoming considerably more sheared in its wake.
This will result in the fairly intense lake bands first being driven
back southward and off the long axis of both lakes...with the
activity then quickly breaking up and becoming considerably more
disorganized as it settles across areas southeast of the lakes
overnight. Outside of the main lake effect areas...the cold frontal
passage will largely just generate some additional scattered light
snow showers.

In terms of overall Lake Erie these should peak
in the 8-10 inch range across Southern Erie and Wyoming counties and
northern portions of Chautauqua and Cattaraugus counties...with a
swath of 4-7 inches extending from the immediate Buffalo southtowns
eastward into southern Genesee County. Off Lake
accums of 6-12 inches will be likely across southern Jefferson...
northern Oswego...and Lewis Counties...with up to 15 inches or so
possible across central and northern portions of the Tug Hill.


Arctic boundary should be through the area by Monday morning.
Northwesterly enhanced lake snows will likely linger through the
morning hours. Drier air gradually arriving behind the boundary will
result in weaker snow intensities and less coverage by mid day.
Backing flow may bring some lighter snows into the Buffalo and
Watertown areas during the afternoon, ahead of approaching clipper
low pushing into the upper Great Lakes.

The clipper low will push quickly through the region Monday night
into Tuesday. A surge of Pacific moisture will accompany the passing
low. Wing of warm air advection/isentropic upglide will push across
the area during this time frame for an outbreak of widespread snow.
A narrow wedge of warmer air aloft will wrap into the fast-moving
system, which may suppress snow ratios a bit early on, with higher
ratios in the colder air behind the system Tuesday. Most areas will
see a general 1-3 inch snowfall Monday night and another 2-4 inch
accumulation Tuesday. Higher amounts should be in higher moisture
quality areas east of the lakes.

The synoptic snows will then morph into a potentially significant
lake effect snow event from late Tuesday through Wednesday night.
During this time period, a secondary surge of arctic air will push
across the lower Great Lakes in wake of a short wave crossing the
region. The steering flow will veer to northwest behind the
shortwave directing the more significant lake snows south and
southeast of the lakes. Model soundings showing moderate to extreme
over-lake instability developing with lake induced capes near 1000
j/kg and lake induced equilibrium levels nearing 15k feet. The top
CIPS analogs showing snowfall potential of at least a foot across
southwest New York and from Rochester to the southern Tug Hill.
There will likely be a period of where snowfall rates will be
locally heavy with forecast soundings also showing ample lift
through the dendritic growth zone. Headlines continue to look very
possible during this time period, and will continue to mention the
potential in the Hazardous Weather Outlook.

As the core of the 500 mb low passes over our region we will
experience the coldest air yet this month with lows dipping into the
single digits to teens Tuesday night and only peaking in the teens
to maybe 20 on Wednesday. Lake effect snows appear to begin to
weaken Wednesday night as high pressure over the Ohio Valley may
build across western New York.


A fair amount of uncertainty remains regarding Thursday/Friday, as
the forecast revolves around the timing of another clipper low
progged to drop across the Great Lakes. Southerly flow ahead of this
system should bring an end to lingering lake effect Thursday, with
the aforementioned low bringing more snow showers in the Thursday
night to Friday night timeframe, depending on the model.
Temperatures will remain below average through the end of the week,
as the region will remain embedded within the longwave trough.

As we move into the end of the week, the longwave trough pattern
that has dominated the eastern half of the country of late shows
signs of breaking down as we move into the weekend, with a lower
amplitude flow regime allowing milder Pacific air to wash across the
northern tier of the country and potentially bringing temperatures
back above freezing and closer to climatological normals in the
mid/upper 30s next weekend. However, longer range guidance suggests
that this break may only be temporary, as the longwave trough
becomes re-established over the East in the week following.


Through sunrise...bands of lake effect snow will produce areas of
IFR conditions east of both lakes...with the Lake Erie lake snows
likely affecting the KJHW terminal at times. Elsewhere conditions
will be much drier with just some scattered light snow showers
possible as a modest cold front crosses the region...and flight
conditions ranging between MVFR and VFR.

As we move into the daylight hours...generally weak/disorganized
lake snows east-southeast of the lakes this morning will re-
intensify and lifting back northward this afternoon as steering
winds back to the west-southwest...and overall conditions become
more favorable. Off Lake Erie...the organizing lake snows along the
Lake Erie shore south of Buffalo will move north to a point just
south of KBUF by mid afternoon...before slowly drifting back
southward again this evening. Off Lake Ontario the lake snows will
intensify across the Tug Hill region...then move north to near KART
later in the afternoon... before moving back south to the Tug Hill
later this evening. Both of these bands will produce areas of LIFR
to IFR conditions... with largely dry MVFR/VFR conditions prevailing
outside of them.

After that time...the lake effect snows east of the lakes will sink
southward to areas southeast of the lakes and weaken following the
passage of a second cold front. Expect continued LIFR to IFR
conditions within the lake snows for much of the night...with
scattered snow showers and localized MVFR found elsewhere.

Monday...Lake effect snow showers with local IFR southeast of the
lakes early, then improvement to VFR.
Monday night through Wednesday...Areas of IFR with periods of snow.
Thursday...Spotty MVFR/IFR with scattered snow showers.


A tight pressure gradient across the Lower Great Lakes will bring
Small Craft Advisory conditions to both Lakes Erie and Ontario through
late tonight and/or early Monday...before high pressure brings about
a return to  lighter winds on Monday.


NY...Lake Effect Snow Warning until 7 AM EST Monday for NYZ006>008-
     Winter Weather Advisory from 1 PM this afternoon to 1 AM EST
     Monday for NYZ010-011.
MARINE...Small Craft Advisory until 7 AM EST Monday for LEZ040-041.
         Small Craft Advisory until 10 AM EST Monday for
         Small Craft Advisory until 7 AM EST Monday for LOZ042-045.



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