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Area forecast discussion
National Weather Service Buffalo New York
955 PM EST Mon Dec 9 2019

Synopsis...
low pressure will move across the central Great Lakes, followed
by a cold front which will sweep across the region late tonight.
Behind the front, expect gusty winds and some lake effect snow
showers Tuesday afternoon through Thursday. High pressure will
move across the region and bring a return to dry weather for
Thursday night and Friday.

&&

Near term /through Tuesday/...
an upper level trough will dig across the Great Lakes region
tonight while a surface low strengthens as it moves from the
Georgian Bay northeast into southwestern Quebec overnight.
The warmest airmass aloft is over wny this evening, with the
last surge of moisture moving east-northeast from the Ohio Valley into the New York
southern tier. This will probably skirt by much of the Niagara
Frontier while moving toward central New York overnight. Cold air
advection will follow with much less steady precipitation, but
with more wind as colder air aloft begins to more effectively
mix to the surface.

A still deepening surface low will move across central and
northern Quebec on Tuesday with a trailing cold front crossing
the eastern Great Lakes late tonight into Tuesday. There will be
a few hours with gusty winds just behind this front. These will
be strongest east and northeast of the lakes where gusts to 45
mph are expected. Tuesday morning will remain windy, then winds
will gradually diminish Tuesday afternoon.

The actual cold front will only produce a few brief showers as
it moves east across the area early in the day with most areas
seeing only a ~1hr-long rain to snow shower as the front moves
east-southeast. Those that don't measure will probably see some scattered
flurries as the temperature drops through the 30s. In all
areas, accumulation should nil to about an inch over higher
terrain, but with little or no impacts due to short duration and
still cooling ground and Road conditions from the past 24+hrs
in most areas.

A secondary cold front will drop across Lake Ontario late
Tuesday afternoon. This will all lapse rates to steepen and
become supportive of lake effect processes, albeit with only low
level moisture resulting in mostly disorganized banding.

&&

Short term /Tuesday night through Friday/...
low pressure will continue to head northeast and deeper into Quebec
Canada late Tuesday. Behind the above noted secondary front,
lakes snows will likely weaken despite a colder air mass
advecting into the eastern Great Lakes with h850t falling to
-12c/-14c. Additionally, lake effect snows will be hampered by
the fairly dry air mass filtering into the region, along with
wind shear which will not provide a good environment for
efficient lake snows. So with that said, lake snows off Lake
Ontario will not completely shut down. It's possible that an
upstream connection to the Georgian Bay will maintain some lake
snows. Although, accumulations will be minor Tuesday night. Off
Lake Erie, it will be a completely different story and without
any upstream moisture available. The short fetch and dry air
mass will likely shut it down almost completely, at least for a
period. Expect only a few snow showers and minimal
accumulations, if any at all.

Wednesday really is The Ball game, this is when there will be the
potential for a brief window of decent accumulating lake effect
snows. A short wave trough and clipper low will race by to the north
of the area. It will send its cold front through the eastern Great
Lakes by mid to late in the afternoon. The cold front itself will do
little, with nothing more than a few light snow showers. However, it
will introduce a more favorable lake effect environment. Convergence
along the advancing cold front will merge with pre-existing lake
induced convergence over both lakes, resulting in intensifying bands
of lake effect Snow. Lake induced equilibrium levels will briefly
rise to 10-11k feet with the passage of the clipper as moisture
improves and a pool of cold air aloft crosses the eastern Great
Lakes. A favorably deep mixed phase layer will be located within the
cloud bearing layer, yielding dendritic growth.

Off Lake Erie...

A weak lake band will move back north across the Buffalo Metro area
very early Wednesday morning ahead of the front. As the front
approaches the area the band will intensify rapidly across northern
Erie and a portion of Niagara County. It will then cross again the
Buffalo Metro area with an intense burst of lake effect snow from an
added boost of synoptic support from the front, and increasing lake
induced instability. The intense band will then move from Buffalo
all the way down to Chautauqua County and push inland through the
southern tier through the late afternoon. The band of snow will
continue for a few hours across the western southern tier Wednesday
evening before quickly weakening as inversion heights lower, shear
increases, and moisture decreases with high pressure building over
Lake Erie.

The fast pace of the southward band drift will greatly limit
accumulations, even though the band of snow may become quite heavy.
Right now, estimates suggest 1-4 inches from the Buffalo Metro area
into portions of Genesee County, and 2-5 inches across the higher
terrain of southern Erie/Wyoming counties into the Chautauqua ridge.
Even though amounts will be limited, if the brief burst of heavy
snow materializes it would produce very difficult travel for a few
hours. Winds will also gust 30-35 knots around this time, producing
blowing and drifting snow.

Off Lake Ontario...

Expect a similar trend on Lake Ontario. The disorganized lake effect
snow showers over the lake will push to the northeast end to near
Kingston Ontario, possibly clipping Cape Vincent in northern
Jefferson County. The band of snow will then intensify during the
early to mid afternoon as the front approaches and synoptic
support/instability improve. This intensifying band of snow will
then move southeast across Jefferson County, crossing the Tug Hill
region during the evening. The band will reach maximum intensity
during this time frame as the Best Lake parameters line up with a
period of upslope flow across the Tug Hill. The band will continue
to March steadily southeast overnight, with a west-northwest to east-southeast oriented
band of weakening lake snow moving into northern Cayuga/Wayne
counties. A few snow showers may also clip the South Shore farther
west overnight from eastern Niagara to Monroe counties as boundary
layer flow veers.

Similar to Lake Erie, the short residence time of the band in any
one location will limit accumulations. Expect 2-4 inches across the
lower elevations of the Eastern Lake Ontario region, with up to 6
inches across the Tug Hill plateau. Wayne and northern Cayuga
counties may see a few inches overnight Wednesday. While amounts are
not overly impressive, this band may produce difficult travel for a
few hours with heavy snow and 30-35 knot wind gusts producing
blowing and drifting.

High pressure builds overhead Thursday with any lake snows in the
morning quickly diminishing off both lakes. Any additional
accumulations in the morning will likely be minor, with the best
chances off Lake Ontario. This is where an additional inch or so
could be found. Other than that, expect one more cold day across the
eastern Great Lakes with highs peaking in the 20s areawide. Thursday
night, the surface high exits off to our east and then relocates off
the coast of Nova Scotia by Friday. Lows Thursday night will fall
back into the upper teens to low for western New York into The Finger
lakes region, while single digits to low teens will be found east of
Lake Ontario by daybreak. Friday, return flow around the high will
initiate a fast warming trend, with temperatures returning to near
normal.

&&

Long term /Friday night through Monday/...
a complex system will then cross our region this weekend. A southern
stream trough will support low pressure developing over the Gulf
Coast Friday. The resulting low will move almost due north, with one
low center moving up the west side of the Appalachians and into the
eastern Great Lakes by Saturday, with a second weak Lee side low
moving up the eastern Seaboard. This southern stream trough will
phase with a northern stream trough moving towards the western
lakes, allowing an expansive area of deep layer ascent to move north
across our area late Friday night and Saturday. This will produce
another round of rain areawide. There is a small chance the rain may
start as a wintry mix across the north country if precipitation
begins early enough, otherwise this looks to be an all rain event as
temperatures warm into the 40s on Saturday.

The northern stream portion of this phasing system will then cross
the eastern Great Lakes Saturday night and Sunday, producing
additional chances of rain and wet snow showers as colder air moves
back into the region. Highs Sunday will be in the mid to upper 30s.

Sunday night, dry weather will returns as high pressure moves east
and builds into the eastern Great Lakes. Dry and fair weather will
likely continue Monday as high pressure moves over the area.

&&

Aviation /03z Tuesday through Saturday/...
an upper level trough will dig across the Great Lakes region
tonight while a surface low strengthens as it moves from the
Georgian Bay northeast into southwestern Quebec overnight.
The warmest airmass aloft is over wny this evening, with the
last surge of moisture moving east-northeast from the Ohio Valley into the New York
southern tier. This will probably skirt by much of the Niagara
Frontier while moving toward central New York overnight. Cold air
advection will follow with much less steady precipitation, but
with more wind as colder air aloft begins to more effectively
mix to the surface. Just ahead of the frontal boundary, winds
may briefly diminish and allow kbuf/kiag to briefly drop to IFR
in lower cigs and possibly fog. Fog or a low cloud deck also is
likely at kjhw, with LIFR conditions are possible as noted with
nearby kole. The also will be a period with low level wind shear ahead of this
boundary at most taf sites.

Winds will shift to the west-southwest and rapidly increase behind the cold
front which will move through the area between 07z and 12z.
There may be some lingering rain or snow showers across higher
terrain. Cigs will gradually lift to the MVFR/VFR flight
category through Tuesday afternoon.

Outlook...

Tuesday night...disorganized lake effect on a west flow.
Wednesday...IFR within lake effect snow east-northeast of the lakes.
Thursday...VFR with localized IFR in lake effect snow showers
east of the lakes.
Friday...VFR.
Saturday...MVFR. Rain likely.

&&

Marine...
a cold front will move across the lower Great Lakes late
tonight, and this will produce gusty southwest winds which will
require small craft headlines for the nearshores and Buffalo
Harbor. There was some concern that 40 knot 925 mb winds would
result in gales on Lake Erie, but with surface temperatures
still near 40 behind the front expect these winds aloft will not
effectively mix to the surface.

Winds will briefly diminish Tuesday night when a surface ridge
builds across the region. Then another cold front will move
across the waters on Wednesday, and this will benefit from
strong cold air advection and much colder surface temperatures.
This may produce low-end gales on Lake Ontario where a gale
watch was issued as outlined below. Elsewhere is likely to
require another round of small craft headlines.

High pressure will then move across the lower Great Lakes
Thursday and then into New England on Friday.

&&

Hydrology...
a lingering warm airmass will cause continued snow melt east of
Lake Ontario which will add to runoff from rain through
Tuesday. For most basins, rainfall amounts will be less than an
inch and the snow pack in place is limited, resulting in little
risk for flooding.

There is a bit more snow pack in place in the Black River basin,
with snow water equivalent (swe) values in the 1-2 inch range.
Model guidance shows a bit more quantitative precipitation forecast here, with 1-1.5 inches
across localized favorable upslope regions. Latest rfc forecast
has Boonville reaching action stage by late Tuesday, with
Watertown approaching action stage Thursday. Mesoscale guidance
has overdone upslope precipitation early in the event,
suggesting the higher amounts this has shown will be a bit
overdone. Mmefs ensembles show a fairly high likelihood that
Black River forecast points will reach action stage, but very
low probabilities of flooding.

Based on this, have discussed this risk in the severe weather potential statement. The most
likely outcome remains action stage, but the situation bears
watching. For now there is not enough confidence to issue any
flood headlines.

&&

Buf watches/warnings/advisories...
New York...none.
Marine...Small Craft Advisory until 4 am EST Thursday for lez040-041.
Small Craft Advisory until 10 am EST Tuesday for lez020.
Gale watch from Wednesday morning through Wednesday
evening for loz044-045-062>065.
Small Craft Advisory from 4 am Tuesday to 1 am EST
Wednesday for loz043-044.
Small Craft Advisory from 1 am to 10 PM EST Tuesday for
loz042.
Small Craft Advisory from 4 am to 10 PM EST Tuesday for
loz045.

&&

$$

Synopsis...apffel
near term...apffel/zaff
short term...AR/Hitchcock

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