Area Forecast Discussion
Issued by NWS Buffalo, NY

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122
FXUS61 KBUF 182005
AFDBUF

Area Forecast Discussion
National Weather Service Buffalo NY
305 PM EST Sat Nov 18 2017

.SYNOPSIS...
A strengthening storm system approaching from the mid western states
will spread widespread rain across our region through this evening,
then a strong cold front will plow across the Lower Great Lakes to
produce mixed precipitation late tonight. Gusty winds in the wake of
the powerful front will introduce a notably colder airmass into the
area for the second half of the weekend. This colder airmass will
generate accumulating snows southeast of both lakes Sunday into
Monday.

&&

.NEAR TERM /THROUGH SUNDAY/...
Low pressure currently pushing northeast into northwest Ohio.
Leading edge of moderate rain along associated warm frontal boundary
pushing into the Southern Tier. This area of rain will continue to
work northward through this evening as the deepening surface low
tracks across Lake Erie/Niagara Frontier on its way into the Saint
Lawrence Valley. There may be a very brief break in the rain behind
the warm front, but another shot of rain will work across the area
associated with a powerful cold front.

The forecast track of the surface low continues to be too close to
be a legitimate high wind threat, although it will certainly become
windy in its wake. The strongest winds tonight will come after
midnight, and will be focused across the immediate lakeshores and
higher elevations where winds will likely gust to 45 mph.

It will take a little while before the colder air deepens in the
wake of the cold front. The rain will eventually taper off as a mix
of light rain and wet snow. There could still be enough snow for
some of the higher elevations to pick up a light coating by
daybreak. This will mainly be the case across the Southern Tier
where the synoptic driven precipitation will transition to lake
effect by morning. Overnight temperatures will only drop to the
lower to mid 30s across higher elevations, and the mid to upper 30s
across the lake plains.

Sunday, a cyclonic flow of much colder air will continue overspread
the lower Great Lakes. Over-lake instability will become more
favorable as the day wears on, with lake induced CAPES and inversion
heights becoming respectable by afternoon, setting the stage for a
period of lake effect snows to the southeast of both lakes in a
predominantly west-northwest flow regime. The best lake parameters
actually develop in the short term period. See discussion that
follows below. During the daytime hours, accumulations will likely
be limited to just a couple of inches or so.

&&

.SHORT TERM /SUNDAY NIGHT THROUGH TUESDAY NIGHT/...
Sunday night the Eastern Great Lakes region will be firmly
entrenched within cold air advecting across the Great Lakes, with
lake effect snow falling southeast of both lakes. Accumulating snow
will be on the minor side through Sunday evening, but as the cold
air aloft deepens, and lake induced equilibrium levels increase
through the night we should see enhancement to the snow rates off
both lakes.

For Lake Erie...

Northwest winds Sunday evening will drive lake effect snow across SW
NYS. As temperatures cool aloft at 850 hPa to -10/-11C lake induced
equilibrium levels will increase to around 12-15K feet and with
ample moisture within the snow dendritic growth zone we should see
some increase in intensity to the lake effect snow around mid-
overnight. The northwest perpendicular flow will allow for a broad
structure to the lake band that will bring snow totals of several
inches Sunday night to the western So. Tier...and extreme southern
Erie County. There remains uncertainty if an upstream connection to
Lake Huron will occur behind the passage of a 500 hPa shortwave
trough, and without this upstream connection...overall snow totals
through this event will remain at advisory levels.

As surface high pressure over the mid Mississippi Valley/Lower Ohio
Valley nudges northeastward, winds will back some, while inversion
heights and overall synoptic moisture diminishes. This will weaken
the lake effect snow Monday as the band lifts northward towards
metro Buffalo. The passage of a shortwave 850 hPa ridge axis Monday
afternoon will end lake effect snow...at about the time the band
reaches near metro Buffalo.

For Lake Ontario...

Northwest winds will also bring lake effect snow southeast of the
lake, with snows focused upon Wayne to Oswego counties. There will
likely be minor upslope lake effect snows on the southern Tug Hill.
Over the longer fetch of the lake, there will likely be a more
concentrated area of snow, with a narrow band of snow setting up
over Oswego County. Temperatures aloft will be a bit colder over
Lake Ontario, dipping to about -11/-12C which will produce extreme
lake instability. The narrow band of snow over Lake Ontario will
increase in strength through the night, and with potential for an
upstream connection to Georgian Bay this band of snow has the
potential to produce snowfall rates of 1-2 inches per hour late
Sunday night and into Monday morning. Behind the passage of a 500
hPa shortwave trough, the band of snow may drop towards Wayne and
northern Cayuga Counties for a brief time mid-overnight Sunday night
before centering back upon far northern Cayuga and across western
and central Oswego counties in the pre-dawn hours Monday. It is at
this time and location that we expect the most intense part of the
snow band, and with the snow band likely to remain over the same
areas for a 4-8 hour window...warning criteria snowfall amounts are
possible. As such a Winter Storm Watch (for Lake Effect Snow) has
been issued for these two counties of northern Cayuga and Oswego
Counties.

Monday morning the band of snow over Oswego County will begin to
lift northward towards the Tug Hill as high pressure from the west
begins to nudge towards the Eastern Great Lakes region. Winds will
continue to back some through the day, and as moisture depletes from
the snow dentritic zone, and inversion heights lower the intensity
to the lake effect snow will diminish, such that it will be in a
much weaker form by time it reaches the central Tug Hill. Here a few
inches of snow will be possible before the band broadens and
dissipates Monday evening.

Later Monday night and into Tuesday a period of dry weather is
expected as an axis of the surface high pressure crosses the region.
Skies will clear Monday night, but a southerly breeze will prevent
temperatures from rapidly dropping, with overnight lows in the lower
30s. Tuesday, these southerly winds will increase further, with
gusts up to 30-40 mph. Highest gusts will likely be across the hill
tops of SW NYS, where a 55 knot LLJ will occasionally produce gusts
30 to 35 mph, and also across the lake plain of WNY where downslope
flow, and under the strong LLJ may bring gusts upwards towards 40
mph. These winds and warming aloft will bring afternoon temperatures
into the 40s to low to mid 50s. Warmest areas will be the Lake
Plains and Genesee Valley, except for SE of Lake Ontario where a
fresh snowpack will slow warming.

Surface low pressure will cut across the Central Great Lakes
Tuesday, deepening to about 985 mb as it crosses James Bay Tuesday
night. Aloft another potent shortwave trough will be crossing the
central Great Lakes, pushing the cold front eastward. Ahead of the
cold front clouds will begin to increase through Tuesday afternoon,
and as these clouds increase we`ll see some diminishment to the
gusty winds.

Rain showers along the front will reach WNY Tuesday evening, and
continue to push inland across the region with the passage of the
cold front. The warm southerly flow ahead of the cold front will
start this precipitation off as plain rain. As temperatures cool
behind the front, rain will mix with and likely change to snow by
late Tuesday night. Not much snow accumulation at this time, with
perhaps an inch across higher elevations.

&&

.LONG TERM /WEDNESDAY THROUGH SATURDAY/...
On Wednesday a colder west-northwesterly flow will again be in place
across our region...however the colder airmass also appears to be
rather dry. This should tend to keep any lake response downwind of
the lakes fairly limited in scope...with a return to largely dry
weather then more likely for Wednesday night and Thanksgiving Day
as surface-based ridging builds across the region..

Another frontal boundary will likely dip southward from Canada
Friday...maintain chances for snow, especially across Lake Ontario
and points eastward which at this time will be closer to the frontal
boundary.

&&

.AVIATION /20Z SATURDAY THROUGH THURSDAY/...
MVFR/IFR restrictions in rain will overspread the region through
this evening. A strong cold front will cross the region in the 06-
09Z time frame with lingering light rain changing over or mixing
with snow before 15Z. Will also see winds shift to the west-
northwest by Sunday morning and increase, with sustained winds 20 to
25 knots and gusts around 35 knots likely.

Outlook...

Sunday-Monday...Lake effect SN, IFR SE of lakes, otherwise VFR/MVFR.
Tuesday...Mainly VFR.
Wednesday...Rain and snow showers with a chance of MVFR/IFR.
Thursday...VFR, but MVFR in lake effect snow showers east of the
lakes.

 &&

.MARINE...
A deepening storm system over the mid western states will approach
the lower Great Lakes this evening, resulting in brisk southerly
winds. While winds will approach small craft advisory criteria, the
highest waves will be directed into Canadian waters.

The real problem during this forecast package will be the strong
winds that will follow the strengthening storm system as it exits to
our north and east late tonight and Sunday. Winds veering to the
west and northwest in the wake of the storm will increase to gale
force late tonight through Sunday. Gale warnings will remain in
place.

These very strong winds will only marginally subside Sunday night
and Monday, as expansive high pressure over the southern states will
make its way off the southeast coast.

&&

.TIDES/COASTAL FLOODING...
Developing very strong onshore winds will build significant wave
action from late tonight into Sunday night along the Lake
Ontario shoreline. The combination of already higher lake levels
and continued wave action will result in increased shoreline
erosion, especially where the lakeshore is already unstable from
erosion earlier in the year.

&&

.BUF WATCHES/WARNINGS/ADVISORIES...
NY...Lakeshore Flood Warning from 5 AM Sunday to 1 AM EST Monday
     for NYZ001>007.
     Winter Storm Watch from Sunday afternoon through Monday
     afternoon for NYZ005-006.
     Winter Weather Advisory from 4 AM Sunday to 1 PM EST Monday
     for NYZ019-020-085.
MARINE...Small Craft Advisory from 1 AM Sunday to 1 AM EST Monday for
         LEZ020.
         Gale Warning from 10 PM this evening to 7 PM EST Sunday
         for LEZ040-041.
         Small Craft Advisory from 1 AM Sunday to 1 AM EST
         Monday for LOZ030.
         Gale Warning from 1 AM to 10 PM EST Sunday for LOZ042>045-
         062>065.

&&

$$

SYNOPSIS...TMA
NEAR TERM...TMA
SHORT TERM...THOMAS
LONG TERM...JJR/THOMAS
AVIATION...TMA
MARINE...TMA
TIDES/COASTAL FLOODING...TMA



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