Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 190651
AFDBUF

Area Forecast Discussion
National Weather Service Buffalo NY
151 AM EST Sun Nov 19 2017

.SYNOPSIS...
A strengthening storm system moving across Western New York will
spread periods of rain across our region overnight, 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 TODAY/...
Low pressure is currently moving away from WNY. The strongest
lift is under the warm conveyor belt along the Appalachians to
the Finger Lakes and within the exit region of an upper level
jet maxed out near 145kts from TN northeast to KY. This is
clearly identified by a band of heavy precip in the same general
region. The surface low is expected to take a track nearly over
KBUF then into central Lake Ontario overnight while continuing
to deepen overnight while the strongest low level jet is
displaced far to the S and E.

Here are a some details for overnight/early Sunday...

Precipitation...The most interesting precipitation area is
associated with the warm conveyor belt/upper level jet.  But the
forecast surface low track will keep the heavy precipitation mainly
east toward the Finger Lakes and into eastern PA from this point
forward. Other areas will receive lighter amounts overnight. A dry
conveyor belt is clearly seen in satellite and radar.  This area
will cross most of WNY and CNY for a good portion of the night with
mainly light/minimal precipitation, followed by a fairly elongated
deformation band entering the OH Valley. That area should make its
way east toward WNY toward daybreak.  By that time, temperatures
will have dropped into the 30s for all but the Eastern Lake Ontario
region, and would not be surprised to see an hour or so of snow
before the main synoptic features move east of the region. This may
put down a dusting/coating of snow over WNY.

Wind...While the strength of the cyclogenic event is admirable, the
track of the low is not favorable for strong to high winds as the
strongest low level winds will be tied to the tight baroclinic region
south and east of the region.  So, while there will be a notable
increase in windspeeds as the low exists to the NE, wind gusts
should max out in the 40-45 mph range, with highest winds in the
typical/wind prone areas along the lakeshores and/or highest
elevation sections of the Southern Tier.  Upstream observations so
far agree with this forecast.

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
later Sunday Night (See short term discussion). During the daytime
hours, accumulations will likely be limited to just a couple of
inches or less due to thin multibands that have yet to coalesce into
a more coherent and impactful band.

&&

.SHORT TERM /TONIGHT 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/...
Good news for Thanksgiving travel is that no significant storms are
forecast surrounding and including Thanksgiving day. Several
shortwaves troughs pinwheeling around a deep mid-level low over
Hudson Bay will occasionally glance the eastern Great Lakes region
with some chances of light precipitation. Overall, the lack of a
zonal flow or ridging in the jet over the Great Lakes will mean
temperatures will average below normal.

This cooler air will be felt on Wednesday behind Tuesday nights cold
front. 850mb temps will slip back to -8C to -10C over the lakes. The
lack of synoptic moisture in this airmass behind the front will
prevent any significant lake effect precipitation. There may be some
lingering rain/snow showers or even drizzle and clouds Wednesday
east of the lakes before the airmass dries out. Models then begin to
show some differences for Thanksgiving day. The 12z ECMWF shows a
shortwave trough and cold front will attempt to shift into WNY
Thanksgiving day while the GFS keeps our area dry with a weak trough
and hardly any moisture not shifting across our region until Friday
morning. Had to lean at least partially with the EC solution while
only including a chance of some rain/snow showers on Thanksgiving.
Temperatures both Wednesday and Thanksgiving Day will be below
normal with highs only expected to reach into the mid/upper 30s
during the day and 20s at night.

The front/trough portrayed on the EC would likely lead in another
shot of cooler air for Thanksgiving night and Friday. Some limited
lake enhancement or upslope showers may develop with best chances,
though still low, would be east of Lake Ontario where the coolest
air aloft would be. Otherwise, dry weather with cool temps expected.
Friday night and Saturday then feature increasing chances of
rain/snow showers as EC/GFS show a deeper low dipping into or
crossing the Great Lakes. Timing differences between these models
have led to holding POPs in the mid-chance range. Temperatures
Friday will remain below normal before rising to near normal in the
mid 40s ahead of this next system for Saturday.

&&

.AVIATION /07Z SUNDAY THROUGH THURSDAY/...
A deepening storm system exiting via the Saint Lawrence Valley will
generate mixed precipitation across the region for the remainder of
the overnight...with most areas experiencing IFR to MVFR cigs. The
lowest cigs will be across the higher terrain of the Southern Tier
and Finger Lakes region.

After daybreak...the widespread synoptic precipitation will give way
to more focused lake effect snow...mainly across the western end of
the state. While cigs will improve to MVFR for most of the TAF
sites...lake snows will reduce vsbys to IFR levels.

Outlook...

Sunday night...VFR...except IFR southeast of both lakes in lake
effect snow.
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 E of the lakes.

&&

.MARINE...
A deepening storm will move from Lake Erie, across Lake Ontario, and
into the St Lawrence Valley by Sunday.  Winds on the back side of
this system will quickly veer from S to WNW and reach gale force
through Sunday. Winds will slacken slowly Sunday night and Monday as
expansive high pressure over the southern states makes 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 early this morning to 1 AM
     EST Monday for NYZ001>007.
     Winter Storm Watch from this afternoon through Monday evening
     for NYZ005-006.
     Winter Weather Advisory until 1 PM EST Monday for NYZ019-020-
     085.
MARINE...Small Craft Advisory until 1 AM EST Monday for LEZ020.
         Gale Warning until 7 PM EST this evening for LEZ040-041.
         Small Craft Advisory until 1 AM EST Monday for LOZ030.
         Gale Warning until 10 PM EST this evening for LOZ042>045-
         062>065.

&&

$$

SYNOPSIS...TMA/ZAFF
NEAR TERM...RSH/TMA/ZAFF
SHORT TERM...THOMAS
LONG TERM...SMITH
AVIATION...RSH
MARINE...TMA/ZAFF
TIDES/COASTAL FLOODING...TMA



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