Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 180645

Area Forecast Discussion
National Weather Service Buffalo NY
145 AM EST Sat Nov 18 2017

A strengthening storm system approaching from the mid western states
will generate occasional rain across our region on Saturday...then a
strong cold front will plow across the Lower Great Lakes to produce
widespread mixed precipitation Saturday night. 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 December like
airmass will generate accumulating snows southeast of both lakes
into Monday.


High pressure will slide to our east while the an area of low
pressure begins to strengthen while tracking to near Saint Louis. A
warm front ahead of this storm will shift into the Ohio Valley. Much
of our area can expect fair/dry weather to prevail for most of the
overnight although under widespread mid and high cloud cover ahead
of this system. Towards daybreak...moisture transport and lift
brought on by a low level jet oriented with the warm front...will;
produce rain for far western New York.

Expect that surface temps will be above freezing for nearly all
locations before the rain arrives early this morning.

Regarding the freezing rain threat east of Lake Ontario on
Saturday AM...precipitation should hold off long enough for
temperatures to rise above freezing while the lower airmass
slowly saturates. With this in mind, have backed off the
freezing rain threat except for over Lewis County. May have to
wait until Saturday before it becomes obvious for or against a
freezing rain threat. Any threat would be short-lived and last
at most an hour or so.

Later today (Saturday)...Model consensus tracks the feature surface
low across Illinois/Indiana to near or just north of Lake Erie while
deepening to the vicinity of 990mb. Strengthening southerly flow
ahead of this will promote rising surface temperatures and
widespread rain spreading from western into central New York. 850mb
temps increasing to between +3C (north country) and +8C (western
SoTier) would normally support high temps pushing into the 60s but
with widespread rain, expect that evaporational cooling will only
support a high of low 50s in WNY and mid 40s in the North Country.
PWATs of between 0.75 and 1 inch is a slight positive anomaly for
this time of year with model consensus QPF yielding a healthy 0.75-1
inch for WNY with 0.25 to 0.5 inch across CNY through 7pm Saturday.
Rainfall will probably come in several waves, with the first
arriving in the AM, possibly followed by a break for a several hours
during the midday.

Heavier periods of rain will then arrive later Saturday afternoon in
the western Southern Tier where the low level jet strengthens ahead
of an approaching cold front. Winds will not be gusty through the
day as warm advection keeps the higher winds aloft from mixing down.


Winter headlines may be needed during this period, as a
transition to accumulating lake snows takes place southeast of
both lakes.

Deepening surface low pressure is expected to track just to the
north of the region Saturday night, which will swing a powerful
cold front through the area. Deepening cold air behind the cold
front will set the stage for some mixed lake effect precipitation
east of both lakes. There could even be a coating of wet snow
along the Boston Hills and Chautauqua ridge and Tug Hill region.

Winds will ramp up within the cold air advection pattern, especially
Sunday. Momentum transfer profiles suggesting 45 to 55 mph wind
gusts at best, with the highest wind gust potential confined to the
immediate shorelines of Lake Erie and Lake Ontario.

A cold cyclonic flow of air will continue across the lower Great
Lakes Sunday into Monday. This will set the stage for lake
effect snows southeast of the lakes in a northwest flow regime.
Over-lake instability certainly looks outstanding with profiles
suggesting moderate to extreme instability developing with lake
induced CAPES over 500 J/KG and inversion heights very
respectable. However, the synoptic moisture fields are far from
outstanding with omega displaced below favorable dendritic
growth zone combined with less than ideal shear profiles all
would suggest this lake effect event likely be a plowable, but
not significant snowfall from the southern Tug Hill over to
east of Rochester off Lake Ontario and the Chautauqua ridge and
Boston Hills off Lake Erie.

Lake effect snows will begin to diminish off of both lakes Monday
in response to a ridge of high pressure quickly building in
across the lower Great Lakes, which will effectively limit
synoptic moisture and lower the capping inversion. As the ridge
builds in, the steering flow will slowly pivot the weakening
snow bands northward toward Buffalo and into the Tug Hill before
ending by Monday evening.


A shortwave trough will dip across the Eastern Great Lakes region
this period, with several additional shortwaves maintaining a deeper
long wave trough over the region through the holiday.

Sunshine Tuesday will fade through the day as a cold front nears the
region...with the cold front bringing a mix of rain and snow Tuesday
night, changing to all snow later Tuesday night and Wednesday as
deeper colder air (-10C/850 hPa) builds over the eastern Great
Lakes. This colder air will generate lake instability with details
of how much moisture and wind direction to still be resolved
relating to lake effect snow potential for Wednesday-Thursday.

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

Temperatures Tuesday will be above normal...but expect at or below
normal temperatures the remainder of the forecast period.


While VFR conditions will remain intact across all of western and
north central New York through approaching storm
system over the mid western states will result in deteriorating
conditons after 10z with the leading edge of rain making it into the
far western counties by daybreak.

The concern through daybreak though will be low level wind shear
over the western counties...including the KBUF...KIAG...KJHW and
KROC Taf sites. While sfc winds will be relatively light from the
southeast...winds as low as 1500 ft will be from the south-southwest
at 40 to 45 knots.

After daybreak today...a swath of rain will cross our region ahead
of a warm front. Cigs will eventually drop to MVFR levels
regionwide...with IFR cigs expected over the Southern Tier and
portions of the Finger Lakes region. This would mainly impact sites
such as KJHW and KELZ.

IFR cigs are forecast to become a little more widespread Saturday
night...otherwise MVFR cigs with rain can be anticipated. The rain
will taper off to mixed rain and snow showers late Saturday night...
except east of Lake Erie where the mixed precipitation will
transition to heavier lake effect.


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.


A deepening storm system over the mid western states will approach
the Lower Great Lakes today (Saturday)...and this will result in
freshening southerly winds. While winds will approach small craft
advisory criteria...the highest waves will be directed into Canadian

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 (Sat night) 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 expansive high pressure over the southern states will
make its way off the southeast coast.


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.


NY...Lakeshore Flood Watch from late tonight through late Sunday
     night for NYZ001>007.
MARINE...Gale Warning from 10 PM this evening to 7 PM EST Sunday for
         Gale Warning from 1 AM to 10 PM EST Sunday for



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