Area Forecast Discussion
Issued by NWS Upton, NY

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FXUS61 KOKX 210258

Area Forecast Discussion
National Weather Service New York NY
1058 PM EDT Tue Mar 20 2018

A major winter storm will impact the tri-state area Wednesday
into Wednesday night as a deepening coastal low moves off the
Mid Atlantic coast to south and east of Long Island. High
pressure builds in from southeast Canada through the weekend.
Another storm system may pass to our south late in the weekend.


An upper low over the Tennessee and Ohio Valleys translates
east tonight ahead of a northern branch shortwave trough moving
into the upper Great Lakes. This will maintain a downstream
polar high across eastern Canada with an anomalously cold
airmass in place for the start of the spring. At the same time,
a coastal low will take shape off the Mid Atlantic coast
overnight in response to the forcing from the approaching upper
low. It is this storm that could bring snow or a snow/sleet mix
mainly for the city, Long Island and NE NJ towards daybreak,
which will be the start of what could be the biggest March
snowfall recorded at Central Park since 1960. Farther north,
drier air extending from high pressure to the north likely keeps
things dry through the night.

As for temperatures, lows will range from the upper 20s across
the interior to the lower 30s at the coast. This is nearly
seasonable due to the insulating affect of the cloud cover.


Snow will overspread the area from the coast into the interior
during the morning hours becoming heavy with snowfall rates of
1 to 2 inches an hour in the afternoon. This could possibly move
in as early as late morning at the coast. As for the guidance,
since the 06Z run there has been a shift to the southeast with
the axis of heaviest precipitation, which now appears to be from
NYC northeast across Long Island and southern Connecticut. This
area has liquid equivalent amounts around 1.5 inches, with
lesser amount to the NW. Much of the area by the time the event
ends Wednesday night will receive between 10 to 16 inches of
snowfall with localized higher amounts. Briefly, there could be
a mix with sleet at the coast in the morning.

Unlike previous storms, this airmass is colder and drier and
with the onset of the moderate to heavy snowfall, temperatures
will likely remain at or below freezing for much of the event.
Thus, expect a higher snowfall ratio, closer to 10:1 or even

There is some uncertainty with the ECMWF solution pointing to
lower liquid equivalent amounts, but it appears to be an
outlier from the respect of a weaker closed low off the New
England coast by Wednesday night. The exact placement of the
heavy snow band is also a difficult thing to pinpoint, but a
well defined deformation zone and frontogenesis to the NW of
the low track points to this signal.

The winds will be gusty throughout the day on Wednesday, with
the strongest gusts across coastal Connecticut and coastal Long
Island. The winds will gust up to 30 mph across New York City,
and 35 to 45 mph across coastal sections of Connecticut and Long
Island. This will result in periods of near-blizzard conditions.


The coastal storm will continue to pull away from the area Thursday.
There will be some lingering snow showers, mainly for Long Island
and southeastern Connecticut in the morning, with only minor
additional accumulations possible. Winds will diminish as well as
the pressure gradient relaxes, though it will still be rather breezy
Otherwise, dry conditions should return for Thursday afternoon as
high pressure builds in from southeastern Canada.

Strong high pressure continues building in at the surface through
the weekend. Upper level low moves through on Friday. Little
moisture to work with, with only weak lift noted, so any light
precipitation looks unlikely, but a passing shower or flurry cannot
be ruled out just yet.

Low pressure over the Mid-West will move east late Saturday into
Sunday. With strong surface high pressure building south from
southeastern Canada, it will suppress the storm to the south,
ejecting off the Mid-Atlantic/Southeast US coast on Sunday. An upper
level trough/upper level low coupled with weak surface troughing may
provide more in the way of lift for some snow showers or flurries.

The storm system that ejects off the coast is forecast to stall out
over the Western Atlantic through the middle of next week, while
surface high pressure gets squeezed between the offshore low and
another approaching storm system to the west, with ridging aloft.


Major Winter Storm to Impact Terminals Wednesday into Wednesday

A strong low pressure area will intensify off the DelMarVa
Coast Wednesday, gradually moving well southeast of Long Island
late Wednesday through Wednesday night.

Spotty light wintry mix for NYC/NJ metro and LI terminals
overnight. Steadier snow expected to develop early Wednesday
morning, becoming heavy at time late Wed morning through Wed eve
with LIFR/VLIFR conds. During periods of lighter precip, could
see sleet mix in for eastern terminals. Isolated thunder
possible during the afternoon/evening. Snow should taper from
west to east after midnight, with conds gradually improving to
VFR late.

Total forecast snow accumulations in inches:

KSWF: 6-10 inches

Snowfall rates of 1 to 3 inches per hour at times late
morning/early afternoon and continuing through the late

N/NE winds 15-25 kt and gusts 25-35 kt likely Wed AM into Wed
Eve, highest along the coast. Peak gusts 35-40 kt possible.
Winds back to the NW Wed Eve, but remain gusty.

.Wednesday Night...Snow tapers off from west to east after
midnight, with conds improving to VFR. NE winds G25-35KT.
.Thursday-Friday...VFR. N winds G20-30KT on Thu.
.Saturday night-Sunday...MVFR possible in chance of rain/snow.


Any Small Craft and Gale conditions will yield to Storm conditions
on the ocean waters overnight and last through Wednesday.  Gale
conditions will prevail by early Wednesday morning for the remaining
western waters and prevail through the day on Wednesday.
By Wednesday night the Storm conditions will reside only in the
eastern ocean waters and last until Thursday morning.  Gale
conditions will continue for the eastern sound also until Thursday
morning.  Gusts over the eastern ocean waters may get up to 55 kts
during Wednesday and Wednesday night with seas 10 to 15 ft.

Waves on the ocean diminish on Thursday, with waves heights in the
morning generally from 8-12 ft. Waves fall below 5 ft late Thursday
night and remain below SCA criteria through Saturday. Waves build
again Saturday night as a storm system passes well south of the
coastal waters, allowing for an increase in northeasterly flow.
Waves then remain above SCA criteria through the beginning of the
new work week as the coastal storm remains over the western Atlantic.

Winds also diminish Thursday, coming below 25 kt late Thursday night
and then remain below SCA criteria through early Sunday morning.
Winds of 25 kt or higher are then possible through Monday morning in
response to the offshore coastal storm.


A significant precipitation event will impact the area Wednesday
into Wednesday night with liquid equivalent amounts from 0.75
to 1.50 inches, highest from NYC and points east. This will fall
mainly as snow, so no hydrologic impact expected.


Prolonged NE-N flow will push water levels up tonight into Wed
night. Departures of about 2-3 ft for moderate flooding, and
1-1.5 ft are needed for minor flooding for many locations.

Just localized minor impacts expected with tonight`s high tide
cycle as NE winds are just beginning to ramp up.

With NE gale to storm force winds going into the late Wednesday
morning/early afternoon high tide cycle, widespread minor to
moderate impacts are likely for the Wed late morning/early afternoon
high tide. Values for the north shore and twin forks of eastern
Long Island were increased where winds will be strongest and
wave action will cause additional runup.

Another round of minor to moderate coastal flooding likely for
the Wed night high tide cycle. Coastal flood watch continues for
the south shore bays of NYC and western Long Island where
impacts appear to be the greatest. There is continued guidance
spread with this tidal cycle, due to competing factors of tidal
piling and energetic wave energy build water levels along the
coast, versus N/NW gales pushing water away from the coast. Will
have to continue to monitor this tidal cycle, as guidance
typically has a low bias on the back end of multiple tidal
cycle surge events.

Water levels expected to fall considerably for Thu afternoon
high tide cycle, with continued offshore winds behind the
departing low. Some localized minor coastal impacts possible.


NYC NOAA Weather Radio Station KWO-35 (162.55 MHz) is off the
air for an extended period of time.


CT...Winter Storm Warning from 6 AM Wednesday to 6 AM EDT Thursday
     for CTZ005>012.
     Coastal Flood Advisory from noon to 6 PM EDT Wednesday for
NY...Winter Storm Warning from 6 AM Wednesday to 6 AM EDT Thursday
     for NYZ067>075-078>081-176>179.
     Coastal Flood Advisory from noon to 6 PM EDT Wednesday for
     Coastal Flood Advisory from 10 AM to 2 PM EDT Wednesday for
     Coastal Flood Warning from 9 AM to 3 PM EDT Wednesday for
     Coastal Flood Watch from Wednesday evening through late
     Wednesday night for NYZ075-080-178-179.
NJ...Winter Storm Warning from 6 AM Wednesday to 6 AM EDT Thursday
     for NJZ002-004-006-103>108.
     Coastal Flood Advisory from 10 AM to 2 PM EDT Wednesday for
MARINE...Gale Warning from 2 AM Wednesday to 6 AM EDT Thursday for
     Gale Warning from 6 AM to 8 PM EDT Wednesday for ANZ335-338.
     Storm Warning from 2 AM to 8 PM EDT Wednesday for ANZ355.
     Storm Warning from 2 AM Wednesday to 6 AM EDT Thursday for


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