Area Forecast Discussion
Issued by NWS Upton, NY

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FXUS61 KOKX 200432

Area Forecast Discussion
National Weather Service New York NY
1232 AM EDT Tue Mar 20 2018

High pressure builds down from south central Canada into
Tuesday as a coastal low develops off the mid Atlantic coast by
Tuesday morning, then tracks east as it weakens. Another
stronger coastal low develops along the mid-Atlantic coast
Tuesday night, then lifts northeast to south of Nova Scotia by
Thursday morning, then into the Canadian Maritime Provinces
through Thursday night. High pressure will then build from
southern Canada from Friday through Saturday. Another storm
system will may impact the area late in the weekend.


Forecast remains on track with only minor updates needed to
reflect current temperature and dew point trends.

Weak northern stream ridging builds in tonight keeping things
dry. Will see an increase in high clouds from SW to NE through
the night. Lows tonight will be around 5-10 degrees below


Details are coming in to better focus on a couple of coastal
lows that will impact the Tri-State Tuesday-Wednesday night.

The first low is in response to a 700-500 hPa shortwave being
sheared by a cutoff low over Canadian Maritimes. As a result the
low weakens as it exits the mid-Atlantic coast, so only expect
light warm advection induced precipitation over mainly the S 1/2
of the area. It does appear that a warm nose works in
around/just above 850 hPa so do expect some sleet to mix in with
a snow/rain mix (rain mixed in mainly due to March sun angle).
Expect any accumulations on Tuesday to be limited to mainly far
SW parts of the CWA

This warm nose lingers over most of the area into Wednesday
morning (except for maybe far NW zones), so do expect sleet to
limit accumulations Tuesday night, limiting them to generally
less than 1 inch.

The second coastal low forms off the mid Atlantic Coast Tuesday
night in response to a combination of a shortwave diving SE
from the Canadian Plains and another shortwave coming in from
the Four Corners region. As the coastal low deepens Wednesday,
it taps into colder low level air to the north, coupled with
dynamic cooling, should change the precipitation to all snow
across the region. There is the potential for a period of heavy
wet snow across the region from late Wednesday morning into
Wednesday evening. In addition gusty winds of 25 to 40 mph could
combine to bring down tree limbs and power lines. The snow
tapers off from W to E Wednesday night as the coastal low pulls
away to the NE.

As a result have issued a winter storm watch for the region from
6pm Tuesday-midnight Wednesday night for NYC/NE NJ/Lower HUdson
Valley for 5-11 inches of snow and from 4am Wednesday-6am
Thursday for Long Island and S CT for 4 to 9 inches of snow.


High pressure builds into the region from southern Canada on
Thursday, providing generally dry conditions through the first half
of the weekend. Some weak forcing may spark off a few snow showers
or flurries late Saturday with an upper level low off the New
England Coast, but not enough confidence to put it in the forecast
as of yet. However, the bigger story will be the potential for
another low pressure system to affect the region for the second half
of the weekend.

There are significant differences in the model solutions, both in
the deterministic and ensemble models, for the second half of the
weekend and the potential low pressure system to affect the area.
The main differences stem from orientation of a strong 1043 mb
surface high pressure over southern Canada Saturday that digs all
the way south into Florida and the Southeast US Coast. The 19 12Z
ECMWF pinches off a piece of this high Saturday night, giving an
area of low pressure developing over the Mid-West a path between the
two highs to move toward our region. The 19 12Z GFS does not pinch
off this high as notably as the ECMWF, with more of a cold air
damming signature noted on Sunday, when the ECMWF pushes the low off
the Carolina Coast, with our region on the northern fringes of the
precipitation field. The low is then forecast to pass well south of
our area and then east out to sea Sunday night. The GFS suppress the
low well south and weakens it. This latest model run of the ECMWF is
more in line with the GFS. Given forecast uncertainty, will not make
much in the way of changes.

Proximity of the low to the coast will determine precipitation
types. If the ECMWF is to be believed, the track is a bit different
to previous storms, at least with its approach to the area, with a
more west to east track, then head northeast east, as opposed to
development or redevelopment off the Mid-Atlantic coast and heading
northeast. However, it will be cold enough for some snow.

Temperatures during the long term will average below normal with a
persistent northwesterly flow.


High pressure gives way to a series of lows passing to the
south and east of the area, the first Tuesday evening, and then
another on Wednesday.

VFR. Light northerly winds will become NE overnight and then
gradually ramp up during the day Tuesday. Gusts to around 20 kt
are expected at the coastal terminals in the afternoon and
increasing to 25-30 kt Tuesday night. Inland gusts will be a bit

There is a chance of a light wintry mix developing Tuesday
evening. Confidence is too low at this time to mention in TAFs.
The brunt of the precipitation is currently forecast to develop
on Wednesday.

.Tuesday night...MVFR conditions possible with a light wintry
mix. NE winds G20-30KT NYC metro/coastal terminals.
.Wednesday...Snow and sleet with IFR or lower conditions
likely. NE winds G20-30KT.
.Thursday-Friday...VFR. N winds G15-20KT on Thu.
.Saturday...MVFR possible late in chance of rain/snow.


Winds and seas will remain tranquil early tonight as high pressure
builds in from the northwest. However, the gradient between low
pressure off to the south and west and high pressure to the
northwest will increase. Therefore, winds will then gradually begin
to strengthen late tonight and into Tuesday morning.  This will
allow winds and seas to increase to SCA-levels Tuesday
afternoon and evening for the ocean and eastern sound, with Gale
to storm conditions developing Tuesday night into Wednesday.

Have issued a storm watch for the ocean for Tue night into Wed, as E-
NE winds rapidly increase in response to the development of the
coastal low. Expect gusts mainly 35-45 kt during this time, and peak
gusts of 45 to 50 kt may be possible on the outer ocean waters. Gale
conditions are expected for the Eastern Sound and the bays of Long
Island overall during this timeframe. Gales should then linger on
the ocean through much of Wed night.

Winds diminish Thursday as low pressure heads away from the coastal
waters. SCA winds possible early in the morning for the ocean waters
and Peconic and Gardiner`s Bay. Thereafter, winds should remain
below 25 kt through Saturday night. Winds increase late Saturday
night into Sunday morning as another low pressure system approaches
the coastal waters. A period of SCA winds are possible from Sunday
morning through Sunday night, mainly on the ocean waters as the low
pressure moves toward the region.

Waves diminish on Thursday as well, with ocean waters coming down
below 5 ft Thursday night. Waves then remain below 5 ft through
Saturday, building again Saturday night in response to approaching
storm. 5 ft waves possible late Sunday morning.


It will be mainly dry through tonight.

There is the potential for a prolonged period of precipitation from
Tuesday through Wednesday night. Total liquid equivalent precip
should range from 0.5 to 1.25 inches, falling mainly as snow and
sleet, so no hydrologic impact expected.


A prolonged NE/N flow will have water levels building Tuesday
night into Wednesday. Tidal departures of generally only 1 to 1
1/2 ft are needed for minor flooding for many locations Tuesday
night through Wednesday night, and 2 to 3 ft for moderate.

At this point, mainly minor impacts are expected during the
Tuesday Night high tide as NE winds are just beginning to ramp
up to SCA/Gale force.

With NE gale to storm force winds continuing 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. Latest guidance has trended
upward with surge for this tidal cycle. At this point, highest
confidence is in the southern bays of Nassau and Western Suffolk
county reaching moderate flood thresholds, but this threat may
need to be expanded a bit depending on model/surge guidance

Another round of minor to locally moderate coastal flooding
likely for the Wed night high tide cycle. There is more 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 multi tidal cycle
surge events.


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


CT...Winter Storm Watch from late tonight through late Wednesday
     night for CTZ005>012.
NY...Winter Storm Watch from late tonight through late Wednesday
     night for NYZ078>081-177-179.
     Winter Storm Watch from this evening through Wednesday evening
     for NYZ067>075-176-178.
     Coastal Flood Watch from Wednesday morning through Wednesday
     afternoon for NYZ080-179.
NJ...Winter Storm Watch from this evening through Wednesday evening
     for NJZ002-004-006-103>108.
MARINE...Gale Watch from this evening through late Wednesday night for
     Small Craft Advisory from noon today to 6 PM EDT this evening
     for ANZ330-340-350-353-355.
     Storm Watch from this evening through Wednesday evening for
     Storm Watch from this evening through late Wednesday night for


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