Special Weather Statement, NWS New York, NY

Area Forecast Discussion
National Weather Service New York NY
949 AM EST Sat Jan 7 2017

.SYNOPSIS...
Snow today as low pressure off the Carolina coast tracks to
southeast of Long Island this evening and then to the Canadian
Maritimes on Sunday. Cold high pressure builds in Sunday into
Monday. Another storm system, mainly rain, affects the region
Tuesday night into Wednesday morning, with yet another weather system
slowly approaching the area Thursday through Friday.

&&

.NEAR TERM /THROUGH TONIGHT/...
Light snow over-spreading the region on schedule as of 14Z. No
changes to forecast.

Concern is for higher amounts due to the "fluff" factor and 0.4
to 0.5 QPF for NYC and 1.0" on eastern LI - which would put them
into the 6+ amounts. Also NE Jersey, N. Westchester and Putnam
will likely need an advisory.

Otherwise, 180 KT Jet in the 12Z OKX Sounding - not a coupled jet
structure, but a heck of an entrance region driving a textbook
ageostrophic direction circulation (see Uccellini and Johnson,
1979).

Overall, a very powdery snow will lessen impacts (and make for
easier shoveling).

From Earlier...

Models in good agreement with a
vigorous southern stream shortwave...currently moving through the
southern states...pivoting up the coast today around a polar low
digging south of Hudson's Bay. Resultant southern low pressure
will track up the coast today and about 50-75 miles se of the
40/70 lat/lon benchmark by this evening. Although low pressure
will not be overly strong and well offshore...the combination of
approaching shortwave energy and right rear quad of a 160+kt
phased n/s stream jet will provide deep lift of Gulf and Atlantic
moisture wrapping around low pressure.

00Z model guidance had shifted low pressure a bit farther west
than 24 hours ago. This still keeps the strongest low/mid-level
frontogenetic forcing to the se/e of LI...where snow amounts in
excess of 1 ft are expected. Concern is this westward model shift
could continue even a bit more based on potential downstream
ridging ahead of convection over Gulf and off SE US coast this
morning. The convectively induced vorts are actually being picked
up and ridden up the coast by the models for this
afternoon...which should result in enhanced lift/snowfall rates
for this afternoon for E LI/SE CT. Even a hint of conditional
symmetric instability aloft with this energy.

As for sensible weather. Light snow has worked in to south central
NJ as of 630am...with expectation for snow to develop across most
of the region from s to n by mid to late morning...becoming
moderate to heavy for E LI/SE CT during the late morning/
afternoon...and then tapering off from W to E this evening. A
period of moderate snow possible for NYC metro and SW CT late
morning into afternoon. Based on SPC SREF...NCAR ensemble and
HRRTLE snowfall rate probs...estimated maximum snowfall rates of
1/2 to 1 inch per hour are expected for NYC metro/SW CT and 1 to 2
inch per hour for E LI/SE CT during the late morning/afternoon.

In terms of snowfall amounts...challenges exist in banding
location...snow ratios...and location of sharp western cutoff to
precip. Consensus QPF for LI/CT has increased on the order of 1/10-
1/4" for LI/CT...with a general 1/2 to 3/4 inch looking likely.
Meanwhile...along and to the west of the NYC metro/Hudson River
there will likely be a sharp tapering down of qpf amounts as is
typical to the west of a strong deformation axis. Will increase
QPF slightly for this cutoff zone...with around 1/4 inch...with
areas to the west tapering to a dusting or less by the time you
reach the far NW hills. Finally...with temps holding in the lower
to mid 20s during the day a higher snow ratio (15-20 to 1) than
the typical (10-11 to 1) would be expected...but the best
sustained lift may lie above the ideal dendritic snow growth
zone...so this could temper ratios on average bit.

Based on above...have expanded warnings westward to Nassau County
on LI and New Haven in CT for 6+ inches snowfall...with as much as
7 to 12 inches for Eastern LI/SE CT. To the west...winter wx
advisories for NYC metro...Southern Westchester...and Fairfield in
CT...for on average a 2 to 5 inch snowfall. 1 to 2 inches for
much of NE NJ and Lower Hud...with perhaps only a trace for far NW
Orange. Strengthening N/NW winds in the late afternoon/evening
will also cause some blowing and drifting of snow across the open
areas of E LI/SE CT.

Clearing skies from west to east after midnight in wake of low
pressure...with breezy NW flow ushering in polar airmass and
dropping lows into the teens. Windchills expected to fall into the
single digits above zero for city/coast and just below zero for
interior.

&&

.SHORT TERM /SUNDAY/...
Polar trough approaches the region Sunday morning and swings across
during the day. Shortwave lift...cyclonic flow and residual/lake
moisture should result in plenty of afternoon strato-cu and
scattered snow showers. Breezy NW flow will continue between high to
the west and departing low pressure. Highs will be about 15 degrees
below seasonable in the lower to mid 20s...with windchills only
rising into the single digits to lower teens.

&&

.LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/...
High pressure and cold temperatures Monday.

Temperatures will begin to rebound Tuesday as high pressure departs
and an upper trough approaches from the west. Increasing mid-upper
level moisture may support a few flurries during the afternoon,
while winds strengthen from the southwest by evening. Given the
antecedent arctic air mass and strong warm advection, it is possible
that there could be a period of freezing rain and/or sleet across
interior portions of the forecast area before transitioning to rain
overnight. The upper trough and attendant cold front move through
Wednesday, with a period of moderate rain possible.

Thursday through Friday a blocking pattern sets up as a high
pressure ridge builds over the Atlantic and low pressure trough
builds across the central U.S. Southwest flow between the two
systems will advect a combination of Gulf and Pacific moisture
northeastward. Heavier precipitation will primarily fall north of
the forecast area, though chances of rain will increase each day as
we move towards the weekend. Broad southwest flow will keep
temperatures mild for the latter half of the week.

&&

.AVIATION /14Z SATURDAY THROUGH WEDNESDAY/...
Low pressure passes to the SE today into tonight.

Deteoriating conditions from south to north as -SN begins this
morning. Intensity will increase late this morning into the
afternoon, with heaviest snow expected from the city terminals on
east. Categories will quickly lower to IFR in visibility, then in
ceilings.

N winds around 15 KT with gusts 20-25 KT, backing and diminishing
slightly this evening.

Max snowfall rates may approach 1 inch per hour at the New York City
terminals, with higher rates likely at times at ISP, BDR and
GON. Lesser rates are expected at SWF. Timing of highest rates
still appears to be this afternoon.

Expected snow totals:
6-10 inches...KISP/KBDR/KGON
3-5 inches...KLGA/KJFK
2-4 inches...KHPN/KTEB/KEWR
1 or less inches...KSWF

There will be blowing snow due to dry nature of the snow tonight
into Sunday.

.OUTLOOK FOR 12Z SUNDAY THROUGH WEDNESDAY...
.Sunday...VFR. NW gusts 25-30 kt. Isolated snow showers/flurries.
.Monday...VFR.
.Tuesday...VFR. Slight chc MVFR late in light snow/wintry mix.
.Wednesday...MVFR or lower in rain AM / VFR PM. SW gusts around 25
kt.

&&

.MARINE...
VSBY of 1/4-1/2 miles or less in snow into this evening. Small
Craft Advisory (SCA) for all through Sunday afternoon. On the
ocean waters, SCA will probably need extension into Sunday night
with future updates. On the ocean, there is also the possibility
of a few gale force gusts for a short period during the late
afternoon and early evening for both today and Sunday. Not enough
confidence in this being a widespread event to go with a Gale
warning.

Winds diminish through the night on Sunday as the high pressure
ridge nears the region. All waters are forecast to fall below
advisory criteria by Monday morning. Sub-SCA conds then continue
through Monday as the ridge axis passes over the waters. Marginal
SCA conds then follow Monday night and Tuesday, possibly increasing
to gale-force by Wednesday as the pressure gradient tightens once
again ahead of the next storm system.

&&

.HYDROLOGY...
No hydrologic impacts are expected the next 7 days.

&&

.OKX WATCHES/WARNINGS/ADVISORIES...
CT...Winter Storm Warning until 1 AM EST Sunday for CTZ006>008-
     010>012.
     Winter Weather Advisory until 1 AM EST Sunday for CTZ005-009.
NY...Winter Storm Warning until 1 AM EST Sunday for NYZ078>081-177-
     179.
     Winter Weather Advisory until 1 AM EST Sunday for NYZ071>075-
     176-178.
NJ...None.
MARINE...Small Craft Advisory from 1 PM this afternoon to 6 PM EST
     Sunday for ANZ330-335-338-340-345.
     Small Craft Advisory until 6 PM EST Sunday for ANZ350-353-355.

&&

SYNOPSIS...MD/NV
NEAR TERM...Tongue/NV
SHORT TERM...NV
LONG TERM...MD
AVIATION...JC/DS
MARINE...JC/MD
HYDROLOGY...MD/NV

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