Area Forecast Discussion
Issued by NWS Upton, NY

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871
FXUS61 KOKX 090247
AFDOKX

Area Forecast Discussion
National Weather Service New York NY
947 PM EST Fri Dec 8 2017

.SYNOPSIS...
An area of low pressure along the southeast coast tonight
tracks south and east of Long Island on Saturday, and off the
New England coast on Saturday night. High pressure will return
Sunday into Monday. Another area of low pressure impacts the
region on Tuesday and Wednesday, followed by high pressure
Thursday. Another low pressure system approaches for the end of
the week.

&&

.NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
Forecast looks right on track with only minor tweaks late this
evening. Quick peek at new NAM supports forecast.

Models in good agreement with strong shortwave energy diving
down the backside of Great Lakes trough, helping to amplify this
trough and begin lifting an intense southern stream shortwave
up the coast. This will also intensify a phased SW upper jet
165-175kt west of the region. At the surface, low pressure
over the Gulf of Mexico will track up off the SE US coast by
Saturday morning.

Increasing lift and moisture due to this scenario will result in
lowering and thickening clouds, with increasing potential for
light precipitation development from SE to NW along the coastal
plain after midnight. Initially quite a bit of dry air to
overcome, but with a southern stream energy feed and subtropical
moisture connection have leaned towards a quicker saturation
and slightly quicker precip development along the coastal plain
late tonight into Saturday morning. Already observing snow
accums in Delaware. Thermal profiles would support snow across
almost all the areas, outside of some initial mixing with rain.

&&

.SHORT TERM /6 AM SATURDAY MORNING THROUGH SATURDAY NIGHT/...
Models continue in good agreement with northern stream
shortwave energy diving down through the Great Lakes into Ohio
Valley on Sat, and then pivoting into the NE Sat night. The
resultant amplifying Eastern US trough will send a southern
stream shortwave up the coast on Saturday, with at least the
eastern 1/2 to 2/3rd of CWA under a favorable right rear of
165-175 kt jet dynamics. At the surface, low pressure tracks
from the SE US coast Sat morning to near the 40/70 lat/lon
benchmark by Sat Evening.

This deep lift tapping into a sub-tropical moisture feed
offshore should allow for snow to overspread the entire region
from SE to NW late tonight into Saturday morning, becoming
steadier later Saturday morning. Models also indicating some
weak mid-level frontogenesis and even some negative epv aloft
across LI/CT which signals potential for some moderate snow
banding in the afternoon/evening. This presents a low to
moderate probability of 1/2 to 1 inch per hour snow fall rates
in the afternoon. Meanwhile, NW zones will be on the edge of
frontogenetic and jet forcing, so there will be subsidence and
dry air to combat, resulting in uncertainty on the expanse of
accumulating snowfall across areas NW of NYC.

In terms of ptype, thermal profiles are cold enough for most of
the region to be in the form of snow. But an 850-950 hpa warm
layer appears that it could make inroads into far se portions of
the area Sat aft/eve to result in a mixing with or changeover
to rain as low pressure make the closest approach. The reason
for this is that northern/southern stream phasing of energies
does not start taking place until late Sat/Sat evening as the
low/mid level low is tracking SE of LI, which does not allow for
enough tightening of the thermal gradient to ensure cold air
all the way to the coast through the event. At this point, based
on an ensemble of thermal profiles, the south fork of LI
appears to have the highest probability of seeing the warm nose
impinge and result in a period of mixing Sat afternoon into
evening.

In terms of QPF, models have trended higher with QPF across he
region, likely better sampling the 3-4+ std PWAT sub-tropical
convection riding up the warm conveyor belt. Generally 1/2 to
3/4 of an inch of across LI/CT, highest east, 3 to 5 tenths
across NYC/NJ metro and SW CT/SE NY border, tapering to 1/4 inch
for far nw zones. With boundary layer and surface temps
holding around freezing along the coast during the event, and
upper 20s to lower 30s across interior, snow ratios not
expected to be too much above 7-10 inches of snow to 1 inch of
liquid.

In terms of snowfall, based on P-type and dynamics mentioned
above, confidence in 6 inches or more of snow for SE CT and
Eastern LI (except for the south fork) is high enough to upgrade
to warning. Farther east a solid advisory level snow expected,
with 3 to 6 inches to at least the Hudson River. The caveat in
terms of snowfall amounts will be that snow accumulation on
pavements may be limited until solar insolation decreases in the
mid to late afternoon through the evening due to marginal
surface temps.

In term of uncertainty, the subtropical moisture feed and SE US
convection strengthening Atlantic ridging are two players that
could either increase QPF across the area or result in a slight
westward shift in the storm. This would have the implication of
increasing potential for warning level snow westward towards the
Hudson River, Advisory level snows in Orange County. On the
other end this would also increase potential for mixing and
reduce snow amounts for the Eastern LI and SE CT. These are low
chance probs at this time, and w ill have to monitor mesoscale
models, satellite and radar trend tonight.

Snow should taper from NW to SE Sat evening/night as shortwave
energy slides NE of the region and low pressure departs into the
Gulf of Maine.

&&

.LONG TERM /SUNDAY THROUGH FRIDAY/...
Snow showers expected Sunday morning as a low pressure system lifts
into the Canadian Maritimes on Sunday. A tight pressure gradient
develops on Sunday resulting in a gusty westerly flow with high
temperatures in the mid to upper 30s and overnight temperatures in
the low to mid 20s. Weak high pressure passes through the local area
Monday with temperatures slightly moderating but, remaining below
normal for this time of year.

A strong upper level low will approach from the Ohio Valley region,
impacting the local area Tuesday through Wednesday. Due to much
uncertainty at this time with QPF and/or snow, have continued with
the chance of snow, possibly mixed with rain at the coast Tuesday
into Tuesday night. High pressure then follows for Thursday ahead of
another clipper low and reinforcing cold shot at the end of the week.

Temperatures Tuesday and through the end of the week will remain
below seasonable levels.

&&

.AVIATION /03Z SATURDAY THROUGH WEDNESDAY/...
Low pressure tracks off the Carolina coast early Saturday, and
into the Canadian Maritimes Saturday night.

VFR through at least 6Z. Light snow develops from southeast to
northwest through 12Z. Please see TAFs for exact timing. Snow
becomes heavier thereafter, with the most intense snowfall
likely during the middle of the day into the early evening. Snow
tapers off after 00Z from west to east.

West winds become light and variable tonight, with northerly
flow generally below 12 kt on Saturday.

Terminal Snowfall Forecast:

KJFK: Snowfall rates could reach 1 inch per hour. Around 5
inches of total snow forecast.

KLGA: Snowfall rates could reach 1 inch per hour. Around 4
inches of total snow forecast.

KEWR: Snowfall rates could reach 1 inch per hour at times.
Around 4 inches of total snow forecast.

KTEB: Snowfall rates could reach 1 inch per hour at times.
Around 4 inches of total snow forecast.

KHPN: Snowfall rates could reach 1 inch per hour at times.
Around 4 inches of total snow forecast.

KISP: Snowfall rates could exceed 1 inch per hour at times. 4-6
inches of total snowfall forecast.

.OUTLOOK FOR 00Z SUNDAY THROUGH WEDNESDAY...
.Saturday night...Snow tapers off from west to east. MVFR or
lower ceilings.
.Sunday...Light snow may linger in the morning near KGON with
possible MVFR/IFR, otherwise becoming mainly VFR. NW-W winds G20-25
kt possible.
.Sunday night-Monday. Mainly VFR, with MVFR possible in isolated
snow showers. WSW-W winds G15-20KT possible.
.Monday night-Wednesday...MVFR or lower is possible with a chance of
snow. A wintry mix or change over to rain is possible Tuesday
afternoon along the south shore of Long Island. SW-W winds G15-25KT
possible Monday night-Tuesday. NW winds G20-30KT possible Tuesday
night and Wednesday.

&&

.MARINE...
A relaxed pressure gradient will remain over the waters through
the overnight. The pressure gradient tightens Saturday into
Saturday night. Marginal small craft gusts are likely on the
ocean Saturday Afternoon into Saturday night, particularly
eastern ocean waters. Ocean seas will begin to build above SCA
Saturday afternoon as low pressure passes makes closes approach
late Saturday into Saturday night.

SCA conditions will continue Sunday into Monday as a low pressure
system moves into the Canadian Maritimes. Winds will gradually
diminish through Monday as high pressure builds across the area
waters. Another low pressure system is forecast to impact the area
waters on Tuesday and into Wednesday resulting in the potential for
SCA conditions to return. High pressure builds across the area
waters on Thursday with seas falling slowly below SCA
conditions.

&&

.HYDROLOGY...
Generally 1/2 to 3/4 of an inch of liquid is expected across
LI/CT( highest east), 4 to 5 tenths across NYC/NJ metro to SW
CT/SE NY border, tapering to 1/4 inch for far NW zones; Saturday
into Saturday night. The precipitation is expected to fall as
mainly snow, so no hydrologic impacts are anticipated.

No hydrologic impacts are anticipated through the extended
period.

&&

.EQUIPMENT...
NYC NOAA Weather Radio Station KWO-35 (162.55 MHz) is off the
air for an extended period of time. See our headline news on
the web at: http:/weather.gov/nyc

&&

.OKX WATCHES/WARNINGS/ADVISORIES...
CT...Winter Storm Warning from 4 AM Saturday to 6 AM EST Sunday for
     CTZ007-008-010>012.
     Winter Weather Advisory from 4 AM Saturday to 4 AM EST Sunday
     for CTZ005-006-009.
NY...Winter Storm Warning from 4 AM Saturday to 6 AM EST Sunday for
     NYZ078>080.
     Winter Weather Advisory from 4 AM Saturday to 1 AM EST Sunday
     for NYZ068-069.
     Winter Weather Advisory from 4 AM Saturday to 4 AM EST Sunday
     for NYZ070>075-176>179.
     Winter Weather Advisory from 4 AM Saturday to 6 AM EST Sunday
     for NYZ081.
NJ...Winter Weather Advisory from 4 AM Saturday to 1 AM EST Sunday
     for NJZ002-004-103-105-107.
     Winter Weather Advisory from 4 AM Saturday to 4 AM EST Sunday
     for NJZ006-104-106-108.
MARINE...Small Craft Advisory from 1 PM Saturday to 7 AM EST Sunday for
     ANZ350-353-355.

&&

$$
SYNOPSIS...Fig/NV
NEAR TERM...NV/PW
SHORT TERM...NV
LONG TERM...Fig
AVIATION...JMC/Maloit
MARINE...Fig/NV/PW
HYDROLOGY...Fig/NV
EQUIPMENT...



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