Area Forecast Discussion
Issued by NWS Upton, NY

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483
FXUS61 KOKX 151756
AFDOKX

Area Forecast Discussion
National Weather Service New York NY
1256 PM EST Fri Dec 15 2017

.SYNOPSIS...
Low pressure develops along the Middle Atlantic coast. This low
will strengthen as it tracks northeast through the evening and
continues out to sea tonight. High pressure then builds in
through Sunday. A series of weak low pressure and frontal
systems will pass through mid week.

&&

.NEAR TERM /THROUGH TONIGHT/...
Expanded the winter weather advisory to include the rest of Long
Island, New York City Metro, and the I-95 corridor into southern
Westchester, and southern Connecticut.

Snow amounts will range from 1 to 3 inches in the advisory area,
with around an inch to just below 2 inches further inland. The
advisory is in effect for more significant impacts to the
evening commute. Special weather statement remains in effect for
north and west of the advisory for lesser impact to the
commute, but still accumulating snow. Will continue to assess
mesoscale trends into the afternoon and fine tune snowfall
forecast if needed.

First reports of light snow are beginning to show up in Maryland
and in SE PA as upper divergence and synoptic lift continue to
increase. Regional radar mosaics are showing increasing
reflectivities to our west, indicative of rapid snow growth
aloft Divergence aloft is impressive as a 170-180 kt jet streak
moves across the Middle Atlantic. Our region will be located in
the favored left exit region for synoptic lift this afternoon
and evening. There will also be some interaction/phasing of the
northern and southern stream which combined with the upper jet
dynamics will intensify a surface low over the Carolinas as it
shifts towards the Middle Atlantic and then south of Long Island
this evening.

Middle level forcing is quite impressive as well on the
NAM, NAM-3km, HRRR, and RAP with a swatch of strong 850-700 mb
frontogenesis across the southern portion of the area. The lift
coincides within the dendritic growth zone which may enhance
snowfall and produce some banding, especially across Long
Island.

Snow will continue to blossom rapidly from west to east this
afternoon into the early evening. The snow should taper off
around 7-9pm in NYC and around midnight on the east end of Long
Island. Liquid equivalent amounts around 0.05 inches inland and
close to 0.20 inches are forecast across Long Island. Thermodynamic
profiles do not show much liquid water saturation aloft for
riming, so think snow will be a dry, high ratio type similar to
what we observed Thursday morning. Ratios may be 15-18:1.
Surface temperatures should drop into the 20s once the snow
begins.

Clearing and increasing winds are expected tonight behind the
departing low as the pressure gradient tightens between the
strengthening departing low, and strong high pressure over the
SE US. Another chilly night with lows in the ranging from upper
teens well inland to mid 20s at the coast.

&&

.SHORT TERM /SATURDAY/...
Mean trough axis shifts offshore on Sat with gradual rising
heights aloft. Could see some lake/ocean effect streamers come
close to the forecast area, although high res guidance is not
indicating we will be impacted by these. Right now, just expect
an increase in cloud cover during the day. Otherwise, a breezy
westerly flow and weak WAA should help temps rebound into the
30s.

&&

.LONG TERM /SATURDAY NIGHT THROUGH FRIDAY/...
Fairly good agreement with the operational models in the long term.
Generally, zonal flow in the upper levels through the middle of next
week, allowing for a progressive pattern.

At the surface, high pressure moves overhead during the day Sunday
and then pushes offshore Sunday night. This allow a warm front to
approach and lift north of the area from Sunday night through Monday
morning. Light overrunning precipitation will occur during this time
frame. Models differ somewhat on timing. Uncertainty will also lie
in precipitation type. Best chances for frozen would be inland
areas, with a rain/snow mix along the coast changing to plain rain
late Sunday night. The rain/snow line will move north Monday
morning, with plain rain expected everywhere by Monday afternoon.
Little, if any accumulation is expected with QPF amounts of only a
few hundredths of an inch.

Surface cold front then approaches Monday night and pushes through
Tuesday. Warmer temperatures expected on Monday and Tuesday with
warm air advection continuing out ahead of the front in the warm
sector of the frontal system. Temperatures will be slightly above
normal on Monday, and a few degrees warmer on Tuesday. Cooler
temperatures for Wednesday and Thursday.

Conditions will become breezy for Tuesday night into Wednesday as
the pressure gradient tightens between the parent low from the cold
front that tracks into the Canadian maritimes and the high pressure
building in from the southwest.

&&

.AVIATION /18Z FRIDAY THROUGH WEDNESDAY/...
***SNOW EXPECTED FOR EVENING PUSH***

High pressure slides east, giving way to an area of low
pressure passing south of the region this evening.

Clouds gradually lower and thicken late this afternoon. Snow
may initially start around 18Z, becoming steadier by around 20Z.
Conditions fall to MVFR then quickly to IFR with steadier snow
late this afternoon and evening. Expect a widespread 1-3 inches.
Highest totals expected for NYC terminals, KISP, KBDR and KGON.

Snow ends and conditions return to VFR between 02z-05z.

Light and variable winds become westerly at 5-10 kt later this
afternoon. Winds increase tonight with some gusts to 20kt after
midnight.

     NY Metro Enhanced Aviation Weather Support...

Detailed information, including hourly TAF wind component forecasts,
can be found at: http:/www.weather.gov/zny/n90

KJFK TAF Comments: Best chance of snowfall expected between
20z-02z with 2-3 inches of accumulation.

KLGA TAF Comments: Best chance of snowfall expected between
20z-02z with 2-3 inches of accumulation.

KEWR TAF Comments: Best chance of snowfall expected between
20z-02z with around 2-3 inches of accumulation.

KTEB TAF Comments: Best chance of snowfall expected between
20z-02z with around 2-3 inches of accumulation.

KHPN TAF Comments: Best chance of snowfall expected between
21z-02z with around 1-2 inches of accumulation.

KISP TAF Comments: Best chance of snowfall expected between
20z-02z with around 2-3 inches of accumulation.

.OUTLOOK FOR 12Z SATURDAY THROUGH TUESDAY...
.SAT...VFR. Chance MVFR after 18Z with isold -SHSN. WNW winds
G20-30KT daytime.
.SUN...VFR during the day. Chance MVFR in -RA/-SN towards
midnight.
.MON...Low chance MVFR in -RA for coastal terminals, light wintry
mix inland.
.TUE...MVFR possible. Slight chance of light -RA.

&&

.MARINE...
A brief period of tranquil conds on all waters today as high
pressure over the NE moves offshore. The pressure gradient will
tighten tonight as low pres passes S and E of the waters. SCA
conds are expected to develop around midnight and continue
through Sat, with the potential for gale force gusts on the
ocean waters late tonight and into Sat. Have issued a gale watch
for this potential, although it currently looks marginal. Could
also have a few hours of 35 kt on eastern LI Sound during the
day Sat, but didn`t have the confidence to include in the watch.

Winds and waves diminish Saturday night as high pressure builds
in. Waves come down below 5 ft across all ocean zones late
Saturday night.

Waves then build to above SCA criteria late Tuesday into
Tuesday night as a cold front moves through and winds increase
behind the cold front. Waves build to 5 to 9 ft on the ocean by
Wednesday and diminish thereafter through Thursday.

Windy conditions expected late Tuesday through Wednesday and at
least part of Wednesday night as the pressure gradient increases.
SCA winds likely across all waters through this time period. Gales
are possible during this time frame as well, but there is still
uncertainty this far out.

&&

.HYDROLOGY...
No significant hydrologic impacts expected.

&&

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

&&

.OKX WATCHES/WARNINGS/ADVISORIES...
CT...Winter Weather Advisory until 1 AM EST Saturday for CTZ011-012.
     Winter Weather Advisory until 10 PM EST this evening for
     CTZ009-010.
NY...Winter Weather Advisory until 1 AM EST Saturday for NYZ079-081.
     Winter Weather Advisory until 10 PM EST this evening for
     NYZ071>075-078-080-176>179.
NJ...Winter Weather Advisory until 10 PM EST this evening for
     NJZ006.
MARINE...Small Craft Advisory from 11 PM this evening to 6 PM EST
     Saturday for ANZ330-335-338-340-345.
     Gale Watch from late tonight through Saturday afternoon for
     ANZ355.
     Gale Watch from late tonight through Saturday afternoon for
     ANZ350-353.

&&

$$
SYNOPSIS...JP/DS
NEAR TERM...DS
SHORT TERM...24
LONG TERM...JP
AVIATION...BC/MD
MARINE...24/JP
HYDROLOGY...24/JP
EQUIPMENT...



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