623
FXUS61 KOKX 202039
AFDOKX

Area Forecast Discussion
National Weather Service New York NY
339 PM EST Fri Dec 20 2024

.SYNOPSIS...
-- Changed Discussion --
A coastal low strengthens as it passes south and east of the area
tonight. High pressure then gradually builds in through early next
week.
-- End Changed Discussion --

&&

.NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
-- Changed Discussion --
The weakening clipper low that was approaching the area from the
west has been absorbed by the strengthening offshore low. Per 18z
WPC surface analysis the offshore low is located east of the
Delmarva about 230nm south of Long Island. The low is progged to
track northeastward and pass just southeast of the 40/70
benchmark. Aloft, the area remains just east of an upper level
trough axis. Shortwave energy rounding the trough will help
strengthen the offshore low as it moves south and east of the
area.

Widespread light precipitation has been observed today. Light
precipitation moved in from the west associated with the weakening
clipper and light to moderate precipitation moved in from the east
associated with the offshore low. Across much of the area this has
fallen as rain/snow or snow. Highest totals so far from grassy
areas across the interior have been around half an inch. Across
eastern Suffolk County and coastal locations it has been mostly
rain. This will transition to all snow for most by late this
afternoon. A lull period might be seen across the western half
of the area, but with guidance trending the offshore low closer
to the coast, higher QPF is now expected across eastern CT and
eastern LI. While the greatest lift is just east of the area,
eastern CT and eastern LI may just be on the western edge of
strong mid level frontogenesis now that the low will likely pass
closer. Since this will fall as all snow across eastern CT,
have bumped up snow totals there to 1 to 2 inches.

PoPs then increase again overnight into early Saturday morning as
another shortwave moves through. This will likely lead to additional
snowfall for at least the western half of the area, but amounts will
be light.
-- End Changed Discussion --

&&

.SHORT TERM /6 AM SATURDAY MORNING THROUGH SATURDAY NIGHT/...
-- Changed Discussion --
Heights slowly rise aloft and high pressure builds in through the
short term period. Snow may be lingering Saturday morning but will
be light and likely end by the afternoon. A very cold airmass starts
to build into the area. 850 temps at 00z 12/22 will be near the
min moving average per SPC Sounding Climatology. Highs Saturday
will be in the 20s and 30s and with a 15 to 20 mph northwest
flow the wind chill values will be in the teens to mid 20s.
Temperatures then drop Saturday night to the teens, with wind
chill values as low as 5 below.
-- End Changed Discussion --

&&

.LONG TERM /SUNDAY THROUGH FRIDAY/...
-- Changed Discussion --
Deep layered ridging occurs over the region Sunday through Monday
morning with a surface ridge axis shifting through later in the day
Monday. The pressure gradient will be tight with gusty conditions
Sunday morning. Below normal high temperatures averaging in the mid
20s across the area for Sunday with afternoon wind chills only in
the teens. The airmass moderates a little on Monday with high
temperatures from the mid 20s to the lower 30s. Winds will be light,
so wind chills will be only a few degrees below the ambient
temperature.

The flow aloft becomes more cyclonic Monday night through Tuesday
night with two separate shortwaves likely phasing to our north. This
will result in a fairly weak surface low and attendant trough
passing either over us or not too far to our north during Tuesday,
then exiting east Tuesday night. PoPs remain no higher than chance
from late Monday night through Tuesday night. Precip types will be
largely diurnally-driven and determined by boundary layer temps.
Therefore expecting snow, rain, or a mix of the two. Any potential
snow accumulation would be minimal. Overall best chances of precip
at this point appear to be during Tuesday.

High pressure builds in for Wednesday and remains in control through
Friday. Some guidance suggest an inverted trough and/or a shortwave
helping to produce some precip during Thursday. Will go with a dry
forecast for all 3 days for the time being and see if any consensus
towards precip emerges before introducing precip chances.

Temperatures Tuesday through Friday will slowly moderate back to
normal, with highs each day a degree or two warmer than the previous
day.
-- End Changed Discussion --

&&

.AVIATION /20Z FRIDAY THROUGH WEDNESDAY/...
-- Changed Discussion --
A clipper low moves across the Appalachians and weakens while
another low deepens off the Mid Atlantic coast this evening. The low
passes well east of the area tonight and up into the Canadian
maritimes on Saturday. High pressure begins to build late Saturday
into Saturday night.

NE winds become N into the evening push. Gusts to around 20 kt for
the city and coastal terminals for the next few hours. The winds
become more NNW towards daybreak. The winds will then become NW
Saturday morning and become gusty towards late morning.

Primarily IFR and MVFR conditions through 0z with -SN. Afterwards
most terminals go back to MVFR as -SN ends. Another period of -SN is
likely towards 09-10Z for city terminals, give or take a couple of
hours west to east. Any -SN ends towards 13-16z with VFR conditions
returning for the remainder of Saturday.

An inch or two of snowfall at KSWF and out towards KGON remains
possible through Saturday morning, with less than 1 inch elsewhere
on runways. NYC terminals generally stay above freezing with
no runway accumulation.

 ...NY Metro (KEWR/KLGA/KJFK/KTEB)TAF Uncertainty...

Amendments likely through late Saturday morning with changing flight
categories with difficult timing with regard to -SN. Precipitation
may be occasional at times tonight. No runway accumulation
anticipated.

OUTLOOK FOR 18Z SATURDAY THROUGH WEDNESDAY...

Saturday afternoon: VFR, NW winds 12-17G25-30kt.

Sunday: VFR, NW winds 10G20kt.

Monday: VFR.

Monday night-Tuesday: Chance of MVFR in light snow or a light
rain/snow mix.

Wednesday: Mainly VFR.

Detailed information, including hourly TAF wind component forecasts,
can be found at: https:/www.weather.gov/zny/n90
-- End Changed Discussion --

&&

.MARINE...
-- Changed Discussion --
Small Craft Advisories are currently in effect for all waters
except the NY Harbor. An offshore low will pass to the
southeast and bring 25 to 30 knot gusts to the waters this
afternoon through tonight. Seas on the ocean waters also rise to
5 to 7 feet and will linger into Saturday night. There will
likely be a lull in winds overnight tonight but northwest gusts
up to 25 knots pick up again during the day on Saturday. SCA
will likely be needed on all waters again Saturday.

Advisory conditions probably continue on the ocean waters into
Sunday afternoon, falling below thresholds by Sunday night. High
pressure then builds across the waters on Monday with a weak frontal
system passing through the area late Monday night into Tuesday. High
pressure builds back in for Wednesday with a weak pressure gradient.
Winds and seas during this time will therefore remain below SCA
criteria.
-- End Changed Discussion --

&&

.HYDROLOGY...
-- Changed Discussion --
There are no hydrologic concerns through the end of next week.
-- End Changed Discussion --

&&

.OKX WATCHES/WARNINGS/ADVISORIES...
-- Changed Discussion --
CT...None.
NY...None.
NJ...None.
MARINE...Small Craft Advisory until 1 AM EST Saturday for ANZ331-332-
     335-340-345.
     Small Craft Advisory from 10 AM Saturday to midnight EST
     Saturday night for ANZ338.
     Small Craft Advisory until 6 AM EST Sunday for ANZ350-353-355.
-- End Changed Discussion --

&&

$$

SYNOPSIS...JC/JT
NEAR TERM...JT
SHORT TERM...JT
LONG TERM...JC
AVIATION...JE
MARINE...JC/JT
HYDROLOGY...JC/JT