okx-afd on 2024-12-20

480
FXUS61 KOKX 210009
AFDOKX

Area Forecast Discussion
National Weather Service New York NY
709 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. A weak frontal system then approaches and moves through the area
Monday night into Tuesday. High pressure builds back into the area
Wednesday and remains in control through Friday.-- End Changed Discussion --

.NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
-- Changed Discussion --Low pressure to the south will continue to strengthen as it heads NE
tonight. Based on model guidance, supported by recent 3-hour
pressure tendencies, the center of the low will pass SE of the
40N/70W benchmark later this evening. Frontogenetic forcing on
the NW side of the low will enhance precip chances early this
evening over the easternmost zones before shifting east with the
storm center. PVA associated with an approaching shortwave from
the west will enhance precip chances for much of the rest of
the forecast area now through the overnight hours.

With the exception of Long Island and parts of NYC, precip type is
expected to be snow from this point on. Based on recent obs, a
surface temperature of 35 seems to be the dividing line between rain
and snow in most cases. Over parts of the city and Long Island, the
overall boundary layer and surface dewpoints are a little higher, so
rain is being observed at 33-34 degrees. It could also in part be
due to some mid-level drying with a lack of ice nuclei, but snow can
form in the coastal areas here as long as there`s saturation as warm
as around -4C aloft. In any case, the mid levels should moisten
up again as the night progresses, promoting chances of snow as
the precip type. Boundary layer and dewpoints will fall here
over the next few hours, so it shouldn`t be too long when all
precip is in the form of snow across the entire area.

QPF for the rest of tonight will be light, so additional snow
accumulation tonight should be under an inch. Snow totals for the
whole event will be mostly under 2 inches, so no advisories needed.
There may be a special weather statement issued later on to address
the potential of slippery road conditions tonight as road temps,
which are still largely above freezing, cool off overnight. Low 2-
meter temperatures overnight mostly 25-30.-- End Changed Discussion --

.SHORT TERM /6 AM SATURDAY MORNING THROUGH SATURDAY NIGHT/...
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.

.LONG TERM /SUNDAY THROUGH FRIDAY/...
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.

.AVIATION /00Z SATURDAY THROUGH WEDNESDAY/...
-- Changed Discussion --Low pressure passes well east of the area tonight and up into
the Canadian maritimes on Saturday. An arctic front moves
through Saturday morning, then high pressure begins to build
late Saturday into Saturday night.

NNE winds become N thru midnight, with occasional gusts to
around 20 kt for the city and coastal terminals. The winds
become more NNW towards daybreak. The winds will then become NW
Saturday morning, increasing to 15-20ktG25-30kt for the aft/eve.

Locally MVFR conditions through 00-03z with -SN. -SN should
briefly end with improvement to VFR possible, then -SN with
MVFR/locally IFR is likely once again towards 06-08Z 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, with less than 1 inch accum
on runways possible. Light dusting possible on untreated
runways for KHPN/KBDR/KGON/KISP. NYC/NJ/LI terminals should generally
stay above freezing with only a slight chance of 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 occasional
-SN. Only a slight chance of any runway accumulation, with most likely
no accum.

OUTLOOK FOR 00Z SUNDAY THROUGH WEDNESDAY...

Saturday Night: VFR. Winds diminishing to NW 10-15g20-25kt late.

Sunday: VFR, NW winds 10-15G20-25kt.

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...
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.

.HYDROLOGY...
There are no hydrologic concerns through the end of next week.

.OKX WATCHES/WARNINGS/ADVISORIES...
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.

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