discussion on 2023-09-20

000
FXUS61 KOKX 202002
AFDOKX

Area Forecast Discussion
National Weather Service New York NY
402 PM EDT Wed Sep 20 2023

.SYNOPSIS...
-- Changed Discussion --High pressure continues to gradually build in from the west
through Thursday, then lifts to the north and east Thursday
night into Friday ahead of low pressure developing off the
Southeast coast. This area of low pressure will slowly work
northward and impact the area over the weekend, and possibly
into early next week. Behind it, high pressure reasserts itself
as it builds south out of Canada.-- End Changed Discussion --

.NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...
-- Changed Discussion --A quiet period of weather is on tap as high pressure gradually
builds in from the west through Thursday. A weak surface trough
over the area will weaken this evening with a more northerly
flow establishing itself, generally 5 mph or less. The
combination of the light winds and clear skies should allow for
decent radiational cooling. Preferred a MET/MAV MOS blend to
account for this with lows ranging from the upper 40s inland, to
the upper 50s across the NYC metro. While this will be close to
normal for the metro area, it will be about 5 degrees below
normal elsewhere.-- End Changed Discussion --

.SHORT TERM /6 AM THURSDAY MORNING THROUGH FRIDAY/...
-- Changed Discussion --Upper trough will continue to retreat from the Northeast and
Canadian Maritimes with heights building aloft. At the surface,
high pressure builds across the Northeast on Thursday and then
off the Northern New England coast on Friday. Low pressure along
a stalled out front off the Southeast coast will begin to
deepen during this time and track slowly northward on Friday.
This low is being monitored by the NHC for possible subtropical
development. Refer to the latest NHC Tropical Weather Outlooks
for updates on this system.

The pressure gradient between the departing high and the
deepening low will result in a gradually strengthening easterly
flow late Thursday night into Friday. Gusts up to 20 mph will be
possible along the coast Friday afternoon. In addition, high
clouds ahead of the system will be on the increase, mainly
during the daytime hours on Friday.

Any rainfall ahead of the low will not arrives until late
Thursday night.

Thursday night will likely be another cool night, but the
aforementioned cloud cover could come into play. For the time,
have leaned toward the cooler MET/MAV MOS again versus the
consensus blends. However, for max temps leaned toward the
warmer MAV based on little change in the airmass associated
with Wednesday`s high temps. Highs Thursday will be close to
normal in the lower to mid 70s. Friday is likely a bit cooler
due to the increasing cloud cover.-- End Changed Discussion --

.LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...
-- Changed Discussion --An active start to the period as confidence grows that a coastal low
impacts the region this weekend.

Developing surface low off the Carolina coast late this week deepens
as it heads north into the weekend. While guidance continues to come
into better agreement on the synoptic pattern, differences with
respect to track and timing still need to be resolved, though a
general consensus is emerging that the weakening low tracks into the
vicinity of the local area this weekend. Regardless of any type of
subtropical classification, impacts would remain the same, bringing
a wet, windy weekend to much of the Tri State.

12Z global ensemble means have nudged the heaviest axis of rainfall
south and east a tad, though plenty of differences remain amongst
individual members. NBM v4.1 probabilities of >2" over the 24 hours
ending 12Z Sunday show a widespread 30-40% across Long Island and
CT, up from about 20-30% last night. However, the NBM v4.1
probabilities of >1" in any 6 hours remains low and not a strong
signal for periods of heavier rainfall. Global ensembles paint a
widespread 0.75" to 2" Saturday into early Monday for the area,
again, with plenty of spread amongst members. The 12Z operational
ECMWF keeps the bulk of the heaviest rainfall offshore, with
practically dry conditions north and west of NYC, though this
solution appears to be an outlier for now. The highest amounts
continue to look to be over Long Island and southern CT, but will
depend on the eventual track of the system. Details should come into
better view over the next few days with additional high
resolution/convective allowing guidance sources available for that
time horizon which will help better diagnose the potential for
locally heavy rainfall on Saturday into Sunday.

Increased easterly flow and precipitation onset looks to begin by
early to mid morning on Saturday, with guidance indicating an
inverted trough feature passing near or just offshore by this time.
A steady rain is likely much of the day for most of the region, and
could fall locally heavy at times. Depending on the surface
features, flow could allow sustained winds 15-25 mph, occasionally
gusting over 30 mph, especially along the coast. The rain and wind
hold down temperatures, perhaps even more so than currently
forecast, with most struggling to get out of the 50s and low 60s
during the afternoon.

By Monday, the weakening low meanders offshore, as the upper low is
potentially absorbed off to the northeast. Some gradual drying out
is likely Monday into Tuesday, with onshore flow continuing as
Canadian high pressure noses south, holding temperatures down a bit
below normal values through mid next week.-- End Changed Discussion --

.AVIATION /20Z WEDNESDAY THROUGH MONDAY/...
-- Changed Discussion --VFR for the TAF period.

High pressure continues to build west of the terminals through the
TAF period.

NW winds 5-10kt, with outlying terminals under 5 kt. Coastal
terminals may back a bit more to the WNW this afternoon/evening due
to sea breeze influence before veering back to NW by 00Z Thursday.
Winds weaken overnight into early Thursday. NNE 5-10kts through the
day on Thursday.

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

The extent and duration of temporary WSW winds for coastal terminals
due to sea breeze influence may need to be amended depending on how
strong or weak the sea breeze becomes.

OUTLOOK FOR 21Z THURSDAY THROUGH MONDAY...

Wednesday Night - Friday: VFR.

Saturday and Sunday: MVFR or lower likely in rain. E gusts 15-25 kt,
possibly higher at the coastal terminals. Strongest winds at this
time look to be on Saturday.

Monday: VFR w/ MVFR possible in rain, depending on how a coastal
system progresses.

Detailed information, including hourly TAF wind component forecasts,
can be found at: https:/www.weather.gov/zny/n900-- End Changed Discussion --

.MARINE...
-- Changed Discussion --High pressure will build in from the west through Thursday and
then passes to the north and east late Thursday night into
Friday. Low pressure along the SE coast will gradually deepen
and start tracking northward on Friday. While winds will be
below SCA though Thursday night, a long period swell will
arrive on Thursday from Hurricane Nigel over the central North
Atlantic. This will allow seas on the ocean to build to around 4
ft. There is a chance they get to 5 ft, but confidence is low.
Easterly winds will then be on the increase late Thursday night
into Friday due to a strengthening pressure gradient between
the departing high and low working up the east coast. Marginal
SCA conditions are likely to develop by Friday afternoon.

Increasing easterly flow as a coastal low tracks north will
result in SCA conditions on all waters early Saturday, with the
potential for gales during the afternoon, particularly on the
ocean.

Winds may then remain above 25 kt on all waters through Sunday
afternoon before lowering. Elevated seas above 5 ft likely linger on
the ocean into Monday. Thereafter, sub SCA conditions expected on
all waters.-- End Changed Discussion --

.HYDROLOGY...
-- Changed Discussion --No hydrologic impacts are anticipated through Friday.

A widespread rainfall event appears likely this weekend, with up to
several inches of rain possible in some areas. WPC has placed NYC
and Long Island in a marginal risk of excessive rainfall through 12Z
Saturday, and across the entire region thereafter through 12Z
Sunday. Hydrologic impacts such as nuisance flooding will be
possible with locally heavy downpours, though confidence in rainfall
rates exceeding flash flood guidance remains low at this time.-- End Changed Discussion --

.TIDES/COASTAL FLOODING...
-- Changed Discussion --There is a moderate rip current risk through this evening for
all the area Atlantic facing beaches. There is a high rip
current risk for the Suffolk County atlantic facing beaches on
Thursday, with Nassau and NYC remaining moderate.-- End Changed Discussion --

.OKX WATCHES/WARNINGS/ADVISORIES...
CT...None.
NY...None.
NJ...None.
MARINE...None.

SYNOPSIS...DR/DW
NEAR TERM...DW
SHORT TERM...DW
LONG TERM...DBR
AVIATION...BR
MARINE...DBR/DW
HYDROLOGY...DBR/DW
TIDES/COASTAL FLOODING...//