Derby, Connecticut 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

134
FXUS61 KOKX 212100
AFDOKX

Area Forecast Discussion
National Weather Service New York NY
400 PM EST Thu Nov 21 2024

.SYNOPSIS...
Low pressure retrogrades northwestward over the area tonight and
will linger nearby through Friday night. The surface low heads
into eastern Canada by Saturday afternoon as high pressure builds
in from the west Sunday and Monday. A fast moving, weaker low
pressure system then skirts to the north of the area late Monday
into Tuesday, before high pressure briefly builds back in on
Wednesday. Another low pressure system then potentially impacts
the area on Thursday.

&&

.NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
Low pressure is currently deepening just offshore as an upper
level low approaches from the west. The surface low can be seen
well on satellite and is located about 100nm southeast of Long
Island. The low will continue to deepen tonight and track
northwestward likely from central Suffolk County up through
southwestern CT and just north of Putnam and Orange Counties.
Light to locally moderate rain continues this evening, but as
cold air gets wrapped into the area a changeover to a rain/snow
mix or all snow is expected for portions of northeast NJ, the
Lower Hudson Valley and southwest CT.

The biggest change with this update is a Winter Weather Advisory is
now in effect for western Passaic and most of the Lower Hudson
Valley. All global guidance is showing a QPF max just southwest of
the center of the low and aligned with strong frontogenetic forcing
as it passes through. There are also hints of banding from the
latest CAM models. The thinking is there will be enough cold air
wrapped in for some of this moderate to heavy precipitation to
fall as snow for the Advisory areas. Although ground temps will
be very marginal (32 to 35 degrees), model soundings support
snow, with only a very shallow layer just above the surface
being above freezing. Tricky accumulation forecast given the
marginal ground temps, low SLRs (under 8:1), and also the
uncertainty of the intensity of snowfall. There is really only a
2-4 hours period overnight sometime between 04z and 10z where
moderate to heavy snow is possible. The thinking is the higher
elevations (above 500 feet) across the Advisory areas will see
the most accumulation, mainly 2-4 inches. Less confidence in
lower elevation areas where amounts could end up short of 3
inches and be more widespread 1-2. Went with the Advisory given
the potential for brief heavy wet snow and the fact that it is
the first snowfall of the season and could produce hazardous
travel in the heaviest rates. Snowfall rates in heaviest
banding could reach .5 to 1 inches per hour.

Further east, over Long Island and southern CT, any heavy precip
will likely fall as rain. No flooding is expected. See Hydrology
section below.

&&

.SHORT TERM /6 AM FRIDAY MORNING THROUGH FRIDAY NIGHT/...
The upper-low to our west captures the surface low on Friday morning
leading to a stacked low that moves into central and eastern PA. The
stacked low then pushes back east and over our area Friday
afternoon. Skies remain cloudy with rain continuing through the day,
with some breaks. Snow or a rain snow mix is possible again Friday
night, but no accumulation is expected.

&&

.LONG TERM /SATURDAY THROUGH THURSDAY/...
**Key Points**

* Gusty NW flow and near normal temperatures behind the departing
  storm system this weekend.

* A weak frontal system moves through late Monday into Tuesday.
  Another area of low pressure potentially nears the region on
  Thanksgiving.

The NBM was followed closely for this update with some lowering of
PoPs for Thanksgiving morning and afternoon based on the 12Z GEFS.

Potent upper low continues to pull away from the local area Saturday
with a midlevel shortwave on its heels into Sunday. Mid and upper
ridging then builds in from the Central Plains through Monday with
another upper low ejecting out of the Central Plains midweek.

At the surface, the pressure gradient begins to relax late Saturday
into Sunday AM as the surface low heads northeast. Saturday in
particular looks to remain blustery, with temperatures in the
mid 40s and northwest winds gusting 25-30mph. Any lingering
cloud cover and AM showers, especially for eastern LI and CT,
should depart by midday. Sunday and Monday look dry and more
seasonable temperature- wise, in the upper 40s to low 50s.
Precipitation chances, rain at this point, looks to enter the
picture late Monday into Tuesday as a weak low pressure system
and associated fronts traverse New England. With increased
southwest flow on Tuesday, highs may be a bit above seasonable,
with upper 50s for most. After a dry and cooler day Wednesday a
more organized low pressure system approaches the area on
Thanksgiving from the southwest. Too early for sensible weather
specifics this far out with timing and strength differences,
with GEFS sfc low positioning anywhere from the Gulf coast to
eastern PA.

&&

.AVIATION /21Z THURSDAY THROUGH TUESDAY/...
Low pressure intensifies south of Long Island this evening. The low
will then shift over the area tonight and meander nearby on Friday.

Mainly MVFR in rain and/or drizzle...with varying conditions likely
at times. IFR conditions with light rain or showers continuing into
the evening. Another period of moderate... possibly heavier rain
this evening/tonight is likely. The rain will mix with or change to
a period of wet snow well NW of the NYC metro terminals, mainly
across the interior of the Lower Hudson Valley, overnight/early
Friday morning. There is a chance of -SN mixing in any
shra...especially KEWR, KTEB, and KHPN early Friday morning. 40-60%
chance of >1" snowfall rates across interior higher elevation for a
few hours.

N-NE winds 10-15 kt. Gusts 20-25 kt will likely be occasional but
more frequent gusts are expected later this evening with the
likelihood of gusts 25-30 kt by mid evening.  Winds shift to the NW
and then W-SW into early Friday morning and continuing into the
afternoon. Gusts to 25kt still likely Friday afternoon.

 ...NY Metro (KEWR/KLGA/KJFK/KTEB)TAF Uncertainty...
Amendments expected for changing flight categories through the
TAF period.

Amendments likely for timing of wind shifts and wind gusts.

Additional amendments likely tonight due to timing of heavier
precipitation and turnover to mixed precipitation north and west of
city terminals.

OUTLOOK FOR 18Z FRIDAY THROUGH TUESDAY...

18z Friday...MVFR-VFR with rain possible. Wet snow may mix in N
and W of the NYC metro. S-SW gusts 15-20kt possible.

Saturday...MVFR possible. WNW winds gusting 20-30kt.

Sunday...VFR. WNW winds gusting to 20-30kt.

Monday...VFR.

Tuesday....MVFR with a chance of -SHRA.

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

&&

.MARINE...
A Gale Warning is now in effect for the ocean waters as there will
likely be a period of 35 knot gusts tonight. A Small Craft Advisory
remains in effect elsewhere for 25 to 30 knot wind gusts. Seas build
on the ocean and likely peak around 10 ft late tonight before
lowering again. Winds fall below SCA criteria by Friday afternoon.
Seas remain elevated over the ocean waters so a SCA will be needed
once the Gale Warning is canceled or expired.

Winds and waves remain elevated across the ocean waters Saturday
into Sunday. SCA seas are likely, with the potential for near Gale
force gusts Saturday through Sunday morning. Thereafter, high
pressure builds in from the west with decreasing winds and seas into
Monday.

&&

.HYDROLOGY...
Additional liquid amounts of 1 to 1.5 inches can be expected. The
bulk of that will fall through early Friday morning, with the Lower
Hudson Valley and southwest CT seeing the higher end of totals.
There will likely be a period of moderate to locally heavy
precipitation overnight mainly across central Long Island and up
into the Lower Hudson Valley and southwest CT. There could be minor
flooding associated with clogged drains. Otherwise, no hydrologic
impacts are expected through the end of next week.

&&

.OKX WATCHES/WARNINGS/ADVISORIES...
CT...Coastal Flood Advisory until 5 PM EST this afternoon for
     CTZ009.
NY...Coastal Flood Advisory until 5 PM EST this afternoon for
     NYZ071-078>081.
     Winter Weather Advisory from 7 PM this evening to 10 AM EST
     Friday for NYZ067>070.
NJ...Winter Weather Advisory from 7 PM this evening to 10 AM EST
     Friday for NJZ002.
MARINE...Small Craft Advisory until 1 PM EST Friday for ANZ331-332-335-
     338-340-345.
     Small Craft Advisory until 6 PM EST this evening for ANZ350-
     353-355.
     Gale Warning until 6 AM EST Friday for ANZ350-353-355.

&&

$$

SYNOPSIS...DBR/JT
NEAR TERM...JT
SHORT TERM...JT
LONG TERM...DBR
AVIATION...IRD
MARINE...DBR/JT
HYDROLOGY...DBR/JT