000
FXUS61 KOKX 212130
AFDOKX

Area Forecast Discussion
National Weather Service New York NY
530 PM EDT Fri Jul 21 2023

.SYNOPSIS...
Low pressure tracks through the Northeast today, with an attendant
cold front moving through the region tonight. Weak high pressure
gradually pushes into the region for the weekend and remains in
control through late next week.

&&

.NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
Broad upper troughing remains over the Northeast and Great Lakes
regions today while a shortwave swings through to our north. At the
surface, low pressure tracks along the Saint Lawrence River Valley,
with a weak secondary to our south along the Mid-Atlantic coast.

Daytime heating and some clearing have allowed scattered showers and
thunderstorms to begin development and should remain until the loss
of daytime heating this evening followed by clearing from a cold
front, west to east. The main concern with this system will be the
potential for flooding, especially with antecedent moist soil
conditions from the recent rounds of rainfall. Moisture deepens with
PWATs increasing on the order of 1.6 to 1.75 inches. Upward forcing
to help maximize the rainfall amount potential will come in the form
of upper level divergence as a the left-exit region of a jet streak
moves in, shortwave lift in the mid-levels, and low level
forcing/moisture convergence along the nearby frontal boundaries and
low centers.

Overall QPF should average between 0.5 to 1.5 inches for most, with
the greatest amounts amounts to the north and west. Given the
convective nature, locally higher amounts are expected. WPC has kept
a slight risk for excessive rainfall for all except far eastern
areas, indicating the potential for scattered areas of flash
flooding. Given recent rains, it will not take much to cause issues,
particularly over the lower Hudson Valley, as well as NE NJ and the
urban centers.

In addition to the heavy rainfall threat, isolated strong to severe
thunderstorms will be possible. There is a veering profile through
the mid-levels with 0-6 km bulk shear of 25-35 kt, supporting
updraft maintenance and the threat of supercell development with
several storms around noon having developed broad rotation. SPC`s
Mesoanalysis is giving a broad area of Connecticut between 2-4 for
supercell composite. With the environmental CAPE and daytime heating
expected to be better this evening, this will be the best chance for
severe weather. A Severe Thunderstorm Watch has been issued
until 8PM for areas of southern Connecticut Putnam and
Westchester counties. Areas of enhanced updraft helicity may
also come into play with the triple point and offshore low
centers nearby. SPC has increased the risk to slight for the
region, primarily for damaging winds, though the risk of an
isolated tornado or some hail remains a possibility.

The cold front looks to move through in the evening hours, around
00z for NYC and the metro, and through eastern areas by 6z, bringing
an end to the threat of showers and thunderstorms as it moves east.
Conditions dry out overnight as the flow turns WNW behind the fropa.
Temperatures bottom out in the 60s by daybreak Saturday.

&&

.SHORT TERM /6 AM SATURDAY MORNING THROUGH SATURDAY NIGHT/...
Upper troughing lingers but drier conditions greet us to start the
weekend, as weak surface high pressure builds in from the west
behind the fropa.

Dew points should drop a tad from previous days with the light WNW
flow, though likely still in the low to mid 60s, with afternoon
highs close to seasonal normals, in the low to mid 80s. Sunshine
prevails with a bit of mid-level cu at times.

Mostly clear skies and light winds Saturday night may allow some of
the far interior and LI Pine Barrens to drop into the 50s overnight,
with the remainder of the region in the 60s. A consensus of MOS
guidance was used for temperatures Saturday night to account for
this.

&&

.LONG TERM /SUNDAY THROUGH FRIDAY/...
The flow aloft will be slightly cyclonic on Sunday with weak high
pressure at the surface. Dry weather and not so muggy with afternoon
dewpoints falling into the upper 50s to lower 60s. High temperatures
will be near normal. For Monday, the flow aloft becomes more
cyclonic, and with boundary-layer dewpoints a little higher this
time, CAPEs will also be higher. Might not take much supplemental
mechanical forcing for diurnal convection to pop up as capping
appears to be weak. Best chances would be from around the city to
points north and west. The flow aloft should be weak, so a limited
severe wx threat in any pulse-type thunderstorms. More of the same
for Tuesday as the axis of the trough aloft moves closer to us. This
axis is progged to shift through during Wednesday, however in a
dampened state. So with more of a flatter flow aloft and stronger
capping beneath, only isolated showers and thunderstorms possible NW
of the city. High temperatures for both Tuesday and Wednesday
remaining near normal.

The flat flow aloft holds for Thursday and Friday with some models
suggesting a shortwave moving through or nearby during Friday into
Friday night. Will go with a dry forecast for Thursday, then low
chances of showers and thunderstorms for Friday. Temperatures at the
top of the boundary layer rise through this period. So in the
absence of significant cloud cover, highs on Thursday average around
90. Even hotter for Friday based on progged 850mb temps of 18-20C.
75th percentile NBM for highs are possible, but with this being a
week away, will go with the cooler deterministic NBM for now. The
first heat advisories of the season may be needed for the Thursday-
Friday period.

&&

.AVIATION /21Z FRIDAY THROUGH WEDNESDAY/...
A warm front will pass east this afternoon, followed by a cold
frontal passage this evening. Weak high pressure will follow
thereafter into Saturday.

Tricky situation regarding thunder especially for the NYC
metros, which sit just W of current radar activity. Opted to go
with vicinity impact this afternoon given current showers/tstms
over Long Island/S CT plus potential for additional convection
to develop elsewhere mainly before 22Z. Also went with vicinity
impact with the cold frontal passage this evening due to
uncertainties in timing/coverage.

Winds before cold fropa S-SE 5-10 kt from JFK east, and W-SW
5-10 kt inland. Winds after fropa tonight shift NW less than 10
kt.

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

AMD possible if direct tstm impact becomes more certain.

The afternoon KJFK/KLGA/KEWR haze potential forecast is YELLOW,
which implies slant range visibility 4-6SM outside of cloud.

OUTLOOK FOR 18Z SATURDAY THROUGH TUESDAY...

Saturday afternoon through Sunday night: VFR.

Monday: Mainly VFR. Slight chance of a late day shower/tstm
KSWF.

Tuesday: Chance of afternoon showers/tstms with brief MVFR/IFR
vsby from the NYC metros north/west.

Wednesday: VFR.

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

&&

.MARINE...
A Marine Weather Statement is in effect for the Long Island Sound.
The National Weather Service in coordination with the United States
Coast Guard have received reports of a large debris field across
Long Island Sound, caused by recent flooding across New England.
This will create some hazards for boaters, swimmers, and watercraft
in the affected area. The debris vary from trees, logs, coolers, and
docks and may not be easily visible.

Sub SCA conditions prevail through the weekend and into early next
week with a weak pressure gradient in place.

&&

.HYDROLOGY...
A frontal system is expected to produce between 0.5 to 1.5 inches of
rainfall across much of the area through this evening, with locally
higher amounts expected. Flash flood guidance thresholds are
lower than normal due to recent rains, with 1-hour thresholds
being most susceptible of being met. The main limiting factors
appear the forward motion of storms and low confidence in cell
training, however some backbuilding is expected. WPC has
maintained a slight risk of excessive rainfall for all but SE CT
and eastern LI, indicating isolated to scattered areas of flash
flooding possible.

No significant widespread rainfall is expected from Saturday through
mid next week.

&&

.TIDES/COASTAL FLOODING...
-- Changed Discussion --
There is a moderate risk of rip currents for only eastern
Suffolk on Saturday with seas near 3 ft and weak offshore flow,
with a low rip risk elsewhere. There is a low risk of rip
currents for Sunday, with weak westerly flow and seas less than
3 ft.
-- End Changed Discussion --

&&

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

&&

$$
SYNOPSIS...JC/BR
NEAR TERM...BR
SHORT TERM...BR
LONG TERM...JC
AVIATION...BG
MARINE...JC/BR
HYDROLOGY...JC/BR
TIDES/COASTAL FLOODING...