000
FXUS61 KOKX 291153
AFDOKX
Area Forecast Discussion
National Weather Service New York NY
753 AM EDT Fri Sep 29 2023
.SYNOPSIS...
An inverted surface trough and weak low move northward along the
coast today into tonight. The low will slowly strengthen and move
east Saturday into Sunday, with high pressure building in its wake
and remaining dominant into next week.
&&
.NEAR TERM /THROUGH TONIGHT/...
-- Changed Discussion --
Most persistent deep convection for the last several hours has
been occurring about 100 miles SE of Long Island, with bands of
moderate to heavy rain (1/2-1" rainfall rates) working north
across the area. 1-1 1/2"/hr band is developing along the NJ
coast and working north into the NYC/NJ metro this morning. In
addition a convergence band is noted developing over NE NJ
which may help enhance rainfall rates across the NYC/NJ metro
and Lower Hud. This is being monitored for potential expansion
and flash flooding concerns over the next few hours, particularly
coinciding with high tide.
High-res model guidance has been struggling with development
and location of potential deeper convergence band over the area,
which would present potential for 1-2"/hr rates for several
hours and a more significant flash flood threat. This type of
persistent intense rain banding may be having trouble
developing across the local area as offshore convection robs
moist/unstable airmass transport into the area. Will have a
better sense of this by late morning. Even if this persistent
high rainfall rate banding does not occur, a widespread 3 to 5"
with locally higher amounts is still likely across much of the
watch area, but over a longer duration, would limit potential
for considerable flash flooding. If this deeper banding does
develop, the potential for considerable flash flooding and 5-7"
amounts will increase. Again, we will have a better sense over
the next few hours.
Previous Discussion...
***Scattered to Numerous Areas of Flash Flooding likely today,
with locally Considerable Flash Flooding possible***
***Moderate Flooding possible along a few quick responding small
rivers and streams***
Closed upper low over eastern Great Lakes will slowly approach
today, gradually sliding across the area tonight into Saturday. At
the surface, an inverted trough sitting off the NJ coast/mid
Atlantic coast will extend into NYC, NE NJ, W LI into Lower Hudson
Valley vicinity this morning. Evidence of a coastal front/convergence
zone across NYC and up Hudson River, which could be a sign of
where the inverted trough/coastal front ends up extending into.
Deeper convection being noted in satellite and radar firing all
along and to the east of this this N/S offshore frontal
boundary this morning, slowly working northwest towards NYC, NE
NJ and W LI.
Continued good model agreement with increasing deep layered
lift from approaching upper low and left front quad of ULJ,
combined with a strengthening SE/E LLJ (35-40kt), transporting a
sub- tropical(+1-2 STD PWATS) and weakly unstable (a few
hundred J/kg of elevated instability) airmass into the region,
converging in the vicinity of the inverted trough/coastal front
(with likely frontogenetic and orographic enhancement). The
environment is certainly primed for deep moist convection and
flash flooding type rainfall in the Flood Watch area.
Deterministic models continue with a strong signal for a large
area of 2 to 5" of rainfall, with localized axis of 5-7" over
the watch area. Much of this rainfall likely falling over a 6-8
hr period. Still spread on exactly where heaviest axis develops
this morning, anywhere from W LI/SW CT to NYC/NJ metro, and
extending as far north as Lower Hudson Valley/SW CT. 00Z HREF
mean QPF through 00z this evening indicating a widespread 3-5"
from NYC, NE NJ, up the Hudson River to Rockland/Putnam and east
into SW CT and W LI. This scenario seems quite reasonable for a
storm total rainfall footprint and most likely area for
localized heavier rainfall swath based on synoptic and mesoscale
setup. 00z SPC HREF PMM, which preserves individual rainfall
maxima, is even more bullish than that with a large area of 5-7"
of rain across the watch area, with locally 10"+ of rain. The
large expanse of 5-7" of rain appears to be a product of
considerable model spread in where the intense rainfall axis
sets up, with the localized 10" likely a localized reasonable
worst case scenario.
In terms of rainfall rates, although 00z HREF 1"hr probs are
not overly impressive, but it continues with a broad 10%
ensemble prob of 3"/3hr across NYC, NE NJ, and Lower Hud, SW CT
and W LI, with an embedded of 30% ensemble prob of 3"/3hr
across the NYC/NJ metro. This 30% 3"/3hr has been a consistent
signal, which have verified well for an elevated signal for
flash flooding(w/ typical location uncertainty). For additional
context, several runs of the HRRR have been producing hourly
rainfall rates of 2"/hr, which seems to be verifying based on
the MRMS estimated rainfall rates this morning. 00Z HRRR has a
widespread area of 3-5" across watch area, with a narrow swath
of 5-7" across W LI/NYC/NJ metro and along SW CT/SE NY border.
So a general consensus exists in a widespread 3-5" with locally
5-7", centered somewhere across NYC, NE NJ, up through Rockland,
Putnam, Westchester, SW CT and W LI. Areas to the NW and E of
this area appears to be in a less favorable areas for persistent
frontogenesis and theta-e convergence.
So expecting numerous bands of moderate to heavy rain moving
from SE to NW (rainfall rates 1-2"/hr) through this morning into
the above mentioned favored area, with a potentially developing
N to S oriented axis of torrential rain (2"+/hr) developing in
vicinity of coastal frontal/inverted trough after daybreak.
Based on high- res guidance, this boundary could sits over the
development area for several hours before moving, with location
becoming more evident in the next 3-6 hrs.
See hydrology sections for more on flood impacts.
In addition to the rain, NE winds of 15-20g25-30mph likely along the
coast Fri AM through Sat AM. A few peak gusts to 35 mph possible.
Lighter winds for the interior
High temps will run several degrees below seasonable in rain and
onshore flow, and near seasonable for overnight lows.-- End Changed Discussion --
&&
.SHORT TERM /SATURDAY/...
As the closed low moves overhead and low pressure becomes more
developed tonight into Saturday morning and gradually slides east,
the inverted trough/convergence axis will break down and slide east
with a more moderate deformation rain on the NW side of the low
continuing Tonight into Saturday morning, gradually tapering from w
to e.
High temps will run several degrees below seasonable in rain
and onshore flow, and near seasonable for overnight lows.
&&
.LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/...
There have been no significant changes to the long term, as
global ensemble guidance is in decent agreement with the overall
pattern. A rather quiescent and generally dry period is
expected along with a warm up into next week.
Closed, positively tilted upper low ejects off the east coast
to start, as a an amplified ridge shifts eastward from the
Central Plains. The ridge builds to the east Sunday through
Wednesday, with the ridge axis overhead by Wednesday. The ridge
axis heads offshore Wednesday night.
At the surface, low pressure heads east/southeast of the 40/70
benchmark on Saturday, with lingering showers across the
southern portions of the CWA though midday. Easterly flow will
continue through the day on Saturday, and into early Sunday,
keeping coastal sections under cloud cover for a good portion of
Saturday, while the interior should see some sun by late
afternoon. The pressure gradient begins to relax Sunday as the
surface low pulls away and winds become more northeasterly by
Sunday afternoon.
A strong high pressure then begins to dominate as it settles over
the northeast through Wednesday resulting in mainly dry conditions
for the region. Temperatures will be on the increase into midweek,
with highs in the mid to upper 70s across the NYC metro, northeast
NJ and the interior. At the coasts, highs in the low to mid 70s are
expected. Either way, these are near or just above average for the
time of year.
&&
.AVIATION /11Z FRIDAY THROUGH TUESDAY/...
-- Changed Discussion --
An inverted trough across the area today will bring periods of
moderate to heavy rain, gusty NE-ENE winds, and IFR conditions.
Widespread IFR ceilings to start this morning for all but KGON
and KBDR which will lower through the morning hours. KGON will
be much slower to drop through the categories. Once locations
get to IFR, they will likely remain there through the TAF
period. KSWF could see some improvement to MVFR late tonight.
Winds will be NE-ENE winds at 10-15 kt with G20-25kt, highest
at the coastal terminals. Gusts will diminish this evening.
The heaviest rain is likely to be across the NYC and Lower
Hudson Valley terminals. However, there is some uncertainty as
to how far east the heaviest rain gets through the day. Embedded
thunderstorms are possible. Confidence in thunderstorm timing
at any one terminal is low. Rain will diminish in coverage and
intensity this evening.
...NY Metro (KEWR/KLGA/KJFK/KTEB) TAF Uncertainty...
Ceiling category changes may vary by 1-2 hours.
Winds may vary 20 degrees on either side due to the close
proximity of a surface trough, especially this morning.
There is a chance of thunder at almost any time, but isolated.
OUTLOOK FOR 12Z SATURDAY THROUGH TUESDAY...
Saturday: MVFR/IFR in lingering -RA in the morning, improving
to VFR by late. NE winds G15-20kt mainly at the coastal
terminals.
Sunday through Tuesday: VFR.
Detailed information, including hourly TAF wind component forecasts,
can be found at: https:/www.weather.gov/zny/n90-- End Changed Discussion --
&&
.MARINE...
An extended period of SCA conds is expected for the ocean into early
next week. E/NE SCA gusts expected thru Sat, with seas elevating to
7 to 10 ft on E/SE wind swells, as well as background SE swells from
a developing offshore low and distant tropical systems impact the
coastal waters. E/NE Marginal SCA gusts expected on the nearshore
waters into Sat AM.
Winds gradually subside Saturday Night through Monday as high
pressure builds in from the west.
&&
.HYDROLOGY...
Potential is increasing for a widespread 3-5" of rain across
the NYC, NE NJ, Lower Hud in vicinity of Hudson River, SW CT,
and W LI with locally 5-7". 00z HREF still indicating a 30%
ensemble prob of 3"/3hr across the NE NJ, NYC, Lower Hud, SW CT,
W LI area btwn 6am and 2pm, which has been a reliable signal
for a scattered to numerous flash flood event.
Another consideration is that the period of heaviest rain this
AM and this PM will coincide with two high tidal cycles where
minor flood thresholds are expected to be reached. This will
limit storm drainage capacity for coastal locations during these
times and exacerbate flash flood threat in these areas during
these times.
Based on the above, scattered to numerous instances of urban,
poor drainage, and low-lying flash flooding are likely in the
Flood Watch area, with locally considerable/life threatening
flash flooding possible.
RFC has the Saddle River at Lodi forecast to go into minor
flood this afternoon and Ramapo at Mahwah forecast to go into
minor flood tonight. Headwater guidance for several of the
flashy streams in NE NJ is around 2-2 1/2" in 6hr for minor
flooding, and 2 1/2 to 3 1/2" in 6 hr for moderate flooding. So,
if the heavy rain axis develops over the the flashy river
basins of NE NJ, Lower Hud, and SW CT, moderate flood levels
would likely be exceeded for several small rivers and streams,
with even potential for isolated major flood levels (LODN4 major
flood headwater guidance is 3" in 6hr).
&&
.TIDES/COASTAL FLOODING...
An approaching full moon will have rising astronomical tides (as low
as 1/4-1/2 ft needed to reach minor thresholds), combined with a
persistent and slightly strengthening E-NE flow and E wind
wave/swell today into Sat Am will have water levels solidly at minor
flood levels (up to 1 1/2 ft inundation) for vulnerable NY/NJ
harbor, Jamaica Bay, western Great South Bay, and coastal
Westchester/Fairfield communities, with localized minor elsewhere
across Long Island and NYC for the high tide cycles through at least
Saturday morning.
A bit more uncertainty on how widespread the minor coastal flooding
will be with the Saturday night and Sunday morning high tide cycles
as astronomical tides peak, but winds turn more to the N-NE and
gradually subside. Highest confidence in additional cycles of
widespread minor flooding is for the southern bays of Nassau and
Queens due to restricted tidal drainage, with perhaps more localized
minor flooding elsewhere.
In addition, continued beach erosion/escarpment will continue
through at least Saturday due to elevated water levels, 4-7 ft
breaking surf, and a strong E-W longshore current. Dune erosion
potential looks to be localized, with overwashes not expected.
Although our surf zone forecast season in over, the risk for
dangerous rip currents will continue into at least early next week
based on the wave conditions.
NOTE: The Peconic River at Riverhead tidal gauge is out of service
(had been reading too high). The gauge is being looked into by our
partners at the USGS.
&&
.OKX WATCHES/WARNINGS/ADVISORIES...
CT...Flood Watch through late tonight for CTZ005-006-009-010.
Coastal Flood Advisory until 2 PM EDT Saturday for CTZ009.
NY...Flood Watch through late tonight for NYZ067>075-078>081-
176>179.
Coastal Flood Advisory until 2 PM EDT Saturday for NYZ071.
Coastal Flood Advisory until noon EDT Saturday for NYZ178-179.
NJ...Flood Watch through late tonight for NJZ002-004-006-103>108.
Coastal Flood Advisory until noon EDT Saturday for NJZ006-106-
108.
MARINE...Small Craft Advisory until 8 PM EDT Saturday for ANZ331-332-
335-338-340-345.
Small Craft Advisory until 8 PM EDT Sunday for ANZ350-353-355.
&&
$$
SYNOPSIS...NV
NEAR TERM...NV
SHORT TERM...NV
LONG TERM...DBR
AVIATION...DW
MARINE...NV
HYDROLOGY...NV
TIDES/COASTAL FLOODING...