FXUS61 KOKX 240244

Area Forecast Discussion
National Weather Service New York NY
1044 PM EDT Mon Oct 23 2017

A slow moving frontal system approaches tonight, impacting the
region Tuesday into early Wednesday with strong winds gusts and
potentially heavy rain. The frontal system slowly moves east of
the area Wednesday night with brief high pressure following
through the end of the week. A cold front then merges with a
developing coastal low Saturday night into Monday.


Made minor changes this update to reflect the latest trends in
observations and guidance. The forecast appears on track.

SSW flow aloft tonight, with a passing 500 hPa shortwave
overnight. Have a dry forecast this evening based on radar
trends on upstream obs. Restrict categorical pops overnight to
areas w of the Hudson River (late) and likely pops to W of the
Ct/NY and Nassau/Suffolk borders. This is based on latest
guidance - which is consistent with idea of best forcing
associated with the shortwave being mainly to the N/W of the

Currently any thunder is limited to SW VA/Central NC - so have
scaled thunder possibilities over far W zones to slight chance
after midnight (with showalter indices progged down to around
2), and confidence has increased in any heavy rain holding off
until during the day on Tuesday. This is consistent with the
timing of the low level jet.

Still could see some patchy drizzle overnight, especially over
eastern zones where will take longer for other than low level
lift to develop.


NWP guidance is in good agreement through Tue night.
A vigorous PAC shortwave will continue to dig the longwave
trough and gets to the Gulf coast by Tuesday Night. The first
feature that will drive surface weather is a low level jet with
high PWs for this time of year. This will result in periods of
rain, with embedded heavier showers and gusty winds. Some CAPE
in model profiles suggest few rumbles of thunder on Tuesday.

The second feature which will drive any convective rain will be
the approaching frontal system for Tuesday night, with models
indicating the best potential for widespread rainfall amounts
above 2 inches to be east of New York City. With amplification
of the longwave trough the frontal system is indicated by NWP
models to slow and track very slowly across the region Tue night
in the longitudinally oriented steering flow. Trend with this
update has been to continue to slow the system`s progression
through Tue night which would only serve to increase rainfall
amounts for eastern sections.

The following potential hazards Tue/Tue night:

Heavy Rain: The slow movement of the front, deep lift, weak
instability, and interaction with an increasingly moist and
tropical airmass (+3 to 4 std) signal potential for multiple
bands of heavy rain as weak waves lift north along the front.
The higher resolution WRF based models continue to indicate the
potential for a localized swath of 3-5 inches. This looks to be
a credible threat based on synoptic pattern, but location/timing
is very much uncertain based on front and frontal wave timing
and location. Would also like to see additional support of this
in some other hi-res models. See hydrology section for
associated flooding threat.

Strong Winds: A wind advisory is in effect for most of our CT
zones, along with coastal zones in NY. A 45-55 kt 925 hpa LLJ
moves overhead Tuesday into Tue night. The GFS BUFKIT soundings
indicating a 12 hr period of southerly winds gusts of 30 to 40
mph, with potential for any low topped convective line/s ahead
of the cold front being able to drag down localized 45-55 mph
wind gusts to the surface. SPC has our area in a slight risk
with damaging wind gusts the main threat. Not seeing the best
low level lapse rates and 0-1 km shear, so not all that bullish
on tornadic activity, with later shifts needing to further


A slow moving upper trough will maintain significant moisture
advection in broad, nearly unidirectional southerly flow into at
least the early portion of Wednesday. Primary update to the
extended forecast was to slow the departure of the surface front
and associated precipitation, mainly across portions of
Connecticut and Long Island. Ensemble forecasts still suggest
some uncertainty with regards to end time of precipitation,
though in general rates should taper off from west to east
through the evening. Depending on how quickly rainfall ends,
adequate mixing in the afternoon and evening with modest west-
northwest flow may support a few gusts.

High pressure then builds through the remainder of the week with a
return to more seasonable temperatures and humidity, though with
some subtle moderation and warming to the air mass as the week

Meanwhile, a long wave upper trough will develop and slowly push
eastward, allowing strong moisture advection again into Sunday. The
increasing difluence aloft will aid in the development of a
coastal low, which is then expected to merge with the frontal
system, increasing chances of rain across the area. At the
moment, the exact timing, accumulations and rates of any
precipitation remain uncertain but the system will continue to
be monitored.


A slow moving cold front will approach the region tonight but
not pass through the region until Tuesday night.

MVFR returns tonight with patchy drizzle developing. Showers
become more widespread from west to east after 06z with MVFR/IFR

With the approach of a low level jet, showers with embedded
thunder are likely Tuesday. The rain will be heavy at times
accompanied by strong wind gusts, especially near the coast.
Compression and low level wind shear are likely, mainly SSW
[email protected] for coastal terminals.

SE winds increase tonight with gusts becoming more frequent
around 25kt. Some gusts 30-40kt possible after 12z Tuesday,
especially the NYC terminals.

.Tue night-Wed...IFR possibly continues in SHRA Tue night, low
prob/sparse TSRA early evening, improving to MVFR and possibly
VFR by midday Wed.
.Wed night-Thu...VFR, with iso-sct MVFR in SHRA.
.Fri and Sat...VFR.


Made minor changes this update to reflect the latest trends in
observations and guidance. The forecast appears on track.

SCA conditions are expected to develop overnight with the
potential for marginal gale force gusts Tue and Tue night on
all waters. Gale watch remains in effect for this potential,
although the higher winds may be more convectively correlated.
As the cold front slowly moves east Wednesday into Wednesday,
winds will gradually begin to subside in its wake, though
lingering swell may maintain SCA- level seas at least into
Friday night. Seas gradually subside into the weekend.


A widespread 2 to 3 inches of rain is likely Tuesday into
Wednesday morning, with locally up to 4 inches. Concern is that
much of this rain could fall in a 6 hr period in slow moving
and/or training bands of low topped convection/heavy rain, but
the predictability on location of where this occurs is low.

Despite antecedent dry conditions, these potential rainfall rates
present an area wide potential for minor urban and poor
drainage, with a localized threat for flash flooding. In
addition, if the heavier rain amounts occur over Northeastern
New Jersey and the Lower Hudson Valley, rapid rises in flashy
small streams and minor flood impacts to adjacent areas would be
plausible. A flash flood watch may be needed as subsequent data
is evaluated.

Heavy rain threat diminishes Wednesday afternoon, with drying
conditions expected Thursday night through Saturday.

The next chance for widespread rainfall across the area
will be Sunday night, though the location and timing of any heavier
amounts are uncertain at this time.


CT...Wind Advisory from 8 AM Tuesday to 2 AM EDT Wednesday for
NY...Wind Advisory from 8 AM Tuesday to 2 AM EDT Wednesday for
MARINE...Gale Watch from 6 AM EDT Tuesday through late Tuesday night
     for ANZ330-335-338-340-345-350-353-355.
     Small Craft Advisory from 2 AM to 6 AM EDT Tuesday for ANZ330-
     Small Craft Advisory until 6 AM EDT Tuesday for ANZ350-353-355.


NEAR TERM...Maloit

NWS OKX Office Area Forecast Discussion