Area Forecast Discussion
Issued by NWS Upton, NY

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FXUS61 KOKX 240901
AFDOKX

Area Forecast Discussion
National Weather Service New York NY
501 AM EDT Tue Oct 24 2017

.SYNOPSIS...
A slow moving cold front approaches the region through tonight.
The front slowly moves across Wednesday and exits east of the
region Wednesday night. A cold front just east of the area
Thursday becomes stationary as a wave of low pressure tracks
northward along the front. The low moves into the Canadian
Maritime Thursday night as high pressure builds into the region.
High pressure dominates into Saturday. A cold front then merges
with a developing low along the southeastern coast Saturday
night into Monday.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
The upper level trough deepens to the Southeast with likewise a
huge dip in the jet stream. The will strengthen a parent low in
the Great Lakes as well as and its associated cold front. The
front approaches the region as the parent low tracks into
Quebec. The high precipitable water of 1.8 to 1.9 from all the
warm and moist air advection will be above the 90th percentile
for this time of year according to OKX sounding climatology. The
low level jet will be a main reason for this with magnitude of
50-60 kt at 2kft. Instability will be increasing as well in the
low levels from this.

In terms of weather and potential hazards, rain showers become
more expansive in coverage today with rain showers becoming
heavy by this afternoon and into tonight out east. There will
also be a chance of thunderstorms. The winds will be increasing
out of the south, with higher winds potentially with the downward
momentum transport via heavy rain or thunderstorms. Low level
jet and surface southerly winds maximized most along the coast
where gusts of 40 to 50 mph are forecast so areal extent of
wind advisory was not changed. Inland zones have a lesser chance
of getting 40 to 50 mph gusts and models reflect this lower
magnitude in their wind gust fields as well.

SPC has placed much of the region in a slight risk for severe
thunderstorms, mainly due to the potential for damaging winds
with a very low percentage probability of a tornado.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH WEDNESDAY NIGHT/...
The front continues to approach tonight and then slowly moves
across Wednesday, exiting Wednesday night. Meanwhile, the region
will still be to the east of the very deep upper level trough
with high precipitable water over the region until late
Wednesday.

Wind advisory still will be in effect much of tonight for
Central and Eastern Long Island and Southern CT while to the
west it ends at midnight. The heavy rain and potential
thunderstorms east of NYC will still keep that downward momentum
transfer going to bring down the low level jet at times. Again
there will be a slight chance for winds to be severe as
indicated in the slight risk area from SPC. Late at night, the
intensity of rain and chances of thunderstorms for Southern CT
and Long Island lowers with the forecast decrease in
instability.

Wednesday will have lighter winds but still some steady rain
mainly across Long Island and Southern CT. Weather dries out
Wednesday night with a light west to northwest flow taking place
behind the cold front.


&&

.LONG TERM /THURSDAY THROUGH MONDAY/...
A negatively tilted upper trough will be moving through the region
Thursday with the axis moving to the northeast Thursday evening.
Meanwhile, at the surface a cold front just east of Long Island and
extending south, offshore of the coast, will have a wave of low
pressure developing and moving along the front Thursday into
Thursday night. There are some spatial differences with the low,
with the GFS slightly farther to the east than the ECMWF. Now
looking like there will be a chance of some light precipitation, so
introduced slight to low chance probabilities for Thursday into
Thursday evening.

Weak upper ridging builds Thursday night and moves through during
Friday. Then Friday night into Saturday the ridge builds and
strengthens across the Western Atlantic as another northern stream
longwave trough moves through the northern plains and into the upper
midwest while digging into the southern plains and Gulf coast region.

All signals still indicting that the full latitude trough will be
slow to progress eastward with the Atlantic ridge in place.
Also a wave pf low pressure will be moving out of the Gulf of Mexico
and move through the eastern portion of the trough, and then merge
with a cold front and low pressure moving into from the Ohio Valley.
So will have a prolonged period of probabilities for rain early
Sunday through Monday, with the coastal low expected to track inland
and then move north through the eastern states. There remains
uncertainty with the location and timing of the low, and the
potential for any heavy precipitation with this system.

&&

.AVIATION /09Z TUESDAY THROUGH SATURDAY/...
A slow moving cold front will approach the region today and
crosses the region tonight into Wednesday morning.

MVFR/IFR cigs have returned this morning along with some scattered
showers. The showers become more widespread from west to east
after 12z.

With the approach of a low level jet, showers with embedded thunder
are likely today. 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 45-55kt@2kft for coastal
terminals.

SE winds have been increasing this morning with gusts becoming more
frequent around 25kt. Some gusts 30-40kt possible after 12z today,
especially the NYC terminals.

...NY Metro Enhanced Aviation Weather Support...

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

KJFK TAF Comments: Amendments are likely for changing flight
categories in rain and possible embedded thunderstorms. Highest
winds gusts expected this afternoon and evening.

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

KLGA TAF Comments: Amendments are likely for changing flight
categories in rain and possible embedded thunderstorms. Highest
winds gusts expected this afternoon and evening.

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

KEWR TAF Comments: Amendments are likely for changing flight
categories in rain and possible embedded thunderstorms. Highest
winds gusts expected this afternoon and evening.

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

KTEB TAF Comments: Amendments are likely for changing flight
categories in rain and possible embedded thunderstorms. Highest
winds gusts expected this afternoon and evening.

KHPN TAF Comments: Amendments are likely for changing flight
categories in rain and possible embedded thunderstorms. Highest
winds gusts expected this afternoon and evening.

KISP TAF Comments: Amendments are likely for changing flight
categories in rain and possible embedded thunderstorms. Highest
winds gusts expected this afternoon and evening.

.OUTLOOK FOR 06Z WEDNESDAY THROUGH SATURDAY...
.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.

&&

.MARINE...
A strong cold front approaching will increase the southerly
fetch with a strong low level jet making for eventual gale force
winds as heavy rain develops and also with the chance of
thunderstorms. The gales are expected to be frequent enough so a
gale warning is in effect for all waters through much of this
evening, remaining for eastern ocean waters, Long Island Sound
and Bays all night.

SCA criteria seas on the ocean linger behind the cold front
Wednesday into Wednesday night but winds overall will become
less than SCA and much lighter as the pressure gradient
decreases.

Small craft ocean seas Thursday will continue to slowly subside as
low pressure moves into the Canadian Maritime and high pressure
builds to the south. During Thursday evening ocean seas will likely
fall below 5 feet and then remain below advisory levels until late
Saturday.

Winds and gusts are forecast to remain below small craft advisory
levels Thursday through Saturday night. Then as a low deepens along
a stalled front offshore Sunday a south to southeast flow will be
increasing with gusts reaching small craft advisory across all the
forecast waters. In additions, ocean seas will be building with
small craft seas possible late Saturday night into Sunday.

&&

.HYDROLOGY...
A widespread 2 to 3 inches of rain is likely through Wednesday
evening with locally up to 4 inches. 6hr rainfall shown by
models maxes out near 1-1.5 inches but locally higher amounts
than that are possible. The location of where this would occur
is of low confidence.

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.

Heavy rain threat diminishes Wednesday into Wednesday night with
a drying trend late Wednesday night. However, there is a chance
of light rainfall Thursday but no hydrologic impacts are
expected. The next chance for widespread rainfall across the
area will be Sunday and Monday. A period of heavy rainfall is
possible, however, timing and location are uncertain at this
time.

&&

.OKX WATCHES/WARNINGS/ADVISORIES...
CT...Wind Advisory from 8 AM this morning to 2 AM EDT Wednesday for
     CTZ007>012.
NY...Wind Advisory from 8 AM this morning to 2 AM EDT Wednesday for
     NYZ078>081.
     Wind Advisory from 8 AM this morning to midnight EDT tonight
     for NYZ071>075-176>179.
NJ...None.
MARINE...Gale Warning until 6 AM EDT Wednesday for ANZ330-335-340-345-
     350-353.
     Gale Warning until 10 PM EDT this evening for ANZ338-355.

&&

$$


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