Area Forecast Discussion
Issued by NWS Upton, NY

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FXUS61 KOKX 181834
AFDOKX

Area Forecast Discussion
National Weather Service New York NY
234 PM EDT Fri Aug 18 2017

.SYNOPSIS...
A cold front will move slowly east across the area late tonight,
then stall just east of Long Island on Saturday. High pressure
will follow Saturday night through Tuesday. A cold front will
pass across the area Wednesday night followed by high pressure.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...

...SEVERE THUNDERSTORM WATCH IN EFFECT FOR PORTIONS OF NORTHEAST
NJ...SOUTHEAST NY...NYC...WESTERN LONG ISLAND AND WESTERN CT
THROUGH 9 PM...

Damaging wind gusts are possible with any thunderstorms that
develop this afternoon into the evening as they approach from
the west.

In addition to the severe weather threat, the threat of flash
flooding remains high due to a combination of earlier rainfall
of up to 4 inches in some areas, anomalously high dew points and
precipitable water values up to 2.2 inches. As such, a Flash
Flood Watch was issued for NE NJ, the Metro NJ/NYC area,
including Rockland, Westchester, and Nassau Counties in NY and
Southern Fairfield County in CT until 11 pm this evening with
the expectation that showers and thunderstorms will increase
ahead of a cold front.

Satellite imagery shows a SW/NE dry slot advecting NE from VA/MD
across SE PA, that should advect NE across the Lower Hudson
Valley. This will increase sfc based and low lvl instability
resulting in the development of organized convection mainly aft
20z this afternoon from SW to NE.

There is a moderate risk of rip currents at ocean beaches,
potentially becoming high this afternoon.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH SATURDAY NIGHT/...
Models at times can struggle with the precipitation forecast
with the departure of large mid level ridges above 580dm. So,
forecast will adjust for this in situation where there is more
forcing and precip is too low and vice versa.

The upper level trough will be approaching the region tonight.
The height falls become more remarkable late tonight into
Saturday morning and this is when the trough moves eastward a
greater distance. The region will be getting closer to the right
entrance region of this jet. The trough amplifies and shortens
its wavelength during the day Saturday. 500mb heights lower
20-30 m from early morning until early evening. Vorticity still
increasing with height tonight and Sat looking at 925, 850, and
700mb levels.

Still have a threat of gusty winds and heavy rain with
thunderstorms tonight. High precipitable waters near 2 inches
still will be in place as will the vertical wind shear.

A cold front will continue to approach tonight. The cold front
moves into the region late tonight into Saturday but does not
totally clear the region until Saturday night.

However with aforementioned dynamics, pops still in place for
parts of the CWA during Sat evening with possibility for
isolated shower activity.

West to Northwest flow behind the cold front later Saturday
night and continued ridging will promote mainly dry conditions.

&&

.LONG TERM /SUNDAY THROUGH FRIDAY/...
Shortwave trough pivots northeast of the region on Sunday, with
generally zonal upper flow over the region for early to mid of next
week, between southern ridging and a closed low moving into Hudson
Bay.

At the surface, high pressure builds into the region Sunday, and
then gradually sinks south and east of the region Monday into
Tuesday. The result will be dry and seasonably warm conditions on
Sunday, giving way to increasing heat and humidity Mon into Tue.
Perhaps some afternoon shower/tstm activity along a lee trough Tue
afternoon depending on any energy sliding through the upper flow
aloft, but low predictability at this point.

Decent model agreement with shortwave energy digging into the Great
lakes Tue into Tue Night and then towards the NE US on Wed. An
associated frontal system will bring the next chance for organized
shower and tstm activity late Tue into Wed.

&&

.AVIATION /19Z FRIDAY THROUGH WEDNESDAY/...
A cold front slowly approaches region late this afternoon.

MVFR/IFR/lIFR ceilings continue in the wake of earlier showers
and thunderstorms. Ceiling conditions have been slow to improve
but some minor improvement is still possible before another
round of showers and thunderstorms arrives around 20-21z.
Thunderstorms are likely this evening until about 01z across the
NYC terminals. Gusty winds and heavy rain possible in any
thunderstorm.

Southerly winds will increase this afternoon, mainly between 10
to 15 kts. Gusts to 25 kt possible along the coast. Higher
gusts possible with any thunderstorm.

For tonight, fog/stratus with IFR or lower conds may linger
through the overnight and especially early morning Saturday.

     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 possible for timing and coverage
of showers and thunderstorms this this afternoon and evening.

KLGA TAF Comments: Amendments possible for timing and coverage
of showers and thunderstorms this afternoon and evening.

KEWR TAF Comments: Amendments possible for timing and coverage
of showers and thunderstorms this afternoon and evening.

KTEB TAF Comments: Amendments possible for timing and coverage
of showers and thunderstorms this afternoon and evening.

KHPN TAF Comments: Amendments possible for timing and coverage
of showers and thunderstorms this afternoon and evening.

.OUTLOOK FOR 00Z SATURDAY THROUGH WEDNESDAY...
.Saturday-Monday...VFR. light NW wind becoming SW by Late
Sunday and Monday.
.Tuesday...Isolated showers and thunderstorms possible with
MVFR or lower conditions...otherwise VFR. Light southwest
wind.
.Wednesday...Scattered showers and thunderstorms with MVFR
possible. Gusty southwest wind to 20 kt.

&&

.MARINE...
SCA on ocean today through tonight with sub SCA on other waters.
Overall sub SCA conditions expected Sat and Sat night with
offshore flow developing behind cold front.

Winds and seas are expected to remain blw sca lvls on all waters
Sun thru Wed as high pressure builds across the waters.

&&

.HYDROLOGY...
Heavy showers and thunderstorms resulted in 1 to 4 inches of
rain this morning from the Metro NJ/NYC area east across Long
Island.

Another round of heavy showers and thunderstorms will be possible
this evening ahead of the cold front. Rainfall accumulations
are expected to be around a half an inch to an inch, with
locally higher amounts possible in thunderstorms.

It still remains difficult to pinpoint where heavier showers
and thunderstorms will develop. There is a chance of flash
flooding with the more likely flooding hazard being minor
flooding of poor drainage and urban areas especially tonight
with places that received heavy rain during the morning.

Impacts from potential showers and thunderstorms Tuesday into
Wednesday are uncertain at this point.

&&

.OKX WATCHES/WARNINGS/ADVISORIES...
CT...Flash Flood Watch until 11 PM EDT this evening for CTZ009.
NY...Flash Flood Watch until 11 PM EDT this evening for NYZ069>075-
     176>179.
NJ...Flash Flood Watch until 11 PM EDT this evening for NJZ002-004-
     006-103>108.
MARINE...Small Craft Advisory until 6 AM EDT Saturday for ANZ350-353-
     355.

&&

$$
SYNOPSIS...GC/JM/NV
NEAR TERM...GC
SHORT TERM...GC/JM
LONG TERM...NV
AVIATION...IRD
MARINE...GC/JM/NV
HYDROLOGY...GC/JM/NV


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