Flash Flooding in New York, September 19, 2016

Area Forecast Discussion
National Weather Service New York NY
1104 AM EDT MON SEP 19 2016

.SYNOPSIS...
A weakening frontal system will move into the area today and
tonight, then slowly pass just to the south on Tuesday. High
pressure from the Ohio Valley will then build in through mid week,
before slowly shifting offshore Thursday and Thursday night.
Another cold front will approach from the north Friday night, and
move across early Saturday. Canadian high pressure will then
settle in for the remainder of the weekend.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
A surface trough and residual outflow boundary across the area
this morning along with deep moisture and marginal instability is
resulting in widespread showers/isolated thunderstorms across the
region. The heaviest rainfall was occurring across Suffolk county
on Long Island and southeast Connecticut where a low-level jet and
backing low-level flow was enhancing the convection. There have
been reports of 1 to 2 inches of rainfall across these areas with
mainly minor flooding. In addition, with high astronomical tides
coinciding with the heavy rain, this may enhance the flood threat
for locations typically more vulnerable to coastal flooding.

The activity is forecast to gradually work east through the day
as the low-level jet passes to the east. However, marginal
instability, deep moisture, and the RRQ of an upper jet will keep
the threat of showers through the day.

It will be warm and rather humid today, with high temps in the
mid to upper 70s. This has been adjusted down a few degrees based
on latest obs and trends. Dewpoints will be in the upper 60s and
lower 70s.

There is a moderate risk for rip currents at the ocean beaches.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM TUESDAY/...
Chances for showers/isolated thunder diminish tonight from west to
east, but cannot rule out periods of heavier showers out east via
any waves of low pressure riding along the weakening boundary.
Lows tonight will be in the 60s to lower 70s.

Still could see isold-sct showers on Tuesday especially in the
morning, especially closer to the coast, until the front passes a
little farther south and washes out. With more sunshine especially
north/west, except temps to rise to the lower and mid 80s in most
places, and upper 70s across far eastern Long Island and SE CT.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/...
Jet stream stays north of the region Tuesday night through Friday
with a quasi-zonal flow in the mid to upper levels. By the weekend,
mid and upper level shortwave with associated jet stream push
southward into the Northeast.

At the surface, winds will shift to a more northwest direction as
high pressure builds in from the Ohio Valley Tuesday night. The high
pressure area centers over the region on Wednesday with sea breeze
development allowing for more onshore flow. The high then shifts
slowly offshore Thursday through Thursday night with a more
consistent return southerly flow developing. An ambient weak
pressure gradient will be in place Friday with a weakening cold
front approaching Friday night. The front moves across early
Saturday with Canadian high pressure to follow to close out the
weekend.

The next chances of rain showers will be mainly associated with the
cold front Friday night into Saturday but the previous run of the
ECMWF model indicated a weak area of low pressure near the region
which could present another chance of rain sometime Friday. However,
the more recent run has backed off of this scenario, keeping the
weak low and associated rain well south of the region.

Otherwise, expecting mainly dry weather with above normal
temperatures with the region being on the eastern side of a longwave
ridge extending from the Central through Eastern U.S. for much of
the long term. Behind the cold front for next weekend, the airmass
will be a much cooler one with temperatures forecast to be closer to
respective normal values. The gusty northwest flow will allow for
more efficient dry and cold air advection.

&&

.AVIATION /15Z MONDAY THROUGH FRIDAY/...
A weak cold front slowly weakens over the area today. High
pressure gradually builds in tonight into Tuesday.

Primarily MVFR/IFR conditions this morning with stratus and
moderate to heavy rain shower activity. The majority of the
convection has shifted east of the NYC metro terminals and should
sink southeast of the eastern terminals through around 15z. This
should end the threat of TSRA...but moderate to locally heavy
rain will continue in its wake. Rain should become more iso- sct
through the afternoon. Widely scattered shower activity could
linger into this evening...particularly for eastern terminals.

Uncertainty in degree of improvement of cigs this afternoon.
Likelihood for coastal terminals to remain MVFR/IFR through the
day...but potential for a period of improvement to VFR in the
afternoon for KEWR/KTEB/KSWF. With light winds and abundant
moisture...likelihood for MVFR/IFR conditions re-developing this
evening and continuing overnight...possible LIFR or lower for
outlying terminals.

Light and variable flow will continue through the overnight hours.

     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: Ceilings likely remain at MVFR...possibly
IFR...levels through the day.

KLGA TAF Comments: Ceilings likely remain at MVFR...possibly
IFR...levels through the day.

KEWR TAF Comments: Brief VFR conds possible, especially in the
afternoon.

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

KTEB TAF Comments: Brief VFR conds possible, especially in the
afternoon.

KHPN TAF Comments: Ceilings likely remain at MVFR/IFR levels
through the day.

KISP TAF Comments: Sparse TSRA coverage through around 15z.
Ceilings likely remain at MVFR/IFR levels through the day.

.Outlook for 12Z Tuesday through Friday...
.Tuesday monring...Improvement to VFR Tuesday morning.
.Tuesday Afternoon-Friday...VFR.
.Friday Night-Sat...MVFR or lower possible in any shra.

&&

.MARINE...
Winds and seas will remain below SCA levels through much of the
period. The next chance of SCA conditions could come Saturday
night and Sunday on the ocean, as a significantly cooler air mass
builds in on a gusty NW flow.

&&

.HYDROLOGY...
There is a threat for localized flash flooding today, with
potential for swaths of 2+ inches not out of the question,
especially across coastal SE CT and Long Island. Otherwise,
rainfall of at least 1-1.5 inches of rain appears likely, with
minor urban and poor drainage flooding.

If heavy rainfall rates coincide with times of high tide today,
the urban flood threat would be exacerbated for low lying locales.

&&

.TIDES/COASTAL FLOODING...
Astronomical tide levels continue to run high. Water levels of
less than a foot above astronomical high tides are needed to
reach minor coastal flooding benchmarks. Some locations along
Jamaica Bay and the south shore back bays of Nassau County may
come close, especially if heavy rain occurs just before or
coincides with high tide.

&&

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

&&

SYNOPSIS...Goodman
NEAR TERM...Goodman/DW
SHORT TERM...Goodman
LONG TERM...JM
AVIATION...FEB/NV
MARINE...Goodman/JM
HYDROLOGY...Goodman/JM
TIDES/COASTAL FLOODING...DW