Area Forecast Discussion
Issued by NWS Upton, NY

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FXUS61 KOKX 250538
AFDOKX

Area Forecast Discussion
National Weather Service New York NY
138 AM EDT Tue Apr 25 2017

.SYNOPSIS...
Low pressure slowly tracks up along the eastern seaboard through
Wednesday. The low eventually tracks from south and east of
Long Island Wednesday night then to off to Nova Scotia by
Friday morning. A weakening frontal boundary then pushes into
the region by Saturday, then stalls out nearby into early next
week.

&&

.NEAR TERM /UNTIL 6 AM THIS MORNING/...
Radar returns south of the Long Island have continued to
increase over the last few hours. However, lower levels are
still quite dry so as the precipitation moves further north
there continues to be weakening of echoes. These returns are
associated with increasing warm advection ahead of low pressure
over the Carolinas.

Have adjusted PoPs for the remainder of the early morning
hours. The low levels should moisten just before day break for
the light rain to become more widespread, especially across the
southern half of the region. Not confident in going with likely
PoPs before 6 am as the dry air seems to be winning the battle
and slowing the timing down for seeing a more widespread
measurable rain. Where light rain does occur, amounts will be
about a tenth of an inch or less through day break.

Rainfall amounts will be on the light side, with about a tenth
of an inch or less through day break and much of this will come
after 3 am when the low levels finally moisten.

&&

.SHORT TERM /6 AM THIS MORNING THROUGH 6 PM WEDNESDAY/...
Low pressure continues to move up the east coast during Tuesday.
Moisture increases with a theta-e ridge shifting in from the south
and PWATs increasing to 1.25 to 1.50 inches. Isentropic and
shortwave lift will help push the likelihood of rainfall northward
through the rest of the CWA. Further lift will then be supplied by
another low level jet by late in the day.

Breezy conditions develop, especially along the coast with a
tightening pressure gradient increasing easterly winds. High
temperatures will be below normal.

With a stacked system, the coastal storm will slowly drift up the
Atlantic Seaboard Tuesday night and Wednesday while weakening as it
does so. Rain remains likely Tuesday night before chances lower
through the day on Wednesday. Models differ regarding the placement
of the axis of greatest qpf. With the low stacked, prefer a more
eastward track for the surface low - which would be closer to the
consensus track of the 500 mb low. This places the strongest lift
over the eastern zones. Will therefore continue with the idea from
the previous forecast and RFCs that the higher amounts of rain are
more likely to fall somewhere over the eastern half of the Tri-State
Area. No thunderstorms are anticipated as elevated instability is
progged to remain to our south, and it appears that the entire CWA
will fall short of flash flood and headwater guidance flooding for
this event. See the hydrology section below for more details. Went
closer to the cooler side of guidance for Wednesday`s high
temperatures.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/...
A gradually filling cutoff low tracks from south of Long Island
Wednesday night to off of Nova Scotia by Friday morning.

The result should be spotty rain Wednesday evening, tapering of to
showers and patchy fog by Thursday morning. For now have gone dry
for the bulk of Thursday and Thursday night, but cannot rule out a
stray shower on the backside of the departing low during this time
frame. Given abundant low level moisture and an inversion forecast
to set up around 850 hPa, do expect some cloud cover, for now going
partly sunny/mostly cloudy however do have possibility of partly
cloudy (optimistic) to overcast (pessimistic). As a result have
decreased the diurnal range in this time frame.

A 700-500 hPa shortwave passes on Friday, with isolated to scattered
showers and thunderstorms (best chance basically area to the N/W of
NYC - where highest CAPES are expected).

Ridging builds in from the South From Friday night into Sunday -
however should also have a frontal boundary stalled out near the
area. The models differ on the strength and orientation of the
ridge, so the amount of precipitation. if any, received during this
time frame is in question.

It will be a balance between increasing subsidence under the
building ridge and vs. lift with the front - for now going dry
Friday night/Saturday then slight chance pops Saturday night-Sunday.

For now have chance pops Sunday night/Monday as shortwaves approach
ahead of a full latitude trough (with a cutoff low at its head).
This might be a little on the aggressive side, especially if ECMWF
idea of a stronger ridge plays out.

Temperatures Wednesday night through Thursday are a blend of MOS and
NAM 2-meter temperatures (with a mix down from 975-950 hPa blended
in for highs on Thursday) and should be around 10 degrees above
normal then 5-10 degrees above normal respectively. Temperatures
Thursday night-Monday were based on the Superblend, and are
forecast to remain above normal.

&&

.AVIATION /06Z TUESDAY THROUGH SATURDAY/...
Low pressure slowly moves up the Mid-Atlantic coastline today
and tonight.

MVFR and IFR cigs expected to gradually develop and prevail
in bands of light rain early this morning. Visibility will take
some time to lower, possibly late morning. Expecting IFR/LIFR
ceilings this afternoon as rain becomes steadier and moderate
to heavy. The low conditions continue early tonight.

E/NE winds increase this morning, with speeds 15-25 kt and
gusts 25-30 kt after 12-14Z. Highest sustained winds for
coastal terminals.

Winds diminish somewhat tonight. LLWS possible this evening
with SE winds 45-50 kt at 2kft.

.OUTLOOK FOR 06Z WEDNESDAY THROUGH SATURDAY...
.Late tonight...IFR/LIFR in rain. NE gusts 20 to 25 KT. Winds
diminish further.
.Wednesday-Wednesday Night...IFR in rain. Potential for
IFR/LIFR in fog Wednesday night.
.Thursday...MVFR/IFR to start...with gradual improvement to
VFR.
.Friday...Chance of MVFR in showers/thunderstorms.
.Saturday...VFR.

&&

.MARINE...
Have started the SCA on all waters a few hours earlier tonight than
what was previously posted with the expectation that winds will
increase more quickly than the previous forecast. Will still go with
the SCA ending Tuesday night after collaboration with the
surrounding offices, and although there could be a few gusts to gale
force on the ocean waters at some point Tuesday or Tues day evening,
confidence to go with a gale warning was not high enough at this
time.

A relatively relaxed pressure gradient over the region Wednesday
night through Saturday will limit winds to 15 KT or less during this
time frame. Sub-small craft conditions are expected on the non-ocean
waters as a result Wednesday night through Saturday. On the coastal
ocean waters, swells from the departing coastal low will keep seas
above SCA levels over at least part of each coastal water zone
Wednesday night through Friday. Seas should fall below 5 ft on all
coastal ocean zones by Saturday.

&&

.HYDROLOGY...
Total rainfall of 3/4 to 2 inches of rainfall is expected tonight
through Wednesday. The highest amounts are most likely to occur
across eastern Long Island and southeastern Connecticut where the
best chances of minor flooding of poor drainage and low lying areas
will be. The heaviest rainfall probably occurs during Tuesday
night.

No significant (0.5 inches or more) widespread rainfall is forecast
from Wednesday night through at least Sunday.

&&

.TIDES/COASTAL FLOODING...
In response to a low pressure system moving northward along the
eastern seaboard, minor coastal flooding (1 to locally 2 ft
above ground) is likely with the Tuesday evening high tide cycle
for vulnerable coastal communities along the southern bays of
western Long Island/Queens/Brooklyn, western Long Island Sound
and portions of lower NY/NJ Harbor. Localized moderate coastal
flooding (2 ft above ground) is possible along the south shore
bays of Nassau County. Only 1/2 to 1 foot of surge is needed to
reach minor flood thresholds Tuesday evening.

Minor coastal flooding thresholds will be approached in the
same areas once again with the lower Wed Am high tide, and
possibly briefly exceeded along the southern bays of Western
Long Island.

More widespread minor coastal flooding is possible with the
higher Wednesday night high tide cycle...with minor inundation
likely along the southern bays of LI. Only 1/3 to 3/4 of a foot
of surge is needed to reach minor flood thresholds Wed night.

In addition...the elevated water levels combined with a
prolonged period of 5 to 9 ft breaking surf Today into Thu will
result in beach erosion issues and may cause some localized
dune toe erosion during the Tue Night into Wed Night high tides.

&&

.OKX WATCHES/WARNINGS/ADVISORIES...
CT...None.
NY...None.
NJ...None.
MARINE...Small Craft Advisory until 8 PM EDT this evening for ANZ330-
     335-338-340-345.
     Small Craft Advisory until 6 AM EDT Wednesday for ANZ350-353-
     355.

&&

$$
SYNOPSIS...JC/Maloit
NEAR TERM...JP/DS
SHORT TERM...JC
LONG TERM...Maloit
AVIATION...PW
MARINE...JC
HYDROLOGY...JC/Maloit
TIDES/COASTAL FLOODING...



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