Area Forecast Discussion
Issued by NWS Upton, NY

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FXUS61 KOKX 250809

Area Forecast Discussion
National Weather Service New York NY
409 AM EDT Fri May 25 2018

A back door cold front over southeast Connecticut and eastern
Long Island will retreat to the northeast early this morning, as
high pressure otherwise dominates into tonight. A low pressure
trough will develop north and west of New York City on Saturday.
Another back door cold front will pass through Saturday night
into Sunday morning, becoming nearly stationary just to the
south and west through Memorial Day. Another cold front will
move through from the west on Tuesday, followed by building high
pressure for the middle of next week.


Fog/low cloud bank remains in place over most of New London
County and also along the CT River. As a result have extended
the dense fog advy until 7 AM. As the back door front retreats
and daytime heating ensues, the fog should burn off rapidly.

A mostly sunny and very warm day still expected today, with
highs 85-90 away from most south-facing shores, at or above the
warmer of the GFS/NAM MOS guidance. Hybrid synoptic scale/sea
breeze regime should lead to gusty conditions along the coast,
and temps dropping through the mid to late afternoon, perhaps
more quickly than fcst indicates.


Mostly clear skies expected tonight, with some patchy ground fog
possible mainly in the interior valleys. Lows tonight should
be near 70 in NYC and in the 60s elsewhere.

As flat upper ridging moves east, chances for tstms should
increase through the afternoon on Sat. GFS way too quick to
initiate storms, almost as soon as a sfc-based theta-e ridge
develops, despite upper ridging and associated subsidence still
overhead. Mid to late afternoon a more likely time frame for
convection as PW quickly increases to near 1.5 inches.
CAPE/shear values are only marginally supportive of severe wx--
more telling are microburst parameters indicating potential for
pulse severe with the strongest cells mostly NW of NYC. Some
cells could produce heavy rain, especially where repeat training
can take place. This may become more likely toward evening and
into Sat night as another back door cold front approaches and
Corfidi vectors shorten.


A shortwave trough moving across the Great lakes and into the
Northeast from Saturday night through Sunday night will
interact with a back door cold front dropping across the region.
Thereafter, heights build across the eastern half of the
country with a tropical low potentially impacting the Lower
Mississippi and Tennessee Valleys. Global models are in very
good agreement this cycle, in particular the GFS and ECMWF. The
GGEM appears to be too aggressive with several frontal waves
developing along the boundary to the south Sunday into Monday.

There is the potential for localized heavy rainfall Saturday
night into Sunday morning as a westerly upper flow and modest
instability raises the concern for training along the cold front
sagging south across the region. Severe weather seems unlikely
due to a weakly sheared environment, however, mixed layer CAPE
value around 1500 J/KG lend to the potential for pulse
strong/severe in the evening. In addition, PW values overnight
Saturday approach 2 inches.

Cold front works south of the area early Sunday morning.
However, post-frontal convection is likely to linger through the
morning as a frontal wave passes to the south. Surface high
pressure then noses in from the north through Sunday night.
Weak warm advection still lends to a low chance of showers and
perhaps drizzle/light rain Sunday night in an easterly flow.

Outside of some scattered late day convection across the
interior ahead of a weak cold front on Monday, the remainder of
the period should be dry as high pressure builds to the north
and offshore by midweek.

Below normal temperatures for highs Sunday and Monday will
return to well above normal levels Tuesday, with a slight
downward tick Wednesday and Thursday as an onshore easterly
flow reestablishes itself.


High pressure dominates through Friday.

Other than lingering LIFR/IFR across SE CT at KGON, VFR
conditions expected.

S-SW winds will continue to diminish and become light and
variable outside of city terminals. SW winds increase to 10-15
kt with gusts 20-25 kt into this afternoon.

.Late tonight...VFR.
.Saturday...Mainly VFR. Showers and thunderstorms likely near
the NYC metro terminals and to the N and W in the mid to late
afternoon, with possible MVFR or lower flight cat and locally
gusty winds. Higher chance of showers and thunderstorms at
.Sunday-Monday...MVFR or lower at times with a chance of showers
and thunderstorms.
.Tuesday...Mainly VFR.


Multiple SCA`s are in effect. First, a tightening pres gradient
between low pres to the north and high pres over the western
Atlantic should bring marginal SCA conds to the near shore
western ocean waters and the Long Island south shore bays from
mid afternoon until around sunset, with gusts up to 25 kt. Then
for tonight, a strengthening sfc inversion will be developing
over the waters, so despite increasing winds aloft mixing
potential will be limited. A 15-20 kt sustained flow should
bring ocean seas up to 5 ft out east tonight.

Generally sub-SCA conditions expected thereafter in a weak flow
regime, but could see a period of E flow gusting to 20 kt on the
ocean waters Sunday into Sunday night.


Localized heavy rainfall is possible Saturday night into Sunday
morning as a cold front passes through.  GEFS means approaching an
inch across the area, but potential for locally 1 to 2 inches of
rain. Minor urban and poor drainage flooding appears to be the
main issue.


Water levels may reach minor flood thresholds along the south
shore back bays of Nassau County and in western Long Island
Sound with the Sunday afternoon/night high tide cycle. Forcing
for increased water levels will be increasing E-NE flow as a
front stalls to the south and strong high pressure sets up E of
New England, along with increasing astronomical high tides with
the full moon on Tue the 29th.


NYC NOAA Weather Radio Station KWO-35 (162.55 MHz) is off the
air for an extended period of time.


CT...Dense Fog Advisory until 7 AM EDT this morning for CTZ007-008-
MARINE...Small Craft Advisory from 2 PM this afternoon to 9 PM EDT this
     evening for ANZ345-353-355.
     Small Craft Advisory from 9 PM this evening to 6 AM EDT
     Saturday for ANZ350.


NEAR TERM...Goodman
SHORT TERM...Goodman
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