Area Forecast Discussion
Issued by NWS Upton, NY

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FXUS61 KOKX 250522

Area Forecast Discussion
National Weather Service New York NY
1222 AM EST Sun Feb 25 2018

A warm front approaches the region this morning, with a wave of
low pressure developing along it, and tracking just south of the
region today. High pressure builds in from the Midwest Sunday
night through Monday night, then slides off the Mid- Atlantic
coast through Wednesday night. High pressure builds to the north
Thursday and Thursday night, as a warm front approaches from
the southwest with low pressure riding along it. This low then
passes near or just south of Long Island Friday, then slowly
drifts to the east of Long Island Saturday.


An upper level jet from Baja California extending northeast
through the Great Lakes will be strengthening and extending
into Northern New England and the Canadian Maritimes this

Expect organized pcpn to overspread the area between 08z and
11z, in response to forcing from increasing 850-700 hPa
frontogenesis and increasing isentropic lift. In addition, a
strong shortwave pivots across the Great Lakes with increasing
positive vorticity advection in the local region late tonight
into Sunday. In addition, the upper level pattern shows
diffluence in the jet levels for early Sunday, helping to
increase lift late tonight.

Precipitable waters increase overnight to between 1.1 to 1.4
inches, which will be above the 90th percentile for February
25th according to OKX sounding climatology.

Raw 2 meter temperatures from the NAM and ECMWF were weighed
more heavily in the forecast for lows tonight, ranging from the
mid 30s to around 40.


For Sunday, the jet core moves closer to the region, with the
right front quad moving near the area by the end of the day. As
previously mentioned, the diffluence and divergence aloft will
be there for Sunday morning into the early afternoon. At the
surface, a low forms along the warm front with signals for
triple point low. Rain continues Sunday and could become heavy at
times and enhanced near higher terrain with orographic lift. Rain
tapers off mid to late in the afternoon. Raw 2 meter temperatures
from NAM and GFS were weighed more heavily in the forecast also
for Sunday without much diurnal temperature range. Forecast highs
Sunday range mostly near 40 to 45.

The core and jet streak maximum passes northwest of the region
Sunday night. The heights at 250mb remain nearly steady Sunday
and then slightly decrease late afternoon into the evening with
a more rapid height fall late Sunday night. The heights at 500mb
follow similarly with a slight decrease Sunday into Sunday
evening followed by a greater decrease late Sunday night.

Confluence occurs aloft Sunday night and the surface low moves
east of the area Sunday night, giving way to high pressure
building in from the Tennessee Valley. There will be a return
to dry conditions and a more spatial spread of lows that will
still be above normal. With moisture laden grounds and for
locations that decouple with winds and hence have lighter winds,
there will be some patchy fog that forms late at night.


A shortwave trough pushes through the area on Monday, but with
dry low levels, it should move through dry. In response to low
level warm advection ahead of the trough, highs on Monday should
be around 10 degrees above normal.

The region then remains on the top side of a deep layered ridge
centered over the N Caribbean Monday night-Wednesday night.
Associated subsidence should keep things dry, with minimal cloud
cover. The exception on cloud cover is on Wednesday and
Wednesday night, where should see a gradual increase in high
then mid clouds ahead of the next system, which tracks into the
mid Mississippi River Valley by Thursday morning. Temperatures
Monday night-Tuesday night should run around 5-10 degrees above
normal. Highs Wednesday should be around 10-15 degrees above
normal and lows Wednesday night around 10 degrees above normal.

The models are in good agreement that the deep layered ridge
axis slides to the east on Thursday allowing a cutoff low to
track into the Great Lakes Thursday then into the northeastern
States Thursday night and Friday. The models then differ on how
fast this cutoff low exits to the east for the remainder of the
week, with the GFS/CMC more progressive than the ECMWF. This
will ultimately be dependent on the location/strength of
blocking downstream over the N Atlantic. As a result, while
there is good agreement that rain develops over the area by late
Thursday, there is not good agreement over when the
precipitation ends (solutions vary from Friday night through
late Saturday at this point). There also is not good agreement
over how much, if any, wintry precipitation the area sees with
this system, with the best chance over interior areas to the N/W
of NYC and Long Island Sound. For now leaned towards a CMC/ECMWF
blend, which were slower than the GFS in exiting precipitation
(given downstream blocking a slower exit makes sense for now).

In terms of sensible weather expect rain to develop from SW to
NE Thursday. Have likely pops for mainly rain Thursday night-
Friday (except for maybe a wintry mix over far N zones friday
morning. For now have chance pops from Friday night into
Saturday with a wintry mix across N/W mainly interior zones
(though it could reach the CT coast). For now, limit chance pops
Saturday to mainly the SE 1/2 of the CWA Saturday afternoon.
Refer to the hydrology and coastal flooding sections of the AFD
for further details on possible impacts from this storm.


A warm front approaches from the south tonight with a weak wave
of low pres developing along it and passing just to the S of
Long Island on Sunday.

MVFR conds are expected to develop through 9z with IFR
returning thereafter and remaining through at least Sunday
morning as the warm front lifts closer and a heavier batch of
rain moves through the area. Rain ends around 21z at western
terminals with conds gradually improving, although this will
depend on how quickly drier air works in behind the system.

NE winds blo 10 kt increase from the E and become gusty late
tonight and Sunday morning. Gusts may diminish a few hours
earlier than forecast.

.Sunday night and Monday morning...Chance of rain with IFR conds in
the evening mainly KBDR/KISP/KGON. Otherwise bcmg VFR with areas of
fog with IFR/LIFR conds possible overnight into the morning.
.Monday afternoon through Wednesday...VFR.
.Thursday...Sub-vfr in rain likely. CHC E winds G30kt and LLWS.


Made minor changes this update to reflect the latest trends in
observations and guidance. The forecast appears on track.

An initially weak pressure gradient with sub-SCA conditions will
increase late tonight in response to deepening low pressure moving
into the Western Great Lakes and building high pressure in Southeast
Canada. Easterly flow increases with SCA conditions becoming likely
overnight into Sunday morning with gales at times for the ocean east
of Fire Island Inlet. The pressure gradient weakens Sunday afternoon
with the close proximity of the center of the low. Expecting the
winds to decrease Sunday afternoon with gusts really starting to
lower mid to late Sunday afternoon. Gales subside after 17Z
Sunday and SCA winds will likely drop off soon thereafter. Aside
from residual ocean swells into the SCA range of near 5 ft,
expecting otherwise sub-SCA conditions with a return of high
pressure for Sunday night.

A relaxed pressure gradient over the waters will limit winds to
around 10 kt or less Monday-Tuesday, with seas/waves 4 ft or
less. The pressure gradient tightens a little Tuesday night and
remains so into Wednesday night, with winds up to around 15 kt,
and seas remaining 4 ft or less. The pressure gradient relaxes
slightly late Wednesday night and Thursday morning, with winds
falling off to around 10kt or less on all waters. The pressure
gradient then begins to tighten through Thursday night with
sustained winds increasing to up to 20 kt with gusts up to 30 kt
possible, highest gusts over the coastal ocean waters. Seas on
the coastal ocean waters build to Small Craft Advisory levels on
Thursday and remain up Thursday night.


Rainfall late today through Sunday is expected to produce
around 3/4 to 1 1/4 inch of rain, through Sunday, with locally
higher amounts possible. With saturated grounds, minor poor
drainage flooding is possible on Sunday with the heavy rain.

It should be dry From Monday through Wednesday night, with no
significant hydrologic impacts expected.

There is the potential for a significant rainfall (1 or more
inches) from late Thursday into Saturday. At this time it is too
early to specify exact impacts, other to note that at a minimum
there is the potential for minor flooding of urban and poor
drainage areas.


Easterly SCA winds will likely result in 1 to 1 1/2 ft surge and
3 to 4 ft waves into Western LI Sound, which may result in some
localized minor flood/wave splashover in vulnerable areas with
Sunday morning high tide, particularly the southwest CT

A coastal low that could be off the Mid Atlantic coast/S to SE
of Long Island from Thursday well into the weekend, could
produce a prolonged period of moderate to potentially strong E-NE
flow over the region. As a result, there is the potential for at
least minor coastal flooding and beach erosion from late
Thursday into the upcoming weekend. Stay tuned as the forecast
track and strength and for this system, and hence the potential
impacts become better resolved with time.


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


MARINE...Small Craft Advisory until noon EST today for ANZ335-338-345.
     Small Craft Advisory until 6 PM EST this evening for ANZ330-
     Gale Warning until noon EST today for ANZ350-353.
     Small Craft Advisory until 7 AM EST Monday for ANZ355.


LONG TERM...Maloit
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