Area Forecast Discussion
Issued by NWS Upton, NY

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FXUS61 KOKX 270235
AFDOKX

Area Forecast Discussion
National Weather Service New York NY
1035 PM EDT Wed Apr 26 2017

.SYNOPSIS...
Weak low pressure slowly departs tonight and dissipates on
Thursday with weak high pressure building in. A cold front
approaches late Thursday night and moves into the region Friday
but will also weaken with time. A cold front will slowly
approach from the north late Friday night into Saturday, and
pass through Saturday evening. High pressure will briefly build
in its wake from southeast Canada on Sunday. A warm front will
approach Sunday night and lift north on Monday, followed by a
trailing cold front late Monday in to Monday night. Weak high
pressure will pass to the south from Tuesday into Wednesday.

&&

.NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...
Forecast is on track with only minor adjustments to T/Td to
better reflect current conds and trends.

Otherwise...the main story tonight will be the low clouds and
fog. The upper level low and surface low both translate east of
the region, getting into the vicinity of coastal New England by
early Thursday. With atmospheric flow staying below 15kt below
10kft and a low level inversion at around 1-2 kft, the low level
moisture stays in place. Most susceptible areas for fog will be
where relatively more rain has fallen, Eastern Long Island and
Southeast Connecticut where just a little cooling this evening
with light easterly flow will help advect and further develop
the fog, and it will eventually become dense. Dense fog advisory
for Eastern Long Island and New London County Connecticut was
issued for late this evening into early Thursday morning. Other
areas farther west will also get fog but not as confident in
widespread dense fog attm. Took relatively warmer MAV guidance
for lows, upper 40s to upper 50s.

&&

.SHORT TERM /6 AM THURSDAY MORNING THROUGH FRIDAY/...
Fog burns off in the morning with daytime heating and as more
westerly flow develops in the atmosphere. The upper level low
moves farther east of the region as a surface low dissipates.
More of a return W-SW flow develops in the region thereafter.
Models agree on a substantially warmer day Thursday, getting
well into the 60s for a majority of the region for highs. Weak
high pressure will briefly reside in the region with dry
conditions during the day and through the first half of Thursday
night.

The next low pressure system and associated upper level trough
will be approaching late Thursday night.The upper level flow
remains SW and the parent low with the system moves northward
Friday within Southeast Canada while its cold front moves in and
dissipates with time. There will initially be enough vertical
lift late Thursday night into early Friday along with some
elevated instability to give the region some showers and
thunderstorms. However, this looks to be brief without producing
much rain. Outside of the rain with the flow ahead of the cold
front turning more southerly, the region will see a return of at
least patchy fog Thursday night into early Friday with more
onshore flow and inherent moisture advection. Dry conditions
return later Friday.

The overall shift will be one of a gradually warming airmass.
Temperatures trend several degrees warmer for both lows
Thursday night and highs on Friday.

&&

.LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...
Big picture shows an amplified patter developing over the eastern
half of the lower 48, with a building upper ridge over the East and
a closed low moving from the Plains states through the Midwest and
into southeast Canada. At first the area will be under zonal flow
aloft between the ridge to the south and an upper trough moving
across southeast Canada, that will send a back door cold front
toward the area late Friday night into Saturday. A shortwave
embedded in the flow aloft could trigger a shower or tstm late Fri
night or early Sat morning, then Sat should be a partly sunny to
mostly cloudy day with temps on the warm side, with highs 80-85 from
NYC north/west and in the 70s across Long Island and southern CT.
Low levels look capped just above the boundary layer until late day
Sat, so do not expect any precip until late day Sat with the cold
fropa, and only and isolated shower or tstm at most.

Cooler air returns for Sat night into Sunday night, with lows in the
40s and 50s, and highs in the 60s to near 70. As the flow aloft
amplifies, a leading warm front well in advance of the surface low
over the Midwest will approach, and move through some time on
Monday, The front should lift through NYC metro and points
north/west by afternoon, nut could take longer across Long Island
and southern CT. Then as upper ridging moves east, cold front
should pass through late day Monday into Monday night, with showers
and a few tstms. Do not expect strong convection with the cold fropa
attm, but a faster timing per 12Z ECMWF could yield a brief window
for this to occur late day Monday from NYC north/west.

Dry wx with near to slightly above seasonable temps expected for Tue
into Wed, with the weakening low passing well north, weak surface
high pressure passing to the south. Zonal flow aloft will also
become re-established by Wed, that could lead to late week precip
chances just beyond the forecast period.

&&

.AVIATION /03Z THURSDAY THROUGH MONDAY/...
Low pressure will continue to pull away to the east tonight.

MVFR to start but will deteriorate tonight to IFR/LIFR with fog
and low ceilings. Winds become light and variable this evening.

Improvement to IFR to start the day on Thursday, with MVFR
likely during the afternoon.

.OUTLOOK FOR 18Z THURSDAY THROUGH SUNDAY...
.Thursday aftn...MVFR.
.Thursday night...MVFR with chc IFR.
.Friday...Chance showers/thunderstorms/MVFR AM, VFR PM.
.Saturday...Mainly VFR.
.Sunday...Mainly VFR.

&&

.MARINE...
No changes to current forecast. Dense fog advsy is in effect
for all waters through 14z Thu due to a weak pressure gradient
and abundant low level moisture. The fog is expected to
gradually dissipate Thursday mid to late morning before
returning again Thursday night into early Friday.

Winds will stay below SCA through Friday but the higher ocean
seas will remain through Thursday night and perhaps into Friday
as well. Non-ocean waters stay below SCA. Ocean seas are more
marginal for SCA Friday, at near 5 ft, so left end time of 6am
Friday for SCA for hazardous seas on the ocean.

Ocean seas above 5 ft should linger into Fri night, and perhaps into
part of Sat into Sat night, depending partly on the strength of SW
flow ahead of a back door cold front that looks to move through Sat
night.

Advy level conditions are also possible on the ocean Mon afternoon
and night after a warm frontal passage and before/just after a cold
frontal passage, with S-SW flow gusting up to 25 kt and seas
building to 5-7 ft per combo of WaveWatch and wind/wave
climatology.

&&

.HYDROLOGY...
No significant rainfall expected through Friday. Rain amounts
through Friday are generally expected to remain below a quarter
of an inch. However, there may be isolated brief heavy
downpours in any thunderstorms that develop.

Combo of a warm frontal passage late Sunday night into Mon morning,
and a cold frontal passage late day Mon into Mon night, could bring
precip amounts of 1/2 to 1 inch, highest NW of NYC. Localized higher
amts possible that could cause typical nuisance ponding.

&&

.TIDES/COASTAL FLOODING...
Due to high astronomical tides and an easterly swell the next
couple of days, water levels may approach minor coastal flood
benchmarks, in particular across the south shore back bays of
western LI. Only 1/3 to 3/4 of a foot of surge is needed for
minor flooding.

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

&&

.OKX WATCHES/WARNINGS/ADVISORIES...
CT...Dense Fog Advisory until 8 AM EDT Thursday for CTZ008-012.
NY...Dense Fog Advisory until 8 AM EDT Thursday for NYZ079-081.
NJ...None.
MARINE...Dense Fog Advisory until 10 AM EDT Thursday for ANZ330-335-338-
     340-345-350-353-355.
     Small Craft Advisory for hazardous seas until 6 AM EDT Friday
     for ANZ350-353-355.

&&

$$



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