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Area forecast discussion 
National Weather Service New York New York
403 PM EDT Wednesday may 27 2015

Synopsis...
a cold front will approach from the west tonight...then move
across Thursday into Thursday night. High pressure will return on
Friday...quickly moving into the western Atlantic late Friday
afternoon into the weekend. A slow moving cold front approaching
from the west will eventually move in Saturday night and stall
nearby for Sunday into early next week. High pressure will build
in from the north late Tuesday into the middle of next week.

&&

Near term /until 6 am Thursday morning/...
Severe Thunderstorm Watch 227 in effect until 7 PM for all areas north/west
of NYC. Line of thunderstorms over eastern PA will move into the area late
this afternoon and this evening...and intensify as it encounters
unstable air mass with MLCAPE 1000-1500 j/kg over northern New Jersey and the
lower Hudson Valley...with middle level shortwave and Lee trough
providing lift/low level convergence. Hrrr also indicates
potential for isolated discrete cells out ahead of the line.
Stronger cells/bowing line segments with the line have potential
for damaging wind gusts and marginally large hail. With corfidi
vectors 10 knots or less and precipitation efficiency over 1.5 inches...will
also have to watch for potential for training of cells with
locally heavy rainfall of 1-2 inches.

Big question is how far and how strong these storms will survive
into the marine layer in place over NYC...Long Island and most of
southern CT. At this time think strongest storms will weaken below severe limits
as they approach the eastern edge of the watch area toward evening...
but depending on later trends it is possible the watch may have to
be locally extended into counties just to its east including New
Haven CT...all of NYC...and Nassau County.

It still appears that the rest of Long Island and southern CT
will be too stable in the low levels for organized convection due
to marine influence...but have carried slight chance pop for any
weakening convection that drifts into these areas later tonight.

After daytime highs 80-85 from NYC north/west...in the 70s most
elsewhere...and only upper 60s for the south shores of eastern Long
Island and southeast CT...should be a humid night with lows in the 60s.

There is some potential for patchy fog late tonight north/west of
NYC where heavier rain falls. Noting that fog has not materialized
the past few nights...still do not have the confidence to mention.

&&

Short term /6 am Thursday morning through Thursday night/...
models differ on coverage/extent of convection with a cold front
on Thursday. Also differ on timing with the GFS/European model (ecmwf) faster than
the NAM/CMC-reg. Noting 700-500 hpa flow has a fairly decent
perpendicular component to the front...leaned towards the faster
timing of the GFS/European model (ecmwf) for probability of precipitation. With a weakening front
though...did not feel comfortable going with higher than chance
probability of precipitation at this time. Probabilities are generally higher from NYC and
areas just north/west.

With less MLCAPE...500-1000 j/kg over the interior than
today...and the best dynamic forcing forecast to the north of the
region...do not think there is as much of a threat for severe
weather. Though an isolated strong to severe storm or two cannot
be completely ruled out.

Highs on Thursday should be in the 80s inland...and in the 70s near the
coast...mainly per warmer NAM MOS guidance. Would not be
surprising if a few locations in NE New Jersey approached 90. Dewpoints
on Thursday should be a tad lower than today...mainly in the lower
60s.

A weak cold front will move across Thursday night. Little to no
precipitation is expected with the front. Lows should be in the
60s in most places...except middle/upper 50s in the valleys well
north/west of NYC.

&&

Long term /Friday through Wednesday/...
ridging trend resumes aloft with jet stream well north
of the region for Friday. This ridge shifts slowly offshore and
becomes less amplified as a longwave trough pushes eastward across
southern Canada. The same pattern remains through the first half of
the weekend and then the jet pushes farther south into the region
for the rest of the weekend into early next week. The middle level
ridge will become more broad Sunday with a shortwave moving in early
next week. The ridging resumes by middle next week with jet stream once
again moving well north of the region.

At the surface...high pressure from the Canadian Maritimes quickly
settles into the western Atlantic late afternoon Friday and into the
weekend. The influence of the high will lessen with time as a slow
moving cold front approaches from the west. The parent low attached
to this front will not be that strong...slowly deepening with
eastward progress through Saturday and then deepens more within the
Canadian Maritimes Sunday through Sunday night. The cold front will
be moving within closer proximity of the region through this time.
The front will then stall near the region with low pressure and
inverted trough developing at the surface to the southwest of the
region along the front. High pressure returns late Tuesday from the
Canadian Maritimes...but once again becomes re-established quickly
offshore into the western Atlantic.

In terms of weather...conditions remain dry Friday and Friday night.
However...with the moist southerly flow Friday night into Saturday
morning...expecting some patchy fog across the region...especially
coastal regions. Some models are showing this moisture as very light
quantitative precipitation forecast. There will be a gradual increasing chance of showers and
thunderstorms first north and west of NYC Saturday ahead of the cold
front. The chances for showers and thunderstorms increase Saturday
night across the whole forecast region with showers becoming likely
Sunday and Sunday night. Thunderstorms will be more limited Sunday
and Sunday night with decreasing instability with a more northerly
and easterly flow developing during the day. With the front stalling
near the region...chances for showers remain into early next
week...before becoming more minimal Tuesday as the front pushes
farther south of the area with mainly dry conditions returning late
Tuesday night into next Wednesday.

The air mass will stay mostly as maritime tropical through the
long term but a little cooler for Monday and Tuesday next week.
The highs are forecast to be around or slightly more than 5
degrees above normal on average Friday and Saturday...near normal
for Sunday...around 5 degrees below normal Monday as well as a few
degrees below normal Tuesday. Temperature return to near normal by
middle next week. Lows at night overall are forecast to be near to
above normal.

&&

Aviation /20z Wednesday through Monday/...
a weak cold front will approach the region tonight and pass across
on Thursday.

Mainly VFR through the evening outside of an thunderstorms. Stratus
to the east should temporarily scour out. As for
thunderstorms...best chance for thunder late this afternoon/early
evening will be at kewr/kteb and kswf. Any storms would weaken as
they move east of the Hudson River.

Return to stratus and fog expected tonight. Better chance east of
NYC metropolitan. IFR to LIFR possible late tonight.

Any stratus should clear Thursday morning.

South to southwest winds continue through the taf period.
Speeds diminish tonight.

New York metropolitan enhanced aviation weather support...

Detailed information...including hourly taf wind component forecasts can
be found at: http:/www.Erh.NOAA.Gov/zny/n90 (lower case)

Kjfk fcster comments: low probability for thunder. Generally expect any
thunderstorms to weaken as they track toward the Airport.

Klga fcster comments: low probability for thunder. Generally expect any
thunderstorms to weaken as they track toward the Airport.

Kewr fcster comments: southeast winds are expected to veer back to the
south/southwest before 00z. Medium probability for thunder. Storms should
weaken as they approach the Airport.

Kteb fcster comments: medium probability for thunder. Storms should weaken
as they approach the Airport.

Khpn fcster comments: low to medium probability for thunder. Storms should
weaken as they approach the Airport.

Kisp fcster comments: lingering stratus should burn off or move east
late this afternoon briefly before developing or moving back in
after sunset.

Outlook for 18z Thursday through Monday...
Thursday afternoon...isolated to scattered thunderstorms possible in vicinity of a weakening
cold front. SW flow diminishing and possibly becoming northwest or vrb.
Friday...VFR with light NE winds veering through the day.
Sat...morning MVFR or lower possible. Chance showers and thunderstorms in the
afternoon and overnight with MVFR or lower. Strong southerly flow.
Sun...MVFR or lower in showers/thunderstorms possible. Shifting winds to
the E/NE.
Monday...possible improvement to VFR if showers/thunderstorms move to the
south. East/NE flow.

&&

Marine...
seas should build to 5 feet over the ocean waters by tonight...with
gusts up to 25 knots east of Sandy Hook with usual late day/evening
southerly sea breeze enhancement. New York Harbor and western Long Island
Sound could experience either direct thunderstorm impacts or a gust front
from weakening inland storms toward evening.

Persistent southerly fetch could build seas on the ocean to 5 feet
Saturday...and remain there for the rest of the forecast period...even
as winds switch to more easterly Sunday into early next week.

&&

Hydrology...
there is the potential for basin average of 1/4 to 1/2 inch of
rain across the lower Hudson Valley/NE New Jersey/SW CT this
afternoon/evening. Noting precipitation efficiency of just over 1.5
inch and corfidi vectors 10 knots or less...there is the potential
for training of cells that could lead to rainfall of 1-2
inches...with minor flooding of urban/poor drainage areas.

On Thursday there is the potential for less than 1/4 of an inch of
basin average rainfall. However...with storm motion of around 10
knots and precipitation efficiency near 1.25 inches...there is still
potential for slow moving storms producing locally moderate to
heavy rainfall...especially over southern CT. This could result in
the minor flooding of urban/poor drainage areas...especially if
it occurs over areas receiving locally heavy rainfall today.

There is potential for heavy rain Sunday and Sunday night
with precipitable water forecast to reach up to near 1.75 inches. There will be
potential as well for an extended widespread rainfall late this
weekend into early next week with a cold front stalling nearby.

&&

Okx watches/warnings/advisories...
CT...none.
New York...none.
New Jersey...none.
Marine...Small Craft Advisory until 6 am EDT Thursday for anz350-353-
355.

&&

$$

Synopsis...Goodman
near term...Goodman
short term...Goodman
long term...jm
aviation...precipitable water
marine...Goodman/jm
hydrology...Goodman/jm

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