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Area forecast discussion 
National Weather Service New York New York
732 am EDT Friday Aug 1 2014

Synopsis...
high pressure remains just offshore through Saturday as a frontal
boundary over the Atlantic coast and low pressure to the south
will impact the area through the beginning of the week. High
pressure then builds into the region on Tuesday...with a cold
front passing through on Wednesday.

&&

Near term /until 6 PM this evening/...
minor adjustments made with this update. Increased the cloud cover
a little as per latest satellite trends. Also delayed onset of
rain chances until shortly before noon.

Deep upper trough continues to rotate in Canada with a surface
high pressure area anchored to the east. A shortwave will ride
north along the eastern side of the upper trough today which will
set off some showers and possibly thunderstorms. Expect these will
stay north and west of NYC where mu cape ranges rom 1500-2000 j/kg
even though there is not a lot of deep layer shear. Not expecting
severe storms but some could be strong with gusty winds and
locally heavy rainfall. With the light south-southwest winds through the
layer...it is possible these storms will move slowly and train
over the same areas.

Kept a slight chance of probability of precipitation over Long Island and southern CT but
confidence is low we actually see any showers over the area with the
instability and lift focused more north and west of NYC.

NAM/met guidance had a better handle on the SW flow warming interior
areas slightly more than coastal locations. 13-14c 850 temperatures lend to
highs in the lower 80s which matched well with guidance and leaned
towards the NAM numbers.

&&

Short term /6 PM this evening through 6 PM Saturday/...
showers and thunderstorms north and west of NYC will likely diminish
in coverage after sunset with waning instability and the shortwave
passing to the north.

Next focus of rain will be the cold front backing in from the east
with the western Atlantic high pressure expanding slightly
westward. Some model differences exist on the timing of when the
precipitation will reach the area with the NAM the slowest and driest for
the day on Saturday. Highest precipitation amounts will be Long Island
and southeast CT with the front positioned just to the south of
the island. Will continue chance probability of precipitation elsewhere though as showers
should still be able to develop with upper jet and middle level
shortwave enhancement. As well...deep moisture streaming from the
south will be in place so any showers will produce locally heavy
rainfall.

Very low confidence on any thunder for Saturday. Little to no
instability or shear...just moderate lift. Expect showers to be very
tropical in nature and maybe an isolated lightning strike or two.
Wind profiles aloft are again light so training of cells is likely.

As for temperatures...NAM 2m temperatures much warmer than all guidance for
lows tonight even with similar cloud cover forecasts. Went with a
blend of mav/mosguide for lows for temperatures in the lower to middle 60s.

Guidance for highs on Sat is lower than previous. Possibly due to
the models coming into agreement for a rainy day. Will lower highs a
few degree from previous forecast.

&&

Long term /Saturday night through Thursday/...
a shortwave weakens at the base of a 500mb trough axis as it
approaches the forecast area...eventually passing through Sunday
night into Monday morning. This will help slowly nudge the offshore
frontal boundary farther out to sea.

Some synoptic lift will be present through Monday with the right
entrance of an upper jet streak in the vicinity. Less certain are
the timing and position of low-middle level shortwaves that pass
through/nearby the County Warning Area before the trough axis aloft passes through.
Probability of precipitation therefore capped at chance...which is close to what a model
blend would produce. Best overall chances of rainfall would be
Saturday night into Sunday morning and closer the offshore frontal
boundary. Monday could even turn out to be completely
dry...especially if the European model (ecmwf) is correct with the trough axis
passing through Sunday night and a lack of low-middle level shortwaves
trailing it on Monday. Do not have enough confidence to remove probability of precipitation
altogether on Monday...so have left the previous forecast of slight
chance/chance probability of precipitation.

Weak high pressure probably keeps Tuesday and Thursday dry...but a
cold front and cyclonic flow aloft could bring an isolated
shower/thunderstorm to the area on Wednesday.

Clouds and an onshore flow will hold high temperatures on Monday
below normal...highs otherwise near normal for the rest of the long
term forecast.

&&

Aviation /11z Friday through Tuesday/...
high pressure will remain offshore today...while a weak front and
wave approach from the southeast late tonight.

Mainly VFR through 00z.

Scattered shower and thunderstorms development likely this
afternoon into the early evening...but mainly n&w of metropolitan
terminals. 17z to 20z window for isolated thunderstorms and rain development across
kewr/kteb. Latest ccfp looks reasonable.

Wind increase out of the S/southeast this afternoon...with seabreeze
development expected at coastal terminals and likely moving inland
of kewr/kteb in after.

Increasing potential for MVFR conds in rain showers activity after
midnight into Sat morning.

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: S seabreeze development expected by 17-18z.

Klga fcster comments: S seabreeze development expected by 18-19z.

Kewr fcster comments: southeast seabreeze development likely by
18-19z. Low confidence whether sparse coverage of thunderstorms between 17z
and 19z will affect terminal.

The afternoon kewr haze potential forecast is green...which implies
slant range visibility 7sm or greater outside of cloud.

Kteb fcster comments: south-southeast seabreeze development likely by 18-20z.
Low confidence in whether sparse coverage of thunderstorms between 17z and
20z will affect terminal.

Khpn fcster comments: low confidence in whether sparse coverage
of thunderstorms between 17z and 19z will affect terminal.

Kisp fcster comments: no unscheduled amendments expected.

Outlook for 12z Saturday through Tuesday...
Saturday-Sat night...rain showers with MVFR or lower conds likely. Low
probability for thunderstorms and rain.
Sun-Sun night...shra/tsra possible...MVFR or lower conds
possible.
Monday-Tuesday...VFR. MVFR possible in isolated rain showers/thunderstorms in the
afternoon/evening.

&&

Marine...
with a weak pressure gradient over the waters...sub-Small Craft
Advisory conditions are expected on the waters through Tuesday.

&&

Hydrology...
locally heavy downpours expected in scattered thunderstorms
today...mostly north and west of NYC. Localized urban/poor
draining flooding is possible due to slow movement.

A similar set up exists for the weekend into Monday with showers
more numerous in coverage across the area and slow movement of
cells. Although thunderstorms are unlikely...the moist airmass will
produce moderate to heavy downpours. This could once again result
mainly in urban/poor drainage flooding.

&&

Okx watches/warnings/advisories...
CT...none.
New York...none.
New Jersey...none.
Marine...none.

&&

$$
Synopsis...jc/ln
near term...line
short term...line
long term...jc
aviation...Nevada
marine...jc/ln/pw
hydrology...jc/ln

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