Scientific Forecaster Discussion
Area forecast discussion
National Weather Service New York New York
403 PM EDT Thursday Aug 21 2014
an upper level disturbance pushes through the northeast into the
weekend...eventually pushing south on Saturday as high pressure
builds down from eastern Canada. This high will remain nearly
stationary over the northeast into Tuesday...then will shift south
of the area and slide offshore through Thursday.
Near term /until 6 am Friday morning/...
upper level trough pivoting through the northeast will bring an
increased chance for showers with isolated threat of thunderstorms.
Scattered activity associated with an embedded vorticity maximum along the trough
already noted from central New York extending down into eastern PA as
of 19z. Flow aloft fairly weak...so anticipating the progression of
this precipitation into the areas west and north of NYC no earlier than
this evening. Decent instability around 1000 j/kg with bulk shear
values between 25 and 30 kts should allow for the continuation of
the development of pulsey thunderstorms into areas of northeastern New Jersey and
the lower Hudson River valley. However...confidence decreases
heading east where the onshore flow throughout the day has
produced a more stable atmo. Confidence is greatest of precipitation west
of NYC and thus have included likely wording this evening to
account for this. Because of the lower confidence heading
east...capped probability of precipitation at chance for the second half of the
night...mainly southeast CT and eastern Li...and did not include thunder
Temperatures right around normal tonight with the cloud cover and
potential for precipitation overnight. Used a blend of met/mav guidance.
Short term /6 am Friday morning through Friday night/...
the region remains positioned under the deepening trough along the
eastern Seaboard as ridging aloft strengthens across the central
US. At the surface...low pressure rides a nearly stationary front
well south of the area...with high pressure dipping south from
southeastern Canada. Some model discrepancy on the extent of precipitation
development during the day and exactly where it forms. Once
again...higher confidence of development over western areas where
a shortwave is expected to move through as it rounds the
trough...so maintaining the higher chance probability of precipitation there...with slight chance
over eastern areas with the thinking that the persistent easterly
flow will work to stabilize the atmo and minimizing the chance for
shower formation. 12z model suite also suggests the stronger
forcing with another shortwave remaining well east of the area
tomorrow afternoon into tomorrow evening...keeping the bulk of that
associated precipitation east of the area.
For tomorrow night...could see lingering showers mainly over
western areas...clearing by 06z as high pressure at the surface
nudges south. Patchy fog also possible with plenty of moisture being
injected in the lower levels by the easterly flow. Temperatures on the
cooler side again with the onshore flow during the day Friday as
well into Friday night.
There is a moderate risk for rip current development along the
Atlantic Ocean beaches during the day Friday.
Long term /Saturday through Thursday/...
models are in good agreement that the 500 mb pattern across noam at the
start of the long term period will consist of an amplifying ridge
extending from south central Canada through the southeast Continental U.S. With a
cutoff low over the western US a slowly weakening blocking pattern
over eastern Canada and the northeast. The anomalously strong ridge
over Quebec will gradually break down and merge with the
aforementioned ridge over the US. This will result in a digging
trough east of New England.
Some uncertainty with the specific details and timing of vorticity energy
tracking through New England on Sat...but the trough axis should
move through Sat night. Have kept the majority of Sat dry with just
a schc of showers early. There could be a few additional showers
during the afternoon if there are any weaknesses aloft...but cannot
pinpoint where and when at this Point. May need to add slight to low
chance probability of precipitation in subsequent forecast packages to account for these timing
differences. Additionally...the amount of cloud cover on Sat is
also questionable as a result.
Otherwise...expect dry seasonable conds through the middle of next
week as heights rise once the trough departs. Surface high pressure will
build into the northeast over the weekend and remain nearly
stationary into Tuesday. This will result in an Ely flow contributing
to below normal temperatures though this time...but with heights
rising Tuesday through Thursday...temperatures will rebound to near normal
Aviation /20z Thursday through Tuesday/...
weak high pressure remains over the area.
Mainly VFR conditions are expected through the afternoon hours.
There will be isolated to scattered showers...and any shower could
reduce ceilings or visibilities to MVFR briefly. Best chance for showers
across NYC metropolitan and north and west...later in the day. Isolated
thunder is a low possibility. By 23z...chance of showers
increases slightly for coastal areas
A higher coverage of showers is expected tonight as an upper level
disturbance moves through. Still...best chance NYC metropolitan and
north and west. VFR ceilings and visibilities may lower to MVFR or even
IFR late at night.
Ceiling improvement on Friday will slow due to moist onshore flow.
Southeast or south wind 5 to 10 kts is expected this afternoon.
Winds tonight should lighten as they turn to the East.
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: amendments possible for timing of ceilings
Klga fcster comments: amendments are possible for timing of ceilings
Kewr fcster comments: amendments are possible for timing of ceilings
Kteb fcster comments: amendments are possible for timing of ceilings
Khpn fcster comments: amendments are possible for timing of ceilings
Kisp fcster comments: amendments possible for timing of ceilings and
Outlook for 18z Friday through Tuesday...
Friday morning...sub VFR possible in low clouds and fog.
generally sub-Small Craft Advisory conditions on the waters through Friday night
with high pressure building south and low pressure remaining well
south of the waters.
Small Craft Advisory for seas will be possible on the ocean waters Sat with a
moderate Ely flow. Current forecast keeps seas just below Small Craft Advisory
levels...but there is the potential for them to be a foot or so
higher. Sub-advsy conds expected thereafter as the pressure gradient
weakens with high pressure building over the northeast.
a few slow moving thunderstorms are possible this evening across the
lower Hudson Valley and interior NE New Jersey and into NYC. These storms will
have the potential to produce locally heavy rain...but basin
averaged totals are forecast to remain light with the isolated
nature of the storms.
No significant rainfall is expected then through the middle of