Scientific Forecaster Discussion
Area forecast discussion
National Weather Service New York New York
205 am EDT Wednesday Jul 23 2014
high pressure will continue to move offshore as a cold front approaches
from the west. The front will move across from Wednesday night into
Thursday...followed by high pressure on Friday. The high will weaken
on Saturday...then low pressure passing to the southeast will impact
the area Sunday into Monday. An associated cold front will approach
late day Monday...and move across Monday night into Tuesday.
Near term /until 6 am this morning/...
southwest winds between a departing offshore high pressure system and an
approaching cold front will continue to advect warm moist maritime tropical
air across the area overnight.
Our local area weather will remain dry as strong middle and upper level
subsidence associated with the high pressure system continues to advect
slowly east across the area.
Short term /6 am this morning through 6 PM Thursday/...
Wednesday...hot and increasingly humid air mass remains over the
area. Surface dew points will rise through the 60s to around 70. Maximum
temperatures will top off in the lower 90s across NYC and NE New Jersey...and
will be in the upper 80s to near 90 elsewhere. Temperatures will be
sightly cooler near the south coasts. The heat index will generally
top off in the low to middle 90s...except for NYC/NE New Jersey...where the heat
index will be in the middle to upper 90s. Since the heat index in NYC
will not reach 95 for 2 consecutive days...and since the heat
index will remain below 100 across the area...will not issue a
As cold front approaches...showers and thunderstorms will develop
north and west of NYC in the afternoon along a pre-frontal trough.
Showers and thunderstorms spread east Wednesday evening and
Wednesday night...and the bulk of the activity should hold off
until Wednesday night as the cold front passes through the region.
With the heat and humidity...ml MUCAPE values will range from
1000-1500 j/kg during the day...and then will drop to around 400
j/kg at night with the loss of the heating. 0-6 km bulk shear will
be weak during the day...and then will rise to 25-35 knots after 00z
Thursday. Cannot rule out strong winds with any thunderstorms...
but primary threat will be heavy rain due to precipitable waters
around 2 inches. Flash flooding of low lying and poor drainage areas
is possible. Will not issue a hazardous weather outlook as the
threat will not be widespread.
Showers and thunderstorms taper off from west to east Wednesday
night as the cold front works its way across the region.
There is a moderate risk of rip currents...mainly Wednesday
after/evening with enhanced sea breeze development.
Long term /Thursday night through Tuesday/...
the surface cold front should be to the east Thursday morning...but with the
800 mb front still yet to move across...a middle level trough still well to
the west and potential for a weak surface wave along the front to slow
its progress even further...will have likely pop to start across southeast
CT and eastern Long Island and chance elsewhere...with chances gradually
ramping down during the afternoon and early evening. Conditions look
mainly dry for later Thursday night and more so into Friday as high pressure
builds from the west.
The high should weaken on Sat as flow aloft becomes more zonal.
As low to middle level moisture return commences...with precipitation
efficiency increasing back to over 1 inch by late day...and as middle
level shortwaves approach in zonal flow...should start to see
increasing chances for showers/thunderstorms beginning late day Sat into
The flow aloft should undergo significant amplification early next
week as an upper ridge builds in the western states and a potent
shortwave trough diving out of central Canada merges with an upper
low over Quebec...carving out a deep longwave trough over the
eastern states. There will be good chances for showers/tstms...
especially Sunday night into Monday morning as an associated wave of
low pressure passes just off to the southeast...then late day Monday into Monday
night with the approach of a cold front from the west.
Could see some leftover showers/thunderstorms on Tuesday mainly in the morning
as the cold front moves to the east...then drier and slightly cooler
air arriving through the rest of Tuesday into Tuesday night.
Aviation /06z Wednesday through Sunday/...
a cold front approaches from the west today...then begins to cross
the tri-state this evening.
High confidence in VFR into this afternoon except for possibly at
khpn with MVFR fog possible 9-13z. For now address thunder
potential with prob30 at all terminals except kgon - for now
appears thunder most likely will not reach kgon until around/just
after 6z Thursday...so not confident enough there to reflect in
tafs. For now went MVFR in tempo groups...however with locally
heavy rainfall possible in stronger convection - would not be
surprised if there were brief periods of IFR or lower with any
Light SW flow gives way to seabreeze at all but kswf this
afternoon. For now cannot rule out occasional gusts 20-25 knots at
city/Long Island terminals with seabreeze this afternoon. However
confidence in occurrence/timing is not high enough to include in
the tafs at this time. Winds diminish to under 10 knots in the
evening...veering to the northwest behind the front this evening across
city and northwest.
Outlook for 06z Thursday through Sunday...
late Wednesday night-Thursday morning...MVFR or lower conditions
in scattered-numerous rain showers/thunderstorms and possible fog.
Thursday afternoon-Saturday morning...VFR.
Saturday afternoon...MVFR or lower possible in isolated rain showers/thunderstorms and rain
mainly northwest of NYC.
Saturday night-Sunday...chance of rain showers/thunderstorms with MVFR or lower
winds will gradually subside through 06z...with sub-Small Craft Advisory conditions
expected overnight. Low probability of fog on the waters based on
forecast dew points near or slightly below water temperatures...so have
removed from forecast.
Winds will increase to 15-20 knots once again on the ocean waters
Wednesday afternoon and to 10-15 knots with 15-20 knots gusts on the
rest of the waters ahead of an approaching cold front. Convection
likely Wednesday night. Seas will average 2-4 feet on the ocean and
around 1 feet elsewhere.
Outside of thunderstorms Thursday and late day Sat have high confidence in
quiet conditions Thursday through Sat...then forecast uncertainty creeps in
beginning on Sunday. Have forecast continued quiet conditions...but if an
approaching wave of low pressure and/or frontal system are
stronger than forecast...hazardous ocean seas could develop if
corresponding S-SW flow is stronger than forecast.
there is the potential for around 1/2 inch of rainfall from late
Wednesday through Thursday with locally higher amounts possible.
Precipitable waters are forecast to be around 2 inches on
Wednesday night...so locally heavy rainfall is possible in any
stronger convection. Areas experiencing locally heavy rainfall
could experience at least minor flooding of urban/poor drainage
area...and possibly localized flash flooding. The confidence in
the latter is currently less than 30 percent so will not highlight
in the hazardous weather outlook.
Low pressure passing just off to the southeast Sunday night into Monday
morning...followed by a cold frontal passage later Monday...have some
potential to produce heavy rain...as precipitation efficiency increases
to over 1.5 inches. This will depend first on the low track...then
on potential for training of cells ahead of the cold front...as
well as other mesoscale/thermodynamic factors. Given these
uncertainties it is too early to outlook this in the severe weather potential statement.