Scientific Forecaster Discussion
fxus61 kokx 222044 cca
Area forecast discussion...corrected
National Weather Service New York New York
444 PM EDT Sat Jul 22 2017
a frontal boundary remains to our south tonight as a wave of low
pressure tracks along it, this front then sags farther to the
south on Sunday after the low passes. Another low then moves
along the front, tracking off the mid-Atlantic coast by Monday
morning. Still another area of low pressure that moves through
the region Monday and Monday night, exits to the east Tuesday.
High pressure builds over the northeast late Tuesday through
Wednesday, then drifts off the coast Wednesday night. Another
wave of low pressure follows for Wednesday night into Friday,
with a cold front moving through Friday. High pressure returns
Near term /until 6 am Sunday morning/...
a few isolated showers are possible before 6pm, mainly across
far west zones, as the main part of a convective complex passes to
the SW, with anvil based precipitation passing mainly to our north.
An area of showers and thunderstorms, currently over north central
PA is progged to move into the NYC Metro area 23-01z, then track
east-southeast along/just south of Long Island through around 4z, with
scattered mainly showers to the north. This area is associated with
a 700-500 hpa shortwave and vorticity maxima that moves through
right behind it. Could be some strong to possibly severe storms
with this cluster, with gusty winds the main threat.
Once this shortwave exits, the overnight hours should be mainly
dry, except for possibly some lingering showers (with a slight
chance of a rumble of thunder) mainly over Long Island.
Will let heat advisory expire at 6pm.
Lows tonight should be around 5 degrees above normal.
There is a moderate risk for the development of rip currents at
ocean beaches through this evening.
Short term /6 am Sunday morning through Sunday night/...
the region should be under locally zonal flow Sunday. With no
shortwaves of note forecast to move over the region in this
flow, it should be mainly dry. However, this is quite a change
from the previous forecast. So to trend things, went with
slight chance pops over western zones and mainly dry over
eastern zones through late afternoon, then increased pops to
chance over far western zones and slight chance throughout
elsewhere (reflecting climatological trends for convection).
A sharpening 700-500 hpa trough/developing closed low moving
into the Great Lakes Sunday night, along with 850 hpa
frontogenesis, will produce an increasing threat of showers,
with a slight chance of thunderstorms. The pops increase from west
to E, to likely throughout by after midnight.
Onshore low-level flow should result in a mostly cloudy sky on
Sunday, limiting highs to near to slightly below normal levels
(mainly around 80-mid 80s). Lows Sunday night should be slightly
above normal due to cloud cover.
There is a moderate risk for the development of rip currents at
ocean beaches Sunday.
Long term /Monday through Saturday/...
an upper level shortwave and accompanying wave of surface low
pressure will be exiting to the east Monday night into early
Tuesday. Showers and isolated thunderstorms will be on-going at the
beginning of the extended forecast period. Instability and cape will
be marginal and increase somewhat during Monday, however may still
not be sufficient to support widespread convection, so will keep
The upper westerly flow will be progressive through the period. A
weak ridge builds Monday night into Tuesday, and Wednesday. Then
another shortwave, with an embedded closed low, over the Canadian
West Coast Monday, opens and digs a more significant trough into
the eastern states Wednesday night into Thursday with a high
amplitude trough, of 1 to 2 Standard deviations above normal,
digging all the way to northern Florida by Friday. The flow looked
to be still progressive with a surface low moving through
Thursday and Friday. However, there are hints that the upper low
will close off again Friday into Saturday and remain along the
northern coast into next weekend. At this time will keep with
persistence and the more progressive flow and keep Friday night
into Saturday dry as upper ridging builds to the north.
Temperatures through the extended period will be near to slightly
below seasonal normals.
Aviation /20z Saturday through Thursday/...
generally VFR through the taf period, outside of any showers and
thunderstorms that are forecast. Sea breezes have moved through the
coastal terminals. A weak cold front will remain nearly stationary
just south of the area through tonight. Low pressure will pass just
south along this front tonight.
Thunderstorms over central and eastern Pennsylvania will continue
to be watched as they move eastward tonight. Best timing for these
thunderstorms is 01z-03z for the NYC terminals. However, there is a
prob30 group to account for more thunderstorms from 04z-10z as exact
timing after 03z cannot be determined, but confidence is medium to
high that we will see thunderstorms during this time period. Chances
are lower, as well as confidence, for eastern terminals such as kisp
and kbdr where only thunderstorms in the vicinity is mentioned from 00z- 10z, and there is no
mention of thunder for kgon, however an isolated thunderstorm is
possible for the same time period.
Any showers and thunderstorms have the potential to lower conditions
to MVFR or lower. Also, gusts associated with thunderstorms 04z-10z
have to the potential approach 30 kt.
Winds will shift from the south to the northeast tonight, then shift
to the southeast on Sunday for most terminals, with speeds generally
10 kt or less.
Outlook for 18z Sunday through Thursday...
Sunday afternoon-Monday night...episodes of rain showers/thunderstorms and rain with
MVFR or lower conditions possible.
Tuesday...an am shower possible at kgon, otherwise VFR.
Tuesday night-Wednesday night...VFR.
Thursday...sub-VFR possible in rain showers/thunderstorms and rain
a light to moderate pressure gradient over the waters around
Long Island through Sunday night, will limits sustained winds
to 15 knots or less, ocean seas to 4 ft or less, and seas/waves on
the non- ocean waters to 1 ft or less through then.
Winds, gusts, and seas will remain below small craft levels Monday
through Thursday. However, ocean seas may approach 5 feet Wednesday
night into Thursday as an increasing southerly flow develops ahead
of a cold front. Waves of low pressure passing through the waters
will bring a chance of thunderstorms Monday and again Thursday into
currently expecting around 1/4-1/2 inch of rainfall across
southern portions of the County Warning Area and less than 1/4 of an inch of
rainfall elsewhere through Sunday morning. Locally heavy
rainfall is possible with any stronger convection tonight,
mainly over southern zones. If the locally heavy rainfall
occurs, it could result in at most minor flooding of urban and
poor drainage areas.
An additional 1/2 to 1 inch of rainfall is likely from Sunday
night into Monday night, with locally higher amounts possible.
There is a small chance for minor flooding or urban and poor
drainage areas, if any experience locally heavy rainfall.
More rain is possible late Wednesday night through Friday.
Significant hydrologic impacts are not expected at this time.
with the approach of the new moon (sun), tides will run high
this weekend into early next week. Positive tidal departures of
1/2 to 1 ft will be needed for minor flooding during the night
time high tides during this time.
Minor flooding is possible starting with the high tide cycle
this evening, mainly in the western South Shore bays of Long
Island. A bit more widespread minor flooding is possible Sunday
night with an east/NE flow expected. The threat for minor flooding
could continue into Monday, with east/NE flow progged to continue.
New York...heat advisory until 6 PM EDT this evening for nyz072>075-176-
New Jersey...heat advisory until 6 PM EDT this evening for njz006-104-106-