Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Mount Holly New Jersey
346 PM EDT Friday Jul 31 2015
high pressure across the Appalachians will give way to a weak
cold front, which is forecast to cross our area late Saturday
into Sunday. A stronger cold front is forecasted to cross our
region late Monday into early Tuesday, then stall offshore on
Wednesday into Thursday. High pressure may arrive on Friday.
Near term /until 6 am Saturday morning/...
a rather pleasant Summer afternoon is under way with light northwest
winds, temperatures in the middle/upper 80s but dewpoints in the 50s. Scattered
diurnal cumulus have formed over most of the area but they will thin out
and dissipate towards evening. Kdix radar shows a sea breeze along
parts of the New Jersey shore but it will not be able to move inland much
against the prevailing northwest flow.
For tonight...light winds and clear skies will allow temperatures to fall
into the 60s most areas...except possibly upper 50s in the Poconos
and low 70s in and near Philadelphia and near the ocean and bays.
Short term /6 am Saturday morning through 6 PM Saturday/...
for Saturday...the flow aloft is broadly cyclonic but without noteworthy
disturbances and the airmass continues rather dry. Oncly expected
clouds are some diurnal cumulus similar to today. The surface pressure pattern
should be similar to today as well so generally westerly wind should develop
in the morning and continue through afternoon. The 850mb winds are forecast to be
somewhat stronger so there could be a little more gustiness with afternoon
mixing. Have included a slight chance for late afternoon T-storms over northern New Jersey
and adjacent PA with some marginal instability expected.
Long term /Saturday night through Friday/...
Dominant middle-level feature through the long-term period is a closed
low in vicinity of hudson's Bay along west/ several shortwave disturbances moving
through it. This feature will be maintained by a developing ridge
over western noam and high-latitude blocking over Davis Strait.
Although there is uncertainty in the timing and strength of the
individual shortwave impulses...and how they affect the amplitude of
the trough...a developing Omega block over noam should lock in the
overall pattern through at least d+7. The most apparent model trends
are to speed the timing of a primary cold front through the area on
Monday night /as opposed to Tuesday night/ ... the front becoming
stationary near the New Jersey coast and Delaware-Maryland-Virginia through Thursday.
The stalled front solution is plausible...given the middle-level
low over hudson's Bay moving slowly northeast through the period...
which will lead to a more zonal flow over the area paralleling
the front by mid-week. This solution is also supported by the
ecens/gefs means...with overall support from the individual
members as well. Therefore...relied more heavily on the Euro
given its recent performance as well as the more stable ecens/
gefs means...and discounted the more progressive GFS after
00z Thursday especially given the more robust noam blocking pattern.
Overall...the main concerns are the potential for convection
and localized flash flooding in the Monday through Thursday time frame.
Temperatures will be at or above normal through at least Wednesday...with above average
uncertainty going forward...due to the questionable location
of the front and associated cloudiness.
Sunday...shortwave ridging aloft west/ high pressure moving in behind the
departing cold front. This will promote a mostly sunny and dry
day. The gradient may relax enough for a sea-breeze to develop
along portions of the coast.
Monday...shra/tsra are possible Monday afternoon as shortwave impulse skirts
to the north and a stronger cold front approaches from the west.
The approach of the shortwave will lead to steeper lapse rates...but
moisture is limited...so probability of precipitation were focused to the west of I-95
where better forcing should reside. The convective potential is
still in question at this point...and may be more focused just
to our west. The models appear to be overdoing vertical mixing
ahead of the front...but there is the potential for 20-25 knot
Monday night and Tuesday...the aforementioned cold front will
move just offshore Monday night where it is expected to become
stationary. There is still uncertainty as to the extent of
rain showers/thunderstorms and rain crossing the County Warning Area Monday night...so low probability of precipitation confined
north/west of I-95 for now. On Tuesday...the County Warning Area is situated between
the stalled front offshore and another shortwave disturbance just
to our north. West/ such a setup...could easily see some precipitation
in Delaware-Maryland-Virginia and especially over the Poconos and Lehigh Valley.
Have low probability of precipitation confined north/west of I-95 for now.
Wednesday through Friday...there remains above average uncertainty during
this time frame. The synoptic setup favors a stationary front
in the vicinity and the models also advertise a wave of low-
pressure traversing this boundary on Thursday. The models are
also suffering convective feedback issues during this period
which further adds to the uncertainty. The main concern is the
potential for some heavy rainfall during this time frame...as
wpc has their maximum quantitative precipitation forecast axis across the southern half of our County Warning Area.
Aviation /20z Friday through Wednesday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg,
kmiv, kacy and surrounding areas.
Hi confidence for VFR conds to continue through tonight and Saturday
at all taf sites. Winds will become light/variable tonight as the surface
layer cools and decouples from winds aloft. Westerly winds will redevelop
on Saturday morning and may be somewhat gusty 15 to 20 knots at times in
the afternoon due to slightly stronger winds aloft.
Sat night and Sunday...VFR.
Monday through Wednesday...shra/tsra are possible...especially each
afternoon. Otherwise VFR.
winds and seas will continue rather light over the forecast waters
tonight and tomorrow morning. Todays northwest flow will turn to the S and
then SW tonight and Saturday as a surface trough becomes more pronounced
along the middle-Atlantic Seaboard. Seas of 2 to 3 feet tonight will
build slightly on Saturday as southwesterly winds increse...gusting to 15 or
20kt Sat afternoon.
Sunday through Wednesday...
Weak high pressure over the central Appalachians will extend over
the waters on Sunday into Monday. A stronger cold front will move
over the waters Monday night and stall in the vicinity through Wednesday.
There may be a period of wind gusts to about 20 knots associated
with the passage of a cold front late Monday into early Tuesday.
This will result in seas increasting...but they are expected to
remain below 5 feet at this time.
Otherwise sub-Small Craft Advisory conditions are anticipated through the period.