Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Mount Holly New Jersey
938 am EDT Tuesday Sep 2 2014
a frontal boundary will move through the region tonight, then
this front will be located south of our region Wednesday as high
pressure builds overhead. High pressure will gradually move from
west to east across the eastern United States through the second
half of the week. On Saturday, a cold front will move through the
region. This front will stall to our southeast for the early part
of next week with high pressure over the Great Lakes.
Near term /until 6 PM this evening/...
930 am update...no changes to the going forecast. Earlier patchy
fog has completely dissipated, and clouds have mostly burned off.
Temperatures still look on track as many locations are already in
Previous near term discussion...while the calendar says
September, it will continue to feel like The Heart of Summer as
the heat and humidity remains across the area. There could be some
slight relief through the day as the surface dew points are
forecast to lower some. Synoptically, high pressure continues to
be centered in the western Atlantic helping to pump up the heat
and humidity within a southwesterly flow. There is however a
notable upper-level trough that will be swinging across the Great
Lakes region and the upper Ohio Valley through the day. There is a
surface front with this feature, however it will remain well to
our west during the day. There should be a pre-frontal trough
developing given the warm to hot airmass. The eventual surface
front may jump into this trough as we work our way into the short
term /tonight/. The forecast challenges for today revolves around
the convective chances and also the heat indices.
As the upper-level trough progresses eastward, the main short
wave is forecast to lift to our north late this afternoon. This
leaves behind the tail end of it sliding into our area. As a
result, the large scale forcing should slide to our northwest
later this afternoon. There will however be instability in place
although this can be affected by the lowering dew points /at least
SBCAPE/. There is some increase in the middle level flow, however the
core /50-60 knots/ of it is forecast to remain well to our west
which is where the slight risk for severe thunderstorms is
indicated. That stronger flow is expected to allow for convective
development well to our west and this is what we will probably
have to wait and see what survives as it tracks eastward. In
addition, some isolated activity could fire along the pre-frontal
trough. As a result, we slowed down the pop increase from west to
east. While an isolated cell could form this afternoon farther
east from local convergence, much of the time will probably be
Regarding the heat, the 850 mb temperatures are forecast to be a few
degrees warmer than yesterday. The forecast soundings indicate the
potential for better mixing being present as the pressure gradient
tightens some and the low-level lapse rates steepen. Therefore this
should result in high temperatures into the lower to even middle 90s
for many inland areas. The mixing however should lower the surface
dew points some in the afternoon, keeping the heat indices a bit
lower. As a result, we are expecting the heat indices to remain
under advisory criteria including the Philadelphia metropolitan area.
High temperatures were a blend of the NAM/GFS MOS, although the
warmer NAM MOS was favored for some areas which also got US close to
continuity. The hourly grids were tweaked using the lamp/lav
guidance, especially this morning to help assist with
temperature/dew point trends thus far.
Short term /6 PM this evening through 6 am Wednesday/...
the aforementioned upper-level trough will generally glance our
area, with a frontal boundary sweeping through. The main forcing and
associated middle level jet maximum is forecast to bypass our area to the
northwest. There still is some increase in the flow with this system
as the tail end of the short wave moves through. This in conjunction
with enough instability should support some convection this evening
before weakening takes place. The amount of shear looks low and
therefore storm organization should be weak and potentially even
disappear as the evening wears on.
The forecast soundings indicate some opportunity for some stronger
cores across our western zones and towards the Delmarva, however
given the best forcing and shear missing our area the severe weather
threat looks rather low as of now. Therefore, no enhanced wording
was carried at this time. We used chance probability of precipitation this evening then these lower
from west to east. Any lingering convection should be done after
midnight as the frontal boundary shifts eastward. The model guidance
shows a plume of precipitable water around 2 inches surging ahead of the surface
front, therefore any convection will have the potential to produce
brief torrential downpours. Any flood threat should be limited
There is no significant airmass change behind the surface front
despite a wind shift to the northwest. As the clouds thin out some
surface cooling will take place, however the dew points look to
remain on the muggy side for much of the night. Some lowering should
start to take place though late mainly over our western zones. The
combination of light winds and lingering low-level moisture, may
allow for some fog to develop particularly where dry air advection
is delayed. We are not expecting a lot of fog, and given lower
confidence we held off including a mention at this time.
As for low temperatures, an even blend of the GFS/NAM was mainly
Long term /Wednesday through Monday/...
Wednesday through friday: dry with few clouds, flow turning
gradually to the southwest by Friday. Think the 00z GFS is to quick
showing some return flow probability of precipitation. Muggy and hot as well but temperatures will
be under heat advisory criteria. Fair shot at 90 Thursday/Friday for phl.
Went at or a touch above met/mav guidance on temperatures based on
the 925/850 mb temperature profiles.
Friday night through Saturday night: a frontal boundary moving south
from the Great Lakes will be a trigger for some scattered showers
and thunderstorms. Clouds should increase from northwest to
southeast Friday night and Saturday. Continued southwest flow will
be in place as well ahead of the front. 925 mb temperatures are likely to
be a touch higher than Thursday/Friday indicating slightly warmer surface
temperatures. Temperatures could possibly making a run at the low
perhaps middle 90's based on how quickly the clouds increase. Went
warmer than wpc/mex guidance in these periods to account for the
warm air advection ahead of the front. In terms of any strong to severe storms. A
modest amount of instability (cape around 1500 j/kg) will be
present with moderately steep lapse rates. However the lack of
shear in place will likely prevent any widespread coverage of
strong to severe storms. Continued thunder in grids for Sat-Sat
night with chance probability of precipitation.
Sunday through tuesday: northeast flow will develop early next week
with the front to our south and high pressure over the Great Lakes.
The 00z GFS is the most aggressive clearing the front during the day
Sunday with only lingering showers across the south. The 00z European model (ecmwf)
and CMC slow the front allowing for additional shower chances focused
across the south, particularly on Sunday but linger isolated showers
into Monday and Tuesday. The European model (ecmwf) appears to be faster than it's
previous run and some members within the 18z GFS ensemble suite
argue for a slower frontal passage. A middle ground will be used
with chance probability of precipitation decreasing from north to south Sunday with slight
chance probability of precipitation across the south Monday. Clouds and showers may keep
highs down on Sunday and lows higher at night. On Monday and Tuesday
ample sunshine may allow most areas to get a touch warmer than what
modeled two meter temperatures or wpc guidance suggest. Radiational
cooling Monday night is likely as well across most of the region
with the favored region just inland off the New Jersey coast and northwest phl
could have a relativity chilly night.
Aviation /14z Tuesday through Saturday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg,
kmiv, kacy and surrounding areas.
Today...some MVFR/IFR visibilities due to fog early mainly at
kabe and kttn, otherwise VFR. A few showers and thunderstorms
should develop later this afternoon mainly west of kphl and
therefore a tempo group will be added to krdg and kabe. A vcsh was
carried elsewhere late. Light to even calm winds, becoming
southwest and increasing to 10-15 knots toward midday. Some local
gusts up to 20 knots should occur this afternoon as enough mixing
Tonight...a frontal boundary will move through with some showers and
thunderstorms arriving from the west, which may tend to diminish
with an eastward extent. A wind shift to the northwest is anticipated
with any convection and also as the front moves through. MVFR
ceilings with visibilities as low as IFR will be possible with any
shower or thunderstorm, otherwise VFR. There is a chance some local
fog develops late as clouds thin and the winds drop off with the
low-level moisture remaining.
Wednesday through Friday night: VFR.
Saturday: MVFR to IFR restrictions possible in any afternoon or
a southerly flow will be maintained today well ahead of a surface
front. There will be some increase later this afternoon and evening
as the pressure gradient tightens some, however these are expected
to remain below advisory criteria. The winds will shift to the west
and northwest later tonight behind the frontal passage. Some
thunderstorms are possible with the front mainly this evening,
however these could weaken before reaching our marine area. Seas
will be in the 2 to 4 feet range over the coastal waters, and 2 feet
or less on Delaware Bay.
Wednesday through Friday night: sub Small Craft Advisory seas and winds expected
at this time.
Saturday: some strong wind gusts possible from thunderstorms,
otherwise sub Small Craft Advisory seas and winds.