Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Mount Holly New Jersey
339 am EDT sun Aug 31 2014
large high pressure well off the East Coast will maintain a
southwesterly flow over the area through the first part of the week.
A weak cold front is then expected to move across the area Tuesday
night and early Wednesday. High pressure will build in behind this
front for the second half of the week. Another cold front may
approach from the west next weekend.
Near term /until 6 PM this evening/...
our region will mainly be caught between systems today, as an
upper-level ridge remains situated to our south and southeast with
surface high pressure to the east. Meanwhile, an upper-level trough
will continue to track across the Great Lakes then across southern
Ontario Canada. This will result in a deep southwesterly flow, which
will pull an abundance of moisture into our region. As a result, a
more humid day is expected.
The presence of some warm air advection combined with Theta-E advection is helping
to produce a decent amount of cloud cover. There may be some breaks
in this as the flow above the surface increases some, however it
will also be fighting an inversion. The initial warm air advection an increase in
moisture may be enough to result in some isentropic lift to pop a
few showers this morning mainly across our western to northern
zones. This scenario is less certain as some model guidance that
were hinting at this seem to have lost a subtle short wave running
out ahead of the main feature. As a result we delayed the slight chance
probability of precipitation some, then gradually expand and increase them eastward with
As we step forward through the day, enough heating combined with
climbing surface dew points should result in enough instability
/MLCAPE up to 1500 j/kg/ to produce showers and thunderstorms. The
bulk of this activity is expected to develop to our west near a weak
frontal boundary with additional development possibly near a
differential heating zone. The height falls are weak this far east
as the short wave trough tracks to our north late in the day.
However, an increase in the middle level flow to near 40 knots will
increase the shear with time. This should be enough to promote some
storm organization with the main Mode being line segments. In
addition, the precipitable water values are forecast to increase to around 2 inches
which will lead to downpours. These downpours may result in brief
poor drainage flooding, however the flash flood threat generally
looks low as there should be enough of a storm motion.
The moist environment combined with enough instability could also
result in some storms producing frequent lightning. Given the
forecast thermodynamic setup, some storms will be capable of
producing gusty to perhaps locally damaging winds. This generally
looks to be from about the Interstate 95 corridor on north and west
by late afternoon. We therefore maintained increasing probability of precipitation from west
to east with the western zones going to likely in the afternoon, and
we also added some enhanced wording. Much of the eastern and
southeastern zones may remain dry for the bulk of the day.
As for high temperatures, we used an even blend of the NAM/GFS MOS
then adjusted down a little bit for some areas with the idea of lots
of clouds prevailing.
Short term /6 PM this evening through 6 am Monday/...
the aforementioned short wave trough is forecast to dampen out as it
tracks well to our north tonight. There is still a trail of short
waves rolling up the backside of the upper-level ridge with also a
weakening frontal boundary nearing the area. The forecast soundings
indicate that enough instability is maintained for awhile for some
showers and thunderstorms to continue through the evening,
especially from the Interstate 95 corridor on north and west. A
somewhat more pronounced short wave may help to focus a convective
cluster overnight, however where this is focused has less certainty
with it at this time. As the precipitable water values remain rather high, local downpours
could put down a decent amount of rain however any flood threat
looks brief and localized.
As mentioned above, some strong convection may occur in the evening
with locally gusty winds possible. This would be more tied to any
line segments that are able to maintain themselves with perhaps some
enhancement from water loaded downdrafts. Otherwise, a wealth of
clouds should be around through the night for much of the area along
with muggy conditions.
As for low temperatures, an even blend of the NAM/GFS MOS was
Long term /Monday through Saturday/...
unsettled weather is expected on Monday with convection still
possible most areas through most of the day. Locally heavy rain is
possible just about anywhere...but latest guidance suggests near and
south and east of the I-95 corridor might be favored. Any flooding
should be more urban and poor drainage flooding. With a continued
southwesterly flow, temperatures will peak 5 to 10 degrees above
normal for the beginning of September.
Monday night and Tuesday...a lull in the precipitation is expected
late Monday night through Tuesday afternoon. If there's enough
sunshine...temperatures could peak out in the low 90s.
Tuesday night...showers and thunderstorms...some with locally heavy
rain...can be expected as a cold front swings through the area from
west to east.
Wednesday through Friday...the air mass behind the cold front really
won't be too much cooler courtesy of a middle-level zonal flow, but it
will be drier and more comfortable. Highs all three days will be
primarily in the low to middle 80s, which is still above normal.
Beautiful early September weather.
Saturday...increasing cloudiness as the day wears on as a cold front
approaches from the west. Showers and thunderstorms are possible
with temperatures still above normal.
Aviation /08z Sunday through Thursday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg,
kmiv, kacy and surrounding areas.
Today...areas of MVFR to perhaps local IFR ceilings will improve to
VFR ceilings by midday. While some light fog may occur, most of the
moisture may result in actual ceilings instead of visibility
restrictions plus a light surface wind is generally maintained
An isolated shower is possible early this morning mainly from near
kabe and krdg, otherwise some showers and thunderstorms are expected
to continue to develop to our west and overspread our area this
afternoon. Some timing was indicated in the tafs, with thunder only
mentioned at kabe to krdg due to higher confidence. This may be
expanded eastward with the 12z issuance. There will be brief times
of MVFR/IFR visibilities especially within showers and thunderstorms
as downpours are anticipated. South-southeast winds increasing to
generally 10-15 knots and becoming south-southwest.
Tonight...showers and thunderstorms should continue eastward with
much of the activity in the evening hours. As a result, there will
be times of MVFR/IFR due to downpours. In addition, some fog could
develop late if some clearing occurs combined with light winds.
Monday into Monday night...IFR conditions expected with showers and
thunderstorms. Coverage of precipitation decreasing during the
Tuesday...generally VFR conditions expected during the day.
Tuesday night...IFR conditions possible as a cold front moves
through the area.
Wednesday through Friday...generally VFR conditions expected.
a southerly flow is expected through tonight ahead of a weakening
frontal boundary as a disturbance tracks well to our north. As this
occurs, the flow is forecast to increase and this opens the door to
some mixing. It appears that the warm air advecting across the area
will keep the stronger winds above the surface. While there can be
some gustiness especially nearshore, it generally looks marginal.
The southerly flow will also allow the seas to build through
tonight, however given the southerly flow direction and fetch, we
kept seas up to 4 feet. Therefore, no Small Craft Advisory has been
issued at this time. Some showers and thunderstorms will arrive late
this afternoon and tonight, and some storms could produce locally
Monday and Monday night...winds and seas should remain just below
Small Craft Advisory criteria. Gustier winds possible in the
vicinity of thunderstorms.
Tuesday through Thursday...winds and seas expected to remain below
Small Craft Advisory criteria. However, some gusty winds are possible with
thunderstorms Tuesday night and early Wednesday.