Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Mount Holly New Jersey
552 am EDT Sat Aug 30 2014
high pressure centered off the New England coast will continue to
move east today and tonight. Meanwhile, a warm front is expected
to lift north late Saturday, then become nearly stationary near
the Canadian border Sunday through Tuesday. A cold front is then
expected to move across the area Tuesday night and early
Wednesday, followed by high pressure for the remainder of the
Near term /until 6 PM this evening/...
our weather will be in transition today as an upper-level ridge
slides across and then shifts to our east. As this occurs, surface
high pressure will continue to retreat to the northeast. This opens
the door for a southerly flow that will bring in more moisture.
The model guidance shows warm air advection occurring through the day, however
there is a more noted increase toward late afternoon. This also
occurs as an increase in the moisture arrives, which is seen in the
Theta-E fields. This increase in moisture will allow the surface dew
points to increase, although this process during the daylight hours
will probably be gradual for much of the area. The moisture increase
should also result in more cloudiness through the day with cumulus
development below some middle to high level clouds on the western flank
of the departing ridge. This increase in moisture within the
southerly flow is already evident early this morning with areas of
cloudiness advecting north-northeastward from Virginia and
Maryland. There are also some low clouds across northeastern
Pennsylvania due to some weak upslope flow combined with boundary
layer cooling. At least some forecast soundings indicate a cap in
place as warming occurs aloft during the day, however moisture
near the base of the cap should help focus some additional cloud
development especially inland.
Low pressure tracking across the Great Lakes region ahead of an
upper-level trough will help assist in warm air advection downstream, which will
organize a warm front to our southwest. In addition, convective
development is expected to our west within the developing warm
sector and near a pre-frontal trough. However, the lift with these
features are expected to remain to our west and therefore today was
As for high temperatures, we used a blend of the NAM/GFS MOS then
made a little bit of an upward adjustment for some inland areas
given the warm air advection. We are not anticipating to obtain deep mixing and the
surface flow looks mostly from the south or southeast. This would
tend to keep temperatures from increasing dramatically, although it
should be warmer than yesterday with some added humidity as well.
Short term /6 PM this evening through 6 am Sunday/...
high pressure at the surface will be situated to our east allowing a
southerly flow to be maintained. Our region will become increasingly
more on the northwestern flank of the upper-level ridge, and the
passage of a warm front will allow much more humid air to overspread
the region. We should see a decent increase in the surface dew
points in the evening as any mixing diminishes. While we still
maintain some warm air advection through the night, this generally turns weak. We
do maintain some moisture advection though, and this combined with a
warmer boundary layer may lead to areas of stratus and even some
The main short wave trough that is forecast to be amplifying across
the Great Lakes is forecast to hang back fairly far to our west.
There does appear to be some lead short waves tracking northeastward
along the periphery of the upper-level ridge. The forecast soundings
do show some increase in the instability tonight, however there may
not be much forcing. There could be some weakening showers/thunder
approaching our northwest zones this evening before they dissipate,
then perhaps some additional activity may try and arrive across our
western zones toward daybreak. The latter may be associated with an
embedded short wave tracking around the ridge. Overall, we think the
greater chance would be to our west closer to the short wave trough and
its associated lift. Therefore attm, we were slow to increase the
probability of precipitation toward morning across the western zones. Otherwise, an increase
in clouds especially late with some stratus development and perhaps
patchy fog. We are not all that certain how much fog develops given
the airmass is still undergoing moistening, therefore we did not
include fog at this time.
As for low temperatures, we went with an initial even blend of the
NAM/GFS MOS then made some mainly minor tweaks.
Long term /Sunday through Friday/...
middle-level ridging over the eastern U.S. Near the beginning of the
long term period gradually becomes zonal toward the end of the
period which translates into above normal temperatures for much of
Sunday - Tuesday...a nearly stationary frontal boundary to our north
and a southwesterly flow both at the surface and aloft will pump in
warm and moist air over our region through the period, bringing a
chance of showers and thunderstorms each day. The best chance for
showers and thunderstorms will be Sunday night into Monday.
Dewpoints will rise into the 70s across the region which will be
indicative of higher than normal precipitable water
values...possibly over two inches. Given the dry conditions of late,
any flooding that does develop would most likely be urban and poor
drainage type flooding. With temperatures a good ten degrees above
normal, and dewpoint temperatures in the 70s, it will feel like the
dog days of Summer.
Wednesday...low pressure system well to the north of the region
sends a somewhat stronger cold front through our area. Then
conditions will dry out behind this front as high pressure builds in
from the west during the day. Dewpoints are expected to drop in to
the middle 60s, and temperatures should drop a few degrees too,
although they are still expected to be above normal for this time of
Thursday - Friday...high pressure continues to build back into our
region with zonal flow aloft, and we should be dry both days.
Temperatures may drop a degree or two compared to Wednesday, but
still should be a little above normal.
Aviation /10z Saturday through Wednesday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg,
kmiv, kacy and surrounding areas.
Today...mainly VFR, with scattered to broken bases around 5000 feet.
Some stratus early this morning mainly north and west of kabe and
krdg. Southerly winds increasing to around 10 knots.
Tonight...VFR for awhile, however an increase in moisture should
result in areas of stratus and/or fog toward morning.
South-southwest winds 5 knots or less.
Sunday...VFR early. Then MVFR possible in scattered showers and
thunderstorms...especially north and west of phl. MDT confidence.
Sunday night into Monday...IFR conditions possible with showers and
thunderstorms. Conditions improving to VFR later Monday. Low to
MDT confidence on precipitation coverage.
Monday night and Tuesday...generally VFR conditions expected.
Tuesday night into Wednesday...MVFR conditions possible in scattered
showers and thunderstorms. Low to MDT confidence.
Late Wednesday and Thursday...VFR. MDT confidence.
high pressure will continue to shift to our northeast and east
through tonight. This will produce a southeast to south flow as a
warm front lifts north of the area by late tonight. While the flow
is mainly on the light side, some nearshore enhancement could occur
during peak heating. While the moisture is forecast to increase with
an uptick in the surface dew points, the water is on the warm side
therefore we may not have much fog development. Overall, the
conditions are anticipated to remain below Small Craft Advisory
criteria through tonight.
Sunday...sub Small Craft Advisory.
Sunday night and Monday...Small Craft Advisory conditions possible as seas build to around
5 feet. Sub-Small Craft Advisory conditions expected again later Monday into
Tuesday. A cold front will approach late Tuesday eventually
crossing on Wednesday. Seas will increase but are expected to
remain below below Small Craft Advisory.
there is a moderate risk of dangerous rip currents through this
evening, as enough of a swell component is still present to
enhance the formation of rip currents.