Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Mount Holly New Jersey
943 PM EDT Thursday Oct 8 2015
high pressure will shift offshore of New England overnight, and as
low pressure tracks to our north during Friday, an associated cold
front will move through our area later Friday. High pressure will
then build into our region on Sunday and Monday. The next cold front
is expected to arrive during Tuesday, followed by high pressure on
Wednesday, and the possibility of another cold front by Thursday.
Near term /until 6 am Friday morning/...
930 PM update...added mention of patchy fog primarily to the
western river valleys as dew points should continue to rise
through the overnight hours. Otherwise...no major changes with the
Previous near term discussion...quiet weather is expected for
much of the overnight as high pressure moves offshore of New
England and loses its grip on our area. Partially clearing skies
early, will eventually fill back in as a warm front approaches the
area and lifts through overnight. The better chance for lower
clouds and precipitation is expected across eastern Pennsylvania
and northern New Jersey.
Short term /6 am Friday morning through 6 PM Friday/...
the warm front is expected to have lifted through the area by
daybreak Friday or shortly after. We will then be in the warm sector
between the departing warm front and an approaching cold front. This
will allow temperatures to warm well above normal, near 10 degrees
in many areas. The warm temperatures, combined with dewpoints in the
upper 50s to low 60s will allow some minor instability to develop
across the area. In fact, MLCAPES may approach between 500-1000 j/kg
in some areas. This instability, combined with enhanced lift from
the approaching cold front and trough aloft will allow for showers
and isolated/scattered thunderstorms to develop along/ahead of the
front late in the day. The shower/thunderstorm activity should be
approaching our western areas by middle-late afternoon, progressing
eastward into the I-95/295 corridor by the early evening hours. With
precipitable water values near 1.50 inches, some rain could be briefly heavy at
times. Also, with bulk shear values forecast around 30-35 knots, a
few stronger winds gusts are possible.
Long term /Friday night through Thursday/...
the mean 500 hpa pattern in d+2 to d+7 period features a ridge in
the west and a trough in the east. The main players affecting the
outcome of the pattern are a short wave over the central Pacific
and another over the Gulf of Alaska. The first short wave carves
out a closed low off the southeastern US coast in the Saturday
through Tuesday period. Meanwhile, the second short wave amplifies
the eastern Continental U.S. Trough on Tuesday, while the resultant height
falls are expected to kick the aforementioned closed low offshore.
A strong cold frontal passage is anticipated Friday night. As the
middle-level flow becomes parallel to this boundary, it will stall
well south of our region Saturday night. At that time, the flow
becomes more anticyclonic aloft across our area, which should
promote high pressure at the surface through Sunday. Meanwhile, positive vorticity advection
aloft will lead to low pressure development off the middle Atlantic
coast. In the Monday night through Tuesday night time frame,
there is uncertainty regarding just how amplified the trough in
the east becomes, which affects the timing of a cold front and
just how far west offshore low pressure tracks. This introduces
even further uncertainty with respect to another potential cold
frontal passage on Thursday.
Overall, the best chance of precipitation will be with the cold frontal
passage Friday night, and then again Monday night through Tuesday
night, although this outcome is more uncertain. Temperature-wise,
above normal values are possible Monday, otherwise near average.
Friday night...a strong cold frontal passage occurs during the
overnight hours. Preceding this, thunderstorms are possible until
around midnight, mainly confined to south Jersey and Delaware-Maryland-Virginia.
The approach of a 500 hpa short wave will lead to middle-level lapse
rates around 6c/km, while higher Theta-E air advects off the
relatively warm ocean waters, leading to greater instability. The
front will be the primary convective forcing mechanism, with
sufficient moisture preceding it. Thereafter, showers may linger
across Delmarva, as the southward progress of the front slows.
Behind the front, cold air advection and modest surface pressure
rises will make for an increasingly breezy night, with northwest
wind gusts of 20 to 25 knots possible by dawn.
Saturday...a wave developing along the front well to our south may
lead to lingering cloudiness over Delaware-Maryland-Virginia early, but otherwise a
mostly sunny day is expected. After a breezy start, winds veer to
the north and gradually subside.
Sunday and Monday...high pressure will crest across the region on
Sunday, followed by a return flow out of the southwest on Monday.
All indications point to good radiational cooling Sunday morning,
which may allow temperatures to dip at least into the middle-30s in
the vicinity of I-80 and points north. If confidence increases,
this may necessitate a frost advisory in later forecasts. After a
mostly sunny day on Sunday, middle and high level cloudiness will
increase on Monday, but dry weather is expected both days.
Tuesday through Thursday...a cold front moves through our area Tuesday
into Wednesday, while low pressure tracks just offshore. There is
uncertainty as to the degree of which these two features interact,
which impacts the timing and chances of precipitation. Precipitation associated
with the cold frontal passage should be relatively light, however,
it could be more extensive if Atlantic moisture is incorporated
into the system. Another cold frontal passage is possible Thursday,
but until the evolution of the aforementioned system is resolved,
this is outcome is uncertain at this time.
Aviation /02z Friday through Tuesday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg,
kmiv, kacy and surrounding areas.
VFR conditions will persist across the taf sites through at least
06z. After 06z, ceilings are expected to lower across Abe/rdg/ttn
areas, but should remain VFR elsewhere. Patchy fog is possible in
these locations as well through 15z. Isolated light showers are
also possible across eastern Pennsylvania and northern New
Jersey after 09z. Winds should remain quite light and be generally
A cold front will be approaching the area through the day Friday.
Scattered showers may develop ahead of the front during the morning
hours, but the main shower and possible thunderstorm activity is
expected just ahead of and associated with the frontal passage. It
should be moving through the Abe/ridge areas around 21z, then the
ilg/phl/pne/ttn areas around 00z, then be near the coast by 03z. A
brief period of lower ceilings/vsbys, moderate to heavy rain, and gusty
winds are possible.
sub-Small Craft Advisory conditions are expected to continue into
tonight as high pressure weakens its affect on the waters. Winds are
expected to increase to 25 knots, and and seas on the ocean are
expected to increase to 5 feet during the day Friday ahead of the
frontal boundary. So we have issued a Small Craft Advisory for the
Friday night Saturday...
a cold frontal passage Friday night will lead to northwest winds
which will veer to the north on Saturday. Wind gusts may approach
25 knots during this time frame, which will lead to building seas,
possibly reaching five feet. Thus, a Small Craft Advisory will
probably be needed for a portion of this time frame.
Sunday through Tuesday...
winds and seas are expected to remain below Small Craft Advisory
criteria at this time.
Marine...Small Craft Advisory from 11 am to 8 PM EDT Friday for anz430-