Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Mount Holly New Jersey
758 PM EDT Friday Oct 9 2015
a cold front will move through our region early tonight, followed by
high pressure moving in from the Ohio Valley, and settling over the
area on Sunday. Meanwhile, low pressure will develop east of Florida
and move northward to a position off the middle Atlantic coastline
on Monday, before being absorbed into a cold front which is expected
to move through our region on Tuesday. Two additional cold fronts may
move through the region during the Wednesday through Friday time frame.
Near term /until 6 am Saturday morning/...
630 PM update...have cancelled the Severe Thunderstorm Watch
update to cancel the remainder of the Severe Thunderstorm Watch.
The main line is moving quickly south into far southeast New Jersey and southern
Delaware. There is a few lingering showers behind the main line, but the
severe threat appears minimal at this time. Once the showers move
offshore, a dry overnight forecast is expected.
Southwest winds will continue ahead of the front, gusting 15-20 miles per hour.
Behind the front, northwest winds will move in and could gust 15-20
miles per hour at times.
Short term /6 am Saturday morning through 6 PM Saturday/...
Saturday should be a pleasant day with seasonable cool temperatures
and mostly clear skies. The only cloud cover expected during the day
Saturday would be mostly high level clouds, except possibly some
lower clouds across the southern areas during the morning hours.
Highs Saturday should be pretty close to normal for most areas.
Long term /Saturday night through Friday/...
the mean 500 hpa pattern in d+2 to d+7 period continues to feature a
ridge in the west and a trough in the east. In particular, two short
waves will have the greatest impact on the evolution of the pattern,
one currently traversing the Rocky Mountains and the other now over
the Gulf of Alaska. The first short wave carves out a closed low off
the southeastern US coast during the Saturday through Monday time frame.
Meanwhile, the second short wave amplifies the eastern Continental U.S. Trough
on Tuesday and Wednesday. The resultant height falls are expected
to lift the aforementioned closed low poleward along the East Coast.
By Saturday night, the flow becomes more anticyclonic aloft across
our region, which will promote high pressure at the surface through
the day Sunday. Meanwhile, positive vorticity advection rounding the closed middle-level low
will induce surface low development off the southeastern US coast.
The main uncertainty in the forecast is with regard to 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
in the Monday night through Tuesday night time frame. This introduces
additional uncertainty regarding two potential cold frontal
passages on Thursday and Friday.
Overall, the best chance of precipitation will be with the cold frontal
passage/offshore low pressure during the Monday night through Tuesday
night period. Temperatures will be at or slightly above average
through the period.
Sunday...high pressure will crest over our region, and with clear
skies and light winds, expect good radiational cooling. There has
been a fairly consistent signal in the guidance for the potential
of frost in the vicinity of I-80 and points north. If confidence
increases, this may necessitate the need for a frost advisory.
Elsewhere, there may be some patchy dense fog, but there is model
disagreement on the amount of surface moisture. At this time, the
best chance is across the Lehigh Valley. As high pressure crests
over our area, winds will gradually veer to the southwest, but a
lingering northeast flow along the coast will keep it cooler. Some
high cloudiness may stream in from the west, mainly over Delaware-Maryland-Virginia.
Monday through Wednesday...a return flow commences around surface
high pressure on Monday. Middle and high level cloudiness increases
as the tropospheric flow becomes meridional in response to the
amplifying long wave trough. A cold front moves through the 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. At this time, the forecast reflects
a slight chance of showers southeast of I-95 Monday, and then
across the entire County Warning Area Monday night into Tuesday.
Thursday and Friday...as mentioned previously, there are two
potential cold frontal passages during this time frame, but
until the evolution of the aforementioned system is resolved,
this is outcome is uncertain at this time.
Aviation /00z Saturday through Wednesday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg,
kmiv, kacy and surrounding areas.
Once the last of the showers passes kmiv and kacy, expect mostly VFR
conditions at all taf sites. Drier air has been a bit slower to
move in, so a few sites, primarily krdg and kabe, could see
reductions in visibility at times through the overnight hours.
After 12z though, expect VFR through the remainder of the period.
Winds will be northwest through the overnight hours, with some
areas gusting around 15-17 knots. Winds will become northeast
during the day Saturday. Wind speeds after 12z, should be light,
generally near or below 10kt.
the Small Craft Advisory remains for the Delaware Bay and coastal
waters. The advisory for the northern Delaware Bay was extended
through 10 PM as winds may remain elevated behind the cold frontal
passage. The remainder of the advisory was left unchanged with and
end time of noon Saturday.
A line of showers and thunderstorms just ahead of the cold front
should affect the coastal waters this evening and may contain some
gusty winds. Winds ahead of the front will be southwest, and switch
to northwest behind the cold front.
Saturday night through Monday...winds and seas are expected to remain
below Small Craft Advisory criteria at this time.
Tuesday and Wednesday...a cold frontal passage Tuesday night will
lead to winds veering from southwest to northwest. At this time,
there is a low probability of winds reaching Small Craft Advisory
thresholds during this time frame.
Marine...Small Craft Advisory until noon EDT Saturday for anz431-
Small Craft Advisory until 10 PM EDT this evening for anz430.