Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Mount Holly New Jersey
415 PM EDT Tuesday Sep 30 2014
a weakening cold front will slowly sag through the region tonight
allowing high pressure to work in from the north later on Wednesday.
The high will slide to our east by Friday before a more potent cold
front sweeps through Friday night into Saturday morning. Another
break between cold fronts is expected through Monday.
Near term /until 6 am Wednesday morning/...
a couple of factors will continue to influence the weather across
our region through tonight. A weakening backdoor cold front will
continue to slowly settle southward across the area this evening and
through the overnight. Meanwhile, an upper level low will continue
to drop southeastward from the eastern Great Lakes and across
Pennsylvania through tonight. The cold pool of air aloft associated
with this feature, along with some surface convergence associated
with the weakening front will produce some scattered showers this
evening and overnight. In addition, a continued easterly to
northeast flow will provide ample moisture. While isolated to
scattered showers are possible just about anywhere through the
forecast area, highest chance probability of precipitation are focused mostly across the
northwestern third of the cwa, as the more elevated terrain will aid
in additional lift and is closer to the incoming upper low passing
overhead. We left a mention of thunder in the forecast for our far
western areas for a few hours this evening, as there is some weak
instability, and there has been some thunder development to the west
of our County Warning Area.
Through the remainder of the overnight then, we have slight chance
to chance pops, highest for our northern zones, with the incoming
upper level low. In addition, guidance was also indicating the
potential for patchy fog, and we kept this in the forecast with the
east-northeast flow and ample moisture. For overnight temperatures,
we mostly took a blend of met/mav guidance and expect lows mostly in
the 50s, except some lower 60s along the coast and in more urban
Short term /6 am Wednesday morning through 6 PM Wednesday/...
through the daytime Wednesday, the previously mentioned upper low
will be swinging by to our northeast, and by late in the day,
surface high pressure to our north will begin to slowly build toward
our region. There will still be the possibility for isolated to
scattered showers, especially through morning to about midday, and
we slowly trail probability of precipitation off through the afternoon. Skies will be mainly
cloudy through midday before some slow thinning occurs late-day.
Maximum temperatures were a blend of met/mav guidance with
continuity, and we expect highs mostly in the low to middle 70s.
Long term /Wednesday night through Tuesday/...
operational models are in pretty good agreement with the large scale
features through the weekend. The first such feature being the
departing upper level cut-off low that should be to our immediate
east. Then we have the potential phasing of a strong northern and
southern stream system to our west that carves out a very large and
deep closed low/trough across the Upper Middle-west by late Friday
Thursday...rather dry day except for some lingering showers early on
across our eastern portions. There could still be some middle-level
instability to work with as the middle-level cold pool scrapes by but
not expecting much quantitative precipitation forecast with. Middle and upper level ridging become more
prominent on Thursday which should give way to late day sunshine.
Temperatures will be in the low-70s; at or slightly below normal.
Friday...model guidance may be breaking down the ridging overhead a
bit too fast, especially given the downstream upper level low is
still lingering by and the approach of a deepening trough upstream,
the ridge should build slightly before breaking. Onshore flow ahead
of the next trough and attendant surface front may allow for
scattered showers to break out along the higher elevation locations
across our western zones by later Friday afternoon. Do not see much
in the way of convection with this activity given the stable lower
atmosphere. We see a increase in shower coverage and possibly
intensity by late Friday night as a stronger cold front inches
closer. Daytime temperatures should remain much the same, low-70s
with increasing cold cover.
Saturday...transition day with a strong front expected to sweep
through during the morning hours. Deep southwest flow ahead of the
front will allow precipitable waters to climb to well above normal making for
heavier rainfall. Antecedent conditions are still pretty dry across
the region so not expecting anything out of the Ordinary to occur
with the possible heavier rain. Lots of cloud cover and cold air
advection occurring dropping temperatures back into the middle to
Sunday - Tuesday...closed middle-level low establishes itself over northern Ontario
during the Saturday through Tuesday timeframe. Preferred the
CMC and UKMET evolution of the middle-level height field over the
GFS and European model (ecmwf)...both of which have a more negatively tilted
trough and placement of the closed low further south and east.
There is still quite a bit uncertainty as shown by both the
gefs and ecens spread in middle-level height fields from the mean
and this thinking is supported by the latest model verifications
which indicate a less amplified solution.
Expect winds to decrease Saturday night as a ridge of high pressure
noses into the region. If the boundary layer decouples enough west/
light winds...could see low temperatures in the 35-40 range in the
northwest interior of our County Warning Area. Sunday is shaping up to be the
coolest day of the period...with 850 hpa temperatures supporting highs
several degrees below normal...middle 50s in the Poconos to middle 60s
in south Jersey. A weak cold front is prognosticated to move through
the region early on Tuesday...presenting the best chance of
precipitation during this timeframe. Temperatures will return
to seasonable levels on Monday and continue through Tuesday
Aviation /20z Tuesday through Sunday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg,
kmiv, kacy and surrounding areas.
Late day and this evening...VFR conditions with continued scattered to broken
clouds with bases at 4000 to 5000 feet. Light east-southeast winds
will continue, under 10 knots. An isolated shower is possible,
mainly in the vicinity of krdg and kabe.
Late evening through overnight...winds are expected to become more
northeasterly through this period, but stay mainly under 10 knots.
Guidance is showing MVFR to possible IFR conditions developing.
Isolated to scattered showers are possible with an upper level low
passing overhead, but confidence and location of showers was too low
to include in the tafs. Best chance for a shower would be in the
Wednesday...possible IFR to MVFR conditions will likely last through
the morning, before becoming VFR around midday to early afternoon
for the taf sites. An isolated to scattered shower remains possible
through the morning. Northeasterly winds are expected, mostly in the
8 to 10 knot range.
Thursday...MVFR early becoming VFR. Easterly winds expected.
Possible lingering shower early at miv/Acy, otherwise dry.
Friday...mostly VFR. Rain, heavy at times late, overspreads the area
from the west. Reduced visibilities and ceilings in heavy showers. Winds back south-southwest.
Saturday...MVFR/IFR early in heavy rain, trending VFR. Winds
continue to back to the west-northwest.
an east-northeast flow will become more northeasterly across our
area waters through this evening and tonight. Speeds are expected to
increase through the overnight, and from early morning Wednesday
through the daytime, some gusts between 20 to 25 knots are possible.
In addition, seas will also be building in the increasing northeast
flow. Seas are expected to build to around 4 feet by around daybreak
Wednesday, and forecast levels should reach 5 feet from about great
egg inlet northward during the daytime Wednesday. We therefore
decided to raise a Small Craft Advisory for our northern Atlantic
coastal waters, from great egg inlet northward to Sandy Hook,
beginning at 15z Wednesday.
For Delaware Bay, winds and seas are expected to remain below Small
Craft Advisory levels through tonight and Wednesday.
Thursday - Friday...Small Craft Advisory conditions will likely persist on our
waters through early Friday as we continue with the easterly onshore
flow. Seas will continue to run around 5 feet. Winds begin to back
towards the south-southeast by Friday ahead of the next cold front.
Saturday...strong cold front moves through. Winds back towards the
west-northwest later in the day and with stronger cold air
advection occurring, we could see a return to Small Craft Advisory conditions.
Marine...Small Craft Advisory from 11 am Wednesday to 6 am EDT Thursday