Scientific Forecaster Discussion
fxus61 kphi 230117
Area forecast discussion
National Weather Service Mount Holly New Jersey
917 PM EDT sun Sep 22 2019
high pressure off the Carolina coast will drift out to sea tonight.
A cold front then approaches on Monday, and moves across the region
Monday night. High pressure returns for the mid-week period,
followed by another cold front late Thursday and Thursday night.
High pressure builds back into the northeast for the end of the week
and next weekend.
Near term /until 6 am Monday morning/...
forecast on track, so will not make any significant changes to
the database. Will adjust hourly grids based on the latest
surface obs, and will adjust hourly temp/dew point grids through
the overnight based on these adjustments.
Otherwise, the main forecast challenge tonight is for fog
development. With subtly increasing near-surface moisture and
another night of light or calm winds, patchy fog is a good bet
in the more susceptible Rural/Valley locations northwest of the
urban corridor, but based on temperature and dew point trends,
will include patchy fog in The Pine Barrens.
A warm night is ahead, with lows in the 60s across the area.
May see some 70-degree readings in the urban corridor and near
Short term /6 am Monday morning through 6 PM Monday/...
a surface trough will become established across the area
tomorrow, with a strong, positively-tilted shortwave trough
progressing from the Great Lakes to the northeast and adjacent
portions of Canada. An attendant cold front generally along the
Appalachians will extend southwest from a weak surface low in
southeast Canada by Monday afternoon. Some convection should
develop along and ahead of the front by afternoon, and may
develop along the surface trough axis as well. The latter is
hinted at by the NAM nest and WRF-arw/nmm simulations, as well
as coarser guidance. Inherited pop forecast looked reasonable
and made few adjustments through the day. Chances are generally
highest northwest of I-95 and dwindle to unmentionable near the
coast and in most of Delmarva.
Model soundings indicate little instability will develop during
the day, even as diurnal heating maximizes, as midlevel
temperatures will be fairly warm owing to antecedent ridging.
Nevertheless, given the warm/humid conditions across the area,
was hesitant to exclude potential of thunderstorms entirely, so
kept mention of slight chances in during the late afternoon.
Monday will be unseasonably warm, with temperatures reaching to
around 90 degrees in the urban corridor and portions of
Delmarva. Most others will be in the 80s, with upper 70s
possible in the Poconos and at the beaches.
Long term /Monday night through Sunday/...
a cold front will cross through the area Monday night. Flow looks to
be pretty progressive with the front working through the area Monday
night and then offshore by Tuesday morning, although the models have
slowed the arrival of the front a bit over the latest runs, which
has the main precipitation arriving late in the day. With enough
surface heating we will be able to have some instability but it will
be limited and with the sun setting earlier this time of year, it
won't hang around quite as long as it does during the Summer months.
Thunder looks isolated at best so will keep a slight chance in the
forecast. Thunder will wane through the late afternoon into the
evening, leaving just showers as we head into the overnight period.
Showers will mostly be right along the frontal boundary and will end
from northwest to southeast, with the bulk of the rain having exited
offshore by around daybreak.
Once the front moves through, high pressure starts to build back
into the region for Tuesday and Wednesday. Northwest flow on Tuesday
will help to rid US of the lingering moisture and will lend to nice
fall days across the area through Wednesday.
A weak cold front will cross through the area on Thursday. The
guidance shows that there there is limited moisture with the front
as it arrives in our forecast area. Some showers may hold together
as the front moves in and it looks like it may mostly be confined to
our northern areas and then possible continue slightly southeast as
the front pushes through, which correlates well with the passage of
the upper trough as well. Not a lot of confidence this far out so
will only include a slight change for showers at this time.
High pressure will then build across the mid-Atlantic on Thursday
night, sliding across our area on Friday, and then offshore late
Friday into Saturday. Expecting a return to dry conditions for
Friday through Saturday. Another weak front may approach our area
late Saturday into Sunday but once again it looks like it may remain
mostly dry as it crosses through our area.
Aviation /01z Monday through Friday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg,
kmiv, kacy and surrounding areas.
Tonight...mainly VFR. However, some patchy fog may develop
late. Winds light and variable. Low confidence on fog
Monday...any fog early in the day will dissipate rapidly.
Mainly VFR with winds increasing from the south/southwest around
10 kt, possibly with some gusts 15-20 kt (especially at
phl/acy). Cannot rule out a few showers late in the day,
especially at rdg/Abe. High confidence.
Tuesday and Tuesday night...VFR. Northwest winds 10-15 knots
during the day, then around 5 knots or less overnight.
Wednesday and Wednesday night...VFR. Westerly winds around 5
knots becoming southwesterly less than 5 knots.
Thursday...mostly VFR. Cold front expected to move across the
area during the afternoon, but likely to remain dry for most of
the area except northeast Pennsylvania and northern New Jersey.
Wind shift from southwest to west, then northwest behind the
front and may gust 15-20 knots in the afternoon.
Thursday night...VFR conditions expected. Northwest winds 5-10
Friday-Friday night...VFR conditions expected. Light and
variable winds early, becoming south to southwest 5-10 knots
late in the day and overnight.
south to southwest winds 10 to 20 kt are expected to develop
tonight and continue through Monday. Some gusts may approach 25
kt by afternoon on the Atlantic waters, with winds expected to
be somewhat lower on Delaware Bay.
In addition, seas will increase to around or slightly above 5
feet by afternoon. As a result, have issued a Small Craft
Advisory beginning at 10 am Monday for the Atlantic waters.
Tuesday...Small Craft Advisory conditions through 10z with southwesterly wind
gusts to 20 knots and seas possibly exceeding 5 feet. Winds
diminishing to 10 to 15 knots and turning westerly throughout
the remainder of the day with seas dropping to 2 to 4 feet.
Wednesday...sub-sca conditions expected with westerly winds
from 10 to 15 knots turning southwesterly through the afternoon
and seas from 2 to 4 feet, though some 5 foot waves may be
possible in the afternoon.
Thursday...sub-sca conditions expected with southwesterly winds
continuing from 10 to 15 knots and gusts to 20 knots. Seas
building to 2 to 4 feet.
Friday...sub-sca conditions expected with northeasterly winds
from 5 to 10 knots turning easterly by the afternoon. Seas to
sit from 1 to 3 feet.
There is a high risk for the development of dangerous rip
currents at New Jersey and Delaware beaches on Monday due to
increasing winds and seas. A rip current statement has been
issued for this risk.
New Jersey...high rip current risk from 6 am EDT Monday through Monday
evening for njz014-024>026.
Delaware...high rip current risk from 6 am EDT Monday through Monday
evening for dez004.
Marine...Small Craft Advisory from 10 am Monday to 6 am EDT Tuesday for