Scientific Forecaster Discussion
fxus61 kphi 161045
Area forecast discussion
National Weather Service Mount Holly New Jersey
645 am EDT Mon Sep 16 2019
a weakening low pressure system will move into the mid Atlantic
region today, pushing a frontal boundary to our south. High
pressure builds across the northeast states on Tuesday into
Wednesday, then shifts southward across the mid Atlantic region
Thursday. The high is forecast to shift to our south Friday,
then offshore over the weekend.
Near term /until 6 PM this evening/...
545 am update: visibility continues to deteriorate in Delmarva,
with fog overspreading much of Queen Anne's County in Maryland.
Have expanded the advisory. Meanwhile, visibility continues to
improve in New Jersey, so have canceled the advisory for the
previously affected counties. Some patchy fog may still occur,
but widespread dense fog is no longer expected.
Additionally, added some slight chances of showers through 8 am
for areas near/north of I-78. Most of the radar returns and
surface observations suggest sprinkles, but have seen a couple
of measurable amounts in northern New Jersey in the past hour.
Dense fog has developed in central/southern Delmarva, with
webcams, surface observations, and satellite imagery all
indicating conditions are deteriorating and with coverage high
enough at this point to warrant a dense fog advisory. However,
the fog has become patchier in New Jersey, perhaps owing to a
subtle shortwave trough moving through at this time. However,
cannot rule out denser fog returning during the next couple of
hours, so will let the advisory ride there for now. Fog should
dissipate shortly after daybreak.
The other concern today is a low-amplitude northwest-flow
perturbation moving through the northeast. Latest model
simulations depict strong enough lift to generate some showers
in the northern cwa, though likely quite light and fairly
scattered. Nevertheless, confidence is increasing with
sufficient coverage to nudge pops upward a little bit (mainly
north of Interstate 78) today. Farther to the south, will likely
observe a sufficient period of partial sunshine to see
appreciable diabatic warming before increased/lower cloud cover
sets in. This makes the temperature forecast somewhat tricky,
with MOS showing a fair amount of spread across the region.
Incorporated a fair amount of hi-res guidance into the highs for
today, as these seem to have reasonable agreement on the
evolution of clouds/precipitation today. This resulted in a
forecast quite close to continuity, perhaps a degree or two
The final concern today is the sea breeze and the timing of the
cold front associated with the perturbation. Temperatures along
the coast will likely be much cooler than areas farther inland,
owing to the sea breeze's push onshore this afternoon.
Additionally, the cold front will bring fairly Stout cold
advection into the area immediately upstream, so should see a
sharp gradient in temperatures (including highs) in the northern
County Warning Area.
Short term /6 PM this evening through 6 am Tuesday/...
cold front will continue south tonight, with large-scale ascent
possibly remaining sufficient for some showers in the southern
half of the County Warning Area. There are subtle indications of increasing
vorticity in the northwest flow overnight, which may explain why
shower coverage in most model simulations appears to increase
south of the front after dark. Not overly sold on this
potential, but did increase pops in Delmarva and far southern
New Jersey to at least mentionable during the evening/overnight
Immediately downstream of the front, may see a resurgence in
low clouds and fog. However, this potential remains rather
unclear owing to uncertainty with the frontal position and
shower coverage. As such, have held off mentioning this in the
forecast. Meanwhile, for areas north of the front, temperatures
should be colder, especially as skies clear. However, there may
be enough of a breeze to keep temperatures from going to values
suggested by the colder guidance, so kept the forecast close to
a statistical blend.
Long term /Tuesday through Sunday/...
a mostly quiet long term period is expected, with precipitation
possibly returning late in the weekend or early next week.
Nice weather is expected for the rest of the week and into the
weekend as high pressure is forecast to affect the East Coast
through the period. High pressure is forecast to build across
New England Tuesday into Wednesday, then shift southward across
the mid Atlantic region on Thursday. Then the high is expected
to build to our south on Friday, then offshore of the southeast
states over the weekend. With the high pressure near the East
Coast for the rest of the week into the weekend, dry conditions
are currently forecast, although there will likely be periods of
cloud cover at times.
As we go into late Sunday into Sunday night, a cold front
begins to approach the area from the west, and a pre-frontal
trough may develop ahead of the front. There is a slight chance
that showers may accompany this trough and frontal passage
Sunday night into Monday.
Temperatures start off a few degree above normal Tuesday, then
near or slightly below normal Wednesday and Thursday, before
rising back above normal Friday, and especially over weekend.
Aviation /12z Monday through Friday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg,
kilg, kmiv, kacy and surrounding areas.
Today...mainly VFR, with cloud cover increasing/lowering from
north to south through the day. Cannot rule out a sprinkle or
shower, especially at ridge/Abe/ttn, but negligible impacts are
expected at the terminals. Light/variable winds becoming
northeast up to 10 kt as a cold front moves through the area
late in the day. Moderate confidence.
Tonight...mainly VFR with cigs around 5000 feet during frontal
passage (possibly somewhat lower, especially at miv/acy), likely
improving from north to south late. Winds generally northeast
up to 10 kt. Low confidence.
Tuesday-Wednesday night...mostly VFR during the day, with a
chance of MVFR conditions overnight. East to northeast winds
Thursday-Thursday night...generally VFR conditions expected.
East to northeast winds early Thursday, become east to southeast
later in the day. Variable winds through the night Thursday.
Friday-Friday night...winds vary between northwest and
northeast 5- 10 knots, before becoming light and variable Friday
545 am update: canceled the dense fog advisory for the New
Jersey coastal waters, as satellite imagery and coastal webcams
indicate improving conditions. However, conditions have
deteriorated on the southern portions of Delaware Bay and the
Delaware Atlantic coastal waters. The advisory continues for
Low clouds and fog may linger through the morning, especially on
Delaware Bay and the adjacent Atlantic waters, so will be
monitoring surface observations closely to determine if the
dense fog advisory needs to be extended.
Regarding winds/seas, sub-advisory conditions are expected
through tonight. However, a cold front will be moving through
during the late afternoon and evening hours. Winds will switch
to northeast, becoming 10 to 20 kt overnight. Though conditions
are expected to remain below advisory thresholds, cannot rule
out an isolated occurrence of 25 kt wind gusts around daybreak.
Tuesday-Wednesday night...Small Craft Advisory conditions
likely as wind gusts increase to 25-30 knots and seas increase
to 5-6 feet for portions of the Atlantic coastal waters.
Thursday-Friday night...winds lower below advisory levels,
however, seas expected to remain above 5 feet.
There is a low risk for the development of dangerous rip
currents at the New Jersey and Delaware beaches today due to
light southerly flow and waves of 2-3 feet in the surf zone.
Delaware...dense fog advisory until 8 am EDT this morning for dez002>004.
Maryland...dense fog advisory until 8 am EDT this morning for mdz015-019-
Marine...dense fog advisory until 8 am EDT this morning for anz430-431-