Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 230123
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
923 PM EDT Thu Oct 22 2020

.SYNOPSIS...
A surface ridge will linger along the East Coast through Friday.
A cold front will move through the Northeast this weekend, with
high pressure building into the region on Sunday. A surface low
will move through the Great Lakes region into southeast Canada
on Monday, bringing a warm front through the Mid-Atlantic Sunday
night followed by a cold front gradually moving through during
the first half of the week. High pressure will slowly return to
the area in the cold front`s wake.

&&

.NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
The synoptic setup remains stagnant as surface high pressure sliding
across the Canadian Maritimes remains wedged south and southwestward
through the Mid-Atlantic region. This is beneath a strong ridge
which is resulting in weaker flow.

Some low stratus along the ern parts of NJ continues to creep
slowly west into ern Burlington county and it also extends NW up
into the Lehigh Valley and southern Poconos. Fog has developed
in other areas with the radiational cooling and the high dew
points in place. Overall, the pattern is similar to the previous
nights with fog, locally dense developing and low stratus along
with a strong low level inversion in place. We have issued a SPS
to highlight the foggy areas through the early overnight. A
Dense Fog Advisory may be needed overnight.

Temperatures will continue to be on the mild side as dew points
are elevated. This will also add to the low-level moisture and
maintain little to no temperature/dew point spreads by late
tonight.

&&

.SHORT TERM /6 AM FRIDAY MORNING THROUGH 6 PM FRIDAY/...
An upper-level trough from the Northern Plains, Midwest and adjacent
Canada will gradually shift eastward Friday. This will drive surface
low pressure up across the Great Lakes into Canada. An associated
cold front will be moving into the eastern Great Lakes and Ohio and
Tennessee valleys by late afternoon. Meanwhile, a ridge remains near
the East Coast with surface high pressure anchored near the Canadian
Maritimes which then extends into the Mid-Atlantic region.

The pattern remains unchanged and therefore we are expecting stratus
and fog (potentially dense) along with some drizzle in the morning
to gradually improve. It appears to be a similar setup up to today
(Thursday), which may result in an area of stratus slow to
dissipate. There is still not much flow in the lower levels given
the presence of the surface high axis near our area plus the ridge
aloft. This means little help from mixing other than boundary layer
heating and heating from above. As a result, we are slow to improve
the cloud cover from south to north. The extent of the cloud cover
will have an impact on high temperatures and hourly temperature
trends, and given the uncertainty did not stray to far from
continuity.

&&

.LONG TERM /FRIDAY NIGHT THROUGH THURSDAY/...
A rather high-maintenance but low-impact medium-range
forecast...

Friday night looks suspiciously similar to the previous several,
with a surface ridge offshore allowing for weak surface winds (a
very light onshore component) keeping low-level atmospheric
conditions rather stagnant. As a result, expect another round of
low clouds and fog across the area with much-above-seasonal low
temperatures.

By Saturday, the synoptic pattern becomes more active and
progressive, as TC Epsilon ejects northeastward well off the
coast as a potent northern-stream vort max moves through eastern
Canada. An attendant cold front will move through the Northeast
during the day, with large-scale ascent weakening as the front
impinges upon the ridging surrounding Epsilon. Most
deterministic guidance indicates precipitation along the front
weakening before reaching the area, with little or no QPF in our
CWA as the front moves through. I am not quite sold on a dry
forecast, but nothing more than slight-chance PoPs are
warranted, as the strongest lift will be well north of the area.

A weak southern-stream perturbation will move through the
southern Mid-Atlantic and adjacent areas Saturday night and
Sunday, and this may generate a resurgence in shower activity
near the front. Any impacts in our area would mainly be confined
to the far south, with QPF remaining quite light. Better chances
for precipitation are south of the area, so only have slight
chances in the grids.

Temperatures on Saturday and Saturday night will be a challenge,
as frontal timing, pre-frontal insolation, and post-frontal
winds will all promote considerable uncertainty. There are
indications winds will stay somewhat elevated Saturday night as
cold advection spreads into the area via northwest winds.
Typically, models are too aggressive cooling the low levels in
these regimes, so did not quite go as cold as the latest
consensus (several degrees lower than continuity). However,
would not be surprised if the Poconos and northern New Jersey
fall well into the 30s, especially if cloud cover diminishes.

By Sunday, winds will become northeasterly to easterly in the
wake of the cold front as cold high pressure shifts eastward
through the Northeast. This will likely keep considerable
cloudiness in place in the southern portions of the CWA
(especially downstream of the enhanced lift attendant to the
aforementioned perturbation moving through the southern Mid-
Atlantic). The northern CWA is more questionable, as descent
associated with the surface high (and midlevel ridging) may keep
cloud cover from increasing, at least early in the day.
So...another day with a difficult temperature forecast. It will
certainly be much colder, with guidance generally in the 50s
(upper 40s in the Poconos). It may end up being colder (typical
model bias in post-frontal onshore regimes), especially if
thicker clouds persist/develop in the north.

The next northern-stream trough moves into southeast Canada on
Monday, but models are still somewhat variable regarding the
timing. Stronger lift will once again remain north of the
area, at least initially, as midlevel ridging in the western
Atlantic remains persistent. The front`s progress will also
slow as it approaches the ridge. Upstream perturbations will
continue to migrate east-northeast through the northeastern
U.S., which will eventually act to shift the faster flow
slightly southward. As the front gradually moves southward with
the evolving upper pattern, stronger lift downstream of the
perturbations should become more favorably positioned in our CWA
with time.

The result is slowly increasing chances of precipitation during
the first half of the week. High pressure should briefly build
into the area thereafter, but given large uncertainty with
timing, felt that chance PoPs were warranted Sunday night
through Wednesday. The whole period will not be wet, and
precipitation will likely be benign in nature, however.
Regarding temperatures, expect slightly above-seasonal values
early next week, at least in the prefrontal regime.

&&

.AVIATION /01Z FRIDAY THROUGH TUESDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Tonight...Conditions will vary initially, generally from VFR to IFR,
but will deteriorate to IFR/LIFR by midnight. Cannot rule out
localized VLIFR conditions. Light and variable to calm winds. High
confidence.

Friday...IFR/LIFR in the morning in low clouds and fog, then
conditions should improve to MVFR late morning and afternoon.
Conditions may improve to VFR at some terminals toward mid or late
afternoon. Calm to light and variable winds, becoming southeast 5-7
kt. Low confidence.

Outlook...
Friday night and Saturday morning...Restrictions probable with
another round of low clouds and fog. Light winds. High
confidence.

Saturday afternoon and Saturday night...Mainly VFR with winds
becoming northwest 5 to 15 kt (possible gusts to 20 kt during
the late afternoon and early evening). A few showers are
possible, but any associated impacts at the terminals would be
brief. Moderate confidence.

Sunday and Sunday night...Restrictions possible with low CIGs,
especially near the coast. Northeast to east winds up to 10 kt.
Low confidence.

Monday through Tuesday...Restrictions possible with a chance of
showers, especially Monday afternoon onward. Southwest winds 5
to 15 kt on Monday becoming light and variable Monday night and
Tuesday. Low confidence.

&&

.MARINE...
Through Friday, a Small Craft Advisory remains for the ocean zones
as swell energy from very distant Hurricane Epsilon arrives. This
will result in building seas/swells. Some fog is expected especially
nearshore for the New Jersey Atlantic coastal waters.

Outlook...
Friday night through Saturday night...Small craft advisory in effect
for the Atlantic waters with high seas continuing (generally 4 to
7 feet). Southwest winds 5 to 15 kt becoming northwest and increasing
to 10 to 20 kt on Saturday night. Fog probable on Friday night.

Sunday...Lingering seas 4 to 6 feet on the Atlantic waters with
northeast winds 10 to 20 kt.

Sunday night through Tuesday...Sub-advisory conditions expected
with increasing chances of showers Monday and Tuesday.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Small Craft Advisory until 6 AM EDT Sunday for ANZ450>455.

&&

$$
Synopsis...CMS
Near Term...Gorse/PO
Short Term...Gorse
Long Term...CMS
Aviation...CMS/Gorse/MPS
Marine...CMS/Gorse


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