Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 280147
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
947 PM EDT Tue Oct 27 2020

.SYNOPSIS...
Weak high pressure will build in through tomorrow. Broad low
pressure, including the remnants of Tropical Cyclone Zeta, is
anticipated to move off the Middle Atlantic coast on Thursday night.
High pressure is expected to build over the region by Saturday.
Another cold front is expected to arrive Sunday night.

&&

.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
Current forecast is in good shape this evening as a shortwave
will traverse the 500mb flow and enter our area. Additionally, a
weak surface trough/low will develop along the stalled boundary
and move into our forecast area overnight. Sky cover will start
to increase once again and we should be overcast through the
night. As a result of the waves moving through, we will see a
round of light rain cross the area, mainly after midnight. Best
chances for rain look to be north and west of the I-95 corridor
with light amounts, less than a quarter of an inch.

With the skies being so cloudy, we should remain fairly mild
overnight. Overnight lows will be in the upper 40s to lower 50s,
with some lower 40s across the southern Poconos.

&&

.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH 6 PM WEDNESDAY/...
The surface trough is slow to cross the region and eventually
tries to push offshore by the late afternoon. Some rain will
remain around the region through the morning, but should start
to clear out through the day, exiting offshore by the afternoon.
Cloudy skies will remain across the region with some thinning
or clearing of clouds late in the day, especially in our
northern zones. Winds remain pretty light all day but should
start to become more westerly as the day goes on.

Daytime highs will warm back into the 60s through much of the
area with some 50s remaining across the southern Poconos and
northwest New Jersey.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH TUESDAY/...
Overview: Will be keeping a close eye on the Thursday and Friday
time period as the combination of the remnants of Zeta and a cold
front could bring heavy rain to our region (and possibly some snow
to the higher terrain in NW NJ and E Central PA). Outside of that,
the long term looks relatively quiet.

Details:

Thursday and Friday: The remnants of Zeta should cross our region
late Thursday and the secondary low pressure system will slide
through Thursday night. Consequently, expect several rounds of
precipitation (mostly rain) Thursday through Friday.

Changes from the previous forecast: The trend for the remnants of
Zeta to be more progressive continues. Consequently, it appears
there will be separation between the remnants of Zeta and the second
low and upper level low. The net result is that the precipitation may
come more in the form of waves rather than steady rain through the
entire period. Another change is that some of the operational models
show considerable dry air advection in the mid levels as early as
Friday morning. This could have impacts on the potential for mixed
precipitation in the Poconos and NW NJ.

Heavy rain: Precipitable water values should be well above normal
(1.5 inches or higher). Also, portions of the region (primarily for
Delmarva and South Jersey) a deep warm cloud layer is expected,
increasing the risk for heavy rain. The caveats are that the rain
(storm total generally 1 to 3 inches, with locally higher amounts
possible especially in Delmarva) should be falling over a long
enough period that the flooding threat could be limited. Also as
mentioned above, the heavier rain could come in waves, with a few
lulls through out this period. For now, will continue a mention in
the HWO, but will hold off on a watch.

Snow/mixed precip: If the cold front arrives early enough and if
cold air filters in fast enough behind it on Thursday night, snow
could mix with rain in the higher terrain of the southern Poconos
and NW NJ. There is still a question if this will occur as several
models show the boundary layer temperatures not decreasing
significantly until the precip moves out. Even if it occurs, little
(if any) snow accumulation is expected as temperatures in the
boundary layer are expected to barely drop to freezing and earlier
rain and warm ground should limit accumulation potential. The other
complication is that (as mentioned above) some operational models
show considerable dry air advection as early as Friday morning.
Consequently the southern Poconos and NW NJ could have only drizzle
by the time the temperatures drop into the mid 30s. For now, there
isn`t much guidance depicting this, so will hold off on including
this in the grids.

Friday night: strong cold air advection will continue in the
wake of the mid level trough crossing off shore. Areas that have
not yet seen the first killing freeze of the season could have
it on Friday night as most of the area is forecast to be at or
below freezing with the exception of coastal areas and portions
of Delmarva.

Saturday through Tuesday: High pressure should be the dominant
feature through much of the weekend, resulting in tranquil, but cool
conditions on Saturday. By Sunday, southwesterly winds could kick
start a brief warming trend. Another cold front is expected Sunday
night or Monday. Moisture advection ahead of this appears limited,
so not sure we will see much precipitation associated with this
front.

&&

.AVIATION /02Z WEDNESDAY THROUGH SUNDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

Tonight...VFR to start the night but conditions will drop to
MVFR around 05-06Z then down to IFR around 08-09z. Light rain
will move through the terminals dropping both ceilings and
visibilities, with better chances to the north and west of KPHL.
Light and variable winds. Moderate confidence but lower
confidence on timing of category changes.

Wednesday...IFR through much of the morning becoming MVFR as rain
exits the region. Ceilings and visibilities will improve
gradually, becoming VFR in the early afternoon. Low confidence
on specific timing.

Outlook...

Thursday and Thursday night...MVFR/IFR conditions in rain. The rain
may be heavy at times resulting in visibility restrictions. Winds
could start light and variable, but should settle out of the
northeast by Thursday evening at 10 to 20 kt with gusts up to 25 kt
possible, primarily at KACY. Non-convective low level wind shear is
possible primarily Thursday afternoon, but still uncertain. Moderate
confidence on the overall pattern, but low confidence on the
potential for LLWS and details of flight category changes like
timing.

Friday...Conditions should gradually improve to VFR by mid day.
Northerly winds 10 to 20 knots with gusts to 25 kt. Moderate
confidence on the overall pattern, but low confidence on the details.

Saturday and Sunday...Mostly VFR conditions expected. Winds will be
shifting from northeasterly Saturday morning, to easterly and
southeasterly Saturday night, to southwesterly during the day
Sunday. Wind speeds should generally be 5 to 10 kt. Moderate
confidence.

&&

.MARINE...
Relatively quiet conditions expected on the area waters through
Wednesday. Seas around 2 to 4 feet. Light north to northwest
winds becoming south to southwest overnight, then west to
southwest around 10 to 15 knots on Wednesday.

Outlook...
Wednesday night and Thursday...Winds and seas should stay below SCA
criteria through this time.

Thursday night and Friday...Winds and seas are expected to build as
a low approaches the region. Gale force gusts are possible
especially Friday morning.

Saturday...Winds and seas will gradually decrease, but we will
likely have SCA conditions continuing at least on the ocean waters
for most, if not all of Saturday.

Sunday...Southwesterly winds and seas on the coastal waters could
build again to SCA criteria.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...None.

&&

$$
Synopsis...Johnson
Near Term...Deal/Meola
Short Term...Meola
Long Term...Johnson
Aviation...Deal/Johnson/Meola
Marine...Johnson/Meola


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