Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 220225
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
925 PM EST Tue Jan 21 2020

.SYNOPSIS...
High pressure will remain over the region through Thursday night. On
Friday, the high will start to shift offshore. A storm system will
affect the area over the weekend as an area of low pressure tracks
into the Great Lakes Friday night with a secondary low developing
along the coast by Saturday. That storm system will move away on
Sunday. High pressure is likely to build back in for the start of
next week.

&&

.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
An expansive high pressure system continues to influence the
weather across the Middle Atlantic region this evening and will
continue overnight.

Clear skies, much of today, have been replaced with a batch of
Ci/Cs clouds across the area this evening. The hourly temperatures
which began to drop quickly have slowed now that the clouds have
arrived. It is envisioned that this will only be temporary however
with the back edge of the clouds already arriving across the NW
areas. We have decided not to adjust the overnight lows but instead
just modified the hourly temps a little these evening hours. The
quicker drop in temps will occur once the clouds move away after
midnight. Lows in the teens will occur in many areas with single
digits in the lightly snowed valleys up north/west and some 20s
in the metro areas and right near the shore. Winds will remain
light overnight.

&&

.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH 6 PM WEDNESDAY/...
A ridge axis is forecast to slide across our region during the
course of Wednesday. Some warm air advection will continue however
the flow will be light given the ridge aloft overhead and surface
high pressure arriving. After a very cold start, a `milder`
afternoon is expected however mixing looks to be limited given the
weak flow. As a result, we went a little lower than what a
MOS/continuity blend showed but this resulted in high temperatures
close to where they should be for this time of the year.

Looks like another day of ample sunshine as the ridge axis moves
across our region. Some high level cloudiness cannot be ruled out
though cresting over the ridge well ahead of a Midwest and Great
Lakes system.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH TUESDAY/...
Overview...

Some familiar themes are present heading into the end of this week
and beyond. A strong +AO remains a prominent feature in the current
pattern. This will continue to limit the intensity and duration of
cold air affecting the region as most of the Arctic air remains
bottled up near the pole. This will have implications over the
weekend for the main system of interest in the long term, as we face
another setup in which any frozen precipitation will likely be
confined to interior and higher elevation locations. Leading into
that, residual high pressure looks to carry us through the end of
the work week as ridging over the central US progresses eastward.
This will also yield a steady warming trend. By the weekend, low
pressure will track towards the Great Lakes, with secondary low
development likely along the coast. This will yield our main weather
event of the period with Saturday in particular looking unsettled.
Heading into early next week, high pressure is likely to build back
into the region as ridging redevelops upstream. Rinse and repeat.

Dailies...

Wednesday night-Friday... Dry and quiet weather with high pressure
almost directly overhead before it starts to shift offshore on
Friday. A gradual warming trend in temperatures with highs several
degrees above average both Thursday and Friday as mid-level heights
rise. Wednesday night will be another cool one with good radiational
cooling conditions, but the lows will also start to moderate by
Thursday night.

Friday night-Sunday... This is the main time period of focus for the
long range. At the synoptic level, multiple shortwaves moving in
from the Pacific will phase over the Plains on Thursday and into
Friday. The development of a closed H5 low over the central US is
indicated by model guidance, and that closed low will drift eastward
through the end of the week while a weak surface low tracks towards
the Great Lakes. As this system progresses eastward, secondary
cyclogenesis is likely as a coastal low begins to develop. This will
bring a precipitation event to our area from late Friday night
through at least Saturday night. As usual, there are some model
differences in how this process will unfold, though nothing out of
the ordinary in terms of the spread between models. Interestingly,
in terms of timing, most of the 21.12z guidance trended towards the
faster GFS solution in bringing precip into the region during the
night on Friday. Usually the GFS has a progressive bias in these
cases but since the air mass ahead of this system isn`t overly dry
the faster onset may be more reasonable.

In terms of precip amounts and types, that will depend on the track
and evolution of the primary and secondary lows. The preceding air
mass is not very cold, and even if the coastal low develops a little
further southeast than currently modeled it will still be tracking
very close to overhead of us. So ptype will be an issue and all
indications still suggest a mainly rain event along and inside of
the I-95 corridor, with better chances for snow and mix to the north
and west, especially towards the Poconos and northern NJ where there
is certainly potential for accumulating snow. Too early to talk
specific amounts but it is possible winter weather headlines will be
needed for our northern tier or two of counties over the weekend.
Precip looks to come in two batches but without much of a lull
between the two. There is an initial round of warm advection driven
precip during the day Saturday, then potentially a shield of wrap-
around precip (especially to the north) on Saturday evening into
Sunday morning as the coastal low strengthens and wraps up. Somewhat
reminiscent of the winter storm in early December, though this
system has an overall warmer and more progressive look to it.
There are some timing differences in the guidance on how fast
the storm will depart, but most indications are that we will be
drying out on Sunday, with the best chance for lingering rain or
snow showers being to the north.

Sunday night-Tuesday... As the weekend storm pulls away, the start
of next week looks fairly quiet as high pressure will likely build
back in. Expecting temperatures to be mostly above average as even
on northwest flow behind the weekend low pressure system, there is
no Arctic air to our northwest to advect into the region.

&&

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

Tonight...VFR with high clouds thru the early overnight. Light
and variable winds. High confidence.

Wednesday...VFR. Light and variable winds, becoming north-northwest
around 5 knots. The winds should locally become west-southwest in
the afternoon. High confidence on VFR occurring; moderate confidence
with the winds.

Outlook...
Wednesday night-Friday... VFR. Winds light and variable, becoming
light easterly on Friday. High confidence.

Friday night-Saturday... Initially VFR Friday night, but sub-VFR
conditions will likely develop by late Friday night and continue
through the day on Saturday. Easterly winds of 10 to 20 kt, with
gusts to 30 kt possible especially near the coast. High confidence
in overall trend for this to be an unsettled period, moderate
confidence in details.

Saturday night-Sunday... Sub-VFR conditions will likely linger at
least through Saturday night and potentially into the day on Sunday,
though the trend will be for gradual improvement. Winds shifting to
westerly on Saturday night at 10 to 15 kt. Northwest winds of 10 to
20 kt expected on Sunday. Moderate confidence.

&&

.MARINE...
The conditions are expected to be below Small Craft Advisory
criteria through Wednesday as high pressure gradually builds across
our region.

Outlook...
Wednesday night-Thursday night... Mainly sub-SCA conditions
expected, though seas may approach 5 ft by late Thursday night
especially to the south. Northeast winds around 10 kt.

Friday-Sunday... SCA conditions are expected for the duration of
this period. Seas will likely build to 5 ft on Friday morning and
remain 5 to 8 ft through most of this period before gradually
diminishing on Sunday. Increasing easterly winds from Friday into
Saturday. Winds may approach gale force late on Saturday, though any
gale force conditions should be brief. A shift to southerly then
westerly flow is likely Saturday night into Sunday with gusts
continuing to reach 25 kt.

&&

.TIDES/COASTAL FLOODING...
The combination of a new moon on Friday, along with an onshore
flow may lead to areas of coastal flooding with the high tide
Friday night.

&&

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

&&

$$
Synopsis...O`Brien
Near Term...Gorse/PO
Short Term...Gorse
Long Term...O`Brien
Aviation...Gorse/O`Brien/PO
Marine...Gorse/O`Brien
Tides/Coastal Flooding...


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