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000
FXUS61 KPHI 060740
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
340 AM EDT Thu May 6 2021

.SYNOPSIS...
Weak high pressure will be across our area today before shifting
offshore tonight. Low pressure develops off the North Carolina coast
late tonight into Friday, then it tracks well off the Mid-Atlantic
coast through late Friday. Weak high pressure builds in Saturday
night into Sunday, but another low pressure system tracks over or
near our area late Sunday into Monday. A stronger area of high
pressure builds into our area later Tuesday into Wednesday.

&&

.NEAR TERM /THROUGH TONIGHT/...
A mid level low over northern Ontario will be the main weather
driver through the day today. Mid level zonal flow will keep the
airmass roughly in place while northwest winds continue behind the
cold front that passed through the region last night. With cool and
dry air advecting into the region and upstream RAOBs in both PBZ and
BUF showing PWAT`s sitting in the 10th percentile, we`ll keep mostly
clear skies through much of the earlier morning.

A mid level shortwave should cross through the region by mid morning
and will bring an increase in cloud cover heading into the evening
as high pressure starts building over the region. Another wave will
move through during the evening as the nose of 250mb jet pushes into
the Mid Atlantic. The strongest portion of that wave looks to drive
south towards the Carolinas and should keep the area dry overnight.
Clouds will continue to thicken through the overnight hours and
we`ll see lows once again in the mid 40s albeit maybe a degree or
two warmer given the increased cloud cover.

&&

.SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/...
A deep upper-level trough will be in place over the East Friday
right into the weekend. Several shortwaves at various latitudes will
rotate through the broad cyclonic flow aloft. Surface cyclogenesis
is forecast off the Southeast coast by Thursday night as one
shortwave tracks near the base of the longwave trough and over an
offshore baroclinic zone. Steady intensification of this low is
expected as it moves into the right entrance region of an
unseasonably strong 130 knot upper jet streak off the Northeast
coast. The location of the jet streak and upper-level steering flow
will keep the center of the low well offshore of our region.

As that low makes its closest approach late Friday, another
shortwave diving out of the Great Lakes will approach our region,
adding some synoptic lift. Meanwhile, at the surface, onshore flow
is expected Friday thanks to the low pressure offshore. Northwest
winds will develop to our west as the upper-level disturbance
approaches. This sets up a convergence zone over the Mid-Atlantic,
which combined with lift from the shortwave will likely lead to a
band of showers developing to our west on Friday. This band will
gradually shift east in response to the eastward progression of
surface convergence. Showers will likely arrive from west to east
during Friday afternoon.

The band of showers continue into the overnight, though they may
weaken some with time as we start to lose the better convergence. It
will also continue to slowly shift to the east and northeast as
westerly flow overspreads more of our area. Overnight PoPs are
mainly high end chance to low end likely. Lows mostly in the 40s.

For Saturday, a band of convergence driven showers will be ongoing
across portions of the area in morning. This band should steadily
lifts off to the northeast with time as the convergence zone weakens
and shifts northeastward in response to the departing coastal low
well offshore. Some clearing is possible by afternoon. However, with
very cold air aloft, any clearing could yield some additional pop up
showers and small hail. High temperatures are only forecast in the
upper 50s to low 60s, a good 10 degrees below average thanks to the
morning clouds and showers and the unseasonably cold air mass
overhead. Dry conditions should prevail across the region by the
overnight, with lows in the upper 30s and low 40s.

&&

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...
Summary...Unsettled at times especially early on, along with cooler
(below average) temperatures.

Synoptic Overview...A large upper-level trough is forecast to
dominate eastern Canada and the eastern U.S. through the first half
of next week. Some amplification is forecast to occur at times with
this trough, however no significant storm systems are anticipated.

For Sunday...Dry weather and weak high pressure should be in control
at least through the first half of Sunday. It will be another cool
day with highs only in the low 60s. By later in the day, we will be
watching an area of low pressure moving in from the Midwest. There
is some spread in the guidance as to the track and timing of this
low, however a period of rain is looking likely. The general idea is
that rain chances increase later Sunday and especially Sunday night,
therefore PoPs were increased into the likely range Sunday night
into early Monday morning.

For Monday through Wednesday...Upper-level trough continues to
linger over the East. However, current indications are that high
pressure building in from the Great Lakes will keep this period at
least mostly dry. Some lingering rain is expected Monday morning as
the system from Sunday departs, though as mentioned there is some
uncertainty in the track of that system. From there, high pressure
should become more established with a dry Tuesday into Wednesday. A
system cruising across the southern states may try to toss some
showers into Delmarva, however this is less certain and therefore
only have slight chance PoPs there for the second half of Wednesday.
High temperatures look to range through the 60s during this period,
a good 5 to 8 degrees below average. Overnight lows look to be
mainly in the 40s, and it does not appear there will be any frost
concerns, save for possibly in the Poconos.

&&

.AVIATION /07Z THURSDAY THROUGH MONDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

Rest of Tonight...VFR, with a few lingering clouds around 5000 feet.
Northwest winds around 10 kt overnight. High confidence.

Today...VFR with northwest winds 10 to 15 kt with gusts to 20 kt
through mid afternoon, becoming more light and variable heading
towards late in the day. Cloud cover will begin to increase and
thicken through the afternoon with high clouds ceilings expected
overnight. High confidence.

Tonight...VFR. Winds mostly light and variable, but the prevailing
flow should gradually shift from northwest to northeast. High
confidence.

Outlook...

Friday...Mainly VFR with ceilings gradually lowering especially in
the afternoon. A chance of rain/MVFR conditions by late afternoon
mainly west of PHL. Southeast winds 5-10 knots. Moderate confidence.

Friday night and Saturday...A period of MVFR or IFR Friday night as
a band of rain crosses the area. Improvement to VFR from southwest
to northeast should occur Saturday morning. An additional shower and
brief/localized restrictions are possible Saturday afternoon. Mostly
light and variable winds Friday night, then becoming northwest
around 10 knots with gusts to 20 knots Saturday. Moderate confidence.

Saturday night and Sunday...VFR. Northwest wind around 5 knots
Saturday night, then becoming southwest 5-10 knots Sunday. Lowering
ceilings and some rain possible by late Sunday, especially to the
west but should remain VFR. Moderate confidence.

Sunday night and Monday...Sub-VFR and a period of rain probable.
Light and variable winds, becoming northeast to north-northwest 5-10
knots. Low confidence.

&&

.MARINE...
Northwest winds continue through the morning and we`ve seen a few
WeatherFlow gauges gusting to around 25 knots. Thus the Small Craft
Advisory remains in effect through the overnight hours. Winds will
diminish through the morning over the waters to below SCA as high
pressure builds toward our region. Once the SCA comes down this
morning, no additional headlines are expected through tonight.

Outlook...

Friday through Sunday...The conditions are anticipated to be below
Small Craft Advisory criteria. Several wind shifts are forecast but
gusts mainly 20 knots or less.

Monday...The conditions should remain below Small Craft Advisory
criteria.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Small Craft Advisory until 10 AM EDT this morning for
     ANZ450>453.
     Small Craft Advisory until 8 AM EDT this morning for ANZ430-
     431-454-455.

&&

$$
Synopsis...Gorse/O`Brien
Near Term...Deal
Short Term...Gorse/O`Brien
Long Term...Gorse/O`Brien
Aviation...Deal/Gorse/O`Brien
Marine...Deal/Gorse/O`Brien