Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 071043

Area Forecast Discussion
National Weather Service Mount Holly NJ
643 AM EDT Fri May 7 2021

Low pressure off the North Carolina coast will track northeastward
and well off the Mid-Atlantic coast through tonight, then to near
Nova Scotia Canada during Saturday. Weak high pressure builds in
Saturday night into Sunday, then low pressure tracks across our area
Sunday night and Monday morning. A strong area of high pressure
builds into our area later Tuesday and Wednesday before weakening
some and shifting offshore into Thursday.


High pressure starts to give way through the afternoon today as a
shortwave will bring showers to much of the region heading into the
late afternoon or early evening. Expect the broad 500mb trough
centered over the Great Lakes region to start to take on a negative
tilt through the afternoon. We`ll see an area of showers develop and
track across central PA through the morning approaching the Lehigh
Valley and southeastern PA by late afternoon. The showers will then
slowly track east into NJ and DelMarVa through the evening. There`s
some modest instability aloft and MUCAPE`s aren`t terrible so I`ve
kept mention of thunder in generally across the southern portion of
our region were temps should warm into the mid 60s. As we head late
into the day, the 250mb jet dives south of the region and we`ll
start to lose our lift so I think the slow moving showers will wane
as the area of showers track towards the coast.

One thing that we`ll need to keep an eye on is the threat for heavy
rain across SEPA. Soil moistures are running high there and the 1
hour FFG from MARFC suggests that we`ll only need an inch of rain to
see flooding. I dont think we`ll get that, but some of the hires
CAM`s are suggesting that the slow moving storms could have decent
rain rates. PWAT`s aren`t high (0.7") but with slow moving storms
and the potential for higher rain rates we may see some ponding of
water and or flooding in areas of poor drainage.

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. High temperatures are a tricky forecast and I`m
relying on the idea that we`ll likely stay cloudy through much of
the afternoon as a shortwave tracks aloft. Anticipate highs in the
mid to upper 50s for most of the region.


Cyclonic flow associated with an eastern Canada and eastern U.S.
upper-level trough will weaken Saturday night into Sunday as one
storm continues to lift across the Canadian Maritimes, and another
storm quickly arrives from the west. Any lingering showers are
expected to rapidly end early Saturday evening as a brief area of
drying occurs with weak high pressure arriving.

A closed low centered just north of the Great Lakes Sunday will
start to amplify south and eastward. A southern shortwave tracking
along a baroclinic zone is forecast to quickly translate eastward
during the course of Sunday. This will result in increasing clouds,
however an area of drier lower level air should keep much of the
rain to our west for much of the daytime Sunday. Some guidance
however is a little faster with the rain arrival, however we leaned
on the slower guidance given the drier air initially in place across
our region. Looks like a decent shot of warm air advection and
overrunning overspreads much of our area Sunday night. This is
expected to result in an area of rain arriving from the west with
much of this centered across the northern half or so of the area. A
surface low will also be arriving into our area Sunday night, and a
strong southwesterly low-level jet will also enhance the lift along
and north of the warm front/surface low track. Our Delmarva zones
may be mostly removed from the stronger forcing and therefore
lowered the PoPs some there. It is possible that temperatures rise
some during the night especially across the southern zones, however
this will depend on the northerly movement of the warm front.


Summary...Unsettled mostly to start, along with cooler (below
average) temperatures mostly dominating.

Synoptic Overview...A large upper-level trough across eastern Canada
is forecast to amplify southward across much of the eastern U.S.
Monday into Tuesday, then lift out some Wednesday. Another upper-
level trough from the Plains is then forecast to shift eastward and
approach the East Thursday. At the surface, the initial trough
amplification will drive low pressure quickly across our area then
offshore Monday morning. Strong high pressure is to follow for much
of the week, although it should weaken especially into Thursday as
it shifts offshore. The secondary incoming trough Thursday should
result in low pressure tracking across the southern states. We may
still remain protected enough from the weakening surface high
despite it shifting more offshore.

For Monday...The guidance agrees that the center of a closed low is
just north of the Great Lakes Monday, then it amplifies as it slides
east-southeastward through Tuesday. Some guidance is a little
stronger with the trough and therefore has the surface low placement
somewhat different. Given the placement of the baroclinic zone, the
surface low should quickly track across our area Monday morning
before shifting offshore. Since it looks to track across our area,
at least a portion of the region should get briefly into the warm
sector although the northward extent of this will depend on the
surface low track. This would result in some areas getting into the
70s Monday afternoon, assuming enough sunshine can return. It looks
like any steady rain ends early Monday, then depending on the
amplification of the incoming trough there could be some additional
showers developing through the afternoon associated with the daytime
heating. This may have to battle some incoming drier air though
which would act to limit the development, and therefore any thunder
potential is of low confidence. We will continue to hold onto some
chance PoPs during Monday (decreasing some through the day from west
to east) following the early morning steady rain in parts of the
area. We should then dry out more Monday night.

For Tuesday through Thursday...Much of this time frame is forecast
to be dominated by high pressure, which initially is strong but then
weakens as it shifts eastward and eventually offshore by Thursday.
This should result in dry conditions but also temperatures will be
on the cooler side, although a few showers are possible Tuesday
mostly up north as the core of the upper-level trough swings
through. Opted to keep it dry for now as much of the forcing is
farther to our north and drier low-level air may win out. We will
have to watch energy sliding across the southern states as an upper-
level trough marches eastward from the Plains. The bulk of this
system is currently anticipated to track well to our south, however
given the weakening and offshore surface high some moisture may try
and extend into portions of our area late Wednesday and Thursday.
The northern extent of this system will depend on the incoming upper-
level troughs strength and timing.


The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

Today...VFR with increasing cloud cover. A chance of showers
developing in the afternoon from west to east could briefly
bring restrictions to MVFR. East to southeast wind 5 to 10
knots. High confidence.

Tonight...Generally MVFR cigs across the region as a a band of rain
affects the Mid Atlantic. Winds generally light and variable.
Moderate confidence.

Saturday...General improvement to VFR from southwest to northeast
through the morning with additional showers and brief/localized
restrictions are possible in the afternoon. Northwest winds around
10 knots with gusts to 20 knots. Moderate confidence.


Saturday night and Sunday...VFR with increasing clouds and
gradually lowering ceilings through much of Sunday, then sub-VFR
conditions Sunday night with some rain. Northwest winds around
5 knots becoming southwest around 10 knots during Sunday, then
becoming southeast to east Sunday night. Low-level wind shear is
possible later Sunday night. Moderate confidence.

Monday...Times of sub-VFR conditions possible along with showers
around, especially in the morning. South to southwest winds around
10 knots, becoming west to northwest in the afternoon and at night.
Low-level wind shear possible early in the morning. Moderate

Tuesday...VFR. Northwest winds 10-15 knots with gusts 20-25 knots,
diminishing in the evening. Moderate confidence.


Below SCA thresholds through Saturday as winds will generally be
onshore through the day today 5 to 10 kts shifting to offshore
through the overnight into Saturday and increasing to 10 to 15 kts.

Waves on our ocean waters should be 2 to 3 feet. Waves on Delaware
Bay are expected to be 2 feet or less.


Saturday night and Sunday...The conditions are anticipated to be
below Small Craft Advisory criteria.

Sunday night and Monday...A period of Small Craft Advisory conditions
possible. Southwesterly winds increase with gusts 25-30 knots
possible, however some milder air moving in over the cooler
waters may limit the mixing somewhat. Seas on the ocean should
build into the 5-7 feet range, before subsiding later Monday.

Tuesday...Small Craft Advisory conditions possible, mainly due to
northwesterly winds gusting to 25 knots at times.




Near Term...Deal
Short Term...Gorse
Long Term...Gorse
Marine...Deal/Gorse is the U.S. government's official web portal to all federal, state and local government web resources and services.