FXUS61 KPHI 191053

Area Forecast Discussion
National Weather Service Mount Holly NJ
653 AM EDT Sun May 19 2024

An area of low pressure moves offshore to our south today. High
pressure then builds across our area through Wednesday. A cold
front approaches and swings through the area on Thursday into
Thursday night. High pressure returns for Friday with another
low pressure system approaching by next weekend.


While showers have mostly dissipated, low clouds continue to
shroud much of the area early this morning, though there are
some breaks showing up near and east of the I-95 corridor.
Patchy fog does still pose a hazard mainly north and west of
I-95 primarily, but it should start dissipating shortly.

Weak easterly flow to the southwest of high pressure centered
over northern New England will keep the marine influence in
place today, so while strong mid-May insolation should help
clouds lift and break up through the day, it may not get much
better than partly sunny across much of the region by late this
afternoon given the plentiful low-level moisture to work with.
Towards the western flank of the marine layer, could even be a
pop-up shower, most likely out in Berks County. The insolation
and general lack of rain should allow most areas to pop back
into the 70s, except near the shore.

Short-term guidance suggests a new push of marine clouds will
move into the region tonight, so have another mostly cloudy
forecast tonight. If clouds are less widespread, fog may become
an issue, but for now have kept it patchy in the forecast. Lows
mostly in the 50s again.


The short term period will be focused mainly on the mid to
upper level ridge that approaches and builds overhead through
the period. At the surface, broad high pressure over New England
will expand southwest and become elongated into the Appalachian
region Monday. The high will sink south of the region on Monday
night before moving offshore late Tuesday into Tuesday night.
During this period, dry conditions are expected with mostly
sunny skies during the day and mostly clear skies at night.

With the clear skies and light winds overnight, expect a
favorable setup for radiational fog Monday night and possibly
again on Tuesday night. Its less clear if there are any more
likely areas for fog development, so have a mention of patchy
fog across the region Monday night. Have left out a mention of
fog for Tuesday night at this time, as there may be some
increasing clouds if a prefrontal trough starts to approach our
region then.

In terms of temperatures, it is looking like we will return
close to normal on Monday with highs in the mid to upper 70s and
lows in the low to mid 50s. For Tuesday, winds become more
southerly as the high shifts offshore resulting in warmer temps
being brought north. Generally looking at the upper 70s to low
80s for Tuesday. However, with rather light surface winds, this
should allow a sea breeze circulation to develop each day, so do
anticipate cooler high temperatures closer to the coast. Given
how cold the water temperatures still are, there is potential
for the sea breeze to develop early and then reach far enough
inland that highs for most, if not all, of the coastal plains
could be slightly lower than the current forecast. However, hard
to determine those details this far out, so have stayed close to
a blend of guidance for high temperatures both days.


Summary: Main focus through the long term is on a cold front
which is expected to cross through the region Thursday into
Thursday night bringing a chance for thunderstorms. High
pressure will return in the wake of the front with another
disturbance that approaches by next weekend.


Wednesday...Upper ridge axis will shift offshore on Wednesday.
There continues to be poor run-to-run consistency on if there
will be a pre-frontal trough in the region as early as
Wednesday afternoon. For now, have kept any mention of
thunderstorms (and just a slight chance at that) confined to
Berks County, Lehigh Valley, and the southern Poconos through
the day, and generally along and west of the 95 corridor
Wednesday night.

Thursday...There actually appears to be less model agreement on
the timing of the front as compared to previous model runs. Some
models (most notably the GFS) depict the cold front passage as
early as Wednesday night into Thursday morning. Given how late
we are in the season, and the fact that the mid and upper level
low should be slowing as the surface front approaches, I think
this is the more unlikely scenario. Thus, have stayed close to
the previous forecast and a blend of other guidance with the
cold front generally arriving from Thursday afternoon into
Thursday night. With the location of the mid and upper level
trough over the Great Lakes, this keeps the better forcing north
and west of the area. However, with enough surface instability
aided by diurnal heating, this should support at least scattered
thunderstorm develop across the area. This is supported quite
well by analog- based guidance. Temps should be above average
again and close to Wednesday temperatures, assuming the later
timing of the front holds.

Friday and Saturday...The forecast for Friday and Saturday is
quite uncertain and depends on how far the front makes it past
our area. Unsurprisingly given how quickly it is depicting the
arrival of the front, the GFS remains the most aggressive with
the progression of the front, bringing in high pressure into the
area on Friday and most of Saturday. ECMWF keeps the front
stalled just south of our region through Saturday. As a result
of the uncertainty in the timing and evolution of the front,
have used a blend of guidance, which keeps at least slight
chance of showers through the weekend. However...the upcoming
weekend will probably not be a washout as almost all guidance
shows at least some period of dry weather. However, due to the
differences in the timing, it is hard to determine when the dry
period could be.


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

Today...Primarily a mix of VFR and MVFR this morning with
intervals of IFR possible either due to cigs dropping or patchy
fog. Cigs and vsby should rise to VFR this afternoon. Light
northeasterly winds will be variable at times. Moderate

Tonight...MVFR cigs/vsby looking likely again with continued
light easterly flow. IFR possible with any patchy fog or lower
clouds. Low confidence.


Monday through Wednesday...Mostly VFR. Some visibility
restrictions possible in BR and FG Monday night and Tuesday

Wednesday night through Thursday...MVFR or even IFR possible
with thunderstorms especially on Thursday.


Conditions over the waters will be sub SCA through tonight with
seas 3 to 4 feet. Northeast may gust to around 20 knots but
stay below SCA levels.


Sunday night through Thursday...No marine headlines are expected.
Fair weather expected through Wednesday with a chance for
thunderstorms on Thursday.

Rip Currents...

A prolonged period of N to NE winds at 10 to 15 mph during the
day and 5 to 10 mph at night will continue today and Monday.
Along with 3 to 4 ft seas, this will result in a MODERATE risk
for the development of dangerous and life threatening rip
currents today and Monday at New Jersey and Delaware beaches.
The rip current risk may be low for most of the day Monday for
southern New Jersey beaches.





SHORT TERM...DeSilva/Johnson
LONG TERM...DeSilva/Johnson

NWS PHI Office Area Forecast Discussion