177
FXUS61 KPHI 181917
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
317 PM EDT Sat May 18 2024

.SYNOPSIS...
An area of low pressure slides by to the south tonight before moving
offshore. High pressure then builds across our area on Sunday
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.

&&

.NEAR TERM /THROUGH SUNDAY/...
310 PM...As of mid afternoon, there`s still some rain ongoing across
portions of the area...mainly over eastern PA into NJ but some
additional showers have also developed farther to the west over
Maryland. This is all in association with a decaying system as it
move across the area from west to east. We will continue to see this
light rain and showers affecting the area through the afternoon
before the precip diminishes through early this evening as the best
upper forcing moves off the coast. In its wake though, there really
won`t be much of a push of drier air as the low levels winds remain
light out of the east. This will keep low cloud cover socked in for
tonight. There could also be some patchy fog around as well as some
drizzle and maybe a few sprinkles around. Expect overnight lows
tonight will be generally in the 50s.

As we head into the day Sunday, weak surface ridging will try to
gradually build back in over the area as ridging in the upper levels
starts to build back into the north. Despite this occuring, the
surface winds will remain onshore out of the east/northeast and I
expect this will keep the overcast stratus deck locked in at least
through the morning. It`s possible that by later in the afternoon we
could start to see some partial clearing occur, especially over
NE PA into NW NJ. Some of the models are showing some light
precip occuring in the afternoon around Chester north into Berks
Counties but there really just doesn`t appear the forcing there
so continue to keep POPs for Sunday below 15 percent. Forecast
highs will range from the 60s near the coast to low 70s near the
urban corridor with mid 70s over our western zones across
Chester and Berks Counties.

&&

.SHORT TERM /SUNDAY NIGHT THROUGH TUESDAY NIGHT/...
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 on Sunday night
will expand southwest and become elongated into the Appalachian
region. 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. There may also be some
patchy fog, more so on Sunday night depending on how quickly skies
clear. For now, best chances appear to be north and west of the I-95
corridor.

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 temps
closer to the coast.

&&

.LONG TERM /WEDNESDAY THROUGH SATURDAY/...
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.

Details:

Wednesday...Upper ridge axis will shift offshore on Wednesday.
Guidance has overall slowed to progression of the pre-frontal trough
so Wednesday now appears to remain dry. Increasing clouds late with
temperatures above average. Shower chances increase Wednesday night
with a chance of thunder as the pre-frontal trough approaches.

Thursday...Guidance remains persistent with the timing of the cold
front during the afternoon hours on Thursday. 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. Cold front then moves
offshore by Thursday night. Temps should be above average again,
depending on timing of the cold front.

Friday and Saturday...The forecast for Friday and Saturday is quite
uncertain and depends on how far the front makes it past our area.
At this time, the 12Z/GFS is more aggressive with the progression of
the front, bringing in high pressure into the area on Friday and
most of Saturday. In comparison, the 12Z ECMWF is much slower with
the progression of the front, keeping the front stalled over our
area into next weekend with shower chances. For this reason, have
used NBM guidance which keeps most of the area within a 20-30%
chance of showers/thunderstorms for Friday and Saturday.

&&

.AVIATION /19Z SATURDAY THROUGH THURSDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas...

Rest of this afternoon...Generally MVFR conditions with some light
rain, stratus, and onshore winds this afternoon with the
exception being around ACY and TTN where it could stay mainly
VFR. Winds generally favoring an easterly direction but
remaining very light (around or under 5 knots). Moderate
confidence.

Tonight...Rain/showers ending but conditions should be MVFR for the
most part (mainly due to cigs) with IFR cigs likely overnight at
ACY, MIV, and RDG. Winds light out of the east to northeast at less
than 5 knots. Moderate confidence.

Sunday...MVFR/IFR to start the day with some improvement occuring
from NW to SE in the afternoon. This should bring most sites up to
VFR by late day with the possible exceptions being at MIV and
ACY where it could stay MVFR. Northeast winds 5-10 knots.
Moderate confidence.

Outlook...

Sunday night...VFR with clearing skies. Patchy fog is possible late
resulting in visibility restrictions.

Monday through Wednesday...VFR with mostly clear skies.

Wednesday night through Thursday...Sub-VFR possible especially on
Thursday with a chance for thunderstorms.

&&

.MARINE...
Conditions over the waters will be sub SCA through Sunday with seas
mainly around 4 feet. Northeast should gust to around 20 knots
Sunday but stay below SCA levels.

Outlook...

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 5 to 10 mph through the
weekend along with 3 to 4 ft seas will result in a MODERATE risk
for the development of dangerous and life threatening rip
currents today and Sunday.

&&

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

&&

$$

SYNOPSIS...Desilva/RCM
NEAR TERM...Fitzsimmons
SHORT TERM...Desilva
LONG TERM...Desilva/Johnson
AVIATION...Desilva/Fitzsimmons
MARINE...Desilva/Fitzsimmons