666
FXUS61 KPHI 192010
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
410 PM EDT Sun May 19 2024

.SYNOPSIS...
High pressure across the Northeast builds across the Mid-Atlantic
through Tuesday before moving offshore on 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 MONDAY/...
315 PM...As of mid afternoon, low pressure was located well offshore
with weak surface ridging trying to build in from the north. However
this set up is resulting in continuing onshore flow leading to
abundant low level moisture persisting over the area. The diurnal
heating from the strong May sun has acted on this moisture to bring
considerable strato-cu across the area this afternoon. That said,
there are some sunny breaks and even some areas where skies have
locally become partly to mostly clear. Expect that as we go through
the latter part of the afternoon into early this evening that this
trend towards less cloud cover will continue. Also, outside of a few
light showers that could pop up over western parts of the forecast
area around western Berks County, expect it will stay dry this
afternoon.

As we head into tonight, it should be mainly clear to partly cloudy
for most of this evening under the influence of surface ridging.
However most of our forecast guidance indicates another round of low
stratus is likely to develop overnight along with some mist and fog.
This is actually the most challenging part of the near term forecast
in terms of whether it will be more of a stratus set up vs. fog for
tonight. Areas where there`s more clearing and it stays clear for
longer could see more in the way of fog. Expect overnight lows
ranging from the low/mid 50s over the coastal plain to the upper 50s
to near 60 over portions of eastern PA.

The trend towards brighter and warmer weather will continue as we
head into the day Monday. High pressure centered right near the area
early in the day will slowly nudge southward through the day and as
this occurs, there will also be upper level ridging building over
the mid Atlantic into the northeast. As mentioned above, expect some
fog and stratus around to start the day but this should give way to
increasing amounts of sunshine by late morning through the
afternoon. For most of the area temperatures will warm some compared
to today as highs reach the 70s to near 80 in some spots. However
light onshore winds will be persisting and this will keep it cooler
near the coast where highs will be stuck in the 60s.

&&

.SHORT TERM /MONDAY NIGHT THROUGH WEDNESDAY/...
Overall, little changes were made to the short term period as the
mid to upper level ridge essentially lies overhead through Tuesday
night before beginning to breakdown and shift offshore on Wednesday.
Down at the surface, broad high pressure over New England will
expand southwest while becoming elongated into the southern
Appalachian region. The high will sink south of the region on
Tuesday and eventually settle offshore the Mid-Atlantic coast by
Tuesday night into Wednesday. This will yield dry conditions
throughout the period.

With clear skies and light winds expected at night, this should
favor the development of radiational fog on Monday night as dew
point depressions lower. It`s a bit unclear how much and where fog
actually does develop but with enough of a southerly wind, it should
remain more in the way of patchy than widespread. It is possible
another round of fog may develop on Tuesday night, but confidence in
this is less than Monday night so have left the mention out of this
out of the forecast for now.

In terms of temperatures, it is looking like we will return close to
normal with respect to low temps both nights with lows generally in
the 50s to around 60 degrees. For high temperatures, it is likely
that much of the area will range about 5-10 degrees above normal on
Tuesday and as much as 10-15 degrees above normal on Wednesday.
Meaning high temps will top out 80s both days as southerly flow
ushers warmer air north. The exception will be for our coastal
locales where a sea breeze circulation looks to develop each day.
Given how cold the water temperatures are, there is potential for
the sea breeze to develop early enough in the day, that the sea
breeze reaches far enough inland that highs for most, if not all, of
the coastal plain could be slightly lower than what is forecast.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH SUNDAY/...
Summary: The main focus through the long term period is a cold front
that is expected to cross through the region on Thursday bringing
the chance for thunderstorms. High pressure looks to return on
Friday with another disturbance approaching over Memorial Day
Weekend.

Details:

Wednesday night...While run-to-run consistency remains poor,
indications are for a pre-frontal trough to approach our
northwestern areas on Wednesday night. This will result in a slight
chance of some thunderstorms across the Poconos and Lehigh Valley
for Wednesday night. PoPs are only around 20% so while the
probability of occurrence is low, it cannot be ruled out. Elsewhere,
dry conditions will continue other than an increase in clouds.

Thursday...Deterministic global guidance between the GFS/ECMWF/CMC
now all lie in close relation to one another with respect to the
timing of the cold front. Based on current depiction, still thinking
that this will be a late afternoon/early evening type of event with
the cold front pushing offshore on Thursday night. While the
location of the upper trough looks to remain over the Great Lakes,
there should be enough surface instability aided by sufficient
diurnal heating with temps rising into the 80s. This is supported
quite well by both analog-based and machine learning probabilities.
Considering this notion, it warrants some monitoring of severe
thunderstorm potential Thursday afternoon into Thursday evening.

Friday through Sunday...The forecast for Friday and much of Memorial
Day Weekend is highly variable and depends on how quickly the upper
flow pattern evolves. The GFS remains the most aggressive with the
upper level pattern with an upper ridge building overhead on Friday
before another shortwave trough passes by on Saturday into Sunday.
Meanwhile, the ECMWF/CMC are much slower with the progression of
upper level features, keeping the upper ridge in place through
Sunday and shortwave trough delayed until early next week. Due to
such drastic timing differences, continued to use a blend of global
guidance which keeps a slight chance of showers (~20%) through the
holiday weekend. However, the weekend certainly does not look to be
a washout by any means, and there will likely be a period of dry
weather...just exactly when that will be is yet to be seen.

&&

.AVIATION /20Z SUNDAY THROUGH FRIDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas...

Through this afternoon...Mainly VFR with scattered to broken
cigs around 4000 feet. Light northeasterly winds will be
variable at times. Moderate confidence.

Tonight...MVFR cigs/vsby expected with continuing light winds
generally out of the east. IFR possible with any patchy fog or lower
clouds. Low confidence.

Monday...MVFR cigs/vsby (with locally IFR) likely to persist
through at least the first half of the morning for most sites
with conditions expected to improve to VFR through the 14-17z
timeframe. Light winds generally out of the east continuing.
Moderate confidence.

Outlook...

Monday night through Wednesday...Mostly VFR expected. Some
visibility restrictions are possible on Monday night and Tuesday
night due to mist/fog.

Wednesday night through Thursday night...Sub-VFR conditions probable
especially on Thursday and Thursday night. A slight chance of a
thunderstorm on Wednesday night with scattered thunderstorms on
Thursday into Thursday night.

Friday...VFR. No significant weather expected.

&&

.MARINE...
Sub Small Craft Advisory conditions over the waters through tonight
and Monday although there could be some gusts of 15 to 20 knots
through this afternoon. Expect seas generally around 3 to 4
feet.

Outlook...

Monday night through Friday...No marine headlines are expected. Fair
weather expected through Wednesday night with a chance for
thunderstorms on Thursday into Thursday night. Fair weather is
expected to return on Friday.

Rip Currents...

A prolonged period of NE to E winds at 10 to 15 mph during the day
and 5 to 10 mph at night will continue through 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.

By Tuesday, the winds will be veering to more of a southerly
direction at 10 to 15 mph which will reduce the onshore component at
most beaches. However we`ll also be heading closer to the Full Moon
on the 23rd so for this reason expect that most of our beaches will
continue to see a MODERATE risk for the development of dangerous rip
currents.

&&

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

&&

$$

SYNOPSIS...DeSilva
NEAR TERM...Fitzsimmons
SHORT TERM...DeSilva
LONG TERM...DeSilva
AVIATION...DeSilva/Fitzsimmons
MARINE...DeSilva/Fitzsimmons