871
FXUS61 KPHI 132110
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
510 PM EDT Sat Jul 13 2024

.SYNOPSIS...
High pressure returns late tonight and persists through Tuesday
night. A cold front approaches from the northwest on Wednesday
before crossing our area during Wednesday night. High pressure
then builds back in from the west to end the week.

&&

.NEAR TERM /THROUGH SUNDAY/...
Diurnally driven showers and some thunderstorms through this
evening, especially in parts of Delmarva and far southeastern
New Jersey. A shower cannot be ruled out early this evening in
the Poconos.

Tonight, high pressure builds in from the west. Abundant low
level moisture will remain in place, and it looks like we will
have a similar set up to last week when we had several days in a
row for marine fog encroaching on the coast. Already has
developed in some areas on the waters based on coastal web
cameras. Not confident yet on how far inland dense fog will
get, so no dense fog advisories on land at this time, but will
be watching trends closely.

For Sunday, zonal flow aloft and very light flow in the low
levels. With moisture still in place, we`ll have another chance
for isolated or scattered showers. Think the coverage will be
limited as we will lack a focus for surface convergence.
Additionally, some model soundings are depicting a mid level
inversion which would limit instability.

As for the heat, no significant changes at this time with the
temperatures and heat index values. Although by strictest heat
index criteria, we are forecasting to barely get to heat
advisory criteria, this is the start of a multi-day excessive
heat risk, combined with relatively warm lows Sunday morning
means little in the way of relief.

&&

.SHORT TERM /SUNDAY NIGHT THROUGH TUESDAY/...
Early in the new week will feature excessive heat and humidity,
increasing with each passing day through Tuesday or Wednesday.
Bermuda high pressure and broad low pressure across the Great Lakes
will funnel heat and humidity northward across the East Coast and
into the Mid-Atlantic. Quasi-zonal flow will exist aloft, though
with a trough and jet feature approaching gradually with each day.
Meanwhile, MCS remnants from upstream across the Midwest will stream
into the area Monday and Tuesday, yielding potential for convection
each day. The convection should generally be favored during the
afternoon and evening time frame, mainly near and northwest of I-95
where diurnal instability is greater. SPC does not have our area
outlooked for any day, however we`ll need to monitor the potential
for severe thunderstorms. Any severe threat (and greater coverage
thunderstorm threat) will be dictated by upstream convection, so
predictability in our area remains low at this time.

On Wednesday, a cold front associated with the trough will begin
approaching the area. A prefrontal trough may develop on the
leeward side of the mountains as well. This combined with
greater synoptic forcing and greater shear should result in
greater coverage of thunderstorms, and could lead to an
increase threat for severe thunderstorms Wednesday afternoon and
evening as well. But again, details remain a bit unclear on
this threat at the moment. Nevertheless, we`ve included the
greatest PoPs for the week on Wednesday.

As far as the heat and humidity go, it`s indeed going to be another
excessively hot and humid stretch of days. Temperatures Monday will
be a bit hotter than Sunday with highs mainly in the mid to upper
90s outside of the higher terrain and away from the coast. However,
dewpoints should mix out a bit more, so resulting heat indices will
be similar to Monday across the board, generally in the upper 90s to
low 100s. Therefore, the Heat Advisory was extended through Monday
night (replacing the Excessive Heat Watch for that time frame). Low
temperatures will be quite warm as well, mainly in the low to mid
70s Sunday night and Monday night, which will not offer much relief
from the heat.

By Tuesday, temperatures increase even more along with lower chances
for convection. Tuesday is shaping up to be the hottest and most
oppressively humid day of the heatwave, with highs largely in the
upper 90s to near 100 degrees in some spots. Dewpoints remaining in
the low 70s will support heat indices ranging from near 105-110
degrees across much of the area. Lows Tuesday night will likely be
the warmest of the heatwave, ranging from the mid 70s to near 80
degrees. Wednesday remains a bit more uncertain with regard to just
how hot it will get. The increasing clouds and approaching
convection could limit more efficient heating as will be seen on
Monday and Tuesday. however, a slower evolution of the system
will probably result in hotter temperatures in the mid to upper
90s, and thus heat indices near 105-110 degrees yet again as
dewpoints will be a bit higher. A faster solution would yield
slightly cooler temperatures, say in the low to mid 90s. Either
way, another very hot and humid day regardless of the specific
numbers. Therefore, the Excessive Heat Watch was extended
through Wednesday. The Watch was also expanded to include Morris
and Warren Counties in New Jersey, and all Delmarva zones. The
southwesterly flow will likely result in 100-105 degree heat
indices near the New Jersey shore and Delaware Beaches as well
on Tuesday and Wednesday.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/...
The approaching cold front on Wednesday will begin to make it`s way
through our area Wednesday night into Thursday, albeit slowly.
Showers and thunderstorms likely lingering into the night,
especially near and southeast of I-95. We`ll finally get some
relief from the early week excessive heat and humidity by
Thursday with cloud cover and lingering convection around, again
mainly southeast of I-95. Dewpoints will fall into the 60s with
a northerly breeze for a change, with high temperatures in the
mid 80s. Dewpoints fall even further into Friday. Lows will be
in the 60s. The stalled boundary just south of our area will
likely result in some scattered showers or storms late in the
week across the southern forecast zones.

&&

.AVIATION /21Z SATURDAY THROUGH THURSDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Through tonight...VFR through this evening with any showers
south of KMIV/KACY. After about 06z, fog on the ocean should
spread at least a little inland. Additionally, we could see fog
develop in the river valleys. Lowest conditions (LIFR) are
likely to be with any marine fog that moves inland, which could
get to KACY and KMIV. Otherwise, expect MVFR or IFR visibilities
for most TAF sites once the fog develops. South to southwest
winds 4-8 knots or light and variable. Low confidence on when
the fog will develop and how low the visibilities will be.

Sunday...Once the fog dissipates, VFR. A few showers or
thunderstorms possible primarily after 16z (could result in
brief local MVFR conditions). Given the limited coverage, a
mentioned is not included in the TAFs at this time. Winds
favoring southwest 5-10 knots, but may be more south-southeast
at KACY. Low confidence if a thunderstorm will directly impact
a TAF site.

Outlook...

Sunday night and Monday...A chance of some showers and
thunderstorms, mainly later Monday.

Tuesday...VFR overall.

Wednesday and Thursday...Some showers and thunderstorms.

&&

.MARINE...
Winds and seas will stay below Small Craft Advisory Conditions
through Sunday.

Marine fog has developed, especially closer to the coast, and
is expected to persist through at least Sunday morning.
Therefore, have issued a dense fog advisory on the Atlantic
coastal waters. It may need to be expanded to include the
Delaware Bay, and may need to be extended further out in time,
but low confidence on that at this time.

Outlook...

Sunday night through Tuesday night...Southerly winds remain near 10-
15 kts with some gusts near 20 kts possible at times. Seas 3-4 feet.
Some areas of fog possible at times, which may warrant additional
Marine Dense Fog Advisories for the Atlantic coastal waters.

Wednesday...Southerly winds increasing closer to 15-20 kts with some
gusts near 25 kts possible at times. Seas increasing to 4-5 feet.
Small Craft Advisory may ne needed. Showers and thunderstorms likely
developing by the evening hours. Areas of fog possible.

Rip Currents...

For Sunday and Monday, weak flow out of the south/southwest will
continue with wave heights only 1 to 3 feet and a 5 to 7 second
period, therefore a LOW risk for the development of dangerous rip
currents for both the New Jersey Shore and the Delaware Beaches.
Fog at times will cause reduced visibility along the shoreline
through Sunday afternoon.

For specific beach forecasts, visit weather.gov/beach/phi

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Heat Advisory from 10 AM Sunday to 6 AM EDT Tuesday for
     PAZ060>062-070-071-101>106.
     Excessive Heat Watch from Tuesday morning through Wednesday
     evening for PAZ060>062-070-071-101>106.
NJ...Heat Advisory from 10 AM Sunday to 6 AM EDT Tuesday for NJZ009-
     010-012-013-015>022-027.
     Excessive Heat Watch from Tuesday morning through Wednesday
     evening for NJZ007>010-012-013-015>022-027.
DE...Heat Advisory from 10 AM Sunday to 6 AM EDT Tuesday for DEZ001.
     Excessive Heat Watch from Tuesday morning through Wednesday
     evening for DEZ001.
     Excessive Heat Watch from Tuesday morning through Wednesday
     evening for DEZ002>004.
MD...Excessive Heat Watch from Tuesday morning through Wednesday
     evening for MDZ012-015-019-020.
MARINE...Dense Fog Advisory until noon EDT Sunday for ANZ450>455.

&&

$$

SYNOPSIS...DeSilva/Wunderlin
NEAR TERM...Johnson
SHORT TERM...Staarmann
LONG TERM...Staarmann
AVIATION...Gorse/Johnson/Staarmann
MARINE...Hoeflich/Johnson/Staarmann