786
FXUS61 KPHI 161950
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
350 PM EDT Sun Jun 16 2024

.SYNOPSIS...
High pressure at the surface becomes centered offshore of New
England through tonight and then gradually shifts southeastward
through this week. A warm front lifts across our area on Monday, and
with building high pressure aloft all of this week will result in a
prolonged stretch of increasingly hot and dry conditions.

&&

.NEAR TERM /THROUGH MONDAY/...
Surface 1027 mb high pressure over New England this afternoon
will migrate eastward to be just offshore by this evening.
Though we will still feel the influence of the surface high
offshore through tonight, a warm front from the southwest will
lift northwards and approach with time. This warm front is posed
to cross through our region sometime Monday morning.

A dry forecast with quiet conditions is on tap for the near term.
Though the warm front will approach with time tonight, no
precipitation is expected thanks to the strength and proximity of
the surface high.

A building 500mb ridge across the Southeast U.S. will continue to
expand north through the Mid Altantic on Monday. Maximum
temperatures on Monday will be in the 80s to around 90 with
dewpoints in the lower 60s.

&&

.SHORT TERM /MONDAY NIGHT THROUGH WEDNESDAY NIGHT/...
Excessive heat is expected to start building during this period.

A building 500 mb ridge across the Southeast U.S. will continue to
expand north through the Mid-Altantic through Wednesday as surface
high pressure shifts more offshore. As the warm air advection
increases, the heat will build with highs Tuesday and Wednesday into
the low to mid 90s for much of the area.

As the ridge builds, subsidence starts to become enhanced and
temperatures at the surface will therefore be hot. The placement of
the ridge aloft though and surface high pressure offshore but
lighter southerly flow does not favor much in the way of increasing
low-level moisture. This is seen in the dew point forecasts from
several of the models, which show dew point values on the lower side
Tuesday especially. Due to ample heating of the boundary layer and a
continued drying of the top layer of the soil tends to result in
lowered dew points especially during peak heating. Therefore, the
heat indices forecast for Tuesday afternoon and Wednesday afternoon
are lower than previously forecast. There also looks to be some
slower development in the building heat with at least some guidance
a little cooler both days. There should be some moisture recovery
Wednesday however even then some lowering of the dew points during
peak heating should occur given the significant heating of the
boundary layer. The southerly flow will continue and it will be on
the light side and therefore sea and bay breeze circulations are
expected. This will provide cooling closer to the coast, although it
is possible that these make their way well inland by later afternoon
or early evening.

This heat will be several consecutive days and factoring in more of
the impacts and not solely on the heat index criteria values, Heat
Risk for Pennsylvania and inland New Jersey is forecast to increase
to moderate with portions of the I-95 corridor increasing to Major.
The concern with the heat is that the temperatures will be building
throughout the week and externally, this is early in the season for
an extended heat event with increasing numbers of vulnerable
communities/events due to school years ending and a federal holiday
in the middle of the week. Despite some lowering of the temperatures
and dew points, some high temperature records Tuesday may be
challenged Tuesday. We continued the Heat Advisory for the I-95
corridor to the north and west, especially as the urban corridor
from Wilmington to Philadelphia to Trenton has a slightly lower heat
index criteria for an advisory (96 degrees). Wednesday may also not
be quite as hot given the ridge still building, however the highest
heat looks more likely once we get into the long term portion below.
Given that this is a multi-day heat event and while heat index
warning criteria may not occur, opted to go with an Excessive Heat
Watch starting Wednesday for the same areas of the Heat Advisory
based on increasing impacts (increasing Heat Risk categories) from a
prolonged excessive heat event. The initial core of the heat looks
to start across our northern to central areas before eventually
sliding southward some later in the week, therefore no heat related
headlines farther south at this time.

&&

.LONG TERM /THURSDAY THROUGH SUNDAY/...
Summary...Excessive heat probable with little to no rain chances.

The model guidance continues to show that the upper air pattern
supports an extended period of heat across much of the area (high
temperatures in the 90s). There`s been almost no change to the
forecast of an expansive and deep-layer ridge that will be centered
across our region with additional warming expected through Friday.

With warm air advection in place during the early potion of the week
and strengthening of the 500 mb high towards the end of the week,
the latter half of the week is more likely to be the hottest. The
heat dome will maintain surface high pressure to our east with the
surface flow more out of the south. The southerly flow looks to be
fairly light and the moisture advection is not much, and therefore
surface dew points should not be crazy with this heat. In addition,
many days of hot temperatures tends to lower the dew points some
during peak heating and this can be enhanced some by a dry to drying
ground. There will also be less relief at night as time goes on
especially in the urban centers with lows dropping only into the
70s. The southerly flow however should keep the coastal areas
noticeably cooler than inland, and if the flow is light enough
especially under this ridge then a sea/bay breeze many days could
get farther inland during the later afternoon hours. The ridge will
feature plenty of dry air aloft which tends to limit cloud
development and therefore rain especially given warm air aloft. As a
result, convective chances are little to none.

While the model guidance does differ some on the timing and extent,
the large ridge is forecast to start pushing southward some and
elongates more west to east Friday through Sunday as a trough in
south-central Canada starts to flatten the northern side of the
ridge. The main concern for this event is that the multi-day heat
tends to have more impacts especially in urban centers when combined
with less relief at night. Heat Risk impacts show Major or higher
impacts through next Sunday.

Confidence is high in widespread heat related impacts across the
region through the week, however as we get into the weekend the
ridge should be flattening some on the north side which lowers the
heights some and also the temperatures aloft. However despite this,
the potentially dangerous heat looks to continue and the Excessive
Heat Watch runs through Saturday. There may also be some opportunity
for a few showers or thunderstorms later in the weekend especially
for our northern zones which will be closer to the Canadian trough.
Given the presence of the ridge though, PoPs are only in the 20-30
percent range mainly across the northern into the central parts of
the region at this point.

&&

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

This afternoon...VFR. Winds becoming SE 5-10 kts. Moderate confidence.

Tonight...VFR. SE winds 5-10 kts becoming S overnight. High
confidence.

Monday...VFR. S wind 10 to 15 knots. Moderate confidence.

Outlook...

Monday night through Friday...VFR with no significant weather.

&&

.MARINE...
No marine headlines expected through Monday. SE winds 8-12 kts
this afternoon, S winds 5-10 kts tonight, and S winds 10 to 20
knots Monday. Seas 2-4 feet.

Outlook...

Monday night through Friday...The conditions are anticipated to be
below Small Craft Advisory criteria.

Rip Currents...

With lighter winds around 10 mph and lower waves of 2-3 feet
this afternoon, there is a LOW risk for the development of
dangerous rip currents for both the New Jersey Shore and
Delaware beaches.

On Monday, winds will increase back to 10-15 mph but look to be
out of the south. This wind direction would result in largely
shore parallel winds except for the more south facing beaches in
Cape May and Atlantic Counties. Additionally, waves in the surf
zone are forecast to remain 2-3 feet with a short-medium period
swell. Combining all these factors together, we are forecasting
a LOW risk for the development of rip currents for now.

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

&&

.CLIMATE...
Record high temperatures Tuesday.

Location          Record High (6/18)

Philadelphia, PA      96/1957
Allentown, PA         95/2018
Reading, PA           97/1957
Mount Pocono, PA      88/1957
Trenton, NJ           96/1957
AC Airport, NJ        95/2014
AC Marina, NJ         94/2014
Wilmington, DE        95/1957
Georgetown, DE        97/2014

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Heat Advisory from 10 AM Tuesday to 6 AM EDT Wednesday for
     PAZ054-055-060>062-070-071-101>106.
     Excessive Heat Watch from Wednesday morning through Saturday
     evening for PAZ054-055-060>062-070-071-101>106.
NJ...Heat Advisory from 10 AM Tuesday to 6 AM EDT Wednesday for
     NJZ001-007>010-012-013-015>020-027.
     Excessive Heat Watch from Wednesday morning through Saturday
     evening for NJZ001-007>010-012-013-015>020-027.
DE...Heat Advisory from 10 AM Tuesday to 6 AM EDT Wednesday for
     DEZ001.
     Excessive Heat Watch from Wednesday morning through Saturday
     evening for DEZ001.
MD...None.
MARINE...None.

&&

$$

SYNOPSIS...Deal/Gorse
NEAR TERM...Franklin/Wunderlin
SHORT TERM...Deal/Gorse
LONG TERM...Deal/Gorse
AVIATION...Franklin/Gorse/Wunderlin
MARINE...Franklin/Gorse/Wunderlin
CLIMATE...PHI