Excessive Heat in Delaware, August 2, 2024

Area Forecast Discussion
National Weather Service Mount Holly NJ
912 AM EDT Fri Aug 2 2024

.SYNOPSIS...
Hot and humid conditions expected through the weekend high
pressure eventually gives way to a slow moving cold front over
the weekend. High pressure builds again to start the week before
another cold front then settles across our region Wednesday
into Thursday.

&&

.NEAR TERM /THROUGH TONIGHT/...
Latest radar imagery shows a few showers/thunderstorms across
northern NJ. But overall, activity should remain on the low-
end for the rest of the morning.

NJDEP issued an Air Quality Alert a short time ago for northern
NJ, so we pushed an AQA.

Another dangerously hot and humid day in store for the greater
Delaware Valley. Excessive Heat Warnings are now in effect for
the urban corridor and down towards the Delmarva peninsula.
Heat Advisories have also been expanded to include Sussex and
Cape May counties in NJ. While overall air temperatures may be a
degree or two "cooler" than on Thursday, dew points are expected
to be higher, pushing heat indicies into dangerous territory.
Unfortunately, the timing of clouds and storms today do not look
to arrive in time to offer any relief during the hottest part of
the day. Heat safety protocols should be urgently followed.

In terms of heavy rain and thunderstorms, an approaching
amplified wave will bring increases chances for scattered to
numerous showers and thunderstorms Friday afternoon. An upper-
level trough will be deepening to our west, making the overall
upper- level synoptic setup more favorable. This will also
increase southwesterly flow aloft, drawing even more moisture
into the region. PWATs are forecasted to increase to 2-2.5
inches across much of the area. With the more favorable dynamics
and higher PWATs, there will be an increased risk for localized
flooding with heavy downpours from any thunderstorms. However,
model soundings show that storm motions should be sufficient to
keep storms from sitting over one place too long. Additionally,
some CAM guidance indicates that the convection may take the
form of a semi-organized cluster or line along a surface trough
moving through the region. If this scenario occurs, it would
increase the risk for damaging wind gusts across the area. Model
soundings also indicate an inverted V with steep low- level
lapse rates, so damaging wind gusts from downbursts will be a
threat even if convection is more scattered. As a result, a
Marginal Risk (1 of 5) for severe weather is in place across the
area and a Marginal (1 of 4) to Slight risk and (2 of 4) for
excessive rainfall mainly for the area near and northwest of the
I-95 corridor, with the Slight risk limited, mainly near and
north of the I-78 corridor.

&&

.SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/...
An upper-level trough shifts across the Great Lakes and Ohio Valley
Friday night into Saturday, then gets to our area Saturday night.
This system will bring an increase in showers and thunderstorms
within a very moist air mass. Heat will again be the concern
initially as the forecast is for temps to quickly warm into the
upper 80s to low 90s with high humidity. The question will be how
high do the temps actually get with respect to the need for a heat
advisory again for Saturday. Looking at the ensemble guidance there
are roughly two clusters in the guidance. The first cluster has more
of a weaker trough, with cooler temps as the southerly flow isn't
quite as strong.  The second cluster within the ensembles shows a
sharper trough with enhanced southerly winds. In the second
scenario, temps from the GEFS suggest that 2m temps have a 75%
chance of exceeding 90 degrees whereas in more of the weaker wave
system from the ENS and the GEPS, the more likely scenario is
topping out in the upper 80s.

Using the location of the current trough as a proxy for tomorrow,
the GFS has initialized the wave a bit weaker today and the EC
matches quite well. So I've trended the forecast more towards the
ENS/GEPS solution for tomorrow but we'll continue to monitor.

Concerning the storms for tomorrow, with a trend towards the
EPS/GEPS solution, I've opted to keep that going for convection.
That leads to strong moisture advection over the region ample
instability. Forcing will be sufficient and lapse rates in the
low levels while not as steep as it could be, will still be
ample strong enough to generate downburst potential through the
day. In addition, with pwats forecast to be in the 90th
percentile, and warm cloud depths close to 12,000 ft, I
anticipate the rainstorms to be highly efficient and have the
potential to cause flash flooding.

For Sunday both the heat and convective potential will be more mild
as cloud cover and lower temps should be prevail however as the
previous shift mentioned, with high humidity it will still feel
fairly unpleasant outside.

&&

.LONG TERM /MONDAY THROUGH THURSDAY/...
Summary...The hot and humid conditions ease during the week, however
chances for some showers and thunderstorms remain.

Synoptic Overview...An upper-level trough across eastern Canada will
extend down across the Great Lakes and Northeast with our area being
on the southerly periphery. This trough may shift far enough
northeast Wednesday and Thursday to result in more of a zonal flow,
however a west-northwest flow aloft is forecast to be in place. As
shortwaves slide east or east-southeast within the mid level flow,
at the surface a front will try and settle into our area from the
north Tuesday although this will be more parallel to the flow aloft.
A weak low may track nearby Wednesday into Thursday, pulling a cold
front across our area.

For Monday and Tuesday...As our region remains along the southern
periphery of an upper-level trough, a weak cold front nearly stalls
just offshore. The cooling effect from this trough looks delayed
therefore Monday is forecast to be hot with highs expected to be in
the upper 80s to low 90s for much of the area. Some drier air
however is forecast to advect in from northwest to southeast and
therefore while still humid the heat indices are forecast to be
below Heat Advisory criteria. The gradual drying may limit
convection especially as we may be between weak systems, however
some isolated mainly diurnally driven convection cannot be ruled out
mainly north/west of I-95. Some more influence of the trough is
forecast on Tuesday along with a cold front approaching from the
west. Some additional cloudiness along with some cooling aloft
should translate to lower temperatures. Dew points are forecast to
be high enough for a humid feel for much of the area, however not to
excessive levels. The overall forcing looks week given not a strong
look to the trough aloft and therefore the chances for convection
Tuesday also looks to be isolated at this point.

For Wednesday and Thursday...The flow aloft may turn a bit more
zonal, and therefore a cold front moving through may tend to do so
on the slower side. Some shortwaves however embedded within this
flow may enhance the local lift at least some and provide the
chances for some showers and thunderstorms. Given the uncertainty
with the details, kept PoPs in the lower chance range (30 percent).
High temperatures are currently forecast to be at or even a little
below average.

&&

.AVIATION /13Z FRIDAY THROUGH TUESDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Today...Prevailing VFR conditions. Chances for showers and
storms increase, particularly after 18Z, which may result in
times of sub-VFR conditions. Southerly winds around 5-10 knots,
higher gusts possible in the vicinity of any thunderstorms. High
confidence in prevailing conditions.

Tonight...Organized showers and thunderstorms early in the
evening may keep terminals sub-VFR, but even as storms move out,
fog and low stratus look to keep many terminals in sub-VFR
territory, especially anywhere that saw rainfall earlier in the
day. Gusty winds possible in and around TSRA, but winds should
calm to less than 5kts overnight once storms depart.

Outlook...

Saturday through Sunday...Scattered showers with embedded
thunderstorms will have the potential to impact the terminals
resulting in heavy rain, reduced visibilities, and locally
strong wind gusts on Saturday. Showers will still be possible on
Sunday although the intensity should be weaker.

Monday and Tuesday...Mostly VFR with a chance for a few mostly
afternoon and evening showers and thunderstorms.

&&

.MARINE...
Conditions are forecast to remain below advisory conditions
Friday. Southerly winds increasing to 10-15 kts, with some
gusts near 20 kts possible in the afternoon and evening. Seas
2-3 feet. Thunderstorms with gusty winds possible in the late
afternoon and into the nighttime hours. Otherwise, fair weather.

Outlook...

Saturday through Tuesday...The conditions are anticipated to be
mostly below Small Craft Advisory criteria. However, the seas
may build to 5 feet on the ocean zones late Saturday into
Sunday. Some stronger thunderstorms are possible Saturday and
Sunday afternoons and evenings.

Rip Currents...

For Friday, there is a LOW risk for the development of dangerous rip
currents for both the Jersey Shore and for the Delaware Beaches.
South winds will average 10 to 15 mph. Wave heights in the surf zone
will be 2 feet with a period around 7 seconds.

For Saturday, South to southwest winds increase to 15 to 20 mph with
waves in the surf zone increasing to around 3 feet. We will also be
within a day of the new Moon. For these reasons, will forecast a
MODERATE risk for the development of dangerous rip currents for the
NJ shore with the exception of Monmouth County where the risk will
be LOW. The Delaware Beaches will also have a LOW risk for
dangerous rip currents.

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

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Excessive Heat Warning until 8 PM EDT this evening for PAZ070-
     071-104-106.
     Heat Advisory until 8 PM EDT this evening for PAZ060>062-
     101>103-105.
NJ...Excessive Heat Warning until 8 PM EDT this evening for NJZ010-
     012-013-015-017>019.
     Excessive Heat Warning until 8 PM EDT this evening for NJZ016-
     021.
     Heat Advisory until 8 PM EDT this evening for NJZ001-007>009-
     020-027.
     Air Quality Alert until 11 PM EDT this evening for NJZ001-
     007>010.
     Heat Advisory until 8 PM EDT this evening for NJZ022-023.
DE...Excessive Heat Warning until 8 PM EDT this evening for DEZ001.
     Excessive Heat Warning until 8 PM EDT this evening for DEZ002-
     003.
MD...Excessive Heat Warning until 8 PM EDT this evening for MDZ012-
     015-019-020.
MARINE...None.

&&

SYNOPSIS...Deal/Gorse
NEAR TERM...MJL/Kruzdlo
SHORT TERM...Deal
LONG TERM...Deal/Gorse
AVIATION...Deal/MJL/Kruzdlo
MARINE...Deal/MJL/Kruzdlo