NWS Philadelphia/Mount Holly wrote its last area forecast discussion before the first event at Sep 8, 11:31 AM EST, 3 hours 53 minutes earlier.
Area forecast discussion, Sep 8, 11:31 AM EST, 3 hours 53 minutes before
Area Forecast Discussion
National Weather Service Mount Holly NJ
1231 PM EDT Fri Sep 8 2023
.SYNOPSIS...
A weak cold front will become nearly stationary across eastern
Pennsylvania today and tonight. The cold front is then forecast
to slowly shift eastward over the weekend before finally moving
offshore during early next week. Another cold front looks to
arrive later Wednesday into Thursday.
&&
.NEAR TERM /THROUGH TONIGHT/...
A mid level trough will extend from the Great Lakes to the
southeastern states today and tonight, with a mid level ridge
remaining over the western North Atlantic. The pattern will
likely keep us under a southwesterly flow aloft.
Short wave energy traveling in the mid level flow is forecast
to aid in the development of afternoon and evening showers and
thunderstorms again today. While the overall synoptic setup
across the Northeast and Mid-Atlantic is fairly similar to
yesterday, and the Storm Prediction Center has expanded the
'Slight Risk' of severe thunderstorms southeastward across the
I-95 corridor, there are some notable differences in the forcing
mechanisms and thermodynamic setup. A surface front draped down
the Appalachians has actually started to drift close to our
northwestern zones, with notably cooler conditions there
compared to yesterday. Temperatures around midday are running
6-10 degrees lower there than midday on Thursday, and 2 to 5
degrees lower across the rest of our region. We still have a
very humid airmass in place with dewpoints into the 70s,
highest south and east of the Fall Line, and that is
contributing to CAPE of 2500-4000 j/kg there, with quickly
waning convective inhibition. There is still some decent deep
layer shear around 20-25 kt, however this is mainly over our
northwest zones, not co-located with the higher CAPE like we had
in place there yesterday. Mid-level winds are somewhat weaker,
owing in part to a weaker shortwave passing by and ridging to
our east actually trying to build back slightly toward the
coast. That portends a weaker forcing mechanism, especially this
afternoon, which suggests that convection will not be very
focused at least initially.
If visible satellite is any indication, thunderstorms may pop
up over New Jersey around Burlington or Ocean Counties, possibly
extending down into southern Delaware. The hi-res CAMs suggest
that may be where we see convection limited to this afternoon
into early evening, perhaps building into northern New Jersey.
This evening, CAMs suggest that larger scale forcing from
shortwaves lifting northward along the western edge of our
region will trigger more numerous showers and thunderstorms over
similar northwestern portions of our region that saw activity
Thursday evening, moving in from Berks County to the Lehigh
Valley around 7 or 8 PM and then gradually progressing toward
I-95 tonight, while weakening. While any of this activity will
bear watching for severe weather, with the forcing and best
instability not lining up like yesterday, the overall severe
threat may be more limited. Storms this afternoon closer toward
the coast may carry either marginally severe hail or strong
winds, while the threat with storms this evening coming in from
the northwest will be more limited to damaging winds. Storms may
produce heavy rain rates, but all indications are that they will
also tend to not linger in any one area for very long, so am not
overly concerned about flash flooding today or tonight.
Otherwise some isolated showers and thunderstorms may linger
later tonight, but will diminish in coverage and intensity.
Areas of fog are possible, especially at locations that receive
rainfall, with a cloudier start to the day on Saturday for most
of our region.
&&
.SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/...
Temperatures trending downward however it will remain humid,
with at least some convection around especially each afternoon
and evening.
The temperatures are expected to be cooler Saturday however it
will still be humid with some showers and thunderstorms. As an
upper- level trough remains to our west, a weak cold front will
be stalling across our region. The coverage of convection is
less certain Saturday as there could be a lull in the larger
scale forcing despite a surface front in the area. Given
temperatures forecast to be cooler, the magnitude of the
instability is also forecast to be lower. In addition, the shear
looks a little less however ample boundary layer heating should
lead to steepening low-level lapse rates once again. This would
tend to still support some convectively generated stronger
surface wind gusts if enough convection develops and especially
if it organizes some. In addition, the precipitable water values
will be in the 1.5-2.0 inches range Saturday and therefore
convection will produce locally heavy rain. While some much
needed rain has recently fallen across parts of eastern
Pennsylvania and northern New Jersey, the entire region can
certainly use the rain, therefore the overall flash flooding
risk should be limited outside of the more urban areas.
As we go through Sunday, the axis of a gradually weakening
upper- level trough is forecast to arrive. This will keep a weak
surface front draped nearly north to south across our area.
Given some cooling aloft and some additional forcing for ascent
with the trough aloft arriving, some increase in convective
development should occur especially in the afternoon and early
evening hours. While there will be cooler temperatures it will
remain humid, however the instability will be held down some
which should mostly keep the thunderstorms intensity in check.
Locally very heavy rain will be probable once again with high
precipitable water values and potentially slower storm motions.
&&
.LONG TERM /MONDAY THROUGH THURSDAY/...
Summary...Cooler temperatures however a slow return to lower
dew points. A continuation for some possible showers and
thunderstorms at times.
Synoptic Overview...The axis of an upper-level trough is
forecast to work across our area during Monday as it weakens. A
much stronger upper-level trough is forecast to move across the
Great Lakes region and adjacent Canada during the first half of
next week before gradually shifting into the Northeast. The
timing of this feature will depend on the amplitude of it and if
it closes off or not. At the surface, a weak cold front is
forecast to finally get pushed offshore Monday. The next cold
front looks to arrive later Wednesday into Thursday.
For Monday...The axis of a gradually weakening upper-level
trough is forecast to cross our region. This will take a weak
cold front offshore. Given some cooling aloft and some forcing
for ascent with the weakening trough aloft, some convective
development should occur especially in the afternoon and early
evening hours. While there will be cooler temperatures it will
remain humid, however the instability will be held down some
which should lower the severe thunderstorm risk. Locally very
heavy rain will be probable due to high precipitable water
values and potentially slower storm motions, however the region
needs the rain and this could limit the flash flooding threat.
For Tuesday through Thursday...Some drier air should work in
during Tuesday in the wake of the weak cold front, however there
is no significant push of drier low-level air and therefore a
humid air mass should still be in place. This may provide a
window for no convective chances though, however the next upper-
level trough working across the Great Lakes and toward the
Northeast will drive a cold front into our area later Wednesday
into Thursday. The details with this front and the extent of
convection will depend on the amplitude and timing of the trough
and associated surface cold front. While it remains humid for a
while, the dew points should lower some during peak heating
each day, although a more notable drop looks to occur late in
the week.
We continue to monitor the track forecast for Hurricane Lee.
The exact track remains uncertain especially tied to when it
makes a northward turn. Therefore it is way to soon to get
specific on any possible direct impacts. However, large swells
and the development of dangerous rip currents into our area will
become more likely during next week.
&&
.AVIATION /16Z FRIDAY THROUGH TUESDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.
Rest of Today...Mainly VFR. A chance of showers and
thunderstorms, mainly after 21Z. Storms may be limited around
Delaware and NJ this afternoon, then focus on eastern
Pennsylvania early this evening before possibly reaching I-95
later in the evening. Wind favoring the southwest and south 5
to 10 knots. Medium confidence.
Tonight...A chance of showers and thunderstorms, diminishing in
coverage after midnight. Some low clouds and fog are possible,
reducing conditions to MVFR or IFR, mainly north and west of
I-95. Variable wind 5 knots or less. Medium confidence.
Outlook...
Saturday and Sunday...VFR much of the time, however some
showers and thunderstorms around especially each afternoon and
evening can lead to local periods of sub-VFR conditions.
Southerly winds 5-10 knots each day, diminishing some each
night. Low confidence.
Monday...Times of sub-VFR conditions probable especially with
some showers and thunderstorms. Southwest to westerly winds 5-10
knots, becoming northwest 5 knots or less at night. Low
confidence.
Tuesday...Mostly VFR. A low chances for some showers and
thunderstorms. Northerly winds 5-10 knots, becoming
southeasterly. Low confidence.
&&
.MARINE...
A southerly wind 10 to 15 knots today is forecast to decrease
to 5 to 10 knots tonight. Waves on our ocean waters are expected
to be 2 to 3 feet, with waves on Delaware Bay at 2 feet or
less.
Outlook...
Saturday through Monday...The conditions are anticipated to
remain below Small Craft Advisory criteria. Some thunderstorms
possible mostly each afternoon and evening, however the greater
chances look to stay inland.
Tuesday...Seas may start to build to near 5 feet on our ocean
zones, otherwise the conditions should be below Small Craft
Advisory criteria.
Rip Currents...
A southerly wind 10 to 15 MPH is expected for today along the
coasts of Delaware and New Jersey. Breaking waves are forecast
to be 1 to 2 feet with an underlying long period easterly swell
(around 11 or 12 seconds). There is a LOW risk of rip currents
for today.
A southerly wind around 10 MPH is anticipated for Saturday with
breaking waves of 1 to 2 feet. An underlying long period
easterly swell (around 10 seconds) is forecast. Being that
conditions will likely be similar to those of today, we will
keep the mention of a LOW risk of rip currents for Saturday.
&&
.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...None.
&&
SYNOPSIS...Gorse
NEAR TERM...Dodd/Iovino
SHORT TERM...Gorse
LONG TERM...Gorse
AVIATION...Dodd/Gorse/Iovino
MARINE...Gorse/Iovino