Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 270840

Area Forecast Discussion
National Weather Service Mount Holly NJ
440 AM EDT Tue Jul 27 2021

Weak high pressure will shift south of the region today. A
couple of weak disturbances will then cross the region between
tonight and Wednesday. A stronger cold front will move through
by late Thursday. Strong high pressure then follows for Friday
and Saturday. Another cold front may approach by the end of the
weekend or early next week.


The weak cold front which we`ve dealt with for the past couple of
days is finally settling offshore this morning. Considerably drier
air has filtered into the northwestern zones, while closer to the
coast it is still more on the humid side. Patchy fog is possible
this morning, mainly in the New Jersey and Delmarva zones where
moisture is better. This will be most likely between sunrise and
8AM, but is unlikely to be very widespread or dense, and will mix
out quickly this morning.

Otherwise, today will be very much on the warm side as weak high
pressure slides to our south. Plentiful sunshine is expected, along
with deep mixing and some downsloping effects in a light west to
northwest flow regime around the north side of the high. This
combined with a seasonably warm air mass in place will yield highs
mainly in the lower 90s, a degree or two warmer than yesterday in
most areas. With the deep mixing, dew points should drop out
significantly, especially to the northwest, which will at least take
the humidity out of the heat.

Mainly a dry forecast through tonight, with a couple of caveats.
First, a sea breeze is likely to develop today. With the heating of
the day and the more humid air lingering near the coast, can`t rule
out a couple of showers or storms developing in the zone of
convergence between the sea breeze and the light westerly synoptic
flow. Have included some slgt chc PoPs over portions of southeast
New Jersey and Delmarva to account for that, but definitely not
expecting widespread activity.

Next, we will watch a mid-level trough and associated cold front
approach this evening and tonight. This is a fairly vigorous
disturbance, but is on track to deliver only a glancing blow to the
northern portion of the area. Ahead of it, thunderstorm development
is likely to our northwest later today, possibly taking the form of
an MCS or a couple of broken lines. CAM guidance suggests the bulk
of this activity will remain north of us, where forcing and shear
are better. However, convection may clip our northern zones as it
approaches the area heading into the evening hours, and can`t rule
out a couple more showers or storms forming on the outflow overnight
a little further south. The SPC SWODY1 has a marginal risk just
barely clipping our northernmost zones, and feel this adequately
describes the situation. With a rather dry air mass in place and the
timing not being very favorable, this system is unlikely to be a big
deal for us. Particularly for the urban corridor and areas south,
any rain tonight is unlikely. Overnight lows should fall to near
seasonal averages, mid 60s to near 70.


A very complicated forecast exists for Wednesday through
Thursday with many mesoscale features in play that will likely
drive how and where convection develops. As was the case last
week, there will likely be ongoing convection across upstate NY
Tuesday evening which will send an outflow south through the
region Wednesday morning. This will turn the winds from the
north, while the cold front in actuality will likely be over
northern NJ. During the afternoon hours, surface high pressure
over New England will head east, which will veer the winds from
the east over the coastal plain of NJ with northern winds
remaining from the outflow over eastern PA. Meanwhile, moisture
values will be sufficient, but not overly impressive with PWATs
around 1.5". Mid-level dry air will be in particular harsh to
any updrafts with 500/300 MB RH forecast to be around 20%. Still
though, there does appear to be sufficient moisture and surface
convergence across central NJ and south for a slight increase
in PoPs. High temperatures will also be tricky Wednesday as the
low level front will likely be north of most of the region.
These type of regimes generally support an over performance in
high temperatures. Due to this, have raised highs to near 90
degrees for Wednesday.

Wednesday night, a weak perturbation on the northeast side of
the mid-level ridge located across the central United States
will likely help to initiate convection over MN/ WI. This
convection is currently forecast to organize into an MCS and
move southeast. How well organized and if the MCS forms is still
in question, but the overall thinking is that if an MCS does
form it will move around the ridge and head towards VA or just
south of the region. As of current there are chance PoPs in the
forecast, but if an MCS forms and goes just south of the region,
this could actually hinder convective development on the
northern side of the MCS. If the MCS doesn`t materialize (or
tracks further north across PA/ NJ/ MD/ DE), the convective
potential looks higher. Either way, the confidence here remains
very low.


An amplified ridging pattern over the central United States with
a deepening upper-level trough across the eastern United States
will persist through the weekend bringing an progressive weather
pattern to the region. The primary items of interest for the
long term will be a cold front on Friday and another cold front

On Friday, a cold front will be crossing the region with winds
turning from the northwest. CAA will again be seasonably strong
with the cold front allowing for some transient wind gusts
Friday afternoon. There is some discrepancy on the timing of
the frontal passage, with some guidance showing a slower frontal
passage. This indeed could happen, and have trended PoPs this
direction. Saturday morning, surface high pressure will build in
from the west with much drier air moving overhead. This will
lead to pleasant conditions, with temperatures below normal for
this time of year. High pressure will quickly move offshore,
allowing for a quick return of higher PWAT air for Sunday.

Sunday into Monday, another closed low embedded in the longwave
trough axis will head southeast towards southern Quebec. Latest
EOF guidance indicates there is widening uncertainty in the
strength and position of this low, with the EPS appearing to be
under-dispersive here compered to its GEFS and GEPS counterparts.
The latest deterministic GFS also brings a strong cold front
through the region Monday, which this too appears to be an
outlier compared to the ensemble envelope. Have kept the
forecast with the blend here given the uncertainty.


The following discussion is for KPHL, KPNE, KTTN, KABE,
KRDG, KILG, KMIV, KACY and surrounding areas.

Through 12z... Mainly VFR. Patchy fog development is possible near
and just after sunrise, mainly near MIV and ACY. This could bring
some localized MVFR VSBYs. Winds light and variable or calm. High
confidence overall, moderate confidence regarding the fog.

Today... Mainly VFR with any fog lifting by 13z. An isolated
afternoon shower or storm is possible near MIV and ACY, but coverage
is expected to be too low to include in the TAF. Winds mostly light
and variable this morning, becoming northwest by late this morning
then west-southwest this afternoon at 5 to 8 kt. Sea breeze
development is likely this afternoon, shifting the wind to south or
south-southeast near MIV and especially ACY. High confidence
overall, except moderate confidence in winds.

Tonight... Mainly VFR. Showers and isolated thunder are possible
especially near RDG and ABE, but there is uncertainty regarding how
prevalent they will be. Winds light and variable or calm. Moderate


Wednesday...Prevailing VFR. An outflow boundary will dive south
in the morning hours from Tuesday evening convection over
upstate NY. As this occurs winds will turn from the northeast.
Wednesday afternoon, winds will veer from the east. As this
occurs, high res guidance indicates showers and thunderstorms
forming along the remnant outflow. These type of events are low
predictability, but has the potential to be a high impact to
aviation concerns. Low confidence.

Thursday...Initially VFR with showers and thunderstorms developing
during the afternoon and/or evening. Winds southwesterly around
5-10 kts. Low confidence.

Friday...VFR. Winds northwesterly around 10-15 kts. High confidence.

Saturday...VFR. High pressure overhead with winds light and
variable. High confidence.


Through tonight... Sub-SCA conditions are expected. Winds mostly
light and variable this morning. By afternoon, a south or south-
southeast winds of 10 to 15 kt will develop, shifting to southwest
overnight. Seas 2 to 3 ft.


Wednesday... Sub-advisory conditions. Northerly winds Wednesday
morning will veer from the east around 10 kts by Wednesday
afternoon. Seas 2 to 4 feet.

Thursday... Winds will be out of the south, and then out of the
southwest around 10 to 15 kts ahead of an approaching cold
front. Showers and thunderstorms will be possible over all of
the waters. Seas 2 to 4 feet.

Friday... A cold front will cross the waters Friday evening with
winds turning from the northwest around 15 kts. Wind gusts are
currently forecast to be just below SCA criteria late Friday
evening. Seas 3 to 5 feet.

Saturday... Surface high pressure moves overhead with light
northerly winds turning from the south. Seas 2 to 4 feet.

Rip Currents...
With generally light winds and seas, the risk for the formation of
dangerous rip currents is expected to remain low for New Jersey and
Delaware through at least Wednesday. However, for today, a fairly
long (~10 second) swell period becomes dominant this afternoon. With
a late afternoon low tide, the rip current risk could increase a bit
this afternoon. Seas will still likely be too low to warrant a
moderate risk, but conditions will be monitored.




Near Term...O`Brien
Short Term...Haines
Long Term...Haines/ JL
Marine...Haines/O`Brien is the U.S. government's official web portal to all federal, state and local government web resources and services.