Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 210736

Area Forecast Discussion
National Weather Service Mount Holly NJ
336 AM EDT Fri Jul 21 2017

A weak frontal boundary will move off the coast later today, with
high pressure from the midwest building in behind it through early
Saturday. Another front and associated low pressure will cross the
area Saturday night through Monday. High pressure is expected to
move in on Tuesday and remain into the latter part of next week.

Lee troughing will persist in vicinity of the I-95 corridor, but
there is a noticeable lack of large-scale lifting mechanisms across
the northern Mid-Atlantic today, particularly as the perturbation
moving through last evening leaves subtle transient shortwave
ridging in its wake. With the region remaining to the south of the
stronger west-northwesterly midlevel flow, hot and humid air will
remain entrenched. However, mixing should be somewhat more effective
today in drying the boundary layer (given influence from the
aforementioned shortwave ridging). Surface dew points will likely be
a degree or two cooler yesterday, though my suspicion is that
temperatures will be quite similar. Moreover, with effective mixing
may come a somewhat more pronounced cloud deck this afternoon, which
should prevent temperatures from increasing any more than those seen

Given these factors, I do not think a heat advisory is required
surrounding the excessive heat warning, particularly since observed
heat indices yesterday marginally reached criteria, at best. Of
course, it will still be hot and humid, and heat stress/exhaustion
is quite possible from prolonged outdoor exposure. Given the
duration of the current hot spell, the excessive heat warning for
the urban corridor is still warranted and did not change, though
afternoon heat indices may not exceed 100 degrees in most locations.

Convection cannot be entirely ruled out today, but think the best
chances are in the Delmarva Peninsula, in closer proximity to a
subtle perturbation approaching from the west late this afternoon.
Think most of the region will remain dry, though.

Lastly, with fairly light winds today, sea breeze penetration inland
may be somewhat more pronounced, which poses a risk for some max
temperature errors in coastal portions of New Jersey.


The main concern tonight is precipitation chances, primarily in
Delmarva. A perturbation will move through the southern Mid-Atlantic
during this period, and subtle large-scale ascent will occur in
advance. With the proximity of surface troughing aiding in (albeit
weak) near-surface convergence and potential for buoyant parcels
within the residual mixed layer, showers and/or storms cannot be
ruled out entirely in this region overnight. The 00Z NAM Nest, with
some support from the WRF-ARW/NMM simulations, is indicative of
scattered precipitation breaking out after midnight in this region.
Kept the highest PoPs during this time frame, though think an
isolated storm is possible during the evening as well. The rest of
the region is expected to be dry, with no apparent lifting
mechanisms aiding in the development of convection.

Another sultry night is anticipated, with temperatures and dew
points mainly in the 70s (60s north of I-78). Some spotty fog may
occur in favored locations, but with the lack of rain expected
today, not confident enough to include in the forecast at this

A mid-level zonal flow over the northern portion of the U.S. early
Saturday will eventually yield to a more trough-like pattern Monday
through Tuesday, then evolve into a west-northwest flow for the
middle and latter portions of the week. This means the heat wave
will continue into Monday, but with temperatures more in line with
normal for this time of year beginning Tuesday.

At the surface, medium range models indicate a warm front will lift
north through the area late Saturday, bringing with it increasing
humidity and the chance of showers and thunderstorms later Saturday
into Sunday. SPC has placed a large portion of the CWA under a
slight risk for severe weather during this time period. A strong
cold front is then expected to move through the area Monday night
into Tuesday, along with showers and thunderstorms. High pressure
behind this front will bring with it more seasonal temperatures for
this time of year and lower humidities for the latter half of next

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

Mostly VFR through the period, though there may be some
patchy/transient visibility restrictions through early morning in
favored more rural valley locations and where rain fell last
evening. Expecting SCT-BKN cumulus around 5000-8000 feet today with
winds transitioning from northwest to west or southwest as the day
wears on. Speeds should remain around or below 10 kts. No storms are
expected through the TAF period.

Saturday morning...VFR expected.
Saturday afternoon thru Monday...Mostly VFR. Sct showers and
tstms with lower conditions possible.
Tue and Wed...Mostly VFR.

Looks like a quiet day for the coastal waters, with light and
variable winds this morning becoming south to southwest 5-15 kts
this afternoon and this evening. There may be a chance of showers or
storms tonight, primarily for Delaware Bay and the Delaware coastal
waters, but confidence is rather low.  Seas should generally be
three feet or less today.

Sub-SCA conditions overall. The main hazard will be sct tstms
mainly between Saturday afternoon and Monday. Locally higher
winds and seas with tstms. Bring your NOAA weather radio along
to get alerts.


The primary wave period for the past several hours has been around 7
seconds. However, there is some concern that an underlying 10-12
second period will become more dominant during the afternoon today.
Should this occur, the rip current risk may become moderate.
Observations will be monitored closely the next few hours to see if
an upgrade is required, but for now, the forecast is for a low risk
of rip currents today.

The KDOV dew point readings are unrepresentative of the area
and should not be used when diagnosing humidity/heat indices and
meteorological analysis of meteorological items which use
dewpoint in a calculation.

PA...Excessive Heat Warning until 8 PM EDT this evening for PAZ070-
NJ...Excessive Heat Warning until 8 PM EDT this evening for NJZ015-
DE...Excessive Heat Warning until 8 PM EDT this evening for DEZ001.


Near Term...CMS
Short Term...CMS
Long Term...Miketta
Equipment...CMS is the U.S. government's official web portal to all federal, state and local government web resources and services.