National Weather Service Weather Forecast Office

• Home
• Site Map
• News
• Organization
• Search for: NWS All NOAA 

-- Discussion containing changed information from previous version are highlighted. --
000
FXUS61 KPHI 161313
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
913 AM EDT Tue Jul 16 2019

.SYNOPSIS...
High pressure over the area continues to move offshore this
morning, and then a warm front will lift through the region this
afternoon. The remnants of Barry will pass through the region
late Wednesday through Thursday. Bermuda high pressure becomes
entrenched off the Southeast coast as several weak cold fronts
pass through the East Coast into this weekend. A stronger cold
front approaches early in the new week.
&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...

910 AM Update...Based on latest obs and trends, made some
slight adjustments to T,Td grids for this morning but otherwise
no changes at this time.

Previous Discussion...Surface high pressure currently just off
the VA Coast will continue to move away from the area today with
increasing southwesterly low- lvl flow behind it. At mid-lvls a
low- amplitude shortwave ridge will initially be centered
upstream of the area, but will be pushed over and then
eventually east of the area today/tonight in response to the
mid-lvl trough associated with Barry`s remnants. A weak warm
front associated with a low passing well to the north of the
area (over northern Quebec) will move across the area through
the day.

Deep moisture will increase considerably today with current
PWAT values near 1 inch increasing to around 2 inches by day`s
end (with afternoon dewpoints running about 10 degrees higher
than yesterday). Temperatures will also warm a few degrees (with
highs generally in the lower 90s outside of the Poconos and
immediate coast (will a sea breeze likely developing).
Unfortunately in terms of heat index today will actually likely
be the most "pleasant" day of the upcoming week or so.

Some diurnal showers/thunderstorms will likely develop towards
the middle of this afternoon over the higher terrain west of the
area and these will likely drift eastward towards the area.
These storms will be fairly weakly-forced given that the best
dynamic lift (right entrance region of upper jet, ejecting
energy ahead of Barry`s remnants) will remain to our north and
west. Consequently generally went with Slight Chc to low end Chc
PoPs focused on our western areas. Can`t rule out a microburst
or two given inverted-V profiles and but think storm coverage
and still modest instability will be limiting factors for any
more notable threat.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM WEDNESDAY/...
The threat for showers and thunderstorms over primarily the
western half of the area will continue into the evening.
Instability remains sufficiently elevated to support thunder for
a few hours after sunset but still expect limited coverage due
to lack of forcing (am discounting the NAM-nest at this point
which seems to be over- convecting). Overnight lows will be
considerably warmer than Monday morning, thanks to increased
dewpoints and mid-lvl cloud cover, with lows generally remaining
in the 70s.

&&

.LONG TERM /WEDNESDAY THROUGH MONDAY/...
An Excessive Heat Watch is now in effect from Wednesday through
Sunday.

A tropical airmass spreads into the Northeast and Mid-Atlantic
ahead of the remnant low of Barry on Wednesday. Bermuda high
pressure will be anchored offshore, and a hot and humid airmass
begins to spread into the region. Surface dewpoints will be in
the mid 70s or so, along with highs in the low to mid 90s. This
results in Heat Index values generally 100 to 105 for all but
the southern Poconos.

With the airmass becoming quite unstable due to the heat and
humidity, and PWATs increasing to over 2 inches, showers and
thunderstorms will become likely as weak low pressure
associated with the remnants of Barry passes through northern
zones. Models are showing several bullseyes of high QPF, but are
not consistent among the models as to where the bullseyes line
up. A max of 2-3 inches of rain is possible Wednesday afternoon
and Wednesday night, but right now confidence is low as to
where. Flooding is possible.

Thursday will remain humid with dewpoints well in the 70s, but
temperatures may not be quite as high due to abundant cloud
cover and rainfall associated with the remnants of Barry passing
through. Highs will generally be in the 80s to around 90 in
Philadelphia. Thursday will feature the lowest Heat Indices of
the stretch, but will still be around 100 in Philadelphia, and
from 100 to 105 in Delmarva.

Another round of moderate to heavy rain is possible with the
passage of the low on Thursday, with another round of 1-2 inches
of rain and flooding are possible.

From there, a prolonged period of excessively hot and humid air
will be in place from at least Friday through Sunday. Highs will
be in the low to mid 90s Friday, then mid to upper 90s on
Saturday and Sunday. Surface dewpoints will be in the 70s,
however, there are some indications that dewpoints may mix out
in the afternoon hours. This results in dewpoints generally in
the mid to upper 70s at night and in the morning, and in the low
to mid 70s during the peak heating of the day. In terms of the
heat index, the max heat index values look to be up to 105 on
Friday and Sunday, and as high as 110 to 115 on Saturday.

Due to high confidence in this occurring, and the dangerous
impacts of a prolonged heat wave, will go ahead and issue a
rare prolonged Excessive Heat Watch that starts on Wednesday and
continues through Sunday for the Philadelphia area. For areas
outside of the urban corridor, confidence is no less in this
occurring, but there is still some time, as values do not look
to reach criteria until Friday. Criteria for Excessive Heat
Watches and Warnings are a bit more stringent for the urban
corridor due to the cumulative effects of a heat wave on an
urban center. It is important to note that overnight lows in the
Watch area will be in the 70s to around 80. So there will not be
much, if any, relief from the oppressive heat at night.

Some weak mid-level shortwave energy may spark off some
afternoon convection Friday through Sunday, but nothing
widespread or organized. Will cap PoPs at slight chance. If
convection does fire up, it may briefly result in some relief
from the high temperatures, but the rainfall will also result in
locally higher humidity values.

A cold front approaches on Monday, and this should bring
convection into the region along with some relief from the heat
and humidity for the start of the work week.

&&

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

Today...Generally VFR with SW winds around 5-10 kts. Showers
and Thunderstorms will be possible late in the afternoon around
RDG and ABE. A sea breeze will be possible at the coastal sites

Tonight...VFR with lingering showers and thunderstorms possible
early. Winds generally 5 knots or less and favoring a southerly
direction (if not variable).

Outlook...

Wednesday...Mostly VFR conditions expected. MVFR or lower in
late afternoon/early evening SHRA/TSRA. South to southwest
winds around 5 to 10 knots.

Thursday...MVFR or lower in SHRA/TSRA. South to southwest winds
around 5 to 10 knots.

Friday through Saturday...Mainly VFR conditions expected. MVFR
or lower possible in isolated SHRA/TSRA in the afternoon and
early evening. West to southwest winds around 5 to 10 knots.
Excessive heat index values expected.

&&

.MARINE...
Sub-SCA conditions are expected today/tonight with seas around
2-3 feet and SW winds 10-15 kts gusting to 20 kts or less.

Outlook...

Wednesday through Thursday...Generally sub-SCA conditions,
however, showers and thunderstorms will result in dangerous
conditions on the waters.

Friday through Saturday...Sub-SCA conditions.

Rip Currents...

Due to seas around 3 feet, a somewhat long wave period of 9
seconds, and southerly winds 10-15 kts there will be a moderate
risk of rip currents today over the northern NJ beaches. Over
the southern NJ and DE beaches waves and winds will be a bit
less so there is a low risk of rip currents forecast at this
time. However the risk could be higher if winds and/or waves
materialize stronger than what is currently forecast.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Excessive Heat Watch from Wednesday morning through Sunday
     evening for PAZ070-071-102-104-106.
NJ...Excessive Heat Watch from Wednesday morning through Sunday
     evening for NJZ015-017>019.
DE...Excessive Heat Watch from Wednesday morning through Sunday
     evening for DEZ001.
MD...None.
MARINE...None.

&&

$$
Synopsis...MPS
Near Term...Carr/Fitzsimmons
Short Term...Carr
Long Term...MPS
Aviation...Carr/MPS
Marine...Carr/MPS