Area Forecast Discussion
Issued by NWS Mt. Holly, NJ
FXUS61 KPHI 262003
Area Forecast Discussion
National Weather Service Mount Holly NJ
403 PM EDT TUE JUL 26 2016
A weak cold front will continue to slowly drift south across the
region tonight before stalling nearby to our south Wednesday into
Thursday. Several waves of low pressure developing along this
boundary will result in periods of showers and thunderstorms late
in the week and this weekend.
.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
The cold front has finally moved into the forecast area this
afternoon. The front was located near the I-80 corridor at 19Z and
was marked by a subtle wind shift from more westerly to more
northerly and a slight drop in dewpoints. The front will weaken as
it continues to progress slowly southward through the region
tonight. A pre-frontal trough was become increasingly diffuse as the
front catches up to it, but it was located near the Delmarva coast
back into southeastern VA.
Surface observations reveal a dewpoint boundary near this residual
surface trough with an axis of higher dewpoints in the 70s and
instability along and south of the Delaware Bay. The Heat Advisory
continues through 7 PM for Kent and Sussex Counties in DE and Queen
Annes, Carolina and Talbot Counties in MD, where current heat
indices are in the 101-108F range.
Mid afternoon objective mesoanalysis from SPC shows about MLCAPE
1000-1500 J/KG over the mid and lower portions of the Delmarva (it
actually shows a pocket of 2000+ J/kg, but that is likely tainted by
the spuriously high 81F dewpoint at KESN). A few showers and
thunderstorms are developing across these areas in concert with peak
heating. Locally heavy rainfall cannot be ruled out with the airmass
remaining moist (PWATs still around 2 inches), slow storm motion
(under 15 kt) and the potential for multiple storms to develop along
the quasi-stationary surface trough. The low coverage of storms
(mainly isolated) and sandy Delmarva soils will limit the overall
flash flooding risk. One or two storms may also contain locally
strong winds given they are forming in an environment conducive for
Expect showers and storms to generally dissipate around or shortly
after sunset with the loss of daytime heating. However, we`ll have
to monitor radar trends across WV and northern VA this evening. The
HRRR continues to advertise a mid-level shortwave trough and
accompanied upstream convection moving into our Delmarva and far
southern NJ zones overnight. Models generally show dewpoints and
elevated instability increasing overnight across these far southern
zones. This signal supports the idea that this convective activity
could be maintained well into the night. Reintroduced scattered
showers and storms into the forecast for the Delmarva late tonight.
Farther north into southeastern PA and central NJ, high clouds will
stream into the area. Ideal radiational cooling conditions under
mainly clear skies and calm winds will allow for patchy fog to
develop before sunrise and temperatures to fall into the lower 70s
across northeastern PA and northwestern NJ.
.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH 6 PM WEDNESDAY/...
Some of the model guidance shows the cold front moving south of
Delaware Wednesday morning while others keep the front stalled over
our southern zones. Given it`s the middle of the summer and zonal
steering flow aloft, we favor a more northern solution with the
front stalling in the Delmarva. Expect additional showers and storms
to develop along the lingering boundary during peak heating hours
Clouds near the front will likely keep temperatures down a few
degrees in DE and eastern MD (upper 80s) while stronger heating
under mostly sunny skies in eastern PA and NJ will allow for
temperatures to climb back into the 90s. However, we are looking at
low 90s for highs and humidity levels will be comfortable with
dewpoints in the 60s.
.LONG TERM /WEDNESDAY NIGHT THROUGH TUESDAY/...
The front will remain fairly stationary over the Delmarva
Wednesday night into Thursday before starting to push back north
as a warm front Thursday night into Friday. This boundary will
act as a focus for weak low pressure development. These waves will
then ride along the boundary and will result in a flare-up of
showers and thunderstorms. Specific timing for this can be tricky
and subject to change, but current thinking is for Thursday night
into early Friday and then again on Saturday. Outside of these
times, expect generally diurnal driven climatological threat of
showers and thunderstorms.
After a bit of a reprieve with lower humidity levels mid-week, dew
points will increase back to 70-75 over the Philadelphia metro
area, southern New jersey and the Delmarva by Thursday and
Thursday night. This will push the heat index values into the
upper 90s, almost 100 on Thursday. After that, expect lower heat
indices for Friday and the weekend as high temperatures will be
suppressed by increased cloud cover.
It appears that the weather will remain warm, muggy and unsettled
through the start of the next week, before the frontal boundary
finally sinks far enough south to allow a marginally cooler, but
drier airmass the infiltrate the entire region.
.AVIATION /20Z TUESDAY THROUGH SUNDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.
Mainly VFR through the valid TAF period. The one thing we`ll have to
watch for is patchy fog to develop late tonight in the fog-prone
terminals (e.g., RDG, MIV). Showers and storms Wednesday afternoon
will likely stay south of the terminals but it will could be very
close by to ILG-MIV-ACY).
W to NW winds around 10 kt are gusting 15-20 kt this afternoon. The
winds will weaken after sunset before becoming light and variable
tonight. Winds generally W-NW 5-10 kt on Wednesday but the sea
breeze will likely result in a wind shift out of the S-SE at ACY and
MIV. The sea breeze front could make it to the I-95 terminals toward
Wednesday night through Sunday...Mainly VFR conditions expected.
Scattered showers and thunderstorms Thursday, Friday and Saturday,
with MVFR/IFR conditions possible with any shower and thunderstorm
NW winds will continue to shift out of the south late this afternoon
and evening as the sea breeze moves toward the coast. The wind are
then expected to turn around toward the north late tonight as a cold
front moves southward. Expect a similar wind shift to occur again
tomorrow. Speeds should remain under 20 knots. Wave heights on our
ocean waters are expected be 2-3 foot through the period. Waves on
Delaware Bay should be 2 feet or less.
Wednesday night through Sunday...Sub-advisory conditions are
expected to continue on the area waters through the forecast
period. Expect higher winds and waves in and near scattered
showers and thunderstorms that will be more likely at the end of
The risk for formation of dangerous rip currents is
low this afternoon and again Wednesday.
The last time the temperature reached or exceeded 98F at PHL was
three years ago on July 18, 2013. The last 100-degree day at PHL
occurred four years ago on July 18, 2012.
Record high temperatures for Wednesday and Thursday are below.
Site 27th 28th
---- ---- ----
PHL... 101-1940 100-1941
ABE... 98-1955 97-1949
ACY... 99-2005 98-1999
ILG... 100-1894 101-1894
TTN... 100-1894 101-1894
GED... 98-2005 99-1949
RDG... 98-1955 99-1941
MPO... 91-1955 93-1949
DE...Heat Advisory until 7 PM EDT this evening for DEZ002-003.
MD...Heat Advisory until 7 PM EDT this evening for MDZ015-019-020.