Area Forecast Discussion
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FXUS61 KPHI 281740
Area Forecast Discussion
National Weather Service Mount Holly NJ
140 PM EDT THU JUL 28 2016
High pressure will be anchored to the north through Friday.
Meanwhile, a weak stationary front will be situated over the area
with a wave of low pressure expected to track along it. A weak
front will stall to our south and areas of low pressure will
develop along this front through the weekend and bring periods of
showers and thunderstorms.
.NEAR TERM /UNTIL 6 PM THIS EVENING/...
930 AM ESTF Update...The Flash Flood Watch (FFA) now goes into
effect at 2 PM for E MD, DE, SE PA and S NJ. A rather diffuse WSW-
ENE oriented frontal boundary bisected the CWA this morning.
Dewpoints were in the 60s north of the front and 70s south of it.
Following strong heating and boundary layer destabilization
during peak heating, scattered showers and thunderstorms are
expected to develop this afternoon where the axis of greatest
moisture and instability will reside along and south of this old
boundary. These storms will like contain torrential downpours as
they develop in a moisture laden airmass where PWATS are forecast
to be 2.0-2.25 inches. There is a signal in the hi-res CAM
guidance (including the HRRR and NCAR ensembles) for locally
heavy rainfall that would exceed 3-h FFG this afternoon and early
evening in SE PA, S NJ and particularly across our Delmarva
The start time of the FFA for the tier of counties farther north
into Bucks Co., PA and central NJ is still 6 PM. We will monitor
the latest observations/mesoanalysis and model trends for
consideration if an expansion of the FFA is needed farther north
and west for tonight.
The Storm Prediction Center has included our Delmarva zones in a
slight risk and SE NJ, far SE PA in a marginal risk for severe
thunderstorms due to the expected instability (MLCAPE 1500-2000
J/kg) along with a moderately strong westerly mid-level flow.
Maximum temperatures should favor the lower 90s in our region.
Readings should not get out of the 80s in the elevated terrain of
the Poconos and northwestern New Jersey, and along the immediate
coast. Heat index values are expected to remain several degrees
below the excessive heat criterion for our northern counties and
just below the criteria for the urban corridor and for our
The synoptic wind should be light allowing local influences to
drive today`s wind direction. Speeds should be mostly less than 10
.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM FRIDAY/...
Precipitable water values of 2.2 to 2.4 inches will overspread
our forecast area from the south for tonight. Conditions are
expected to remain unstable, especially across our southern
counties. Also, surface low pressure approaching from the west
will result in the development of a favorable low level wind
profile for organized convection tonight. The surface wind should
begin to favor the east to south quadrant late tonight, especially
on the coastal plain, while winds around 800 HPa will be from the
The very high precipitable water values should result in heavy
rainfall in any organized convection. As a result, the Flash Flood
Watch remains in effect for southeastern Pennsylvania, central and
southern New Jersey, Delaware and northeastern Maryland.
The cloud cover and high humidity will keep temperatures from
falling below the 70s for tonight except in the Poconos and far
northern New Jersey.
.LONG TERM /FRIDAY THROUGH THURSDAY/...
An area of low pres will ride alg the stalled front and bring
rain to the area on Fri. There is still some disagreement in the
mdls as to the extent of the rain and the locations of the
heaviest, with the GFS indicating most rain n and very little s
and the ECMWF having a more widespread event. For now, will just
carry high chc everywhere.
Mdls then clear things out Fri night into early Sat, so have
lowered pops and removed them for the first part of Sat.
Then there is more disagreement in the mdls into Sat as both mdls
develop precip again but the ECMWF is more widespread into early
Sun. However, by later Sun, both the GFS and ECMWF has another
good shot of precip over the area as another wave moves by alg the
After Sun, the ECMWF is much drier through midweek, keeping most
of the period dry, while the GFS keeps Mon with a chc of precip
and diurnal stuff on Tue. The drier ECMWF soln is supported by the
CMC, so have leaned the fcst in that direction attm.
Temps look to be a bit cooler than we have had recently, with
temps not expected to hit 90 degrees in many areas thru the
.AVIATION /18Z THURSDAY THROUGH TUESDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.
Flying conditions will continue to deteriorate through tonight as
thunderstorms with gusty winds and heavy rain will periodically
impact all taf sites, but especially KPHL, KPNE, KTTN, KMIV, KACY
through early Friday morning. Added vrb15g30kt winds to these TAF
sites to account for potential for gusty winds in and near
thunderstorms. Confidence is low for the timing of convection at
any particular TAF site, but high that storms will be in the
region through early Friday morning.
Showers and storms will expand in coverage tonight and early
Friday morning. This will result in ceilings dropping to MVFR at
most sites around or after 04Z.
Southeast winds will back to the east late tonight generally 10
knots or less, except in and around thunderstorms, but shift to
the northwest and increase in speeds, gusting at 10-20 kts after
Conditions gradually improve by late morning/early afternoon
Friday with precipitation ending, and ceilings lifting
Sat through Sun...Shra/tsra will lower cigs/vsbys at times
Otherwise VFR expected. Moderate Confidence.
Mon...Mainly VFR. Moderate Confidence.
The wind direction is forecast to favor the SE for much of the
day, becoming more S late in the day and this evening. Winds
speeds will be light, around 10 knots. The winds will be variable
in some spots tonight but a general light SE wind is favored late
Wave heights on our ocean waters will be 2 feet or less and waves
on Delaware bay should be 1 foot or less.
Scattered showers and thunderstorms are possible for this
afternoon with numerous showers and thunderstorms anticipated for
Fri through Mon...There is a chance of marginal sca conds Fri
night into erly Sat, but confidence is low. Otherwise, no marine
headlines are anticipated thru the pd.
There is a low risk for the development of dangerous rip currents
for today due to the relatively light wind and tranquil seas.
The probably risk for the development of dangerous rip currents
on Friday is also low.
PA...Flash Flood Watch from 6 PM EDT this evening through Friday
evening for PAZ105-106.
Flash Flood Watch through Friday evening for PAZ070-071-
NJ...Flash Flood Watch from 6 PM EDT this evening through Friday
evening for NJZ009-010-012>015.
Flash Flood Watch through Friday evening for NJZ016>027.
DE...Flash Flood Watch through Friday evening for DEZ001>004.
MD...Flash Flood Watch through Friday evening for MDZ008-012-015-