Scientific Forecaster Discussion
fxus61 kphi 280146
Area forecast discussion
National Weather Service Mount Holly New Jersey
946 PM EDT Thu Jul 27 2017
a warm front will move through our area later tonight. Low
pressure will develop along the frontal boundary across the Ohio
Valley, and it will move towards the area early Friday through
Saturday. High pressure will return over our area for the late
weekend and will persist into the week.
Near term /until 6 am Friday morning/...
there was one isolated thunderstorm moving off Chesapeake Bay
toward Talbot County in Maryland. The storm is expected to
weaken gradually once it moves away from the warm waters of the
Bay. Otherwise, we are anticipating isolated showers and patchy
fog overnight in northeastern Maryland, Delaware, New Jersey and
eastern Pennsylvania. Partial clearing may take place for a
The wind is forecast to be light and minimum temperatures
should favor the 60s and lower 70s.
Short term /6 am Friday morning through 6 PM Friday/...
the start of a prolonged period of unsettled weather begins on
Friday. Warm front ahead of the developing low seems to get hung up
across the mid-Atlantic. Several waves of low pressure will develop
and intensify over the Ohio/Tennessee valleys along the boundary as closed 500 mb
low digs into the Ohio Valley from the Great Lakes. The first surface
low will lift into the mid-Atlantic and Delmarva Friday afternoon,
and showers and scattered thunderstorms will move into the Delmarva,
Delaware valley, and southern New Jersey during this time.
Onshore flow ahead of the low will usher abundant moisture into the
region. Surface dewpoints will rise into the upper 60s/low 70s
throughout, and pwats will range from 2-2.25".
This first wave of showers and thunderstorms will produce heavy
rain, mainly to the south of I-78. Hardest hit areas will be across
northeast MD, de, and southern NJ, where between 3/4-1" rain is
possible during the day. Locally higher amounts are possible in
training of thunderstorms. These areas are the areas already hit by
heavy rain early this week.
With 3 hour and 6 hour ffg values on the order of 2-4 inches, do not
think this will be hard to reach, especially given the potential for
training of thunderstorms. Therefore, have issued a Flash Flood Watch
for much of southern NJ, parts of southeast PA, northeast MD, and Delaware
starting Friday afternoon, though the bulk of the rain comes Friday
night and Saturday.
Storm Prediction Center has most of southern New Jersey and northern parts of the Delmarva in a
marginal risk for severe weather on Friday, and southern portions of
the Delmarva in a slight risk for severe weather. Thunderstorms with
damaging winds are possible, but big concern will be for the
Long term /Friday night through Thursday/...
Summer Nor'easter this weekend with the potential for flash
flooding increasing across portions of the region...
A shortwave over the upper Midwest as of this afternoon will carve
out an an anomalous cutoff low across the mid Atlantic on Saturday
which will persist into Tuesday. This will be the primary weather
maker during this time frame with a Summer Nor'easter leading to
heavy rainfall. The potential for flash flooding has increased
and a Flash Flood Watch is now in effect generally south of
Heavy rainfall...shortwave energy rounding the base of the mid
Atlantic trough will lead to cyclogenisis along a stalled frontal
boundary near Delmarva from late Friday into early Saturday. As
the mid-level trough closes off, the system will become vertically
stacked on Saturday, with the surface low stalling off the
Virginia CAPES, before moving gradually northeastward on Monday.
While there is model consensus through Saturday with the track
of the low, the Canadian and NAM take the low further south and
east on Sunday, while the GFS and European model (ecmwf) keep it locked in closer
to the coast. We favored the GFS and European model (ecmwf) over the Canadian
and NAM, as they have a better handle on the shortwave impulse
associated with this system.
That said, there is still considerable spread in the gefs and
eps 500 hpa heights with the placement of the cutoff low, and this
is leads to uncertainty in the track of the surface low. Both
the gefs and eps means low positions are close to the op runs at
00z Sunday. This is further evident in the ensemble spread in
mean precipitation amounts, which is most acute in the I-195 to
PA Turnpike corridor. We have higher confidence in the potential
for flash flooding southeast of this line, and lower to the
Quite an impressive setup develops across the Flash Flood Watch
area on Saturday with an easterly 850 hpa jet of 50 kts
developing (+3 to +4 Standard dev above normal), transporting deep
later moisture into the region with precipitable water values
in excess of two inches (+2 to +3 Standard above normal). There are
two potential bullseyes of heavy rainfall, one near the New Jersey
coast, and another extending from southeast PA into Delmarva.
With a frontal boundary stalled across Delmarva, rainfall across
this area into southern New Jersey will also be convective in nature,
as a broad area of instability sets up, as evidenced by mu
cape, along with negative lifted and showalter indices.
Additional rainfall is possible on Sunday, mainly across
southern New Jersey and Delmarva.
A return to fair weather is expected in the Mon - Wed time
frame, with the next chance of showers on Thursday with an
approaching front. Temperatures will be below normal from
Saturday thru Monday, especially closer to the coast with an
onshore flow. A return to near normal is expected thereafter.
Aviation /02z Friday through Tuesday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg,
kmiv, kacy and surrounding areas.
The patchy MVFR ceilings should give way to VFR conditions for
a time overnight before patchy low clouds and fog develop toward
daybreak. The patchy low clouds and fog should lift and break on
Friday morning with a return to VFR.
Showers and thunderstorms are forecast to spread into our
region from the southwest mainly during the afternoon.
Conditions will likely deteriorate, especially late on Friday
as the rain becomes moderate to heavy.
A light and variable wind overnight is forecast to settle into
the northeast to southeast quadrant for Friday at less than 10
Saturday...widespread IFR expected in low clouds and heavy
downpours. Northeast winds may gust up to 25 knots and there is
the potential for low-level wind shear across the terminals.
Sunday...IFR may continue at Acy, miv, and ilg. Improvement to
MVFR elsewhere. Northeast winds may continue to gust up to
around 20 knots.
Monday and Tuesday...MVFR conditions may persist into Monday at
Acy and miv, otherwise VFR conditions. North winds generally
around 10 knots both days.
south to SW winds 10-20 knots this evening will become light by Friday
morning, then will become east-southeast 10-15 knots Friday afternoon. Occasional
gusts to 25 knots possible at Delaware and southern New Jersey ocean waters this
evening, but do not think they will be widespread or frequent enough
to warrant a Small Craft Advisory. Seas will range from 2-4 feet.
Widespread showers and thunderstorms will impact the waters Friday
afternoon with reduced visibilities, gusty winds, and rough
we issued a gale watch for the New Jersey waters, including the
waters east of the mouth of Delaware Bay, from Saturday morning
into Saturday night. Small Craft Advisory conditions may persist thereafter from
Sunday into Monday.
Based on the latest trends and guidance, we will carry a low
risk for the development of dangerous rip currents on Friday.
Wind and wave conditions are expected to be relatively light.
A high risk is possible from Saturday through Monday.
a Flash Flood Watch has been issued for much of southern NJ, parts
of southeast PA, MD, and Delaware. 3 hour and 6 hour ffg values range from
2-4", and with widespread showers and thunderstorms producing heavy
rain, these values are likely to be met.
Will forecast a storm total of 2-4" inches of rain from tonight
through Saturday, however, with training of thunderstorms,
significantly higher rain amounts are likely, especially in areas of
Delaware and southern New Jersey that already received heavy rain and flooding
early this week.
Please be aware of Road conditions, and be aware for flash flood and
flood warnings that may be issued, especially Friday night and
minor coastal flooding is possible Saturday night and Sunday
morning along the oceanfront. While astronomical tides lower,
a surge potential of 1-2 feet may put some oceanfront sites
into the minor category at the time of high tide.
There is the potential for high surf with breakers of at least
8 feet along oceanfront beaches on Saturday and Sunday. A high
surf advisory may be needed. Beach erosion is also possible.
record daily rainfall
Acy 2.05 2012 2.21 1884
Phl 8.02-2013 3.53-1980
Ilg 2.34-1914 1.85-1913
Abe 3.00-1969 1.64-1979
Ttn 2.35-2012 2.84-1961
Ged 2.80-2016 1.07-1969
Ridge 3.57-1969 2.51-1961
Mpo 2.15-1969 4.59-1969
Abe 8.21 #8 wettest 10.42 is the record in 1969
Ridge 8.02 13.85 is the record in 2004
July average temps: projecting, for now, phl 0.9 above normal,
losing 1.3 positive departure from the values through the 26th.
Ditto approximately the same loss at our other cli sites will
show mpo a little below normal, ttn and Acy near normal, and all
other stations above normal.
dew point readings at kdov continue to measure too high compared to
surrounding locations and should be treated as unrepresentative of
PA...Flash Flood Watch from Friday afternoon through Saturday
afternoon for paz060-070-071-101>106.
New Jersey...Flash Flood Watch from Friday afternoon through Saturday
afternoon for njz012>027.
Delaware...Flash Flood Watch from Friday afternoon through Saturday
afternoon for dez001>004.
Maryland...Flash Flood Watch from Friday afternoon through Saturday
afternoon for mdz008-012-015-019-020.
Marine...gale watch from late Friday night through late Saturday night