Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 141955
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
355 PM EDT Mon Aug 14 2017

.SYNOPSIS...
High pressure will weaken tonight. A weak warm front positioned
across the western Atlantic and Carolinas will slowly push northward
toward the region tomorrow and tomorrow night. At the same time and
much further offshore, Tropical Storm Gert is expected to pass to
our south and east. A cold front will approach the region from the
northwest on Wednesday and track as far south as Reading, Trenton,
and Belmar. The front will retreat Thursday into Friday, but may be
slow to clear the area as high pressure blocks its progression
northward. A cold front, from the west, is then expected to cross
the region Friday night into Saturday. High pressure will build on
Sunday and Monday.

&&

.NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...
Southwesterly midlevel flow poleward of TC Gert in the western
Atlantic is aiding in the progression of several perturbations
moving into the Mid-Atlantic region during the next several hours.
The most prominent vort max is approaching the area now, with light
rain and/or showers attendant to the strongest (not to be confused
with strong) lift just downstream of it. Models have really
struggled with the evolution of the precipitation, both in terms of
timing (overall, too slow) and intensity (overall, tending to
diminish the precipitation too quickly). The HRRR and 12Z NAM Nest
have generally converged on a solution, bringing light showers
through and northwest of the I-95 corridor during the next few
hours. The main question mark will be the intensity of the showers
on the southwest fringe of the precipitation. Here, the showers may
have marginal buoyancy to work with as the stronger instability
approaches the area from the south on the upstream side of the
precipitation shield. The last several runs of the HRRR and the 12Z
NAM Nest indicate stronger convective cells on the southwest side,
and current radar observations indeed suggest such an evolution is
occurring. The question, however, will be if enough instability is
present for lightning. Model soundings are not promising in this
regard, with very thin positive buoyancy present (owing to modest
low- and midlevel lapse rates and a rather shallow layer through
which parcels may rise). With this in mind, think the chances of
thunder are too low for mention at this point...though it certainly
cannot be ruled out entirely.

The QPF trend has been upward with the main band of precipitation as
it moves through this evening/overnight, but with large-scale ascent
generally weak (and weakening as upper-level confluent flow
mitigates effects of differential cyclonic vorticity advection) and
the mode/intensity of the showers unclear ... not biting too much on
the heavier precipitation amounts at this point. Think a tenth to a
quarter inch in the main swath of precipitation is possible, with
locally higher amounts if stronger convective cells develop.

A weaker perturbation upstream and poleward of the evening vort max
will sweep through the Northeast and northern Mid-Atlantic tonight.
As this perturbation moves through, there may be some enhancement of
showers in the northern CWA (generally north of I-78) later tonight.

The forecast generally features slight chance to chance PoPs across
the area, highest in the I-95 corridor to the Lehigh Valley this
evening through around midnight or so, then confined to the Poconos
and vicinity after midnight, tied to the aforementioned
perturbations.

Conditions will be increasingly humid tonight, with lows near to
slightly above seasonal averages. Skies should be cloudy, and winds
will be light. Some fog may develop late tonight, but confidence was
too low for inclusion at this point. However, this may need to be
added in later forecasts once the evolution of this evening`s
precipitation is determined.

&&

.SHORT TERM /6 AM TUESDAY MORNING THROUGH 6 PM TUESDAY/...
As TC Gert makes its move northward and then northeastward through
the western Atlantic, a well-timed longer-wave trough will be
swinging southeastward into the Great Lakes region and the adjacent
Northeast. This will be enough to keep Gert safely away from the
coast. Meanwhile, the southern stream will continue to eject
perturbations northeastward into the Mid-Atlantic on the northwest
fringe of the subtropical high in the Atlantic. With onshore flow to
the northwest of Gert, low clouds will likely be stubborn to
rise/dissipate during the day in the region. MOS is considerably
lower with max temperatures on Tuesday, and with onshore flow
expected to continue much of the day, think the lower temperatures
are a decent bet. Meanwhile, large-scale lift in advance of the
southern-stream perturbations should generate scattered showers and
storms during the afternoon. Instability will increase (even with
the potential for low clouds to hang around through the morning, at
the very least), as large-scale lift will cool the midlevels fairly
effectively. This will be especially true south of an equivalent
potential temperature gradient that will slowly lift northward into
the Delmarva Peninsula and adjacent areas during the day (just how
far north this baroclinic zone reaches will be a big forecast
concern during this period). With the above in mind, slight chances
for storms are present tomorrow afternoon with any showers that form.

Models are indicating the strongest precipitation signal south of I-
76, and this is generally where the highest PoPs reside. The mode of
precipitation is a bit of a question (some have fairly isolated
convective cells or convective clusters -- e.g., the 12Z WRF-
ARW/NMM; others have fairly light showers with little in the way of
convection -- e.g., the 12Z NAM Nest). With such varying solutions,
obviously not confident of tomorrow`s precipitation evolution in
general and specifics like thunder chances and QPF in particular.

The forecast is a general broadbrush of slight-to-chance PoPs,
slight chance of thunder, and cloudy skies for the region tomorrow,
with gradual improvement in the northwest CWA as dry air advection
approaches upstream of the main trough swinging through the
Northeast during the afternoon. Forecast max temps are slightly
below seasonal averages and are generally a blend of MAV/MET
guidance.

&&

.LONG TERM /TUESDAY NIGHT THROUGH MONDAY/...
Overview...The period starts with a stationary front draped across
the western Atlantic and Carolinas, a cold front across the Great
Lakes and TC Gert well east of Cape Hatteras. The cold front
will work to the south and east and arrive in our region
Wednesday night. It will pause across the CWA on Thursday.
Confidence is low as to how far south this front will get, but
right now it looks like a Reading to Trenton to Belmar line. The
front will retreat north on Friday as another cold front works
through the Great Lakes and Ohio Valley. Confidence is once
again low on how far north the front retreats. The cold front to
our west will work through the region Friday night or Saturday.
Surface high pressure behind this front will cross into the
region on Sunday and anchor up into Monday.

Temperatures....For reference, KPHL normals are mid 80s and upper
60s. Temperatures will be above normal on Wednesday, about normal on
Thursday, above normal on Friday, about normal on Saturday, Sunday,
and Monday. The warmest and most humid day of the week appears to be
Friday just ahead of the cold frontal passage. No extreme
temperatures are expected through the period with mainly just
slightly above or slightly below if not normal.

Precipitation...POP confidence is low into Saturday and depends on
how far the coastal warm front edges up into our area from the south
Tuesday night, how far south the cold front drops into the area on
Wednesday night and how far the same front retreats northward
Thursday into Friday. The closer the proximity and duration of these
boundaries, the more unsettled it will be.

For now, we`ll paint small POPS Tuesday evening into Wednesday night
across portions of the CWA. Coverage should be more widespread on
Thursday into Friday night. Rainfall rates will also increase
Thursday night into Friday as PWs exceed 2 inches. Small POPS will
continue on Saturday and Sunday, then nil POPS Monday.

Winds...Benign through the period, mainly 15 mph or less. Highest
speeds right along the coast.

Impacts...TC Gert is forecast to pass well to our south and
east on Tuesday and Tuesday night. Swells from the system will
likely bring at least a moderate risk for the development of
dangerous rip currents Tuesday into Wednesday as well as the
possibility of SCA conditions on the ocean. These concerns are
addressed below.

&&

.AVIATION /20Z MONDAY THROUGH SATURDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

Increasing confidence that patchy light rain or showers will move
through most of the terminals through this evening. Given the patchy
nature of the rain/showers, VCSH and TEMPO groups are included for
now along/northwest of I-95. General timing is through 06Z. Mostly
VFR expected, even with the precipitation, but CIGs are forecast to
lower slowly to 5-10 kft during this period. Sub-VFR CIGs are
expected near/after 06Z as somewhat variable winds become more
easterly. Sub-VFR conditions may prevail through Tuesday morning.
Winds below 10 kts are forecast through the period.

OUTLOOK...

Tuesday night through Wednesday night...Mainly VFR. MVFR possible in
late night and early morning low clouds and fog. Small chance of a
late afternoon or evening Thunderstorm across southern NJ and the
Delmarva both nights.

Thursday through Friday...Mainly VFR, but MVFR possible in showers
and thunderstorms. Late night and early morning low clouds and
fog possible.

Saturday...Some morning MVFR possible, then VFR behind an exiting
cold front.

&&

.MARINE...
Seas will be slowly building during the next 24 hours as Tropical
Storm Gert makes its closest approach to the area. It will remain
well offshore, but longer-period swells should continue to increase
during the period. Seas will approach advisory criteria late
tomorrow afternoon, but uncertainty with onset and the somewhat
marginal nature of the seas preclude the issuance of an advisory at
this point.

A chance of showers exists tonight and tomorrow, and there is a
chance of lightning on Tuesday. Winds/seas will be erratic near any
showers/storms that occur. Additionally, there may be some
visibility restrictions late tonight, though widespread/dense fog is
not anticipated. Winds should generally be from the east or
southeast around or under 10 kts.

RIP CURRENTS...

The rip current risk remains moderate through this evening as
onshore winds around 10 kts combine with gradually increasing swell
during the period thanks to Tropical Storm Gert. Conditions are
expected to worsen somewhat tomorrow, as longer-period swell with
higher seas will occur.

Swim only in the presence of lifeguards where safety prevails.
Otherwise, you`re on your own with any rescue potentially critically
delayed. There have been numerous drowning fatalities this year
along the NJ coast.

OUTLOOK...

Tuesday night...SCA seas possible due to Gert Tuesday night and
Wednesday. Otherwise sub-SCA conditions expected through this week.

Rip currents...TC is forecast to pass well to our south and
east Tuesday night into Wednesday. Swells from the system will
likely bring at least a moderate risk for the development of
dangerous rip currents Tuesday night into Wednesday.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...None.

&&

$$
Synopsis...Kruzdlo
Near Term...CMS
Short Term...CMS
Long Term...Kruzdlo
Aviation...CMS/Kruzdlo
Marine...CMS/Kruzdlo


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