Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 160153
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
953 PM EDT Sun Apr 15 2018

.SYNOPSIS...
A strong low pressure system and its associated fronts will
affect the region tonight and Monday. The low will move away
allowing high pressure to build over the area for Monday night
and through Tuesday. A low pressure system and its associated
fronts will affect the area Wednesday through Thursday. An upper
low will remain over the region into Friday. High pressure will
build down across the area from Canada beginning Friday night
and remaining into next weekend.

&&

.NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
For the 930 PM update, made some minor adjustments to the hourly
temperature and dew point grids (increased the winds a bit
more). A low-level jet above the low-level wedge of colder air
is assisting in lift resulting in quite a bit of shower
activity. This is expected to continue, then we await the
arrival of the frontal forcing very late tonight from the west
and southwest. A mesoanalysis shows a little bit of instability
spreading northward across parts of Delmarva and south/eastern
New Jersey. This is indicated on the MUCAPE analysis thus far
and therefore not enough to result in lightning production. We
therefore removed the thunder for this evening, and focused it
more with the incoming frontal forcing late tonight and more
toward morning.

Some light freezing rain is being reported at Mount Pocono
where the temperature is down to 31 degrees. It appears this
near or slightly below freezing air is above 1800 feet in
elevation, therefore rather localized. No advisory or
statements issued at this time as any light icing should be in
the highest elevations and given the gusty winds any accretion
may be limited (roads should be just wet). Temperatures are
expected to slowly rise from south to north overnight as the
low-level jet works its magic and starts to dislodge the colder
air currently stuck at the surface (surface winds start to veer
from east-northeast to southeast).

Otherwise, high-resolution models place the front in portions
of Delmarva and far southern New Jersey toward daybreak, but do
not have a lot of confidence on this placement. Models tend to
progress these fronts too far northward, and suspect this is the
case here as well. Did slow its progress somewhat from hi-res
model consensus (HRRR/RAP, NAM Nest, WRF-ARW/NMM, RGEM), but not
a whole lot from the previous forecast (not enough confidence
to do that yet). Low-CAPE/high-shear setups are always a
challenge, and the placement of the front and the quite marginal
surface-based instability are certainly adding to the
challenge. Nevertheless, with shear profiles exceptionally good
(given the pronounced veering in the low levels downstream of
the front and resultant very large low-level SRH), sufficient
instability would lead to potential for locally severe storms
toward daybreak.

QPF looks quite substantial during the 06Z to 12Z period, especially
near/northwest of the urban corridor, where 6-hour totals of
0.75-1.5 inches are likely, with locally higher totals probable
with the strongest convective cores and with any training
convection that occurs. See the hydrology section below for more
details.

&&

.SHORT TERM /6 AM MONDAY MORNING THROUGH 6 PM MONDAY/...
Front moves through the area early in the day, which is not entirely
favorable for generation of surface-based instability and a more
appreciable severe threat as the strongly-forced convection moves
through. Nevertheless, the degree of shear is large (0-6 km bulk
shear in excess of 60 kts and 0-3 km SRH in excess of 300 J/kg)
near/south of the retreating warm front. Certainly cannot rule out
damaging gusts or even a tornado (though the threat of tornadoes
seems low given the marginal surface-based instability). Main
convective line should move out of the area by late morning.

Threat of training convection and locally heavy rain exists, with an
isolated instance of flash flooding plausible. Flash flood watch
continues through 6 pm, though the threat will likely be over before
then. Main threat appears to be with small streams and in urban
areas, and the timing is concerning given the Monday morning
commute. For these reasons, decided a watch was prudent despite the
somewhat marginal setup. See the hydrology section for more details.

As the surface low scoots to the northeast near/west of the I-95
corridor during the morning, cold air aloft will move into the area
as the midlevel low approaches the region. Instability showers
(perhaps even with thunder) will likely develop upstream of the
convective line by afternoon, as depicted in most all of the
guidance. Maintained chances of this along/northwest of the I-95
corridor in particular, given this area`s closer proximity to the
midlevel vort max.

Temperatures will probably begin to fall during the afternoon as
colder air filters in upstream of the cold front. Used solely hi-res
guidance for temperatures during this period, as precipitation
effects, frontal placement, and potential for rapidly changing sky
conditions in the afternoon warrant little use of statistically-
derived guidance.

&&

.LONG TERM /MONDAY NIGHT THROUGH SUNDAY/...
The strong low which brought the significant weather in the near
and short term portions of the forecast will be N/NE of the region
Monday night. While the fronts will have passed through the area
and all organized precip will be done, there is still the possibility
for some scattered showers under the upper low. We therefore have
slgt chc or low chc pops for the much of the area Monday night and
Tuesday. This scenario is being shown by both the GFS and EC models.
Below normal temperatures are expected this period with lows in the
30s and highs Tuesday only in the mid/upper 40s (north) and low 50s
for the Delaware Valley, srn NJ and Delmarva. Gusty winds 15 to 25
mph expected.

Fair weather is expected from Tuesday night into Wednesday as high
pressure arrives across the area. It will be a bit warmer than
Monday/Tuesday, but still slightly below normal for mid-April.
Highs will generally be in the mid/upper 50s north/west and
low/mid 60s south/east. These values will be 2 o 4 degrees below
normal.

Another low pressure system will approach the area later Wednesday
and last into Thursday evening. The system will be progressive,
so even though the low is depicted by the models as being
rather strong, it will not have alot of moisture to work with.
We therefore have mostly chc pops in the fcst for Wed night/Thu
and slgt chc pops into Thu night. Temperatures will remain
mostly at or a little below normal this period. The warmest day
will be Thursday (near normal) with mid/upper 50s for highs
across the north and low/mid 60s over Delmarva and srn NJ.

High pressure from Canada builds in for late this week and into the
weekend. Mostly fair weather is expected with a continuation of below
normal temperatures.

&&

.AVIATION /02Z MONDAY THROUGH FRIDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

Tonight through Monday morning...MVFR/IFR (possibly LIFR) with
low clouds and showers. Winds east-northeast 15-20 knots with
gusts near 30 knots, becoming southeasterly early Monday morning.
Southeasterly low-level wind shear (40-55 knots) overnight and
part of Monday morning.

A line of heavy showers and some thunderstorms moves through
the area between about 09Z and 14Z Monday with LIFR conditions,
torrential rain and erratic winds likely as the heaviest
portion of the line moves through. Lightning is anticipated
however the coverage is less certain and therefore a VCTS was
included for now. Conditions are expected to improve slowly to
MVFR after the heavy showers/storms pass. Moderate confidence.

Monday afternoon...Improvement to VFR, though scattered showers may
develop near/northwest of PHL. Winds becoming southwest or west
around 10 knots with gusts to 20 knots. Potential for some erratic
winds/gusts around showers. Moderate confidence.

Outlook...
Mon night thru Wed...VFR. Few showers psbl N/W Mon night/Tue.
Wed night and Thu...Mostly VFR. Sct showers.
Thu night/Fri...VFR expected.

&&

.MARINE...
Gale Warning continues, however winds may remain lighter near buoy
44009.

Models show a relative minimum in wind gusts later this evening
before winds turn southeasterly/southerly and strengthen
overnight through Monday morning. Widespread gales are expected
in advance of a line of heavy showers and storms moving through
the area between 4 AM and noon. Erratic/severe wind gusts and
torrential downpours are possible with these storms.

After the line of storms passes, winds should gradually diminish
from south to north, with small craft advisory conditions likely
during the afternoon on Delaware Bay and the Atlantic Delaware
coastal waters. Gale conditions may linger on the New Jersey coastal
waters for most of the afternoon before conditions improve by
evening.

Outlook...
Monday night/Tuesday morning...Low end SCA conditions.
Tuesday thru Friday...Sub-SCA most of the time. Near SCA seas and
a few showers possible Thursday.

&&

.HYDROLOGY...
QPF looks quite substantial during the 06Z to 12Z period, especially
near/northwest of the urban corridor, where 6-hour totals of 0.75-
1.5 inches are likely, with locally higher totals probable with the
strongest convective cores and with any training convection that
occurs. Flash flood guidance is high, and our thinking is that any
flash flooding threat would be isolated. However, nuisance/urban
flooding is probable, and the timing of the main line of convection
moving through the area (generally after 4 AM) means that impacts to
the morning rush may occur. In coordination with WFOs BGM/OKX,
hoisted a flash flood watch from the Philly metro northward into the
southern Poconos and northern New Jersey from 2 AM to 6 PM Monday,
with the main threat being associated with the primary convective
line moving through late tonight and Monday morning.

Flash flood guidance is on the high side (about 1-2 inches in an
hour) in most of the area (lowest in the urban corridor); however,
with anomalous precipitable water, considerable moisture convergence
via a very strong low-level jet impinging upon the approaching cold
front, and contributions from upslope flow, should any training
convection occur within the main line, 1-hr FFG could be met in
spots.

One of the primary concerns is the timing. With the main convective
line moving through just before or during the morning commute, heavy
rainfall may cause some traffic headaches. The nature of this event
suggests flood issues would be with flashy small streams and in poor-
drainage urban areas. Mainstem river flooding is not anticipated at
this time.

Total forecast rainfall accumulations in the watch area are
generally 1.5-3 inches, with 1-2 inches expected elsewhere (possibly
somewhat lower in the far southern portions of the county warning
area).

&&

.TIDES/COASTAL FLOODING...
Increasing astronomical tides associated with the approaching
new moon in combination with increasing anomolies as the result of
an established onshore flow will be the primary drivers for minor
to moderate flooding along the Mid-Atlantic coast into Monday
afternoon. An offshore wind shift later Monday is then expected
to end the threat of coastal flooding across the area.

A Coastal Flood Warning is in effect for the northern NJ coast
(Middlesex, Monmouth, and Ocean Counties) for the high tide this
evening and the high tide Monday morning. It may turn out that
the inlet areas and coastal streams experience slightly higher
water levels with this system, but the ocean front areas should
approach or exceed established moderate flooding levels as well.
Heading south,anomolies are expected to decrease slightly, so
coastal sections in the southern portions of Ocean County may
only see minor flooding, depending on how all the components of
this weather system play out.

Further south, a Coastal Flood Advisory is in effect for the
remainder of the New Jersey coast (Atlantic and Cape May Counties),
the Delaware coast, and all of Delaware Bay, for the high tides
this evening and Monday morning.

There is also the potential of minor flooding along the Delaware
River and upper Chesapeake Bay. For these locals, there is a
greater concern for the Monday morning high tide cycle, given an
increasing southeasterly flow regime between now and then. We will
continue to monitor this situation and issue later Advisories, if
necessary.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Flash Flood Watch from 2 AM EDT Monday through Monday
     afternoon for PAZ054-055-061-062-070-071-103>106.
NJ...Flash Flood Watch from 2 AM EDT Monday through Monday
     afternoon for NJZ001-007>010-012-015-017>019.
     Coastal Flood Advisory until 1 PM EDT Monday for NJZ021>025-
     027.
     Coastal Flood Warning until 1 PM EDT Monday for NJZ012>014-020-
     026.
     Coastal Flood Advisory until 2 PM EDT Monday for NJZ016.
DE...Coastal Flood Advisory until 1 PM EDT Monday for DEZ002>004.
     Coastal Flood Advisory until 2 PM EDT Monday for DEZ001.
MD...None.
MARINE...Gale Warning until 5 PM EDT Monday for ANZ450>453.
     Gale Warning until 11 AM EDT Monday for ANZ430-431-454-455.

&&

$$
Synopsis...O`Hara
Near Term...CMS/Gorse
Short Term...CMS
Long Term...O`Hara
Aviation...CMS/Gorse/O`Hara
Marine...CMS/O`Hara
Hydrology...CMS
Tides/Coastal Flooding...JJM



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