Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 220759
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
359 AM EDT Sat Apr 22 2017

.SYNOPSIS...
A cold front will remain south of the region this weekend while high
pressure builds in from the northwest. An area of low pressure
developing along the boundary to our south will move off the
Southeast coast Sunday night. The offshore low will then turn
northward Monday and track just off the eastern seaboard through the
middle of next week. A cold front is forecast to pass through the
area Thursday or Thursday night but may quickly return back
northward as a warm front on Friday.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
The large-scale environment features a potent vorticity maximum in
southeast Canada and adjacent New England with the vorticity lobe
elongated southwestward to a second potent maximum in the central
plains of the U.S. This second maximum is in the (gradual) process
of being cut off from the polar jet as the northern and downstream
vort max pivots northeastward into the Canadian Maritimes.

Attendant with the New England vort max is an upper-level jet
streak, with the Mid-Atlantic in the right rear quadrant. Combined
with differential cyclonic vorticity advection from the central
plains vort max and low-level isentropic ascent occurring along and
north of an east-west baroclinic zone stretched across the southern
Mid-Atlantic, large-scale ascent will be present for much of the day
across the area, allowing for multiple rounds of scattered showers
and isolated storms, particularly in the southern CWA (which is in
closer proximity to the baroclinic zone). These rounds will be timed
based on small-scale perturbations ejecting from the main vort max
in the central U.S.

Scattered showers and isolated storms are developing in the Delmarva
Peninsula and far southern New Jersey early this morning in an
environment of marginal/thin elevated CAPE with parcels generally
rooted in the 925-850 mb layer. Though instability is weak, deep-
layer shear is sufficient for updrafts to generate small hail (in
addition to decent rain rates). The threat for severe storms is low,
but the threat of lightning is high enough to include in the
forecast this morning generally along and south of the Mason-Dixon
Line.

High-resolution simulations continue to generate scattered
showers/isolated storms through the morning southeast of I-295 with
at least light rain/showers northwest into eastern Pennsylvania and
northern New Jersey. Though the general idea is present in most high-
res guidance, there are major discrepancies remaining, including
timing of the steadier precipitation across the area, locations with
the maximum QPF, and the strength of the convective showers on the
south side of the precipitation shield. Based on very-near-term
trends, there is reason to place somewhat heavier PoPs in the grids
early-mid morning with gradually lowering values by late morning
through mid afternoon. Precipitation amounts look to be light, but
locally heavier amounts are likely in stronger convective showers.

Models are depicting a second round of somewhat steadier
precipitation moving in late this afternoon generally south of I-78.
Hi-res guidance appears to be converging on the timing (generally 3
pm to midnight), with the best chances likely south of the Mason-
Dixon Line. Therefore, kept PoPs elevated late this afternoon in the
southern CWA with this second steadier round.  Notably, the NAM/NAM
Nest timing looks a little faster than other guidance (HRRR, RAP,
WRF-ARW, WRF-NMM), and the coarser operational guidance (GFS, ECMWF)
appears to agree, which would mean most of the precipitation would
be done by early evening.

There is some question regarding storm potential this afternoon,
with the NAM and WRF simulations depicting an environment of
marginal instability remaining present near Cape May and Georgetown.
Other models indicate negligible CAPE present. Felt the need to
include a slight chance of storms in these areas this afternoon
given the models so far underplaying the convective potential north
of the baroclinic zone in general, but not very confident of this.

Temperatures will hold fairly steady through the day given the
overcast conditions and scattered showers.

This is a very low predictability pattern, as the process of upper
lows cutting off is sensitive to several phenomena poorly
resolved/predicted by numerical models (and not well understood
physically). In addition, the aforementioned small-scale
perturbations ejecting from the main vort max are poorly resolved by
operational models and can be modulated by proximity convection (or
other processes). As such, the forecast for today will be highly
reliant on extrapolation of current observations/trends and will be
subject to several potentially significant modifications over the
course of the day.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SUNDAY/...
The cutoff process of the central U.S. vort max continues tonight,
with the upper low expected to be in the vicinity of the Mid-South
by 12Z Sunday. Operational models generally agree that a
perturbation ejecting from this low will be exiting the Mid-Atlantic
tonight, followed by a brief period of subsidence as downstream
ridging commences along the coast of the Southeast. The current
forecast depicts a drying trend across the CWA during this period,
with PoPs generally confined southeast of I-78 Saturday evening to
far southern Delmarva/southern NJ after midnight. There may even be
some partial clearing in the southern Poconos and vicinity by late
in the night.

Overnight lows are expected to be on the chilly side, with
temperatures in the 30s to around 40 northwest of the Fall Line to
the mid to upper 40s in the urban corridor and areas southeast.

&&

.LONG TERM /SUNDAY THROUGH FRIDAY/...
Surface high pressure/low-level ridge axis extends anticyclonically
from the Southern Plains to the northern Mid Atlantic/Northeast
states. Subsidence underneath the ridge should provide an
opportunity for most of the CWA to dry out on Sunday. PoPs were
decreased considerably but there still remains a low chance for
showers from the Delaware Bay southward. Did not want to
entirely close the door on the possibility of lingering precip
across these far southern zones with some of the models still
supporting the stationary front to get hung up nearby to our
south.

A cutoff low developing over the Mid South late in the weekend is
forecast to gradually progress eastward over the Southeast states
Sunday night-Monday then offshore Monday night. The closed low then
looks to transition to an open wave and turn northward Tuesday in
response to strong downstream ridging over the western Atlantic.
Models generally show this system tracking parallel to the eastern
seaboard but remaining offshore during this time. The period
beginning Sunday night for locales south of the Mason-Dixon Line
(Monday farther north into eastern PA and NJ) through Tuesday or
Tuesday night is setting up to be wet though it`s a bit premature to
label this a total washout. Rainfall does not look to be
particularly heavy since the region is located on the stable, cool
side of the frontal boundary.

Persistent onshore flow and clouds will result in below normal
temperatures (particularly for daytime highs) Sunday through
Tuesday. However, highs on Sunday may wind up being near normal
across NE PA and NW NJ if clouds are able to clear out early enough
to allow for stronger heating to take place in the afternoon.

Conditions should dry out Wednesday as the offshore low passes north
of our latitude and a shortwave ridge builds overhead. With surface
winds quickly becoming southerly and subsidence underneath the
ridge, expect a warmup into the 70s on Wednesday (except cooler
along the coast and at higher elevations in NE PA/NW NJ).

The warming trend continues into Thursday with the region situated
deeper within the warm sector ahead of a cold front. Temperatures on
Thursday will ultimately depend on the timing of this front, which
is still uncertain this far out. We will continue to side close to
WPC guidance, yielding highs in the 80s across most of the region
(cooler again near the coast and in the mountains) with the
assumption that the fropa occurs after peak heating. Added a chance
for showers and thunderstorms during the afternoon and evening but
PoPs were generally kept low due to the aforementioned timing
uncertainty of the front.

The next low pressure system is forecast to organize over the
Southern Plains and then track northeastward through the Midwest and
Great Lakes region late in the week. In this pattern, a warm front
would eventually lift northeastward through the region but a large
temperature gradient that may potentially exist across the front and
high uncertainty in fropa timing greatly degrades forecast
confidence for high temperatures on Friday. We would have the
opportunity for 80s again on Friday if the warm front is quick
to progress through, but if it is delayed, onshore flow on the
cool side of the warm front will yield highs 10-20F lower (a
scenario similar to what we saw yesterday). Forecast represents
an in-between solution, which is lower than the latest WPC
guidance.

&&

.AVIATION /08Z SATURDAY THROUGH WEDNESDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

Low-confidence forecast today. Generally IFR CIGs/VSBYs at all TAF
sites through around daybreak before CIGs rise to MVFR or low-end
VFR during the morning. Light rain or showers are expected to move
through the terminals this morning, with potential for occasional
MVFR/IFR CIGs/VSBYs, especially if heavier rain falls. Improvement
to VFR is expected by afternoon, but another round of rain may move
in by late afternoon or early evening, especially south of KPHL.
Conditions should improve by late evening at the terminals. Winds
should generally be north or northwest with speeds around or below
10 kts through tonight.

OUTLOOK...
Sunday...Predominately VFR with showers staying well south of the
terminals. There is some potential for MVFR CIGs during the late
morning/afternoon near ACY/MIV if stratocu becomes BKN. NE winds 5-
10 kt in the morning veer out of the E-SE by late afternoon.

Sunday night and Monday...Rain moves spreads back northward and CIGs
lower to MVFR late Sunday night and Monday. Low confidence in IFR
CIGs, but if it does happen, it would more likely occur at the
southern most zones (ILG/MIV/ACY). Onshore winds continues.

Monday night through Tuesday night...IFR restrictions likely with
period of rain. Persistent NE winds will be strongest (10-20 kt)
Tuesday afternoon and evening.

Wednesday...Improving conditions. MVFR CIGS may linger into the
first part of the morning before trending to VFR later in the
day.

&&

.MARINE...
Scattered showers and isolated storms will continue today across the
New Jersey/Delaware coastal waters and in Delaware Bay. Outside of
gusty/erratic winds near storms, winds will stay below advisory
criteria through tonight, with seas generally 2-4 feet. There may be
some patchy fog this morning, but visibilities are expected to
remain above 1 SM. Chances for showers decrease late tonight.

OUTLOOK...
Sunday...NEly winds in the morning, becoming Ely in the afternoon.
Winds will increase from S to N (10-15 kt in DE Bay and NJ coastal
waters, 15-20 kt in DE coastal waters). Although there is no SCA
headline issued, there is still a chance that seas could reach 5 ft
in our southern coastal waters where the gradient is strongest.

Sunday night...E-NE winds increase to 15-25 kt late (strongest winds
south). Seas will also increase, likely reaching or exceeding 5
ft in DE coastal waters and possibly farther north adjacent to
the southern NJ coast. SCA likely needed for at least
ANZ454-455.

Monday through Tuesday night...SCA expected across the region with
strengthening E-NE winds 20-30 kt. There is an outside chance for
gales Tuesday when GEFS probability of greater than 30 kt sustained
winds increase to 20-30 percent while seas greater than 9 ft
increase to 50-70 percent in our outer coastal waters.

Wednesday...Winds expected to drop below advisory levels, but seas
may remain elevated.

&&

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

&&

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



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