Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000
FXUS61 KPHI 251039
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
639 AM EDT Tue Apr 25 2017

.SYNOPSIS...
Low pressure along the coast of the Carolinas will slowly progress
northward along the Eastern Seaboard through Wednesday before
progressing eastward into the northwest Atlantic late this week.  A
strong subtropical high develops in the western Atlantic Thursday
through the weekend. A front will attempt to approach the area late
this week but will likely stall in the northern Mid-Atlantic
this weekend. A strong cold front will sweep through the eastern
U.S. early next week.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
A well-defined cyclonic circulation associated with an upper
low was seen early this morning in the GOES-16 satellite water
vapor loop with the center of it over SC. The surface reflection
of this vertically-stacked cyclone was a 1000-mb low pressure
centered just off the NC coast. A coastal front was positioned
on the northwestern side of the low over eastern NC.
Additionally, high pressure was centered well to our north over
eastern Canada.

With our area positioned between the low to our south and the high
to our north, east-northeast winds off the ocean were advecting
low clouds inland overnight. Adjusted PoPs to reflect a limited
coverage of showers until the steadier rain moves in from the
south. Expect the leading edge of this steady precip to reach
our Delmarva zones during the mid morning, Phila area around
midday and I-78/I-80 corridors in northeastern PA/northwestern
NJ mid to late afternoon. Models have been in very good
agreement with this timing. Kept a slight chance for thunderstorms
S/E of I-95, where hi-res CAM guidance was highlighting a
tongue of modest elevated instability mainly this afternoon.

There is a risk for locally heavy rainfall. A heavy rain band that
was occurring upstream over NC was juxtaposed with the TROWAL and
easterly low-level jet on the northern side of the low as well as an
inverted surface trough that extends northwestward from the surface
low. Accordingly, these features were used to prognosticate the
timing and location of the heaviest rain for our area later today.
These features appear to provide a favorable setup for heavy
rainfall mainly this afternoon in the coastal plain. Model spread
for QPF has also narrowed with the latest iteration of guidance,
adding to forecast confidence. Official rainfall forecast was a
blend between WPC and the wetter NAM/HRRR/GEM to take into account
the potential for convection to enhance rates this afternoon in
eastern portions of DE and NJ. The highest amounts in these
locations were 1.5-2.0".

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM WEDNESDAY/...
Low pressure near the Outer Banks late this afternoon will
track north-northeastward and just off the Delmarva coast
tonight. The widespread rain should exit the region toward
sunset as both the low-level jet and TROWAL will progress
downstream of us. However, the threat of locally heavy rainfall
will persist through the evening in the coastal plain as the
coastal front sets up near the NJ coast. Elevated instability
associated with the cold pool aloft under the upper low could
support a few thunderstorms in E NJ. Expect showers to end from
SW to NE this evening in E MD and overnight elsewhere.

Moisture trapped beneath a low-level inversion will keep stratus
and fog over the area tonight. There is a potential for fog to
become dense in spots, especially near the coast. The combination
of persistent onshore flow and overcast conditions should
prevent temperatures from falling much more than a few degrees
tonight. Lows range from the upper 40s in the Poconos to the
upper 50s in S DE.

&&

.LONG TERM /WEDNESDAY THROUGH MONDAY/...
By 12Z Wednesday, the slow-moving surface low should be
positioned just off the coast of Delaware or New Jersey, with
models already showing quite a bit of spread in its placement
and movement thereafter. Fortunately, the implications of these
model differences are relatively minor for our area, as
precipitation will be winding down in our region by this point.
Nevertheless, some residual showers are likely to persist on the
northwest side of the low, potentially aided by a coastal front
that will be slow to dissipate and/or move away from the
northern/central NJ coast during the day. As midlevel subsidence
and attendant drying move in during the day, precipitation will
become lighter/spottier, with most of the showers likely out of
the area Wednesday night. Regardless of the coverage of showers,
low-level clouds are likely to stick around for much of the
day, especially north/east of the Delmarva Peninsula. There may
even be some fog on Wednesday morning and again Wednesday night.
Most guidance looked too warm given the expected persistence of
the cloud cover, so went just above the colder MET guidance for
max temperatures, though this may still be too warm.

Midlevel ridging will amplify quickly upstream of the weakening
low, as a potent shortwave trough progresses into central North
America. Models are showing a considerable amount of
disagreement with the small-scale phenomena associated with this
feature, with the 00Z GFS in particular being problematic owing
to what appears to be a substantial amount of convective
feedback. Placed little or no weight on the GFS solution for the
rest of the long-term forecast as a result. However, confidence
is not much better with the rest of the model suite given the
complex interactions associated with the smaller-scale
perturbations revolving around the larger-scale cyclonic flow.
My suspicion is that the models are likely a bit too progressive
with the system in general, particularly given the retrograding
subtropical ridge in the western Atlantic, which will aid in
the amplification of the midlevel East Coast ridging. This is a
blocky pattern, and models tend to progress waves too readily in
such a regime.

A surface low is forecast to progress NNE from the central U.S.
into the Great Lakes/southern Canada Wednesday and Thursday.
The associated cold front will struggle to move eastward as
blocky ridging takes hold. Despite weakening large-scale lift as
the surface low and attendant midlevel vort max move well north
of the region late this week, frontal ascent in a moistening
warm sector should be enough to generate isolated to scattered
convection along the front as it progresses into the eastern
Ohio Valley and adjacent Appalachians Thursday night. The front
makes its run for the CWA Thursday night through Friday night,
with the highest chances for precipitation generally north of
I-78. There will be sufficient instability for thunderstorms.
GFS forecast soundings do hint at a somewhat favorable CAPE-
shear parameter space for severe storms on Friday during peak
heating, but the displacement from the main vort max seems to be
a primary limiting factor. Nevertheless, this will be something
to monitor as the end of the week approaches.

The main question in the long-term period is what happens to
this front over the weekend. The strong subtropical ridge in the
western Atlantic and the displacement northward of the vort max
into far eastern Canada suggests the front will likely stall
somewhere in the Northeast. Unfortunately, this could be
relatively close to our region, which sets up this forecast for
major potential for error over the weekend.

For now, it seems that most of the operational mid-range models
place the front north of the area. This would make for a warm
and humid weekend as moist southerly/southeasterly flow
entrenches itself equatorward of the remnant front.
Perturbations in the west-southwesterly midlevel flow would move
generally parallel to this boundary, bringing occasional chances
for convection near it. Because of the uncertainty in the
placement of this boundary, its general proximity to the region,
and added uncertainty associated with the timing of these
perturbations, felt a broadbrush of slight chance to low-end
chance PoPs were justified through the weekend across the area.
Kept temperatures close to the previous forecast (i.e., on the
warm side) given the higher likelihood of the front not quite
making it through the area. Note that this is not an indication
that it will storm the whole weekend or even very much of it;
instead, it is an indication that there could be some storms at
some point this weekend.

Importantly, the weekend forecast is low confidence. If the
front ends up being closer to or even through the region,
temperatures will be considerably colder than forecast. However,
the trend has been to position this boundary farther north
(i.e., the amplified ridging has been forecast to be stronger)
with time, which makes sense given the blocky nature of the
pattern, so this seems like a fairly low probability outcome at
this point. One other caveat - the closer this boundary is to
the area - the better the chances for more precipitation this
weekend.

By early next week, a deep, negatively-tilted trough approaches
the eastern U.S. with an attendant strong cold front moving
through the region. Widespread/strong convection will likely
develop along the front as it sweeps through the Northeast on
Monday (if the timing of the model guidance is to be believed).
Pattern recognition suggests the potential for a more pronounced
severe threat as this boundary moves through. However, given the
large spread in simulations by this point, too early to get into
the finer details.

&&

.AVIATION /09Z TUESDAY THROUGH SATURDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

Stratus advecting inland will lead to the development of IFR CIGs
from E to W overnight. IFR should reach PHL/ILG by 08-09Z and
then RDG/ABE between 10-12Z. Generally remaining IFR today with
mod to locally heavy rain moving in from S to N between 15-19Z.
Local/temporary LIFR restrictions possible in the heaviest
rain. E-NE winds 10-15 kt with gusts to 20 kt expected at most
terminals today. Stronger winds near the coast will affect ACY
where gusts to 30 kt are possible late this morning and
afternoon.

Steady rain ends from SW to NE between 22-03Z. However, do not
anticipate much improvement in the CIGs behind it with low clouds
trapped over the region. Expect IFR to LIFR conditions for most of
tonight. Some of the guidance is indicating dense fog tonight but
this may be a bit overdone. Nonetheless, restrictions from VSBY
are also expected.

OUTLOOK...
Wednesday...Scattered showers, especially in NJ. Residual low
clouds/fog possible in the morning, but conditions should
primarily be VFR by afternoon. Winds north or northeast 5 to 15
kts. Confidence below average.

Wednesday night...Low clouds and fog (MVFR/IFR) again possible
with light winds. Confidence below average.

Thursday...Winds becoming southeast and increasing to 5 to 15
kts. Potential for some gusts near the coast. Slight chance for
thunderstorms late, mainly north/west of KPHL. Generally VFR,
except locally sub-VFR conditions near any storms. Confidence
average.

Thursday night through Saturday...Generally VFR with
south/southwest winds 5 to 15 kts. Locally sub-VFR conditions
with isolated/scattered storms, especially Thursday night and
Friday. Best chances for storms near/north of KPHL. Confidence
average.

&&

.MARINE...
A Gale Warning remains in effect for the coastal waters and the
lower Delaware Bay today. Do not expect a long duration of gales
today with the period generally confined to 2-4 hours (longer in the
DE coastal waters). It looks to be an increasingly marginal setup
the farther north you go along the coast with it being highly
uncertain if winds get to gale force in our northern coastal waters
of NJ. Seas will quickly build to 7-10 ft today in concert with the
surge of gale-force winds.

A SCA remains in effect for the upper DE Bay though the ending time
of the SCA was pushed ahead to this evening with winds quickly
ramping down as the coastal low approaches.

A SCA will be needed for the coastal waters for tonight after the
GLW expires this evening or is cancelled beforehand.

A Marine Dense Fog Advisory may be needed for tonight with a
very moist airmass advecting over the relatively cool waters.
The potential for dense fog would be favored after the rain ends
this evening.

OUTLOOK...
Wednesday through Friday...Sub-advisory level winds expected.
However, seas will likely remain near or above 5 feet through
the period. A chance of showers on Wednesday, and a chance of
storms late Thursday night through Friday. There may be some
restrictions to visibility Wednesday and Wednesday night from
patchy fog.

Friday night...Sub-advisory conditions expected. A chance of
thunderstorms persists, with locally gusty/erratic winds near
any storms.

Saturday...Some potential for advisory-level winds by afternoon.
A low chance of storms.

&&

.TIDES/COASTAL FLOODING...
A Coastal Flood Advisory was issued for tonight for coastal
zones with minor tidal flooding expected along the DE and NJ
shores, in the DE Bay and on the far lower part of the DE river.
The minor flooding should also affect the back bays and the
estuaries. This tidal flooding episode is a result of the
cumulative effects of onshore flow and the approaching new moon.
Overwhelming majority of guidance (ESTOFS, ETSS and ensembles
from Stevens Institute) continues to predict widespread minor
coastal flooding at all of our tidal forecast points this
evening along oceanfront and DE Bay. The surge on top of the
base astronomical tides is expected to be a foot to a foot and a
half at that time.

With all that said, we will have to monitor the potential need
to upgrade to a warning if ESTOFS verifies, which predicts Cape
May, Atlantic City and Lewes will reach moderate flooding. This
appears to be an outlier solution with the setup not particularly
primed for a moderate coastal flood event.

It appears as though the tidal Delaware River above the Commodore
Barry Bridge area may just reach the minor flooding threshold.
However, the impacts should not be widespread enough there to
warrant a Coastal Flood Advisory.

We are not anticipating any coastal flooding along the upper eastern
shore of Chesapeake Bay.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...Coastal Flood Advisory from 6 PM this evening to midnight EDT
     tonight for NJZ012>014-020>027.
     Coastal Flood Advisory from 9 PM this evening to 1 AM EDT
     Wednesday for NJZ016.
DE...Coastal Flood Advisory from 6 PM this evening to midnight EDT
     tonight for DEZ002>004.
     Coastal Flood Advisory from 9 PM this evening to 1 AM EDT
     Wednesday for DEZ001.
MD...None.
MARINE...Gale Warning until 6 PM EDT this evening for ANZ431-450>455.
     Small Craft Advisory until 9 PM EDT this evening for ANZ430.

&&

$$
Synopsis...CMS
Near Term...Klein
Short Term...Klein
Long Term...CMS/Klein
Aviation...CMS/Klein
Marine...CMS/Klein
Tides/Coastal Flooding...Klein/Iovino



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