Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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474
FXUS61 KPHI 231350
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
950 AM EDT Sun Jul 23 2017

.SYNOPSIS...
A frontal boundary extended from the southern Great Lakes to the mid
Atlantic states this morning with one low pressure system south of
Long Island,  moving seaward. Another low will follow tonight,
passing south of Long Island on Monday. The cold front will trail
down into yet another low over the Delmarva Peninsula Monday night.
Canadian high pressure builds down into the northeast United States
Tuesday and Wednesday. A cold front approaching from the northwest
late Thursday may be very slow to pass through the mid Atlantic
states next Friday and Saturday.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Cloud cover is going to hang tough, owing to some northeasterly
flow that will seep into the region. KDIX is indicating an
outflow boundary from the aforementioned MCV that is sagging
slowly southward through central New Jersey (as of 930 am
located generally from near the Philadelphia International
Airport to Atlantic City). Winds become northeasterly to the
north of this boundary, and dew points are a few degrees lower.
Higher- resolution model output suggests this may continue
southward to approximately the Mason- Dixon Line through a
considerable portion of the day (at least a chunk of the morning
hours) - some showers continue to develop along this boundary
bur remain light. Models are notoriously too eager with
dissipating such boundaries or returning them northward. Bumped
down dew points considerably (around 3-5 degrees) north of the
Mason- Dixon Line today and re-oriented the winds to a more
northeasterly direction for a longer duration for these areas as
well. Guidance is noticeably cooler with highs today, and this
makes sense given the current observational trends. Will be
interesting to see how much clearing, if any, occurs today.

Regarding convective potential, the next in a series of midlevel
perturbations moves toward the region this afternoon and
tonight, and with somewhat more focused large-scale ascent
downstream in a weakly to negligibly capped environment,
scattered to widespread convection should develop this afternoon
to our west. The timing of its approach to our region looks
rather late in the day, and held off on higher PoPs to mostly
the tonight period. However, some storms may move into the far
western CWA by late afternoon. With the approach of stronger
westerlies as a larger-scale trough begins to amplify today,
vertical shear profiles will continue to improve. Think the main
question today will be degree of instability, with remnant cloud
cover and outflow boundaries likely making model depictions of
the thermodynamic environment mere pipe dreams.

However, southwesterly flow to the south/west of the area
should advect warm/moist air northeastward, allowing boundary-
layer based parcels to attain CAPE values 1000-2000 J/kg by
afternoon near/south of the convectively-reinforced surface
boundary in the southern Mid-Atlantic. Where this boundary ends
up is anyone`s guess (mine is south of consensus, based on the
past 24 hours of analysis). Should such instability materialize,
CAPE-shear parameter space is favorable for organized severe,
with downburst winds associated with bowing segments and/or
mesovortices embedded within convective segments/clusters the
main threat. Highly heterogeneous convective environment would
suggest that more organized (lengthier) lines of convection may
be hard to materialize, especially given the very moist boundary
layer hindering the development of somewhat stronger cold
pools. CAM guidance hints at such a scenario playing out, and
given pattern recognition and the past 24 hours as "analog
guidance", this increases confidence in at least the overall
mode of strong to severe convection this afternoon/evening -
again, should the stronger instability pan out.

With PWs 1.75+ inches (somewhat lower than the past 24 hours,
aided by somewhat drier midlevels and a southward push of the
convectively-induced surface boundary) and the potential for
strong updrafts should near-surface based instability
materialize, heavy rainfall is a threat with any of the stronger
storms today.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM MONDAY/...
Tonight`s forecast could be metaphorically described as throwing
darts in the dark.

As convection from the west moves into the region overnight,
there is a clear signal from CAM guidance of developing a
mesoscale convective vortex on the north side of the convection,
with larger-scale influence via a developing/eastward-moving
surface low translating across the Mid-Atlantic during this
period (as the trough in the eastern Ohio Valley continues to
amplify). Some CAM depictions (and even some of the coarser
guidance) produce very alarming QPF during this period (with 3
to 6+ inches indicated in a corridor generally between I-78 and
I-80) as the MCV trudges eastward. This seems way overdone, and
the CAMs have had a history of that in the past 24 hours.

The concern is that the thermodynamic support just will not be
there (or, alternatively, will set up farther south) since the
convection occurring the past few hours has displaced the
surface boundary to far southern Virginia at this time. My
suspicion is that the higher side of the QPF distribution is a
low probability, but I suspect there will be a swath of heavy
rainfall totals (maybe 1-3 inches rather than 3-6) where
convective organization combines with large-scale support.
Regarding the latter, as the trough to the west amplifies, a
downstream 250-mb jet streak in New England provides substantial
upper-level divergence. Considerable warm/moist air will be
lofted above the zonally- oriented convectively-reinforced
boundary ... somewhere ... in the Mid-Atlantic region and
advected northward. Meanwhile, with antecedent heavy rainfall in
portions of the region (the Lehigh to Monmouth County corridor,
for instance), ingredients are in play for a flood event. Too
much uncertainty for a watch at this time, but flooding
potential is definitely a concern tonight. Stay tuned.

Temp forecast is a blend of MET/MAV MOS with some addition of
CAM 2-meter temperatures to account for effects from
precipitation.

&&

.LONG TERM /MONDAY THROUGH SATURDAY/...
500 MB: A trough crossing New York State on Monday reaches eastern
New England Tuesday with ridging height rises to follow Wednesday.
The next trough...complex...moves into the northeast USA for
the end of the week.

Temperatures: Calendar day averages about 5 degrees above normal Monday
near normal Tuesday through Thursday...and then may run several degrees
above normal Friday and possibly into the weekend as well?

Forecast basis: a 50 50 blend of the 00z/23 GFS/NAM MOS was applied
Monday-Tuesday (except used mainly 00z/23 GFS MAV TT/TD), then
00z/23 GFS MEX MOS Tuesday night before relying heavily on the
05z/23 WPC D4-8 elements for Wednesday - Saturday.

The dailies...

Monday... Unsettled conditions into Monday evening. The frontal
boundary is expected to drift south through our area with lingering
shower and thunderstorm potential along and ahead of the front.
We just dont know extensive any showers will be.  Precipitable
water values should be decreasing slowly from north to south...
to an average of 1.5 inches. Partial clearing develops from
north to south later at night, with patchy dense fog possible.
PWAT is modeled to decrease to 1.2 inches by Tuesday morning.

Tuesday...The axis of a mid level short wave trough has passed by in
the morning with height rises expected.  It should be drying out.
A fly in the ointment may be onshore flow and a sprinkle on the
coasts?

Wednesday...Partly sunny, possibly after morning stratus or fog?
Again there is some modeling hint of sprinkles on the coast.

Thursday...Warm air advection develops late in the day. Chance
of showers and isolated tstms late.

Friday...a chance of showers and thunderstorms with the next
cold front, especially morning.

Saturday...Hopefully dry weather returns but models are slowing
the front and possibly stalling it as a tricky complex mid level
pattern develops. One short wave passes to our northeast while
another digs to our west, building the Atlantic ridge westward
and making southwest flow aloft tend to stall the front. If that
happens...showers would be in our area.

&&

.AVIATION /14Z SUNDAY THROUGH THURSDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

MVFR/IFR ceilings this morning generally along and north of the
Mason-Dixon Line should slowly improve to VFR in the 15Z-18Z
time frame with northeast winds 5-10 kts. Thereafter, winds may
become easterly or southeasterly (or even transition to
southwesterly this afternoon for a time), remaining around 5-10
kts. Stray showers may occur this morning, but best chances for
precipitation are after 00Z, where scattered to widespread
strong thunderstorms are expected to move through the area.
Erratic winds/gusts are likely with any shower/storm.

OUTLOOK...

Monday...VFR.  A chance of showers and thunderstorms with
associated sub VFR conditions along a cold front.

Monday night...Conditions lowering to MVFR and possibly IFR in fog
patches with clearing after a chance of evening showers and
thunderstorms.

Tuesday...MVFR and IFR conditions in morning stratus/fog
improving to VFR.

Tuesday night through Thursday...Mainly VFR though some MVFR fog
or stratus possible Tuesday night and Wednesday morning.

&&

.MARINE...
Sub-advisory conditions are expected to continue through
tonight. Storms are in the forecast again tonight, and some of
these may be strong and will likely produce heavy rainfall.
Locally rough seas should be expected near precipitation.
Outside of any precipitation, seas will generally be 2-4 feet
with a northeast to east flow developing this morning veering to
the southeast tonight, generally 5-15 kts.

OUTLOOK...
Monday through Thursday...No marine headlines are anticipated.

RIP CURRENTS...
The moderate risk of rip currents was lowered to low risk for
Delaware, as the underlying swell of 10-12 seconds has lessened,
and the swell direction is less favorable for an increased risk. A
moderate risk of rip currents is expected to continue through
today for the New Jersey coast as the new moon will combine
with an underlying swell of 10-12 seconds to create a more
favorable environment for rip currents.

&&

.TIDES/COASTAL FLOODING...
We have issued another coastal flood advisory for this evening`s
high tide for the Atlantic coastal waters and for adjacent
portions of the coast of Delaware Bay, as the new moon combined
with increased onshore flow today should provide conditions at
least as favorable, if not somewhat more so, for minor coastal
flooding. With last evening`s high tide, some minor flooding
occurred in most of this area (though generally below advisory
levels). Thus, with the somewhat higher forecasts observed with
this evening`s high tide, an advisory seems warranted.

&&

.CLIMATE...
ABE month of July rainfall as of 1 am...6.46",20th wettest in
the POR dating back to 1922. July record 10.42" 1969.

July temperatures will average above normal for most of our
area except possibly KMPO.

Monthly positive departures will slip about a degree from where
they stand the first 22 days of the the month.

PHL is projecting around 80.2 degrees or 2.1 degrees above the
norm of 78.1. That would mean RDG TTN ABE ILG would average 1 to
2 degrees above normal for the month, ACY Marina 0.5 deg, ACY -
Pomona about 2.5 above and GED about 3.5 degrees above normal.
Only KMPO would possibly be slightly below normal.

&&

.EQUIPMENT...
Dew point readings at KDOV continue to measure too high
compared to surrounding locations and should be treated as
unrepresentative of the area.

Hibernia PA and Hamburg NJ transmitters are off the air. Both
have trouble tickets. No known rts.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...Coastal Flood Advisory from 7 PM this evening to 1 AM EDT
     Monday for NJZ012>014-020>027.
DE...Coastal Flood Advisory from 7 PM this evening to 1 AM EDT
     Monday for DEZ002>004.
MD...None.
MARINE...None.

&&

$$
Synopsis...Drag
Near Term...CMS/Meola
Short Term...CMS
Long Term...Drag
Aviation...CMS/Drag/Meola
Marine...CMS/Drag
Tides/Coastal Flooding...
Climate...
Equipment...



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