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Area Forecast Discussion (AFD)

Issued by Philadelphia/Mt Holly (PHI)

FXUS61 KPHI 231056

Area Forecast Discussion
National Weather Service Mount Holly NJ
656 AM EDT Fri Aug 23 2019

A cold front will continue slowly push to our south today and 
move to our south over the weekend as high pressure builds to 
our north through early next week. The high will try to nose 
itself down the east coast, but not fully make its way into our 
area. This high will retreat north and eastward by Tuesday and 
Wednesday as an area of low lifts northward offshore of the east
coast and a cold front approaches from the west around the 
middle to end of next week.


700 am update: Refined PoPs across the area this morning based
on stronger consensus of the 06z hi-res convection-allowing
models and radar trends. Looks like a persistent band of showers
will slowly sag southward through the day, something that a lot
of the models wanted to dissipate much too quickly. Instead,
thinking this band may persist into the afternoon, while
convection develops farther south as the perturbation discussed
below approaches the southern CWA later today. Also, a few
isolated showers may develop this morning southeast of the more
persistent band.

Previous discussion...

A weak midlevel perturbation is moving through the area at this
time, which is generating enough cooling aloft to allow showers
to develop along the increasingly nebulous surface front, which
is eastern PA and and northern/central
NJ. Overnight deterministic models appeared to place the front 
too far south, likely owing to complicating effects from 
antecedent convection. Furthermore, hi-res models are struggling
mightily with the showers in the CWA now. As such, several 
amendments to the forecast have been required based on radar 
trends the past few hours. Main adjustments early this morning 
are to maintain high- chance PoPs along a Reading to Trenton to 
Morristown corridor, gradually shifting this south while 
lowering PoPs as the perturbation moves east of the area.

Nevertheless, the residual effects of these showers will be 
thicker cloud cover for longer north of the front, and this has 
required lowering temperatures several degrees today (generally 
near/north of the I-76 corridor). Additionally, as subsequent 
vorticity maxima move east along the frontal boundary, 
additional showers cannot be ruled but should become confined to
areas south of I-78 by afternoon and south of I-76 by late in 
the day.

Areas near and to the south of the front will still see some 
latent instability and will be in much closer proximity to the 
strongest large-scale and mesoscale ascent. A fairly strong 
perturbation will approach the area this afternoon. Anticipating
storms will develop near the Mason-Dixon Line (and southward), 
riding to the east of the perturbation through the evening 
hours. Hi-res models are generating quite a bit of convection 
from Delmarva southward this afternoon. Strongest instability 
will be south of the CWA, so the best chances of severe storms 
will be south of the area as well. Nevertheless, cannot rule out
the potential entirely. Additionally, stronger storms will be 
capable of producing locally heavy rainfall, though not 
expecting much of a flooding threat today.

The potential exists for showers (and maybe an isolated storm 
or two) to reach as far north as the I-76 corridor, but this 
will depend entirely on the progress of the front today. Model 
trends are a little slower in general, but there will be 
complicating effects from the clouds/showers early this morning 
(among other factors). Expect adjustments to the forecast as the
details on frontal placement and the evolution of this 
afternoon's vort max become clearer.


Storms may be ongoing in southern portions of the CWA during 
the evening hours, but as the attendant perturbation moves 
offshore, the storms will move southeast of the area during the 
overnight hours. Hi-res simulations are indicating the potential
for storm totals over an inch in portions of Delmarva (and 
possibly far southern New Jersey). Cannot rule out a rogue 
severe storm or nuisance instance of flooding, but these chances
appear fairly remote as the stronger instability/lift will be 
well south of the area by this time frame.

Drier air will filter into the area overnight, with sky cover 
improving north to south with time. Lows will be considerably 
cooler tonight (probably by about 10 degrees, give or take) as a
combination of cold advection and some radiational cooling take


Drier and cooler conditions start the long term period, before 
warmer weather and greater chances of precipitation return by 
the middle of next week.

As we start the weekend, Saturday is expected to be a very 
pleasant day. High pressure will be building to our north across
eastern Canada, with our area on the far southern edge. This 
will keep easterly flow across the area, and as a trough/low 
aloft moves across the area, temperatures will cool to near or 
below normal. Dry weather is expected, with mainly 
afternoon/evening clouds due to the trough/low moving overhead.

However, as we move into Sunday and Monday, the easterly flow 
will continue as high pressure builds a little farther southward
across New England. The high will try to nose its way down the 
east coat toward the Mid Atlantic region during this time as 
well. Guidance continues to indicate precipitation developing 
across the area during this period, so we will have a slight 
chance of showers Sunday through Monday. However, it may just 
end up being more cloudy rather than rainy as some stratocumulus
clouds could develop with the persistent easterly flow. 
Regardless, if it does rain, it is expected to be light a PW 
values are mostly an inch or less.

As we move into Tuesday through Thursday, unsettled weather 
could return to the area. An area of low pressure is forecast to
be lifting northward offshore of the east coast Tuesday into 
Tuesday night, while a cold front approaches from the west 
during the day Wednesday into Thursday. There are some timing 
differences with these systems between the GFS and ECMWF. 
However, there will be an increasing chance of showers and 
thunderstorms starting Tuesday and continuing through Wednesday,
and possibly continuing into Thursday, depending on how fast 
the cold front moves through.


The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, 
KMIV, KACY and surrounding areas.

Today...Persistent band of showers will shift slowly southward
today, affecting RDG/ABE/TTN this morning, PHL/PNE/ILG late this
morning into early afternoon, and ILG/ACY/MIV around midday
onward. Meanwhile, more widespread showers/storms may affect
areas south of the Interstate 76 corridor this afternoon. Brief
sub-VFR conditions are expected in proximity to showers, but
MVFR CIGs may occur on occasion for much of the morning outside
of precipitation as well. North to northwest winds around 10

Tonight...Mainly VFR with light northwest or north winds. Any 
evening showers/storms in Delmarva and southern New Jersey 
should move southeast away from the terminals by the overnight 


Saturday-Saturday night...VFR conditions expected. Winds 
generally northeast 5-10 knots.

Sunday...Mostly VFR conditions expected. Winds remain northeast
5-10 knots, with gusts 15-20 knots possible.

Sunday night-Tuesday...MVFR ceilings may begin to develop 
overnight Sunday and continue into Monday for some areas. Winds 
remain east to northeast 5-10 knots Sunday night through Monday 
night before shifting to southeast on Tuesday. Gusts 15-20 knots
possible at times.


Sub-advisory winds/seas are expected through the period, with 
north to northeast winds 5 to 15 kts through this evening 
increasing to 10 to 20 kts overnight. Seas will generally be 2 
to 4 feet.

Once again, there is a chance of storms today, generally for 
Delaware Bay and the Atlantic waters south of Atlantic City. 
Locally stronger gusts and higher waves should be expected in 
their proximity.


Friday-Saturday night...Conditions expected to remain below 
advisory levels, although winds could gust around 20 knots at 

Sunday-Monday...Winds may approach advisory levels and gust 
around 25 knots at times. However, seas are forecast to build to
around 5 feet, so a Small Craft Advisory may be needed.

Monday night-Tuesday...Winds may drop below advisory levels and
gust around 20 knots at times. However, seas are forecast to 
build to around 5 feet, so a Small Craft Advisory may be needed.

A northerly wind of 5 to 15 mph is expected today. Breaking waves 
around 2 to 3 feet are likely along with a medium-period southerly 
swell. A low risk of rip currents is forecast.

A long-duration northeast to east wind is expected for the weekend 
and into the early part of the new week. As a result, we are 
anticipating a prolonged period with an enhanced risk for the 
development of dangerous rip currents from Saturday through at least 


A prolonged northeast to east flow is expected Saturday through
at least Tuesday. This persistent onshore fetch, combined with 
increasing astronomical tides due to an approaching new moon on 
Thursday will likely lead to increasing coastal water levels. 
Some guidance is already indicating that minor coastal flooding 
is becoming more likely, possibly starting as early as Sunday, 
but more likely Monday and Tuesday.




Near Term...CMS
Short Term...CMS
Long Term...Robertson
Tides/Coastal Flooding...Robertson