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Area Forecast Discussion (AFD)

Issued by Philadelphia/Mt Holly (PHI)

FXUS61 KPHI 161814

Area Forecast Discussion
National Weather Service Mount Holly NJ
210 PM EDT Mon Sep 16 2019

A weakening low pressure system will move into the Mid Atlantic
region today, pushing a frontal boundary to our south. High 
pressure builds across the northeast states on Tuesday into 
Wednesday, then shifts southward across the Mid Atlantic region 
Thursday. The high is forecast to shift to our south Friday, 
then offshore over the weekend.



930 AM update:

Fog has generally thinned out across the area, although some 
patchier fog is still evident on satellite over mostly 
Caroline/Sussex counties along with a bit off the Delaware 
coast. This should completely scatter out over the next couple 
hours. Will keep the marine fog advisory going until 10 AM, but 
allowed the over-land advisory to expire (due to visibilities 
improving over 1/2 mile). Otherwise no major changes to previous 

Previous discussion:

The other concern today is a low- amplitude northwest- flow 
perturbation moving through the Northeast. Latest model 
simulations depict strong enough lift to generate some showers 
in the northern CWA, though likely quite light and fairly 
scattered. Nevertheless, confidence is increasing with 
sufficient coverage to nudge PoPs upward a little bit (mainly 
north of Interstate 78) today. Farther to the south, will likely
observe a sufficient period of partial sunshine to see 
appreciable diabatic warming before increased/lower cloud cover 
sets in. This makes the temperature forecast somewhat tricky, 
with MOS showing a fair amount of spread across the region. 
Incorporated a fair amount of hi-res guidance into the highs for
today, as these seem to have reasonable agreement on the 
evolution of clouds/precipitation today. This resulted in a 
forecast quite close to continuity, perhaps a degree or two 
colder overall.

The final concern today is the sea breeze and the timing of the
cold front associated with the perturbation. Temperatures along
the coast will likely be much cooler than areas farther inland,
owing to the sea breeze's push onshore this afternoon. 
Additionally, the cold front will bring fairly stout cold 
advection into the area immediately upstream, so should see a 
sharp gradient in temperatures (including highs) in the northern


Cold front will continue south tonight, with large-scale ascent
possibly remaining sufficient for some showers in the southern 
half of the CWA. There are subtle indications of increasing 
vorticity in the northwest flow overnight, which may explain why
shower coverage in most model simulations appears to increase 
south of the front after dark. Not overly sold on this 
potential, but did increase PoPs in Delmarva and far southern 
New Jersey to at least mentionable during the evening/overnight 

Immediately downstream of the front, may see a resurgence in 
low clouds and fog. However, this potential remains rather 
unclear owing to uncertainty with the frontal position and 
shower coverage. As such, have held off mentioning this in the 
forecast. Meanwhile, for areas north of the front, temperatures 
should be colder, especially as skies clear. However, there may 
be enough of a breeze to keep temperatures from going to values 
suggested by the colder guidance, so kept the forecast close to 
a statistical blend.


Current long range forecast looks on track with a relatively 
tranquil pattern through the weekend.

High pressure is forecast to build across New England Tuesday night 
into Wednesday, then shift southward across the Mid Atlantic region 
on Thursday. The high is then expected to build to our south on 
Friday, then offshore of the southeast states over the weekend. With 
high pressure and its subsidence near the east coast for the rest of 
the week into the weekend, dry conditions are currently forecast, 
although there will likely be periods of cloud cover at times.

A cold front will approach the area from the west on Monday, and a 
pre-frontal trough may develop ahead of the front. There is a slight 
chance that showers may accompany this trough and frontal passage 
late Monday.

Temperatures will be near or slightly below normal Wednesday and 
Thursday, before rising back above normal Friday, and especially 
over weekend into Monday.


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

Today...Mainly VFR, with cloud cover increasing/lowering from 
north to south through the day. Cannot rule out a sprinkle or 
shower, especially at RDG/ABE/TTN, but negligible impacts are 
expected at the terminals. Light/variable winds becoming 
northeast up to 10 kt as a cold front moves through the area 
late in the day. Moderate confidence.

Tonight...Mainly VFR with CIGs around 5000 feet during frontal
passage (possibly somewhat lower, especially at MIV/ACY), likely
improving from north to south late. Winds generally northeast 
up to 10 kt. Low confidence.


Tuesday night-Saturday...Generally VFR conditions with light winds.


930 am update: Dense Fog advisory remains in effect for the
Atlantic waters adjacent to Delaware until 10 AM. Some patchy 
fog may linger after 10AM but is not expected to be dense.

Previous discussion...

Regarding winds/seas, sub-advisory conditions are expected 
through tonight. However, a cold front will be moving through 
during the late afternoon and evening hours. Winds will switch 
to northeast, becoming 10 to 20 kt overnight. Though conditions 
are expected to remain below advisory thresholds, cannot rule 
out an isolated occurrence of 25 kt wind gusts around daybreak. 

Tuesday night-Thursday...Small Craft Advisory conditions likely as 
wind gusts increase to around 25 knots and seas increase to 5-8 feet 
for portions of the Atlantic Coastal waters.

Thursday night-Saturday...Winds lower below advisory levels, 
however, seas will average 6 to 10 feet for the Atlantic coastal 

Rip Currents... 

There is a low risk for the development of dangerous rip 
currents at the New Jersey and Delaware beaches today due to 
light southerly flow and waves of 2-3 feet in the surf zone.




Near Term...CMS/Carr 
Short Term...CMS 
Long Term...Franklin/Robertson