Area Forecast Discussion
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FXUS61 KPHI 281937
Area Forecast Discussion
National Weather Service Mount Holly NJ
337 PM EDT Wed Sep 28 2016
An area of low pressure will move across the Ohio Valley and
combine with another low across the central and southern
Appalachians Thursday into Friday. A cold front will cross the
region around Sunday. High pressure is then expected to build
to our north into early next week.
.NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...
1040 am: Ceilings 800 to 1800 ft spreading southwest onto the NJ
coast now will eventually reach PHL around 19 or 20z. Cold air
damming minimal today since the big push of cool moist boundary
layer air doesn`t arrive most of our fcst area until after max
heating today. Cold air damming does become a much bigger player
tomorrow. Gusty e-ne wind 20-30 mph develops by this afternoon.
Showers and isolated embedded tstms MD e shore and w DE into
extreme sw NJ with diminishing intensity so the 1040 AM pop
adjustment was to account for the mid level rain dying out
midday/early this afternoon. while was not exactly sure how to
handle this CHES Bay organized system that dumped 3-4.5" on the
other side of the Bay and 1-2" parts of the MD e shore, it is on
We have seen the SPC expanded marginal risk into MD E shore but no
action at this time for enhanced wording. Will reevaluate with 12z
GFS guidance. NAM has not handled this system very well so am
inclined to favor GGEM/GFS/EC/UK/WPC blend.
Forecast soundings support a deep enough moist layer for at least
patchy drizzle to develop late this afternoon, toward the NJ coast.
High temperatures range from the mid/upper 60s along and north of
the I-80 corridor to mid/upper 70s across southeastern PA,
southern NJ and Delmarva.
.SHORT TERM /6 AM THURSDAY MORNING THROUGH 6 PM THURSDAY/...
The upper low is forecast to slow down over the Midwest states
tonight as it becomes cutoff. High pressure will continue to
remain anchored to our north while the frontal boundary remains
stalled to our south. The easterly low-level jet between these
systems over the mid Atlantic will strengthen further to around 40
Expect showers to expand northeastward across the entire area
tonight. Expect for the NAM-based guidance, models continue to
show a band of heavy rainfall developing very close to the western
most zones of our CWA. The trend has been to shift the axis of
higher QPF farther south and west, likely as models are more
accurately capturing the magnitude of the cold-air damming
pattern. Additionally, with the greatest low-level convergence
situated at nose of the low-level jet forecast to be positioned to
our west, the greatest risk for excessive rainfall may remain on
the western side of the Chesapeake Bay. QPF amounts of 1-2 inches
are forecast for eastern MD and out toward Chester/Berks Co. in
eastern PA. We opted to not issue a Flood Watch at this juncture.
.LONG TERM /THURSDAY NIGHT THROUGH WEDNESDAY/...
Unsettled weather continues for the first portion of the extended
fcst, with some improvement by the end.
A large area of upr lvl low pres will be located to our s and w at
the start of the extendd pd. This low will spin in place then
gradually retrograde to the Grtlks and sern Canada by the weekend.
At the sfc, an area of low pres will be located over the midwest,
with high pres to the ne over New Eng and sern Canada. The sfc
features will keep a persistent moist nely flow over the area at
the sfc with swly flow aloft. As the sfc low moves by to the nw,
one pd of rain is expected and as several other s/wvs rotate thru
the upr low several other pds will occur.
As is usually the case with upr lows, it will not rain the whole
time. It will be cloudy and dreary with pds of rain. The mdls
differ on which times will be rainy and where those rainy pds will
occur. For example on Friday, the GFS is mainly dry, while the
ECMWF is fairly wet and the NAM somewhere in the middle.
Have tried to make adjustments to pops where there was a decent
signal in the mdls, but these times were limited and overall
confidence on which pds will be rainy are low.
By Sat and Sun, the upr low moves nwd into the Grtlks area and
begins to weaken and fill. Overall precip chcs shud decrease.
However, guid today is wetter than it was yda and once again it
will depend on individual w/wvs. Sat shud be wetter than Sun with
precip chcs decreasing with each passing day. Some guid now even
wants to linger precip chcs into Mon until the upr low finally
passes to our e and high pres builds in behind it.
By Wednesday, all eyes turn to the tropics and any potential
impacts from what is now Tropical Storm Matthew later in the week.
Please see NHC advisories for updates.
.AVIATION /19Z WEDNESDAY THROUGH MONDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.
This afternoon...VFR conditions to start. Cigs 1000-2200 feet advect-
develop southwestward fm the NJ now and spread west to PHl by 18z.
showers vcnty KILG and KMIV may reach PHL but these showers are
tending to dry out as they move northeast. drizzle may develop
KACY late in the day. light n-ne wind early this morning becomes
gusty northeast 20-28 kt by this afternoon.
Tonight...widespread cigs 700-1700 ft with vsby occasionally aob
3 mi in stratus, mist and fog with showers. ne wind with scattered
gusts 20 kt.
Thu night-Fri...A prolonged period of MVFR or IFR conditions with
pds of RA expected to affect the TAF sites through the period.
E to Ne winds are expected to gust 20-25 knots for much of the
area, especially KMIV and KACY. Moderate confidence.
Fri night-Sun...Some improvement, possibly to VFR by Sun. SHRA remain
psbl. Moderate confidence.
SCA in effect for the NJ coast. A Gale Watch was previously in
effect for ANZ450/451 for tonight but we do not anticipate gales
to develop before Thursday. Winds of 15- 25 kt are expected with
gusts to 30 kt through tonight along the NJ coast. Seas in the
coastal Atlantic waters will subsequently build to 5-7 ft by
tonight. SCA continues with a 6 PM start for De Bay and the
Atlantic DE waters though could see a sooner start.
Thursday night...Gale warning in effect early, will likely need to
Fri...Gale wind may remain across the northern half of the
coastal waters. SCA may be needed elsewhere.
Fri night-Sat...Wind expected to drop below SCA, however, seas
will likely remain above 5 feet thru the pd.
Sat night-Sun...Wind and seas expected to drop below SCA levels
and no marine flags anticipated.
There continues a moderate risk for the formation of dangerous
rip currents today as per a recheck of the ene sustained wind
~18kt and dominant 4 second short period swell.
Thursday: The probability for the formation of dangerous rip
currents is at least moderate to perhaps high.
A persistent onshore flow is expected for the rest of the week
into the weekend. The strongest winds are expected Thursday through
Friday. Tides levels are forecast to increase through this period,
and we could reach minor coastal flood levels.
Will add extensive climate information late this morning or this
afternoon as time permits.
MD...Flash Flood Watch through Thursday afternoon for MDZ008-012-
MARINE...Gale Warning from 4 AM Thursday to 2 AM EDT Friday for ANZ430-
Gale Warning from 4 AM Thursday to 6 AM EDT Friday for ANZ450-