Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 230724

Area Forecast Discussion
National Weather Service Mount Holly NJ
324 AM EDT Fri Sep 23 2016

A cold front will cross our region tonight, then high pressure
builds across the Northeast over the weekend into early Monday. A
warm front should lift north of our area later Monday, with a cold
front slowly crossing our region Tuesday and Wednesday. This front
may then stall near the coast Thursday.


Early this morning...clear cool and calm most of the area with
spotty countryside fog. Exception: low clouds will hug the southeast
NJ coast and much of southern DE. coolest temps will be in the upper
40s to around 50 in nw NJ and ne PA where radiational cooling occurs,
per the continued 2m temp fcst of the 00z/23 GFS (am really liking
the GFS 2m temp data for radiational cooling mins in the countryside
and then adding 3-4F to the max 2M temp at 18z for max temps on partly
to mostly sunny non-marine influenced days).

Today... Sunny and very warm with max temps 88 or 89 in PHL/ABE and
mid-upper 80s elsewhere. MAX temps may occur today between 4 and
5PM when the BL temp is warmest. winds become light west. Max
temps are forecast generally 10-15F above normal but below record.
Wont be surprised at isolated 90F this afternoon at the typically
hotter I-95 spots.

Forecast basis is a 50 50 blend of the 00z/23 gfs/nam mos except
as noted which in this case...raising guidance max temps based on
GFS2m temp of 85 at 18z and an expected 17C at 850MB.


Cold frontal passage from north to south. Have doubts about CFP
passage prior to 6 AM fm KESN to KGED southward. Low or mid clouds
and a briefly gusty ne wind to 20 mph accompany the CFP. It`s
possible the max temp for the calendar day of Sept 24 will occur
at 1 am in a few spots around PHL (74-75F). Small chance of a shower
or isolated tstm near and north of I-78 this evening. NCAR WRF
(00z/23) confused display this evening and SPC WRF so far hasnt run.
pwat 1.4-1.5 inches this evening dries to .5 inches I80 north by
morning so that mins kmpo to KFWN and KMPO may again be near 50.

Forecast basis:50 50 blend of the 00z/23 gfs/nam mos guidance.


Summary...A taste of Fall Saturday and especially Sunday and Monday
before perhaps some moderation occurs. The chance of rain next
week highly depends on the strength and timing of a cold front
near the middle of next week.

The synoptic setup is comprised of amplified flow over the weekend
which then carries through much of next week. A strong upper-level
trough will be across the Northeast over the weekend while a closed
low traverses the Plains and Midwest. There continues to be model
differences with the evolution of these, especially the trough
moving across the Plains. This will impact the timing of a cold
front into our area next week. Some ridging should move across our
area Monday, with surface high pressure shifting offshore. Given the
extent of the amplified flow, a slower system would tend to allow
more moisture return northward. We used a model/continuity blend
Saturday into Monday, then blended the 00z WPC Guidance into
continuity thereafter. Some adjustments were then made following
additional collaboration with our neighboring offices.

For Saturday and Sunday...A strong upper-level trough slides across
the Northeast. This will push a cold front to our south to start
Saturday as surface high pressure, situated just north of the Great
Lakes to start the day, builds southeastward. This results in a
northeast low-level flow along with deepening cold/dry air
advection. As the cold air advection increases, there appears to be
enough low-level moisture initially remaining in place. This
combined with warmer ocean water may result in an area of low clouds
sliding southward especially across New Jersey and Delaware Saturday
morning with even a shower. As the day wears on, much drier air
should overtake the region and the clouds should thin out from north
to south. As of now, we increased the cloud cover in the morning but
kept the forecast dry. It may be a bit breezy for a time Saturday
especially along the coast ahead of the building surface high.
Sunday should be a true Fall day across the area. There should be
enough subsidence and drying to promote less clouds at night,
therefore both nights should be cool to chilly. Some guidance is
showing the potential for mid to upper 30s Saturday night and
especially Sunday night across portions of the Poconos and far
northwestern New Jersey. If this occurs then patchy frost is
possible, especially if high cloudiness holds off during Sunday

For Monday and Tuesday...A very amplified pattern is forecast with a
trough departing New England and another strong trough moving across
the Plains and Midwest. In between, a ridge arrives in our area
during Monday. This will drive surface high pressure over our area
to start Monday before shifting offshore into Tuesday. Given some
warm air advection, a warm front may move to our north later Monday
although lift looks limited with it. The guidance differs some with
the evolution of a strong closed low that moves across the Great
Lakes and toward the Northeast. Some guidance breaks off the energy
and keeps a second closed low back across the southwestern states
and into the southern Plains. We leaned more toward the setup of
following a closed low across the north, which is closest to the WPC
Guidance. This evolution will determine the timing and strength of a
cold front that eventually arrives in our area. As of now, we have
it arriving Tuesday however given the uncertainty only low POPs were
carried. Some temperature moderation should occur Tuesday ahead of
the front.

For Wednesday and Thursday...There is the potential for a strong
trough or closed low to arrive in the region during this time frame.
The details are less certain given the rather amplified pattern that
is advertised by the model guidance. If the closed low idea pans
out, the timing could be slower which will also impact the movement
of a surface cold front. As of now, we bring the front slowly
through Wednesday and it may stall for a time near the coast
Thursday. There is the chance for a decent ribbon of enhanced
moisture and lift to shift eastward ahead of the trough/closed low
and associated cold front, however the overall evolution and
therefore details are of lower confidence and thus the POPs were
kept on the low side.


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

Early this morning...cigs 700-1500 ft vcnty KACY and KMIV with
VFR conditions elsewhere.  calm or light north wind.

Today....VFR few cirrus with the wind becoming west or southwest
this afternoon around 5 to 10 kt.

Tonight...VFR to start then sct-bkn layers aoa 2500 ft spread southward
with a wind shift to ne and gusty to 20 kt for an hour subsequent
to a cold frontal passage. There is a slight chance of rain
showers with the front at KABE and KTTN late but not in the TAF
attm due to low confidence.

Saturday and Sunday...MVFR ceilings possible Saturday morning
especially south and east of PHL, then VFR through Sunday as drier
air arrives. North-northeast winds 10-15 knots Saturday, becoming
less than 10 knots later in the day and through Sunday.

Monday and Tuesday...VFR overall. Some showers are possible on
Tuesday. Winds becoming southeast at 10 knots or less Monday, then
southwest on Tuesday.


No marine headlines today. We may need an sca for ne wind gusts 25
kt in ANZ450-51 and seas building to 5 ft there late tonight.

Otherwise winds becoming westerly for a time today (after sunrise) then
south southwest this afternoon (sea breeze influence) before shifting
light northwest this evening and then north northeast gusty to at
least 20 kt behind the cold frontal passage early Saturday morning.

A cold air advection surge in the wake of a cold front within a
north-northeast wind Saturday may result in 25 knot gusts and seas
near 5 feet. However, this may be short in duration and therefore
held off on issuing a Small Craft Advisory at this time. The
conditions overall then Sunday through Tuesday are anticipated to be
below Small Craft Advisory criteria.

The probable risk for the formation of dangerous rip currents
today is low.

Weekend...There is a chance of a moderate risk for the formation of
dangerous rip currents on at least one of the weekend days, as a 3
to 4 foot east-southeast swell from Tropical Cyclone Karl arrives
along with a gusty northeast wind. Karl`s slow development may
result in a lower swell which would help reduce the potential risk
for the formation of dangerous rip currents.


Will update the previous morning climate JAS discussion with new
reality and forecast data.  No major changes anticipated.




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