Area Forecast Discussion
Issued by NWS Mt. Holly, NJ
FXUS61 KPHI 220933
Area Forecast Discussion
National Weather Service Mount Holly NJ
533 AM EDT THU SEP 22 2016
High pressure over our area will gradually weaken as it shifts
offshore into Friday. A cold front will move across our region later
Friday into early Saturday, then high pressure builds in during the
weekend and early next week. A warm front lifts to our north
Tuesday, while a cold front remains well to our west Wednesday.
.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Early this morning...cirrus bands, mostly thin with patchy shallow
fog in parts of ne PA and nw NJ.
Today...Considerable sunshine with remaining bands of thicker cirrus
thinning midday. Max temps 8-13 degrees above normal...greatest
positive departure in the northwest portion of our area vcnty KABE
and KMPO. light wind becomes east this afternoon around 10 kt with
scattered gusts near 15, especially coasts.
Forecast a max temp of 86F in PHL and confidence is very high
that the max will fall in the range of 85-87F.
KABE and KSMQ should be warmest with max temps around 87F,
possibly even 88.
Forecast basis started with a 50 50 blend of the 00z/22 GFS/NAM
.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM FRIDAY/...
Clear or mostly clear with radiational cooling and patches of fog
possible late at night...but now, not the usual suspects of ne PA
and nw NJ. Instead southern NJ, the Delmarva and possibly se PA.
Forecast basis started with a 50 50 blend of the 00z/22 GFS/NAM
MOS. I did lower that blend by 3-5F to fcst mins in the countryside
lower than guidance but within a couple of degrees of the coolest
GFS 2m temps (09z) which has widespread 48F Northampton/ Warren
and Sussex counties in ne PA and NW NJ.
.LONG TERM /FRIDAY THROUGH WEDNESDAY/...
Summary...A taste of Fall later Saturday into early next week before
perhaps some moderation occurs. Not much chance for rainfall through
the middle of next week.
The synoptic setup is comprised of amplifying flow Friday and
especially the first half of next week. This starts as a strong
trough migrates across the Rockies Friday and Saturday with
downstream ridging, while a trough amplifies across eastern Canada
and New England. This pattern change to a much more amplified one
will offer a much cooler airmass into our region, however the
details are less certain especially next week. Overall though the
evolving pattern locally looks to be mostly dry, especially as a
significant trough closes off well to our west. We used a model
blend approach Friday into Sunday, then blended the 00z WPC Guidance
into continuity thereafter. Some adjustments were then made
following additional collaboration with our neighboring offices.
For Friday...An upper-level low near the eastern Carolinas should
open up and and shift offshore. The pattern then starts to amplify
as a trough digs across the Northeast. As this occurs, weak low
pressure tracks from the eastern Great Lakes and then across New
England, with an associated cold front dropping south-southeastward.
This cold front is forecast to settle across our area later in the
day and at night. There is some convergence and lift associated with
the front, although on the weaker side and the moisture return looks
fairly limited. There still could be isolated showers with the front
mainly across our northern zones since they are closest to the
energy sliding across New England. The forecast soundings from the
GFS and NAM could support isolated thunder with the front. We added
slight chance POPs across the northern areas in the afternoon and
evening. Otherwise, the front should come through with a band of
clouds. A much cooler and drier airmass is to follow this front,
however this may lag a bit.
For Saturday and Sunday...A strong upper-level trough slides across
the Northeast Saturday with some additional amplification possible
Sunday. This will drive 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. Decent cold air advection is
forecast to occur under a north-northeast surface wind. While the
airmass should be rather dry, there could be stratocumulus initially
Saturday with the onset of the cold air advection particularly over
the warmer ocean waters. There should be a zone of overrunning from
near the Ohio Valley westward as the aforementioned cold front is
draped back that way, however any precipitation induced by this
should remain there instead of tracking east with the southbound
cold front. Given high pressure building down, we kept a dry
forecast going over the weekend. It may turn a bit breezy for a time
over the weekend especially along the coast Saturday ahead of the
building surface high. There should be enough subsidence and drying
to promote less clouds at night, therefore both nights should be
cool to chilly (upper 30s in the Poconos).
For Monday through Wednesday...A much more amplified pattern is
forecast with a trough in New England and another strong trough
moving out of the West and across the Plains. In between, a ridge
builds and should arrive in our area later Monday and Tuesday. This
will drive surface high pressure over our area Monday before
shifting offshore into Tuesday. Some quick return flow may occur
late Monday as the high slips to the east. Given some warm air
advection, a warm front may lift to our north Tuesday although lift
looks limited. A pronounced amplified pattern then unfolds as the
guidance overall shows a closed low evolving in the Plains. The
location however of where this closed low develops will determine
the placement of the downstream ridge. This should tend to slow down
or even halt the next cold front that originally looked to arrive on
Tuesday. As of now given the closed low forming well to our west
with downstream ridging, this front is held well to our west through
Wednesday. As a result, little in the way of precipitation is
forecast through this time frame.
.AVIATION /09Z THURSDAY THROUGH MONDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.
Today...VFR bands of cirrus thinning. Wind becoming east northeast
around 10 kt this afternoon with scattered gusts around 15 kt.
Tonight...VFR but may degrade to areas of ifr or mvfr fog after
06z, vcnty KMIV/KACY and KILG where a little more shallow moisture
available. Nearly calm wind.
Friday and Saturday...VFR. A cold front moves across the area later
in the day Friday into early Saturday with some clouds. An isolated
shower or thunderstorm cannot be ruled out with the front, mainly
north of PHL. Light southwesterly winds, becoming northwest Friday
night, then north-northeast into Saturday and increasing to 10 to 15
Sunday and Monday...VFR overall. Northerly winds near 10 knots
Sunday, then becoming light east-southeast on Monday.
A Small Craft Advisory for hazardous is in effect from Noon today
to 6 am Friday. Have pulled back a bit on the extensiveness and
duration of SCA conditions and converted to Haz seas. Forecast
seas are a little less than previous guidance. The 18z GWES prob
for >6 ft is southeast of our area, and pulling eastward faster
early Friday. We dont have 44009 data to measure seas so we rely
Elsewhere for the Atlantic waters a 2 to 4 foot easterly swell of
11 seconds occasionally 6 seconds through tonight. De Bay a few
gusts 20 kt today.
Water temperatures continue above normal.
A cold front will move through later Friday into early Saturday. A
much cooler airmass will arrive in its wake, and a cold air
advection surge looks to take place Friday night and Saturday
morning. It is at this time overall where a fairly short period of
gusts to 25 knots can occur. This may also build the seas to 5 feet
for a time before the winds settle some Sunday and especially into
Monday. Otherwise, the conditions are anticipated to be below Small
Craft Advisory criteria.
There is a moderate risk for the formation of dangerous rip
currents along the NJ shore and Delaware Beaches today, mostly
driven by a 2-3 foot east-southeast swell of 11 seconds and
additional onshore wind wave created by an east northeast wind
gusting 15 kt. Swim only in the presence of a lifeguard.
Friday...Statistically a low enhanced risk due the tendency for
more of an offshore wind. However, experience tells us that a 3
ft 8 second swell causes increased rescues and threat to life.
Tomorrow should see swell increasing to 3 ft and 11 seconds by
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 lack of development may
result in a lower swell which would help reduce the potential
risk for the formation of dangerous rip currents.
September continues well on its way to a top 10 warmest September
through most of our forecast area, and...for the 3rd consecutive
So while there may be a 2 or at most, 3 day string of below normal
temperatures between Sunday and Tuesday, that is not likely to be
enough to prevent a third consecutive top ten warmest month for
much of the NWS Mount Holly forecast area. In this case a probable
second consecutive top 5 warmest month in the period of record for
PHL and ABE and possibly ACY too.
We`ve run the actual numbers through the 21st, our forecast from
today through the 29th and then the GFS2m max/min as seen on the
FTPRHA for the 30th.
While there is considerable run to run and model variability on
outcomes Tuesday through Friday...it all averages out as follows
in long term climate sites representing both urban and countryside.
Philadelphia: Projects a near 74 degree average or about 5 degrees
above the monthly normal of 69.1. This will probably be a #4 or #5
warmest September on record for Philadelphia.
Philadelphia September average temperature rankings
73.8 -2016 and 2005 tie #4
Philly ranked #7 warmest July followed by a warmest ever August
(in the POR dating back to 1874).
Atlantic City: where there can be greater variability due to the
sea breeze cooling during the afternoon and the radiative cooling
at night, is still projecting near a 71.5 degree monthly average
temperature, or 4+ degrees above the monthly normal of 67.2. Odds
are locking into a top 4 to 8 warmest September in the period of
record dating back to 1874. Atlantic City recorded a #7 warmest
July and warmest ever August.
73.3 - 1961
72.8 - 1881
72.3 - 1931 and 1930
71.7 - 1921
71.6 - 2015
71.5 - 1933
71.3 - 2016 and 2005 tie #7
Allentown: projects to a monthly average of near 69.7 degrees, or
almost 6 degrees above the monthly normal of 63.9 and a highly
probable 3rd to 5th warmest September on record. Allentown ranked
8th warmest July and #2 warmest August.
70.8 - 1961
70.3 - 1980
69.7 - 2016 and 2015
69.4 - 1931
Allentown and Philadelphia have so far recorded only 1 day below
normal through the first 21 days of September, TTN only 2 days
below normal and RDG 3 days below normal.
Rainfall: The rains of earlier this week were welcome. Still only
ACY and GED of our long term climate sites are above the monthly
normal. Multiple weather forecast models indicate little or no
rain here for at least the next 7 days. The only good news, is
that evaporative rates will be a little smaller due to the
climatology of lower temperatures and shortening daylight hours.
MARINE...Small Craft Advisory for hazardous seas from noon today to 6
AM EDT Friday for ANZ454-455.
Near Term...Drag 534
Short Term...Drag 534