Area Forecast Discussion
Issued by NWS Mt. Holly, NJ-- Remove Highlighting --
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FXUS61 KPHI 210910
Area Forecast Discussion
National Weather Service Mount Holly NJ
510 AM EDT WED SEP 21 2016
High pressure builds into our area through tonight, while low
pressure near the North Carolina coast will weaken as it slowly
shifts eastward through Thursday. High pressure will then shift
offshore Thursday, and a cold front slides across our area from
the north later Friday into early Saturday. High pressure from
Canada builds into our region later Saturday through Monday before
shifting offshore. A warm front should then lift to our north late
Monday with a cold front arriving during Tuesday.
.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Early this morning...considerable and mostly high cloudiness with
patchy dense fog. dewy. calm wind becomes light north to northeast.
low temperatures well above normal.
Today...Sunshine through considerable and variably thick high cloudiness.
northeast wind 5 to 15 mph. Max temps about 10 degrees above normal...a
bit warmer than yesterday.
Forecast basis: a 50 50 blend of the 00z/21 gfs/nam mos.
.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM THURSDAY/...
Considerable high cloudiness slowly thins southeastward late at
night. Patchy fog possible late but with lower dewpoints, the risk
of fog appears less likely. lows a few degrees cooler than that
of this Wednesday morning but still around 5 degrees above normal.
Forecast basis: a 50 50 blend of the 00z/21 gfs/nam mos.
.LONG TERM /THURSDAY THROUGH TUESDAY/...
Summary...Warm days into the weekend, then a taste of Fall Sunday
and Monday before some warming occurs. Not much chance for
additional rainfall through early next week.
The synoptic setup is comprised of zonal flow initially Thursday
that then amplifies Friday and right into early next week. This
occurs as a strong trough migrates across the Rockies Friday and
Saturday with downstream ridging, however a trough also 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 a little less certain although the overall
pattern locally looks to be mostly dry. We used a model blend
approach Thursday into Saturday, then blended the 00z WPC Guidance
into continuity thereafter. Some adjustments were then made
following additional collaboration with our neighboring offices.
For Thursday and Friday...An upper-level low near the eastern
Carolinas will weaken as an initial zonal flow will start to
transition to an amplifying trough across New England. The latter
takes place mostly on Friday. This will result in high pressure,
centered to our northeast, weakening as it shifts offshore. A
generally light northeasterly flow Thursday will keep temperatures a
bit cooler near and along the coast, however inland with 850 MB
temperatures near +15C afternoon temperatures Thursday are expected
to top out in the 80-85 degree range for many areas. As a cold front
arrives later Friday, the flow should turn more westerly with slight
warming. The timing of the cold front Friday varies some in the
guidance, however the main push of cooler air looks to arrive Friday
night and especially beyond. There is not much moisture along and
ahead of the cold front, and forcing looks to be unorganized and
weak. As a result, just carried slight chance POPs across the far
north Friday night as weakening showers are forecast to be tracking
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 Saturday as
surface high pressure, situated 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 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, 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 as the high builds down
and the pressure gradient tightens some along with cold air
advection. Given not much clouds anticipating and some lowering of
the wind especially more inland, both nights should be cool to
For Monday and Tuesday...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
arrives in our area later Monday into Tuesday. This will drive
surface high pressure right over our area Monday before shifting
offshore into Tuesday. Some quick return flow may occur later Monday
as the high slips to the east. Given some warm air advection, a warm
front may lift to our north Monday night. A cold front then moves
through during Tuesday, however its strength is less certain as some
guidance develops a closed low from the Great Lakes to the Ohio
Valley. If this occurs, the front should slow eastward along with
better moisture return ahead of it. Given the uncertainty, went with
slight chance POPs for many areas Tuesday. Some warming is expected
ahead of the aforementioned cold front.
.AVIATION /09Z WEDNESDAY THROUGH SUNDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.
Early this morning...VFR cirrus but patches of LIFR fog is possible
at some of the countryside TAF sites though an increasing northeast
wind toward 10z should mix out the lower visibility to vfr.
Today after 12z...VFR cirrus. East-northeast around 10 knots,
perhaps a wind gust around 15 knots in a couple of spots in the
late morning and early afternoon hours.
Tonight...VFR with cirrus thinning southeastward late. patchy MVFR fog
possible late but less likely than the occurrences of early this
morning. light northeast wind.
Thursday and Friday...VFR overall. Early morning fog is possible at
a few of the rural airports. East-northeast winds less than 10
knots, becoming light Thursday night then turning west to northwest
during Friday and Friday night.
Saturday and Sunday...VFR overall as high pressure gradually arrives
from the northwest. North-northeast winds 10-15 knots, especially
during the day Sunday.
-- Changed Discussion --Water temperatures have reverted to significantly above normal.
Seas on the Atlantic waters will remain steady around three feet
today and rise to 4 feet tonight with a possible need for SCA seas
vicinity DE tonight. Wind gusts from 15-20 knots are possible on
the ocean but are expected to remain below SCA criteria.
An east-northeast wind Thursday into Friday should increase some for
a time, however it appears that gusts should remain below 25 knots.
The seas however especially off the Delaware coast could build to 5
feet for a time. A cold front settles through late Friday into early
Saturday with a surge of much cooler air in its wake. This may
result in northerly wind gusts nearing 25 knots for a time Saturday
Today...low enhanced risk NNJ and Moderate risk s NJ and DE for
the formation of dangerous rip currents, in part dependent on a 1
ft 12 second ESE swell from Tropical Cyclone Karl arriving and in
part dependent on the strength of the onshore flow.
Water temperatures have reverted to significantly above normal
and so while swimming off the NJ and DE beaches these next several
warm days will be fun... be aware that rip currents can be
dangerous to your health. There were rescues this past weekend and
a beach drowning with the cause as yet, unknown. The point: odds
favor being safest swimming within sight of a life guard.
Thursday and Friday...A generally low or low enhanced risk is
Weekend...There is a pretty good chance of a moderate risk for the
formation of dangerous rip currents on at least one of the weekend
days as a 4 to 5 foot ESE swell from Tropical Cyclone Karl arrives
along with a gusty northeast wind.
-- End Changed Discussion --
September is well on its way to a top 10 warmest September through
most of our forecast area, and...for the 3rd consecutive month!
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 20th, our forecast from
today through the 28th and then the GFS2m max/min as seen on the
The following are the predictive sampling of reliable long term
Philadelphia: Projects a 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
74.0 -2016 #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 around 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 - 2016 and 1933
71.3 - 2005
Allentown: projects to a monthly average of near 69.9 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
69.9 - 2016
69.7 - 2015
69.4 - 1931
Allentown and Philadelphia have so far recorded only 1 day below
normal through the first 20 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 days.