Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Mount Holly New Jersey
920 am EDT Wednesday Jul 29 2015
a weak area of high pressure will shift to our east today. A
weakening cold front moves through our region Thursday night,
however it then slows offshore on Friday. The next weak cold front
should cross our area late Saturday into Sunday followed by high
pressure. A potentially stronger cold front is then scheduled to
arrive from the northwest during Tuesday.
Near term /until 6 PM this evening/...
today...the boundary drying becomes evident by forenoon with
highest dew points and MLCAPE this afternoon (400j) confined to the
southern portion of our Delaware-Maryland-Virginia forecast area where am carrying a
slight chance for a thunderstorm with heavy rain. Precipitable water increases to
1.7 inches there late today. This area is the leading edge of a
northeastward developing instability burst with ki by 00z of 34c along md's
Elsewhere...hot hot hot and used the GFS mavmos straight up. Sea
breezes will eventually mitigate some of the pure heat in Delaware and
New Jersey but this will be a toasty day with 18c at 850 and near record
in some places. Please see the climate section for the vulnerable
Kphl...am 80 percent confident of 94. Maximum should be within the range
of 93-95f. Less confident of dew point in the 16z-18z time frame
when maximum hi should occur.
Maximum hi today 98-99f...basically just below urban corridor
criteria of 100...but since forecast near 100 Thursday and warm nights
this morning and again tonight (lows of 76-77)...we issued a two
SW winds this afternoon may gust 15 miles per hour in a few spots with sea breezesdeveloping
along the coasts between noon and 2 PM.
Confidence: above average.
Short term /6 PM this evening through 6 am Thursday/...
any showers on the Delaware-Maryland-Virginia should dissipate by 00z.
Thereafter...southerly flow and more humid with patchy fog and stratus
forming. Precipitable water increases to 1.9 inches by morning along the I-95
corridor. Models have a weak ib thrust across Delaware and S New Jersey
overnight where scattered or isolated heavy showers and thunderstorms might
develop. Confidence on placement and occurrence is below average
and this may need modification in later forecasts, possibly
westward into east PA.
Temperatures well above normal and used a blend of 00z/29 NAM/GFS MOS
with a favoring of the warmer available guidance.
Overall confidence: above average except below average on convection
Long term /Thursday through Tuesday/...
the synoptic setup is comprised of a narrowing upper-level ridge
Thursday that shifts to our east as an upper-level trough slides
across the Midwest and Great Lakes regions. This trough is
forecast to overspread the northeast and into the middle Atlantic
Thursday night through the weekend. Despite this trough in place,
we are mainly on the southern extent of it therefore a rather warm
to hot air mass will be in place. A weakening cold front Thursday
looks to bring the greatest chance for convection, then we await for
the next potentially stronger front to arrive during Tuesday. We
used a model blend Thursday through Saturday, then went with a
wpc/continuity blend thereafter.
For Thursday and Friday...as a narrowing upper-level ridge shifts
offshore, we start to become influenced by an upper-level trough
traversing the Great Lakes region and southern Canada. A
southwesterly low-level flow will pull additional moisture into
the County Warning Area which will be reflected in higher surface dew points
compared to Wednesday. The air mass at 850 mb is forecast to be
similar to Wednesday therefore another hot day is expected.
However the forecast challenge is cloud cover and the timing of
convection especially during the afternoon and evening Thursday.
If this is delayed late enough with more sunshine, then heat
indices will be a bit higher. The influence of the incoming upper-
level trough will offer some height falls, however a rather toasty
boundary layer looks to be maintained.
While there may be some clouds to start Thursday with maybe a
shower or thunderstorm, we think there will be enough sunshine to
get temperatures into the lower 90s for many areas especially the
Delaware valley. This puts heat indices very close to 100 degrees
for a few hours in the afternoon. This is of most concern for the
urban centers where heat remains trapped during the night as well.
As a result, for the urban corridor from Wilmington to
Philadelphia to Trenton and the immediate nearby locations we
opted to carry an excessive heat warning through 6 PM Thursday.
Regarding convection, a weakening cold front arrives later in the
day however there should be enough lift within the hot and humid
air mass to promote storm initiation to our west. It is not quite
clear if we can get some isolated activity in the morning as the
moisture pools, otherwise we focus on the afternoon and evening
hours as plenty of instability will be present. The greatest shear
though is forecast to pass just to our north where a middle level
speed maximum moves through. The signals in the model guidance
indicate a line/broken line or clusters of convection arriving
from the west. If enough organization can occur and also factoring
in precipitation loading of the downdrafts, locally damaging winds
could result. In addition, precipitable water values about 2 inches will promote
torrential rain within convective cores and this could lead to
The convection should wane during Thursday night as the front
weakens but then slows as it moves off the coast. Therefore we
kept the highest probability of precipitation in the evening across the eastern zones. The
front looks to stall offshore Friday with a possible weak wave of
low pressure tracking along it. As of now, it looks like the
precipitation should be offshore with drying occurring across our
area. Another warm to even hot day on Friday, however the dew
points are expected to be noticeably lower for much of the area.
For Saturday and Sunday...an upper-level trough remains across
the northeast although it may weaken some Sunday as a ridge in the
western Atlantic tries to expand northwestward. This keeps a
stalled frontal boundary offshore however it appears that ripples
of energy along it remain to our east. Since the true cooling
affects from the upper-level trough are relegated well to our
north, the very warm to hot conditions persist although dew points
may remain on the lower side. A weak front or surface trough looks
to move through later Saturday, however much of the lift may
remain to our north.
For Monday and Tuesday...the offshore front should dissipate as
the upper-level trough reloads some across the Great Lakes and
into New England. This will send another cold front our way during
Tuesday with the potential for some convection mainly during that
time. It appears that many areas should add to the stretch of 90+
degree daytime temperatures.
Aviation /13z Wednesday through Sunday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg,
kilg, kmiv, kacy and surrounding areas.
Today...VFR with scattered clouds at or above 6000 feet this afternoon. Isolated thunderstorms
possible vicinity kged and kesn late this afternoon. SW wind gust near 15
knots this afternoon except south southeast sea breezes developing
coastal locations 16z-18z.
Tonight....VFR to start then patchy MVFR or IFR conds in
fog/stratus/ haze late, especially northwest New Jersey and portions east PA. Chance of
a shower or thunderstorm vicinity kphl/kmiv toward 10z.
Thursday...some fog/haze possible early, then mostly VFR. Some
showers and thunderstorms are expected from west to east mainly in
the afternoon and evening, possibly in the form of a line. Some
ceiling and especially visibility restrictions are anticipated for
a time as a result. Southwest winds increasing to 10-15 knots,
then diminishing to around 5 knots at night and becoming west-
Friday through Sunday...mainly VFR as a front lingers offshore.
West- northwest winds near 10 knots Friday, becoming southwesterly
near 5 knots Saturday and Sunday.
buoy 44009 sea surface temperature data has returned to service.
No headlines through tonight. Seas generally at or below 3 feet through
tonight. SW wind gusts less 15kt early this morning becoming south-southeast
this afternoon and southerly tonight.
Outlook... Thursday through Sunday...
the conditions are expected to be below Small Craft Advisory
criteria. A southerly flow should increase though Thursday ahead
of a weakening cold front, however gusts should be below 25 knots.
The seas will respond for a time, however they should be below 5
feet. Some thunderstorms are expected late Thursday with the
approach of a weakening cold front, with locally gusty winds
possible. The front then lingers offshore Friday through Sunday.
there is a spotty moderate risk for the development of dangerous
rip currents along the Ocean County portion of New Jersey
otherwise a low risk elsewhere.
Of concern is the continued hot weather and above normal sst's
playing a role in swimmer behavior. Even with a low risk of
dangerous rip current formation, they can occur. It takes only one
to be fatal. For our safety, lets swim in the presence of
lifeguards who are trained to watch, Rescue and otherwise minimize
ocean water related hazards in the swimming surf zone. Thank you.
kacy rainfall 6.58 ranked 14th wettest now with the por dating
back through 1874. Wettest 13.09 in 1959.
Kabe will probably experience near record warmth this afternoon-
July 29. The record there is 96 set in 1954. Right now we're forecasting
Krdg and kmpo may be the other locations within 2f of the record
for this date
Krdg - 97 set back in 2002.
Kmpo - 90 set back in 1949.
PA...excessive heat warning until 6 PM EDT Thursday for paz070-071-
New Jersey...excessive heat warning until 6 PM EDT Thursday for njz015-
Delaware...excessive heat warning until 6 PM EDT Thursday for dez001.