Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Mount Holly New Jersey
944 am EDT Tuesday Jul 28 2015
weak high pressure covers the northeast United States through
Wednesday. A weak cool front passes through the Middle Atlantic
States Thursday night. Another cool front may approach from the
northwest next Monday. Meanwhile, low pressure centers along the
southeast coast of the United States this week will be monitored
for possible northward development next week.
Near term /until 6 PM this evening/...
the last of the low clouds were lifting and dissipating around
The remnants of the frontal boundary that arrived on Sunday night
extended across southern New Jersey and the upper Delaware-Maryland-Virginia
peninsula this morning. The boundary is forecast to drift
northwestward into northern New Jersey and southeastern
Pennsylvania today. Meanwhile, weak areas high pressure should
remain centered over the eastern Great Lakes and off the middle
As the boundary continues to dissipate, we are not expecting much
of a chance of showers and thunderstorms in its vicinity. However,
we will continue to carry a slight chance of showers and
thunderstorms for this afternoon in parts of southern New Jersey,
Delaware and northeastern Maryland. Developing sea and Bay breeze
boundaries may provide enough lift to result in isolated
convection. Mixed layer cape values are anticipated to increase to
about 700 to 1100 j/kg in those areas during the afternoon.
Maximum temperatures are expected to be around 90 at most
locations in our forecast area. Readings should be a bit lower
than that in the elevated terrain and along the coast. Dew point
readings should favor the upper 60s and lower 70s. The resulting
maximum heat index values are in the lower and middle 90s.
Today may be the first of at least seven consecutive days of 90
plus temperature readings in much of our region. The maximum
number of consecutive 90 plus days at Philadelphia is 18 set from
July 29 through August 15, 1988. The 8 to 14-day outlook for the
period from August 4-10 suggests a cooling trend at that time, so
we do not anticipate approaching the record.
Short term /6 PM this evening through 6 am Wednesday/...
a weak surface high is forecast to be over our area tonight as an
upper-level ridge axis slides across the eastern Great Lakes and
Ohio Valley. This still maintains northwesterly flow aloft, but
light and variable flow at the surface. Due to a lack of organized
short waves to provide enough lift, an isolated shower or
thunderstorm initiated probably by any low-level convergence /I.E.
Sea and Bay breezes/ are expected to dissipate early in the
evening. In addition, any cumulus field is expected to flatten and
dissipate with perhaps some high cloudiness occurring at times.
It will be a warm and muggy night for much of the County Warning Area as the
surface dew points should recover from any daytime heating
influence. The combination of light to calm winds, a moist
boundary layer and not a lot of cloudiness could result in patchy
low clouds/fog development late.
As for the low temperatures, a MOS blend was generally used.
Long term /Wednesday through Monday/...
**potential seven day or longer heat wave for phl beginning
today and probably continuing through next Monday or tuesday**
500 mb: high pressure in the Southern Plains will be the dominant
influence here through Thursday. Otherwise a trough tries to
evolve over the eastern states next weekend.
Temperatures: the month of July so far has been near normal
(temps). This extended period will see calendar day averages above
normal every day. Temperatures should average 5 to 7 degrees above normal
Wednesday and Thursday and 3 to 5 degrees above normal Friday
through Sunday and then more than 5 degrees above next Monday
Forecast basis: unless otherwise noted, a 50 50 blend 00z/28
GFS/NAM MOS guidance Wednesday-Thursday, 00z/28 mex MOS Thursday
night and Friday, then the 0521z/28 wpc gridded elements of
maximum/min temp, 12 hour pop, 6 hour dew point-sky-wind from Friday
night through next Monday.
While not in the synopsis due to its potential to cause
confusion... and its lack of significant weather impact, it looks
like there will be a leftover wind shift boundary somewhere near
I-95 through Thursday before a more structured weak cool frontal
passage occurs Thursday night.
Wednesday...relatively dry heat. Precipitable water ~1.35 inches. Heat index
below 100f. Southwest wind gust 15 miles per hour during the afternoon except
weak sea breezes coastal locations beginning around noon. Used the
warmer mav temperatures for the daytime forecast. Forecast maximum temperatures may be a
degree too low. Overall confidence: above average.
Thursday...approaching wet bulb front with the 850 mb hot produce of
18-20c ahead of the cf and a quick start to 90f Thursday morning.
Afternoon dew points are expected to be much higher than
Wednesday (by almost 10f). Raised the 00z/28 NCEP guidance using
the 00z/28 European model (ecmwf) but did so with less than ideal confidence.
MLCAPE builds in central PA to 1400j by 18z Thursday and 600j New Jersey.
Appears unlikely to have any showers or thunderstorms going in our area
before 16z and probably not to the I-95 corridor until near 22z.
That should allow more warming than modeled and hi nearing 100f by
18z. Maximum temperatures should remain below records which are 97-99f except
kmpo 90. South-southwest wind gust 15-20 miles per hour during the afternoon.
What would prevent the heat index reaching 100f? Thursday morning
showers and thunderstorms vicinity kphl south and east would lessen sunshine
and make it much more difficult for the air temperature to exceed
90f. There are 00z/28 ec/GFS model hints of a weak instability
burst trying to shove northeastward into the Delaware-Maryland-Virginia and snj Thursday
morning and produce morning showers.
A scattered-broken band of showers and thunderstorms should develop and pass
across the area Thursday aftn/night. Precipitable water increases to 1.9 inches along
the I- 95 corridor during Thursday evening so heavy rainers and
local poor drainage flooding anticipated and cant rule out
isolated flash flood. While timing is excellent for strong thunderstorm
development, severe unlikely since 0-6km bulk shear is pretty weak
and lapse rates via tt index look pretty poor. Sref modeled precipitable water
back edge of 1.5 inches down to a kdov- kblm line by 12z Friday.
Friday...dew points lower by 10 to 12f on light northerly flow and air
temperatures down 2-3f. Blue Moon and potentially nice hot Summer day for
the end of July. Above average confidence.
Saturday and Sunday...heats up maybe a bit more each day on west-southwest
afternoon wind gusts 10-15 miles per hour. A nice weekend since afternoon
dew points still remain pretty modest...low to middle 60s. Small chance
of a thunderstorm NE PA Sat evening in warm air advection - this per 1400j near khzl midday
reaching kblm by 00z/Sunday. Average confidence.
Next Monday...more humid and continued hot. 2000j MLCAPE i95 southeastward.
Average confidence on the forecast details.
Aviation /14z Tuesday through Saturday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg,
kilg, kmiv, kacy and surrounding areas.
Today...some low clouds will dissipate by late morning, otherwise
VFR with a cloud base mostly between 3500-5000 feet. An isolated
shower or thunderstorm may develop through the day especially
south and east of kphl. Sea/Bay breezes are expected to develop by
middle afternoon and affect kacy, kmiv and kilg with winds turning
more from the southeast.
Tonight...mainly VFR. However due to the moist air mass in place,
some late night low clouds/fog may develop, and our confidence is
on the lower side at this time. Light southerly to calm winds.
Wednesday through Thursday morning...mainly VFR. However, the
high humidity levels may result in some late night and early
Thursday morning MVFR or IFR low clouds, haze and fog. Light SW
Thursday afternoon and Thursday night...a chance of showers and
thunderstorms that may result in MVFR or IFR conditions at times.
Gradient SW wind gusts 15-20 knots Thursday afternoon.
Friday through Saturday...mainly VFR. Light wind, mainly north
Friday morning with sea breezes both Friday and Saturday.
the conditions are expected to remain below Small Craft Advisory
criteria through tonight. A southerly flow will continue with some
enhancement occurring this afternoon and early evening as a sea
and Bay breeze develops. The seas should run in the 1-3 feet range
Wednesday through Saturday...no marine headlines are anticipated.
a long period /around 10 seconds/ swell from the east-southeast
will continue today. A southerly wind at 10-15 miles per hour with some
afternoon sea breeze enhancement along with waves of around 2 feet
in the surf zone are forecast today. The setup is similar to
yesterday and therefore we will maintain a moderate risk of
dangerous rip currents for both the New Jersey and Delaware
kacy rainfall 6.33 ranked 15th wettest with the por dating back
through 1874. Wettest 13.09 in 1959.
Kabe may experience near record warmth on Wednesday July 29. The
record there is is 96 set in 1954. Right now we're forecasting 94.