Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Mount Holly New Jersey
1139 am EDT Thursday Jul 30 2015
a weak cold front will move through our region tonight, then
stalls to our south and east. The next weak cold front should
cross our area late Saturday before stalling offshore Sunday. A
few weak waves of low pressure may track along the front and
possibly move closer to our coast Monday as high pressure tries to
arrive. A stronger cold front is then forecast to arrive during
Tuesday, before also stalling nearby on Wednesday.
Near term /until 6 PM this evening/...
the approaching cold front extended from The Finger lakes across
central Pennsylvania down into West Virginia late this morning. A
line of showers and isolated thunderstorms was out ahead of the
front and it was approaching Scranton, Harrisburg and the western
suburbs of Washington. As the precipitation moves eastward into
increasingly unstable air, the thunderstorms should become more
numerous and they will likely strengthen. Much of our forecast
area remains under a marginal risk for severe weather for this
afternoon into this evening.
The hrrr seems to have a good handle on the timing of the
convective line. It should be into the Allentown and reading areas
by 200 PM and it is expected to reach the Interstate 95 corridor
in our region around or shortly after 400 PM. The showers and
thunderstorms are anticipated to be nearing the coast at 600 PM.
Maximum temperatures are forecast to be near 90 degrees in much
of our area. Readings should not get out of the 80s in the
elevated terrain up north and along the immediate coast. Dew
point readings are expected to be mostly in the lower 70s. We will
continue the excessive heat warning for the highly urbanized areas
along the Interstate 95 corridor.
The wind should remain from the southwest around 6 to 12 miles per hour.
Short term /6 PM this evening through 6 am Friday/...
the aforementioned cold front will continue its trek toward the
coast and should move offshore tonight. The showers and
thunderstorms will continue into tonight and will end from
northwest to southeast as the front pushes off the coast and then
further offshore. As the front pushes away, we will also see some
clearing behind it and areas to the north and west of philly will
be able to cool off a bit and drop down into the lower to middle 60s
overnight. Areas to the south and east of philly will hold onto
the cloud cover a bit longer and will likely remain in the upper
60s to lower 70s for overnight lows. Even with the clearing, low
level moisture may hang around for a bit and patchy fog will
Long term /Friday through Wednesday/...
the synoptic setup is comprised of an upper-level trough from the
Great Lakes to the northeast Friday with some amplification into
the weekend. Despite a warm to hot airmass in place, a substantial
reduction in the dew points is expected Friday and even for the
weekend overall for the County Warning Area. A weak front moves through late
Saturday with some potentially tricky convective chcs, then the
next one arrives during Tuesday before stalling nearby Wednesday.
There is more uncertainty in the details especially next week as
the amplitude of the trough and the positioning of frontal
boundaries will impact our sensible weather. We used a model blend
Friday through Sunday, then went with mostly a wpc/continuity
For Friday...an initial front is stalled offshore as it becomes
trapped between a ridge in the western Atlantic and a trough from
the Great Lakes to the northeast. Any remaining frontal forcing and
associated convection should remain well offshore, meanwhile a
noticeable drier airmass settles into our area. Despite it still
being warm to hot across the cwa, the lower dew points will result
in a much less humid feel /even cooler at night/. The aforementioned
upper-level trough amplifies some especially to our northwest
through northeast, and some upstream convection may try to make a
run at our northwest zones very late at night. Overall, this looks
like a very small chance plus with the initial drier airmass in place
and therefore we opted to leave the night time frame dry.
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 weak ripples of
energy along it remains to our east. A secondary frontal zone is
forecast to move through late Saturday into Sunday, and the greatest
lift and shear is mostly from near our northern zones on northward
into New England. The dew points should start to inch back upward
again later Saturday, however enough drying may limit much
convective development. Some guidance does initiate more convection
by late Saturday from northeastern Pennsylvania to northern New
Jersey then slides it southeastward. There may be enough shear with
increasing instability to allow for a gusty thunderstorm, however
this is dependent on the airmass moistening up sufficiently as the
upper-level trough remains in place. For now, we continued some low
end chance probability of precipitation across the far north Saturday afternoon.
The front looks to stall just offshore given weaker flow with a
southeastern extent, however some weak impulses may track along it.
It appears that any convection leftover with the front is offshore,
therefore we have a dry Sunday. It will continue to be warm given a
similar pattern and the dew points will probably increase a bit
mainly Sunday night across the southern and eastern areas.
For Monday and Tuesday...as the upper-level trough reloads from the
west, it may support weak impulses traversing the offshore front. If
this occurs and with the upper-level flow turning more southwesterly
for a time, the front may inch back into our area to help focus some
convection Monday. This looks weak overall with the main forcing
remaining mostly across the Great Lakes region Monday. Tied to this
forcing, another cold front arrives during Tuesday with the
potential for some convection although the stronger shear may again
slide to our north. We opted to go close to the 00z wpc guidance
which carries probability of precipitation of low end chance or less.
For Wednesday...as the upper-level trough starts to weaken in the
northeast, the frontal boundary from Tuesday should slow or even
stall in our vicinity. This could help focus some convective
development especially during peak heating, with possible
convectively induced impulses arriving from the west. There is much
more uncertainty this far out as the positioning of the upper-level
trough will govern the frontal zone placement and subsequent
Aviation /16z Thursday through Monday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg,
kilg, kmiv, kacy and surrounding areas.
Some MVFR ceilings were lingering in our region around 1530z. The
clouds will continue to lift.
Showers and thunderstorms, associated with an approaching cold
front, are expected to reach krdg and kabe between 17-18z. The
showers and thunderstorms are forecast to reach kilg, kphl, kpne
and kttn around 20z. They should push through kmiv and kacy
between 22-00z. Clearing is expected to occur tonight.
South to southwest winds around 5 to 10 are expected through today,
becoming northwest behind the frontal passage.
Friday...a chance of some fog early mainly at kmiv and kacy,
otherwise VFR. Northwest winds around 10 knots, then diminishing to
near 5 knots at night and becoming southwesterly.
Saturday...VFR overall. A shower or thunderstorm possible in the
afternoon and evening mainly north and west of kphl. Southwest winds
Sunday and Monday...VFR is anticipated.
a south to southwest flow of around 10 to 15 knots with gusts up to
20 knots is expected on the area waters through tonight. Seas will
remain around 2 to 4 feet.
A cold front will approach from the west and reach the area
waters late tonight. Showers and thunderstorms are expected to
precede the frontal passage.
Friday through Monday...the conditions are anticipated to be below
Small Craft Advisory criteria overall. A lingering front offshore
Friday into Saturday, then another front moves through late Saturday
and possibly stalls nearby into Monday.
a moderate risk of dangerous rip currents is forecast for
today, with increasing south to southwest flow ahead of a cold
front and the full moon on Friday.
PA...excessive heat warning until 6 PM EDT this evening for paz070-
New Jersey...excessive heat warning until 6 PM EDT this evening for njz015-
Delaware...excessive heat warning until 6 PM EDT this evening for dez001.