Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Mount Holly New Jersey
402 am EDT Friday Jul 3 2015
a weak high pressure system will build into New England today
and tonight. Meanwhile, a low pressure system over the lower
Ohio Valley will ride along the stalled frontal boundary and
pass east of the Delaware-Maryland-Virginia peninsula on Independence day.
Another area of high pressure will then move into the
region for the end of the Holiday weekend and Monday. A weak
low pressure system might ease north into the area on Tuesday as
another cold front approaches the forecast area by Wednesday
Near term /until 6 PM this evening/...
Patchy fog has developed overnight due to clearing skies and light
winds, but the fog will dissipate after sunrise.
Today should be a fairly nice day for most areas. There are no
strong short wave/vorticity impulses forecast to move across the
area and there is not any instability forecast as a strong cap will
be in place. So we do not expect any precipitation to develop
through the daytime period for most of the area. The only exception
would be across southern New Jersey and Delaware-Maryland-Virginia where there is some
weak channelized vorticity, which could create a few isolated
showers. Otherwise, mainly an increase in cloud cover is expected
Highs are a blend of mav/met MOS, mosguide, and blended 925 mb European model (ecmwf)
Short term /6 PM this evening through 6 am Saturday/...
An area of low pressure will be developing to our west this evening
along the stalled out frontal boundary that is located to our south
and southwest. This low will track eastward through the night along
the frontal boundary and approach our southern areas toward daybreak
Saturday. A couple of short wave/vorticity impulses will be moving
across the area overnight as well. This will lead to increasing
shower possibilities overnight into daybreak Saturday. There is not
much, if any instability forecast for most areas except our far
southern areas. So we only keep isolated thunderstorms for extreme
southern New Jersey and Delaware-Maryland-Virginia.
Lows are a blend of mav/met MOS and mosguide.
Long term /Saturday through Thursday/...
the GFS initialization looked much better than the WRF-nmmb with
the 500mb trough in the Midwest and also better than the WRF-nmmb
at 850mb. Once we get down to 925mb, the WRF-nmmb looks better.
The implication of the 500mb differences (esp in the upper
midwest) is that the WRF-nmm has a slightly more sheared weaker
solution for Independence day. The latest GFS is very close to
the previous two European model (ecmwf) runs and now has the heaviest rain flipped
to the opposite end of our County Warning Area from last night. A deep/dt valid 18z
Saturday shows this transition which started with 06z run last
night. With the trough deeper and hanging back, the next wave on the
front becomes much more muted and the last low on the wave train
becomes the Alpha low. And there GOES Independence day.
Last night it appeared the Euro was too slow with wave number two,
but in reality and unfortunately its looking like it will be right
for the wrong reason as its wave number three that should make it
three consecutive wet saturdays in a row. The warm air advection induced phase
of the system should be underway or on the verge of getting
underway in the western part of our County Warning Area as Independence day
starts. We see decent middle level qvec convergence plus a well
positioned 250mb entrance region of the jet for the cool sector
precipitation maximum. The raw quantitative precipitation forecast may be overdone there because of the lack
of predicted convection, there should be a secondary maximum in that
part of the cyclone.
While the European model (ecmwf) has been steadier, there are still some latitudinal
differences to the extent of convection in our County Warning Area. The GFS is on
the northern envelope. The gefs mean does a north-south-north
meandering around the op surface low position. We spread convective
chances northward as Saturday progresses, but the best chance
remains from the Mason-Dixon and Acy expressway line southward.
This should be a secondary 9and possibly highest) precipitation maximum. GFS
precipitation water values exceed two inches yet again.
As far as severe GOES, predicted 500mb temperatures are rather warm, a few
degrees warmer than 6/30. Thus middle level lapse rates are poor. Dcape
is not strong. We will have more than adequate bulk shear, but mixed
layer convective available potential energy are predicted to remain less than 1k joules except for
possibly Delaware-Maryland-Virginia. So if there is a window for stronger storms, its
far south as of now. The early start to clouds and precipitation is not going
The warm air advection push gets through during the afternoon, so if there is a
Silver lining, the current timing (pmdhmd suggests a non-CMC blend)
would end the highest precipitation chances toward evening. Not confident
about maximum temperatures at both ends of our cwa: northwest clouds/precipitation may
make US too high, southeast, a warm sector squeeze we may be too
low. Full sun supportive of near 90.
If we can bank on current timing, showers and thunderstorms would be done by
the start of any firework festivities Saturday evening. Given the
timing slippage from last night, we carried a low chance early.
Sunday continues to be the better weekend day. A dry forecast with
temperatures recovering to near normal and relatively low humidity
levels. Forecast soundings do show some cape, but have the cap
holding. Its the type of day in which very isolated convection
would affect less than 10% of our County Warning Area.
The transition to a warmer and more humid week then starts on
Monday. This might be tempered slightly on Monday and Tuesday as
yet another weak low pressure on our old frontal boundary is
predicted to form and drift slowly north. Probability of precipitation are gradually
increased from south to north Monday into Tuesday.
No big changes to Wednesday and Thursday forecasts. A slow moving cold front
should be near enough to trigger more thunderstorms on Wednesday
and Thursday. The usual model timing differences are there with
the GFS faster than the European model (ecmwf). Normally slower is better in Summer
(unfortunately for saturday), so we kept relatively high probability of precipitation in
for both days.
Aviation /08z Friday through Tuesday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg,
kilg, kmiv, kacy and surrounding areas.
Patchy mist/fog has developed overnight due to clearing skies and
light winds, leading to MVFR and IFR conditions at some airports.
The fog will dissipate after sunrise, then VFR conditions are
expected through today into the first half of the night. Rain may
start moving into the area late Friday night/early morning Saturday.
Once the rain begins, conditions may lower to MVFR or lower.
Light and variable winds early this morning will become more east to
northeast later this morning, then south-southeasterly by this
afternoon and remain south-southeasterly until daybreak Saturday when
the become easterly again. All speeds should be less than 10 knots
today into tonight..
Saturday...predominately MVFR, with IFR conditions at times
during heavier showers and thunderstorms.
Saturday night...some MVFR conditions possible in fog, especially
at more rural airports.
Sunday and Sunday night...VFR.
Monday and Tuesday...predominately VFR, but decreasing to MVFR/IFR
going forward into Tuesday as more showers and thunderstorms develop.
sub-Small Craft Advisory conditions will remain across the area
today into tonight, although winds may begin increasing and gusting
around 20 knots overnight Friday into early Saturday morning as low
pressure approaches from the west.
Saturday and Saturday night...depending on the strength and track
of the low pressure system along the stalled front, a Small Craft Advisory may be
needed, especially on the northern ocean waters starting
Saturday afternoon. While winds should decrease Saturday night,
seas might still reach Small Craft Advisory criteria.
Sunday through Tuesday...outside of any special marine warnings...no
synoptic scale induced marine flags anticipated.
due to the recent full moon and a steady easterly flow today into
tonight, tides will continue running high. The high tide later
tonight into early Saturday morning may approach the minor flood
level, but right now no sites are forecast to reach levels needed
for a coastal Flood Advisory. The two sites that may get the closest
would be Reedy Point and Philadelphia. We'll continue to monitor
through the day to see if an advisory is needed.
Then on Saturday night, the tidal concern switches to the
Oceanside as a lingering northeast flow should bring actual tides
close to minor tidal flooding levels. The highest concern
is along the New Jersey coast toward New York City.
the combination of an onshore flow and the full moon is expected
to increase the strength of rip currents on Friday and Saturday,
before conditions improve on Sunday. Local regression guidance
shows the rip current risk for the New Jersey and Delaware beaches to be just
below moderate for today, with a greater likelihood for a moderate
risk on Saturday (which may not be a great beach day because of