Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Mount Holly New Jersey
935 am EDT Thursday Jul 2 2015
a cold front will stall to the south of our forecast area
while a weak low pressure wave moves along it today. A high
pressure system will slide across New York state and New
England tonight and Friday as another stronger wave
of low pressure follows in tandem south of our forecast area.
As another high pressure system builds into the Great Lakes
over the weekend, the frontal boundary is expected to
sag farther to the south. The high pressure system should
pass across the forecast area on Monday. Then a cold front from
the northern plains should reach the eastern Great Lakes by
Near term /until 6 PM this evening/...
the cold front has stalled right around the southern Delaware state line.
As the low develops and slides along the front, will continue to
see showers and thunderstorms in the vicinity of the front. For
the most part this precipitation should be confined to the southern half
of the region, and generally only through late afternoon, but
considering the uncertainty, have not made any changes to probability of precipitation at
this time. There is not a significant amount of instability, but
there should be enough to allow for isolated/scattered
thunderstorms to develop during today as well. Precipitable water values increase
to 1.5-2.0 inches, so any showers/thunderstorms will be pretty
efficient rain producers. Although there is a fair amount of
shear, due to the limited instability, we do not expect much/if
any severe weather, although any thunderstorms could produce gusty
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 Friday/...
the front remains stalled out to our south tonight, while the low
pressure that develops during the day moves eastward along the front
and passes offshore overnight. There remains a couple of short
wave/vorticity impulses that will move across the area overnight
which should continue to allow scattered showers to develop across
portions of the area, mainly southern New Jersey and the Delaware-Maryland-Virginia.
Instability wanes even more overnight, but we will keep isolated
thunder in the forecast as there may still be some rumbles of
thunder through the night.
Lows are a blend of mav/met MOS and mosguide.
Long term /Friday through Wednesday/...
the WRF-nmmb initialized better in the east, while the GFS
initialized better in the west at 500mb. At 850mb the WRF-nmmb
was overall better while it was equal billing at 925mb. Deep/dt
the ridge out west continues to verify stronger with overall
more amplitude to the 500mb flow. Conversely the deep/dt on the
Friday surface wave on the GFS was weaker.
There is slightly closer modeling consensus with the late week
system. There is still a way to go as to when, but none of
the models are dry in this period. Applying a correction factor
of timing, the European model (ecmwf) appears too slow given that nearly all of the
players are already in the Continental U.S.. the GFS appears too fast given
the higher verifying amplitude flow, while the WRF-nmmb may be
showing its northward bias. Its now the furthest north model.
Pmdhmd suggests a GFS/European model (ecmwf) blend which we are following to Iron
some of the differences. The latitudinal extent has implications
as there will be some heavy rain closer to the wave train.
Thus on Friday we slowed the GFS solution down by about 6 hours
which is a best fit timing. Thunder was close to predicted 925mb
and 850mb instability aloft (sfc based cape not much at all) and
near swody2 delineation. Because of the slower arrival of pcpn,
maximum temperatures are higher than GFS MOS.
Friday night, greatest northwestward extent of precipitation chances
in our County Warning Area. Mention of thunder was only kept well south again
following showalter and 925mb guidance.
For Saturday we kept chance probability of precipitation in most of our County Warning Area as a timing
hedge of the departing surface low in the morning. Also
because of the short wave in the albeit flattening 500mb flow
for the afternoon. Unless all model timing is too slow, the day
in general should be improving as it GOES along. The onshore flow
would keep coastal areas cloudier longer.
Then the Holiday weekend is expected to improve Saturday night
and Sunday as the wave moves offshore and a high pressure system
builds in from the Great Lakes. The same for Monday as the high
drifts across our County Warning Area.
For the rest of the long term, not much change. Old boundaries
never seem to go away in the Summer and the European model (ecmwf) in particular
brings it northward on Tuesday. Meanwhile, a cold front should
come close enough to US from the Great Lakes to bring a second
source of thunderstorms for Wednesday.
Temperatures at the start of the Holiday weekend should average
below normal. This changes starting with the daytime
highs on Sunday as they should be at or above normal for the
rest of the long term. Given we remain in the westerlies, while
warm, it does not look oppressively hot or very humid.
Aviation /14z Thursday through Monday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg,
kmiv, kacy and surrounding areas.
Patchy mist/fog is developing at many airports early this morning,
reducing visibilities to MVFR range, with some IFR. This should burn off
in the next few hours, and lead to VFR conditions for the rest of
the day. Scattered showers are expected to move into southern New
Jersey and the Delaware-Maryland-Virginia today. A few of the showers may develop
into thunderstorms. The highest risk for lightning will be at
kmiv and kacy this afternoon. Any showers/thunderstorms that do
affect a taf site could temporarily reduce visibilities/cigs, but we have
not included the lower conditions at this time.
Winds early this morning are generally light and variable/calm and
should remain so through much of the morning. Once the wind picks up
a direction, tafs across the west should have more of a west-
southwest wind, while tafs across the east should have more of an
east-southeast wind, and the central taf sites should have more of a
More mist/fog is expected to develop again overnight tonight into
Friday morning as winds should become light and variable/calm again.
Friday...VFR conditions northern airports/terminals, possibly
becoming MVFR southern airports and terminals.
Friday night and Saturday...greatest likelihood for sub VFR
conditions, especially for airports and terminals closer to the
coast and in Delaware-Maryland-Virginia. Improvement as the day continues Saturday.
Saturday night through Monday...primarily VFR conditions expected.
the cold front will continue to slowly sag to our south this
morning, before stalling out through tonight. Winds are expected to
remain below advisory levels, and seas should remain below advisory
levels as well at 3-4 feet.
Friday through Saturday...pending the strength of the low pressure
system on the stalled front, Small Craft Advisory conditions could
occur on the ocean waters. Higher chances southern waters.
Saturday night through Monday...sub Small Craft Advisory conditions
expected. Confidence increases Sunday into Monday.
the onshore flow combining with rather robust Spring tides
following the full moon might cause minor tidal flooding with the
evening high tide cycles on Friday (the more likely one) and
Saturday. This will depend on the strength of the departing waves
of low pressure.
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. We think that one or
both days could reach moderate risk levels.