Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Mount Holly New Jersey
930 PM EDT Friday Jul 3 2015
a weak high pressure system will remain over New England tonight,
before moving offshore on Saturday. Meanwhile, low pressure over
the Ohio Valley will move along a stalled frontal boundary to our
south 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 midweek.
Near term /until 6 am Saturday morning/...
the evening water vapor satellite imagery shows a positively-
tilted upper trough over the Midwest. Moisture is streaming ahead
of the trough from the western Gulf of Mexico northeastward toward
the central Appalachians and middle-Atlantic coast. At the surface, a
stationary frontal boundary was positioned to our south near the
Regional radar mosaic this evening showed a band of light showers to
our south and west over western PA/VA/central Maryland that was
associated with DPVA from a leading middle-level shortwave trough
and isentropic lift. Probability of precipitation through tonight closely follow the latest
hrrr, rap and NCEP WRF-nmm, which seem to have had a good handle
on the progression of these showers thus far. Precipitation is generally
expected to advance northeastward into the Eastern Shore of Maryland
this eve, Delaware close to midnight and eastern PA/New Jersey late
tonight into the early morning.
The best lift with the front and the upper trough will remain
upstream of the area tonight. The County Warning Area will be located on the
cool/stable side of the boundary. There is some residual dry air in
the middle and low-levels. Therefore, do not expect the precipitation to be
steady or heavy tonight in this setup.
Short term /6 am Saturday morning through 6 PM Saturday/...
still lots of uncertainty in this period. There is considerable
disagreement between models and run-to-run on individual model runs.
For the most part, the main deterministic models had a slightly
drier trend with the 12z/18z runs. However, this has been the
case the last few days, with the 00z runs tending to be wetter
solutions. At this point, think the highest chances will still
come before 21z. Think that it will mostly be a steady rain given
a rather stable atmosphere in place. As far as rain amounts, have
favored lighter amounts given the uncertainty of how much moisture
advection we will have. The threat of thunderstorms, heavy
rainfall and flooding looks to be dwindling given these recent
Most model solutions continue to show the precipitation lifting out
quickly from middle to late afternoon. This timing may allow for
some outdoor 4th of July festivities to take place late in the
day. The ground may be damp though in parts of the area that
receive the steadier/heavier rainfall (most likely Delaware-Maryland-Virginia and
With all the cloud cover in place, it looks to be a rather cool day
(highs in the 60s in the mountains and 70s elsewhere).
Long term /Saturday night through Friday/...
any lingering precipitation should be ending Erly Sat evening which should have
a positive impact on any fireworks displays and other Holiday
High pressure will then build in for Sat night through Monday keeping this
period dry over most, if not all of the area. The only location that
guidance implies could see some precipitation would be over the Delaware-Maryland-Virginia come
By Monday night into Tue, another quick WV moves thru, then the GFS
is much faster with a cold front from the W, bringing it through by Wednesday
morning and the associated precipitation. The CMC and European model (ecmwf) are much slower
bring the front through later Wednesday into Wednesday night. If the ec is to be
believed, with the slower frontal timing, Tuesday night into Erly Wed,
could be dry and Wednesday could be on the hot side ahead of the front. Then
the European model (ecmwf) develops what appears to be a very strong wave for Friday
that the GFS doesn't have, and is subsequently dry.
Once again, confidence is lower than average. There will be a cold front passage.
Have shaded this forecast toward the model consensus and the slower
timing, but, not sure on the most likely periods of precipitation as a result
of the uncertainty. For now, am just carrying low probability of precipitation for Fri, as
we have a week to see what, if anything, develops.
Temperatures will start off near normal and rise to above normal values by the
middle and end of the week.
Aviation /01z Saturday through Wednesday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg,
kilg, kmiv, kacy and surrounding areas.
VFR conditions expected through the overnight with ceilings at or above
5,000 feet. Some light rain showers may begin to move into the
region by 05z with chances increasing from south to north
overnight. The steadiest and heaviest showers look to be from
13-18z Saturday, which will correspond to the highest chance for
MVFR ceilings/visibilities from phl/pne to the Southeast. Ridge and Abe
could escape the prospects for any MVFR restrictions. Acy has
highest likelihood to see MVFR conditions with the slowest rise in
ceilings late in the day. Ceilings will likely lift to VFR and
become broken by 19z outside of Acy where an onshore flow will
tend to keep them lower. Winds will be at or under 10 knots from
the south late this evening then shifting to easterly for most of
Saturday. A wind shift to a more westerly direction is likely to
start by Saturday evening.
Sat night...some MVFR fog possible at the more rural arpts, otherwise
VFR. High confidence.
Sun through Tuesday night...VFR sun and Sun night. High confidence. VFR
Monday and Tuesday outside of any rain showers/thunderstorms and rain mainly in the aftns.
Wednesday...predominately VFR, but decreasing to MVFR/IFR with
increasing chances of rain showers/tsra, especially west as cold front approaches.
winds and seas should stay below Small Craft Advisory criteria tonight into tomorrow
morning. By tomorrow afternoon, winds and seas could build to near
25 knots/5 ft, primarily on the northern Atlantic waters. However,
there is considerable uncertainty and have capped winds/seas just
below Small Craft Advisory threshold.
Outlook... Sat night and sun...there is still some uncertainty
with the strength and track of the low, but it appears the best
chance of seas above Small Craft Advisory conds will be early Sat night and possibly some
marginal wind. However, confidence is low enough at the moment to
hold off issuing any flags at this time. The best chances appear to be on the
northern waters. After any Small Craft Advisory lingering into the mrng, the conds should
decrease below Small Craft Advisory later into sun.
Sun night through Wednesday...no marine flags are anticipated.
with continued weak on shore flow, high tides this evening and
Saturday evening could approach the minor flooding level, but
right now no sites are forecast to reach levels needed for a
coastal Flood Advisory. Sat night and sun look to be the same way.
While some sites may approach minor tidal flooding levels, current
data indicates they will remain below criteria.
there is a moderate risk of rip currents on Saturday along the
New Jersey shore given the continued onshore flow and moderate
period swells. The risk is low for Delaware.