Scientific Forecaster Discussion
Area forecast discussion...corrected
National Weather Service Mount Holly New Jersey
409 PM EDT Sat Jul 12 2014
high pressure will continue to move offshore tonight. The heat and
humidity will build Sunday and Monday as southerly return flow
strengthens around the high. A cold front will slowly move through
the area Tuesday and offshore Wednesday. High pressure builds into
the region Thursday and moves offshore by the end of the week.
Near term /until 6 am Sunday morning/...
in general...high pressure prevails across most of the area. The
weather is dry in most areas...but some scattered showers and thunderstorms have
develop across the western edges of the forecast area...and they may
affect there areas into the early evening. A few short term warnings
have been issued due to the slow nature of the cells. We will leave
mostly chance probability of precipitation for those areas...with a low likely pop for Berks
County through 00z. A quiet evening and overnight expected with some
patchy fog overnight and a muggy mild night expected. Low temperatures a
met/mav MOS blend.
Short term /6 am Sunday morning through 6 PM Sunday/...
high pressure will lose its influence on our weather. An approaching
upper trough and surface fronts will bring increasing chances for afternoon
showers/thunderstorms to most areas. Probability of precipitation are in the chance range for the most
part. Another very warm and muggy day expected with high temperatures in
the low/middle 80s north and central and upper 80s and low 90s
Long term /Sunday night through Saturday/...
synoptic setup early next week will feature a digging trough to
our west as an anomalously strong upper low for this time of year
moves southeastward over the Great Lakes. Deep S-SW flow downstream
of the trough and upstream of a Bermuda ridge will strengthen over
the eastern Continental U.S.. an active weather pattern is expected under this
setup through Tuesday.
A surface trough associated with lopres moving just east of James Bay is
forecast to extend southward from the low into upstate New York and then
back to the southwest across western PA prior sunset Sunday evening.
Upstream showers and thunderstorms that develop in western/central
PA along and ahead of the surface trough during the daytime would most
likely progress eastward into the forecast area Sunday night.
Nocturnal convection may be able to be maintained somewhat by a
southwesterly low level jet, but it should weaken later into the evening with
the loss of daytime heating. Probability of precipitation are highest toward the Lehigh
Valley/Poconos (likely) and decrease to the southeast (to slight
chance by the New Jersey/Delaware coast).
On Monday, the entire eastern Seaboard will be positioned within
the pre-frontal warm sector that is well ahead of the Great Lakes
upper low and attendant cold front. Hot and humid are forecast
with southerly return flow around the high and the low level thermal ridge
overhead. However, the extent of leftover debris clouds from
overnight convection is uncertain, but if clouds linger later into
the day than currently anticipated, forecast maximum temperatures in the
lower 90s may not be realized. Assuming sufficient breaks in cloud
cover do occur, moderate to strong instability would develop over
the region (median SBCAPE from the 09z sref are 1500-2000 j/kg).
Convection would likely develop along a sharpening Lee surface trough
during the afternoon. Stronger SW winds aloft will spread
eastward ahead of the trough, helping to increase deep-layer
shear over the area (fcst median 0-6 km shear from the 09z sref
increases to 40 knots across northestern PA late in the day). There is a
potential for severe thunderstorms Monday afternoon and evening
given the forecast environment. The area is outlooked in a slight
risk by Storm Prediction Center for Monday. Damaging winds would be the primary threat
but large hail, isolated tornadoes and flash flooding are also
While the models have come into better agreement with the handling
of the upper low over Great Lakes, there are still some differences
regarding the timing of the cold front on Tuesday. The potential
for another round of severe thunderstorms ahead of the cold front
on Tuesday would be higher in the slower solution that was
depicted by the 00z/12z European model (ecmwf). 12z GFS continues to be more
progressive with the front, shifting the core of the warm, moist
and unstable air south and east of the region (which would limit
the potential for a more significant severe thunderstorm event).
Despite these uncertainties, probability of precipitation were increased to likely for
most of the area Tuesday with favorable synoptic lift along the
front and in the right entrance region of a strong upper-level
The cold front should be offshore on Wednesday. Post-frontal northwest flow
will advect much cooler, drier Canadian air. Both Wednesday and
Thursday both 5-10f below normal. Forecast maximum temperatures ranging from the
low 70s in the Poconos to low 80s along and east of I-95 combined
with dewpoints at or below 60f will be a respite from the Summer heat and
Aviation /20z Saturday through Thursday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg,
kilg, kmiv, kacy and surrounding areas.
VFR conditions continue at the terminals this afternoon. Showers and
isolated thunderstorms continue west of krdg early this afternoon and they
may begin to affect krdg around 19z through 21z so amds there are
possible. Elsewhere...few-sct cumulus and a decent S/SW winds inland and
mostly southeast wind across de/NJ.
Good conditions will continue this evening...then some patchy fog
will develop overnight. MVFR visibilities may develop after 06z across most
areas. The more metropolitan taf sites may remain 6sm haze/br however.
Sunday...a bit of morning fog...then more VFR conditions with
building daytime cumulus. Showers/thunderstorms may develop later during the
afternoon. Winds mostly srly 10-15 kts.
Sunday night through Tuesday night...unsettled weather pattern with the
potential for MVFR/IFR in several rounds of shra/tsra. The best
chance for precipitation will be during the afternoon/evening periods on Monday and
Tuesday. S-west winds ahead of the cold front around 10-15 knots each
aftn, and 5-10 knots each night.
Wednesday-Thursday...Canadian high pressure builds into the area
with VFR conditions.
winds and seas will remain srly or southeasterly overnight and Sun morning.
Winds speeds and seas will be sub-Small Craft Advisory in those periods. Sun
afternoon will see an increase in winds with seas to follow. We will
bring out the Small Craft Advisory for Sun afternoon with wind gusts around 25 kts
and seas increasing to around 4 feet. Scattered thunderstorms possible later sun.
Sunday night through Tuesday...
southerly winds expected in advance of a slow-moving cold front will
become gusty each afternoon and evening. Small Craft Advisory was issued for Sunday
night for the Atlantic coastal waters with gusts to 25 knots and seas
4-5 feet. Small Craft Advisory may be needed again Monday and Tuesday.
the high tides tonight and Sunday night may approach the minor flood
level along the Atlantic coastal areas, but are not expected to
reach high enough levels to require a coastal Flood Advisory.
Marine...Small Craft Advisory from noon Sunday to 6 am EDT Monday for