Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Mount Holly New Jersey
932 PM EDT Tuesday Jul 29 2014
weak high pressure over the area merges with the stronger western
Atlantic high pressure late this week...forcing the offshore front
to back in toward the coast as a warm front this weekend.
Near term /until 6 am Wednesday morning/...
the clouds will continue to dissipate overnight as additional dry
air builds into our region.
The light wind and the clearing sky should allow temperatures to
drop into the 50s at most locations. Some readings in the upper
40s are possible in the elevated terrain. While record low
temperatures are not expected, we should get close. The records
are listed in the climate section below.
Short term /6 am Wednesday morning through 6 PM Wednesday/...
weak high pressure continues to move across the area early in the
day Wednesday. However, late in the day, a weak surface trough may
develop across our western zones, and there are some weak
vorticity impulses that may swing through the broad trough aloft
as well. This, combined with some moisture may help enhance the
shower/thundershower potential late in the day. There is not much
instability across our area, but there may be just enough for an
isolated thundershower to occur for our northwestern areas. This
being said, we expect most of the focused activity to remain to
925/850 mb temperatures increase a degree or two Wednesday, and
thicknesses increase ever so slightly as well. Therefore highs on
Wednesday should be a few degrees warmer than Tuesday. We used a
mav/met MOS, mosguide, and mixed 925mb temperature blend for highs on
Long term /Wednesday night through Tuesday/...
500 mb: the large trough in the eastern United States weakens
late this week, then probably flattens early next week.
Temperatures: calendar day averages several degrees below normal
Thursday then near normal thereafter possibly warming above normal
early next week. This will allow July to average near or slightly
below normal at most locations except at least a degree below
normal at kacy.
Forecast basis: unless otherwise noted the long term period of
this forecast is generally based on a 50 50 blend of the 12z/29
NAM/GFS MOS for the period Wednesday night - Thursday night... the
12z/29 GFS MOS Friday and Friday night and then thereafter
(saturday onward) the 1522z/29 wpc gridded guidance which at times
is blended 50 50 with the 12z/29 mexmos. All this is eventually
checked against the 12z/29 European model (ecmwf) to see if we're in the ballpark
with European model (ecmwf) cyclically persistent 18z 2meter temperatures. The
precipitation probability guidance is checked against the 09z/29
sref probability of precipitation for .01 in 3 hours through 00z Friday and thereafter the
12z/28 gefs 6hr probability of precipitation for 0.05
Wednesday night - Thursday evening...continuing the prior middle
shift forecast. Mostly dry across the region outside the
possibility of isolated instability showers across the northern
portions of the County Warning Area Thursday afternoon. Cold pool interaction with
a passing shortwave may be enough to get air mass showers to occur.
Friday...transitional day with the looks of an inverted trough
sitting off the Atlantic coast...the old baroclinic zone that
pushed through on Monday. Interior instability and warm air
advection associated with short waves moving northeastward through the Middle
Atlantic States appear to make showers probable for eastern PA. It
should be unstable enough for thunder there during the afternoon.
Friday night - weekend...still uncertainty in shower timing
details which will affect maximum temperatures this weekend but it appears
that there has to be a period of showers...some briefly
heavy...with isolated embedded thunderstorms. This in assn with a pair of
short waves moving northeastward through the trough. Again...timing the
primary period of showers, in my opinion, is still as of this
writing...quite uncertain. Maximum temperatures Saturday could differ 5
degrees either way of our current forecast...depending on whether its
raining between 11am and 3 PM. I maintained our prior middle shift
forecast temperatures for this day...not using the cooler wpc guidance, nor
the much warmer GFS guidance. The European model (ecmwf) is even cooler than the
Monday - Tuesday...a slight drying warming trend though not
entirely void of any showers with a few more weak shortwaves
shearing northeastward nearby or overhead.
Aviation /02z Wednesday through Sunday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg,
kilg, kmiv, kacy and surrounding areas.
VFR conditions are expected through the taf period with mainly
scattered cloud cover, especially from 0400z Onward.
A light and variable wind is anticipated overnight with a light
west to southwest wind expected for Wednesday.
Wednesday night - Thursday...mostly VFR. Isolated shower or
thunderstorm possible mainly north of kabe Thursday afternoon.
Friday - Saturday - Sunday...mostly VFR ceilings. IFR conditions in
showers and isolated embedded thunderstorms expected sometime between
Friday afternoon and midday Saturday.
sub-advisory conditions will continue through Wednesday as winds
remain 10-15 knots or less and seas remain 2-3 feet or less.
Wednesday night - Saturday...sub-sca conditions expected. Showers
and thunderstorms should occur sometime later Friday into Saturday.
Next week: we are monitoring the GFS operational run depiction
(for two days of forecast cycles, though todays 12z/29 run backed
off) of a tropical system in the Atlantic Basin for early next
week. The European model (ecmwf) operational version has been much more
conservative. We could be seeing a 1 foot 15 second swell arrive
along our shores next Monday... maybe increase to a 2 foot 13
second southeast swell next Tuesday...again...if a tropical system occurs
as outlooked today.
a cool to even locally chilly overnight is anticipated. This may
result in some record low temperatures being approached Wednesday
morning. As of now, the forecast low temperatures are not record
tying or setting. The record low temperatures for July 30th are
listed below for reference.
Philadelphia, PA....58 degrees set in 1981 and 1914.
Wilmington, Delaware......55 degrees set in 1956.
Allentown, PA.......50 degrees set in 1997 and 1968.
Reading, PA.........52 degrees set in 1981.
Atlantic City, New Jersey...53 degrees set in 1981 and 1968.
Trenton, New Jersey.........54 degrees set in 1997.
Georgetown, Delaware......54 degrees set in 1981.
Mount Pocono, PA....42 degrees set in 1968.