Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Mount Holly New Jersey
354 PM EDT Wednesday Jul 23 2014
a potent cold front will push through the region late tonight into
Thursday morning before moving offshore. High pressure will fill in
behind the departing front through the first half of the weekend
before a complex system approaches late on Sunday spilling into the
new work week.
Near term /until 6 am Thursday morning/...
an upper-level trough will continue to amplify across the northeast
and northern middle Atlantic regions through this evening. As this
occurs, gradual height falls will overspread our region and a cold
front will arrive. The most noted height falls and strengthening
cyclonic middle to upper-level flow is focused more to our north and
some of this even lags behind the cold front. Despite this, ample
instability due to a very warm/hot and humid airmass is in place and
some convection will continue to develop from the west. Some spots
this afternoon will have a heat index very near 100 degrees. The
initial convection appears to be tied mainly to near a Lee side
trough and also terrain influence, and will be slow moving given the
initial weaker wind fields. The convection farthest east has
struggled some so far. A laps sounding at 19z near Philadelphia
showed a cap near 700 mb, however this appears to be weakening /also
a precipitable water value of 1.7 inches/.
As the cold front arrives, additional showers and thunderstorms are
expected to arrive from the west. It appears that any organized
convection is more tied to the cold front and/or the Lee side trough
through this evening. This is when any organized convection should
occur, with the greater chance across our western zones. There should
be at least a broken band of convection propagating eastward across
the County Warning Area through this evening, although it may tend to weaken toward
the coast as it encounters some lower instability beyond sunset.
Given steep low-level lapse rates prior to the onset of boundary
layer cooling, a localized downburst damaging wind gust could occur
given the instability and some drier air aloft. The severe weather
threat still looks rather low given weak shear for storm
organization, however isolated severe weather is possible. The
convection will also be capable of producing very heavy rain and
frequent lightning given the moist lower level environment in place.
We continued with low likely probability of precipitation from west to east, although these
fade to chance probability of precipitation toward the coast and southern areas assuming the
convection weakens at least some. We also continued some enhanced
wording /non-severe/ for the areas especially with the likely probability of precipitation
through this evening.
Once the convection weakens with an eastward extent, some showers or
a thunderstorm may continue through the night especially farther
east as the front gradually shifts eastward and enough instability
is forecast to hang on.
As for low temperatures, we used an even blend of the NAM/GFS MOS
with some tweaks. It will be a warm and humid night, and some local
fog is possible especially where it rains a decent amount and if
some clearing is able to occur. Overall, this does not appear to be
widespread therefore it was not included at this time.
Short term /6 am Thursday morning through 6 PM Thursday/...
an upper-level trough will be across the northeast, however its axis
is forecast to hang back to the west until late in the day. This may
allow the cold front to slowly clear our eastern and southern zones.
The combination of the cold front, instability and enough forcing
should sustain or allow for the redevelopment of some showers and
thunderstorms. This looks primarily focused across the eastern and
southern portions of the County Warning Area. The northwestern areas should see a
drying trend, with even a drop in the dew points through the day.
The cloud cover could be problematic as convection along with the
southwesterly flow aloft could maintain middle to high clouds across a
good part of the area for awhile. Overall, any showers and storms
can produce locally heavy rain given the high precipitable water values being
squeezed to the south and east. As of now while a gusty thunderstorm
cannot be ruled out, severe thunderstorms are not anticipated due to
weaker middle level lapse rates and low-level flow.
We confined the highest probability of precipitation /chc/ south and east of Interstate 95,
with these lowering with time through the afternoon. High
temperatures were based on an even blend of the NAM/GFS MOS, then
some tweaks were made based on the idea of more clouds across the
Long term /Thursday night through Wednesday/...
Friday...just dealing with some lingering showers across our extreme
southern zones late Thursday night with the front just offshore.
There is some reaming instability just behind the front before the
cooler and drier airmass moves in on the nose of an advancing high
pressure but for the most part we will continue to dry out.
Temperatures should be similar to Thursday with neutral thermal
advection and weakly building heights.
Saturday...high pressure will sliding off the coast allowing a weak
return flow to develop with dewpoints climbing back into the low-60s
in a lot of places. The base of the trough continues to flatten with
a zonalish flow developing. Guidance shows a very weak middle-level
impulse moving through to our north during the midday hours without
much in the way of low-level forcing. This should provide more in
the way of cloud cover though the higher ridges could heat enough to
break through the middle-level cap producing isolated showers.
Sunday - Monday...strong piece of middle-level energy dives out of the
Upper Middle-west early Sunday morning and begins to carve a more
defined trough across the eastern U.S. Multiple waves embedded
within the mean southwesterly flow aloft swing through, timing of
the waves is still uncertain, touching off showers and
thunderstorms...very unsettled pattern expected. Surface cold front,
tied to a surface low pressure in southern Quebec, swings through
later on Monday slowly bringing an end to the unsettled period.
Tuesday - Wednesday...continued drying trend expected with surface
ridging and the middle-level trough retrograding, weakening, a bit.
Temperatures should be below normal.
Aviation /20z Wednesday through Monday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg,
kmiv, kacy and surrounding areas.
Rest of this afternoon...VFR with cloud bases around 5000 feet. Some
thunderstorms north and west of kabe and krdg should organize some
with a broken line arriving at these terminals around 22z, then
continue eastward thereafter. There is the potential for some brief
gusty west winds and very heavy rain with the thunderstorms.
Southwesterly surface winds mainly around 10 knots.
Tonight...VFR ceilings overall. A broken band of showers and
thunderstorms moving through the area in the evening, before
weakening toward the coast. A time of MVFR/IFR conditions should
accompany this activity, especially the visibilities. Southwest
winds around 5 knots, shifting to the north-northwest late. Gusty
westerly surface winds with a thunderstorm are possible in the
evening. Localized MVFR fog is possible late.
Thursday...VFR ceilings should thin out from northwest to southeast
through the day. However, some MVFR/IFR conditions may linger with a
few showers or a thunderstorm through middle afternoon especially from
near the kphl metropolitan on south and east. Localized MVFR fog possible
early. North-northwest winds around 10 knots.
Thursday night through Saturday...mainly VFR. Isolated showers
possible west...confidence is low on showers.
Sunday...sub-VFR conditions expected in areas of showers and
no headlines anticipated through Thursday. A cold front will
approach from the west through this evening, then gradually settle
south and east of the area overnight and Thursday morning. A
southerly flow will be maintained ahead of the front, with some
enhancement nearshore into this evening due to a sea breeze. The
winds will then shift to the northwest and north overnight and
Thursday behind the front, as drier air gradually seeps
southeastward. The flow overall is forecast to be weak enough to
keep winds below advisory criteria. However, a few gusty
thunderstorms are possible this evening for all the zones and then
mainly the southern zones Thursday.
Friday through Saturday...high pressure overhead will keep seas and
winds below Small Craft Advisory criteria.
Sunday...sub-sca expected with scattered showers and thunderstorms.
Monday...seas may build to Small Craft Advisory levels under a strengthening
southwest surface flow.