655 
fxus61 kphi 051958 
afdphi 


Area forecast discussion 
National Weather Service Mount Holly New Jersey 
358 PM EDT Sat Jul 5 2008 


Synopsis... 
a nearly stationary frontal boundary will remain in the area through 
Sunday as a couple of more disturbances track along it. 
Gradually though, high pressure in the western Atlantic will build 
into our area early next week. A stronger cold front is forecast 
to arrive on Wednesday and possibly stall just to our south as 
high pressure arrives from the northwest Thursday and Friday. The 
aforementioned front may then tend to lift northward on Saturday 
as a warm front. 


&& 


Near term /until 6 am Sunday morning/... 
a surface analysis this afternoon shows a stationary front draped 
from near Cape Cod to far southern New Jersey and then back across 
Virginia. An upper air analysis places an upper-level trough from 
the northeast back across the Ohio Valley with a 300 mb jet from 
West Virginia into southern New England. At least a couple of subtle 
short waves are embedded in a moist conveyor belt found on the 
east side of the upper-level trough. One of these features moved 
through this morning, with another one forecast to move across the 
County Warning Area this afternoon and evening. These features will be the main 
focus for showers and some thunder, however it should be noted 
that it will probably not take much in this moist airmass to pop 
showers even at night. 


Extensive cloud cover for a good part of the day has held the 
instability down some. However even so, cape values are generally 
between 1000 to 1800 j/kg this afternoon. The flow through the 
atmospheric column is not very strong, therefore cell motion has 
been fairly slow which can lead to locally heavy rainfall /see 
hydrology section below for more details/. The forecast soundings 
indicate the cape profiles are fairly thin, and this coupled with 
a moist sounding and weak winds, the strength of the updrafts may 
not be strong enough to generate a lot of lightning. We will 
maintain the mention of thunder though given a few cloud to ground 
strikes being detected attm, and the instability that is in place 
and linger for awhile tonight. The GFS appears to be overdone 
late tonight with a secondary short wave as the quantitative precipitation forecast bullseye in 
New Jersey does not look quite right. We therefore leaned a bit 
more toward the NAM. Overall the guidance has higher probability of precipitation the 
farther south one GOES and this general idea was continued 
tonight as activity gradually spreads in from the southwest. 
Given the weak flow and amount of moisture in the atmosphere, we 
are expecting a good amount of clouds tonight along with some fog. 


&& 


Short term /6 am Sunday morning through Monday night/... 
Sunday is expected to start out with clouds and some fog along 
with perhaps some showers. Again, the current thinking is that the 
quantitative precipitation forecast bullseye the GFS has in New Jersey ending at 12z Sunday is 
overdone due to some convective feedback /suspicious looking short 
wave/. The aforementioned front looks to try and transition into a 
weak warm front as it may slide northward some on Sunday. This 
will be in response to the upper-level trough sliding a little 
farther to the east along with additional short wave energy 
lifting northward, which should help push the surface boundary 
somewhat northward. Overall, Sunday may be very similar to 
Saturday as clouds will give way to some breaks of sunshine. With 
the upper-level trough, a frontal boundary at the surface and 
some embedded short wave energy, showers and some thunderstorms 
are expected to become more numerous through the afternoon. Some 
of the thunderstorms will tend to be the pulse type as winds are 
not very strong and the cape profiles are generally thin given the 
amount of moisture. The low-level flow may tend to back a little 
more to the southeast and this could lead to some possible upslope 
enhancement across the northwestern zones. The models are hinting 
at this potential with the placement of the quantitative precipitation forecast. If this occurs, 
the coastal areas may not see much in the way of showers and 
storms for awhile Sunday. 


A lot of the showers and thunderstorms are expected to diminish 
fairly quickly Sunday evening as they will mainly be diurnally 
driven. Even with the loss of daytime heating, the flow will 
continue to keep a moist and somewhat unstable airmass across the 
area even at night. Any subtle short waves may easily be able to 
pop some showers especially with any surface boundaries lingering. 
We may once again see some fog develop Sunday night, however at this time 
it was not included in the forecast. 


For Monday, the persistent upper-level trough will still be just 
to our west however it will be in a weakening phase as the large 
ridge across the western Atlantic builds westward some. Still, we 
cannot rule out some showers and thunderstorms especially during 
the afternoon and evening. The airmass is expected to modify some 
on Monday, therefore temperatures were bumped up a little bit. 


&& 


Long term /Tuesday through Saturday/... 
an expansive upper-level ridge will be the main weather player to 
start things off during this period as the persistent upper-level 
trough from the near/short terms lifts across the area and 
weakens. This will allow for hot and humid conditions on Tuesday 
and Wednesday. The overall model guidance is in decent agreement 
with the larger features, which mainly involves a slightly 
amplifying upper-level trough moving across the northern tier 
states Wednesday through Thursday. This will ultimately drive a 
cold front across our area later Wednesday. The Gem global seems 
to be on its own with a faster and more amplified vortex crossing 
the southern prairies of Canada late in the period. HPC used the 
European model (ecmwf) for Tuesday through Thursday, then a 70/30 blend of the 
European model (ecmwf) ensemble mean/European model (ecmwf) thereafter. We followed this guidance 
with mainly just some minor adjustments based on local affects and 
collaboration. We did go slightly lower with the probability of precipitation than HPC, 
given that the main flow should be positioned farther northward 
and with the uncertainty regarding areal coverage/timing at this time. 


Based on the above, high pressure at the surface and aloft 
anchored off the southeastern U.S. Coast Tuesday will pump a hot 
and humid airmass into our area. This will give way to an 
advancing cold front on Wednesday, with the potential for a 
surface trough to develop well ahead of the front. As of now, the 
stronger middle to upper level flow looks to be north of our region. 
However, the advertised pattern could be characteristic of some 
convective complexes that develop in the Great Lakes region and 
slide eastward. This far out, uncertainties with the initiation 
points along with the placement of individual short waves, leads to 
less confidence on the track of some convective complexes if they 
do indeed form. Still, the surface cold front will be moving into 
a hot and humid airmass so something to keep an eye on. 


The aforementioned cold front may then tend to stall just to our 
south during Thursday as it remains beneath a nearly 
parallel/zonal flow aloft. As this occurs, a surface high is 
forecast to build toward our area later Thursday and Friday from the 
northwest. As middle to upper ridging begins to take place to our 
west Friday into Saturday, and the surface high gradually slides 
to our east, the leftovers of the stalled frontal boundary should 
lift northward as a warm front. The placement of this frontal 
boundary in the vicinity may be just enough to keep low chance probability of precipitation 
in the forecast, mainly during the afternoon and early evening 
hours. 


&& 


Aviation /20z Saturday through Thursday/... 
sites were MVFR to VFR this afternoon with MVFR to VFR visibilities and 
scattered convection. Instability looks to be best south and west 
this afternoon into the early evening, then dissipate from north 
to south overnight only to strengthen again on Sunday. The weak 
front draped across the region may be a local focus for some 
convective activity early in the taf period, and a weak middle level 
impulse coming out of the 500 mb trough to our west may ripple through 
to enhance any activity later this afternoon and this evening. 


Models indicate another impulse may move toward the area, especially 
the southern sites, toward daybreak, although the GFS seems to 
suffer from convective feedback and the NAM is not so robust with 
it. Climatologically, with a stalled front and a lot of surface 
moisture around, we generally see a decrease in activity and a 
worsening of conditions from VFR/MVFR to MVFR/IFR in the overnight 
period. Although the models do agree on some approaching middle level 
trigger toward daybreak, we've leaned toward climatology in the 
tafs for now. 


Outlook... 
because of the proximity of the front, and then because of a 
developing warm and humid flow as the Bermuda high begins to be the 
main influence, at least scattered showers and thunderstorms will be 
possible through the outlook period. 


&& 


Marine... 
the quasi-stationary front over our waters is forecast to weaken 
gradually or lift north as a Bermuda high to the southeast and 
associated middle level ridge gradually build. We therefore are 
maintaining continuity and forecasting a southerly flow to be 
increasingly dominant over all the waters with time. Eventually, 
what is left of the middle level low to the west and an associated cold 
front should move through and cause winds to shift to the west 
fairly late in the forecast period. Winds through the forecast are 
prognosticated to be light to moderate and seas are capped below the 
advisory criterion. Scattered showers and thunderstorms remain in the 
forecast given the middle level low to the west and the warm and humid 
air mass. Because of the rising dew points and the water 
temperatures, some fog may begin to become a problem, especially in 
the overnight and morning periods. 


&& 


Hydrology... 
the nearby 12z radiosonde observations had precipitable water values around 1.7 
inches. The water loaded airmass coupled with not much wind 
/mainly unidirectional/ in the column, will lead to rather slow 
forward propagating cells. There could be some backbuilding 
/training/, which would enhance the locally heavy rain potential. 
Some training was observed early this morning from near 
Philadelphia to portions of northwestern Burlington County. On 
Sunday, the precipitable water values are forecast to remain near 
2.0 inches, therefore once again locally very heavy rainfall is 
possible. The areal coverage of the flash flooding threat looks 
isolated attm, therefore no watch will be issued however a 
mention in the hazardous weather outlook will be included. The 
rain is needed across the area, just not all at once. 


&& 


Phi watches/warnings/advisories... 
PA...none. 
New Jersey...none. 
Delaware...none. 
Maryland...none. 
Marine...none. 


&& 


$$ 
Synopsis...gorse 
near term...gorse 
short term...gorse 
long term...gorse/HPC 
aviation...delisi 
marine...delisi 
hydrology...gorse 

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