708 
fxus61 kphi 182018 
afdphi 


Area forecast discussion 
National Weather Service Mount Holly New Jersey 
418 PM EDT Friday Jul 18 2008 


Synopsis... 
high pressure stretching from the western Atlantic to the Tennessee 
Valley will gradually move eastward through Saturday. A frontal 
boundary from New England to the Great Lakes should meander mainly 
north of the area into Sunday. An area of low pressure is then 
forecast to track from the Great Lakes to New England, pushing a 
cold front through our area early next week. This front may linger 
just to our south and east however during most of next week with 
the potential for a few disturbances to track along it. 


&& 


Near term /until 6 am Saturday morning/... 
a surface analysis this afternoon places a high pressure system from 
the Tennessee Valley eastward to the western Atlantic. A frontal 
boundary continues to meander to our north, which is now located 
roughly from southern New England into southern Ontario, then back 
to the Midwest. An area of low pressure is spinning just off the 
coast of Georgia and South Carolina. 


Plenty of hot conditions out there this afternoon and dew points 
have come up as advertised, however a gradual downward trend was 
noted at some reporting sites this afternoon as some drier air 
from aloft mixed down. Despite this, there is a noted increase in 
low-level moisture for most areas compared to the previous two days. 
Given the increased low-level moisture coupled with daytime 
heating, has allowed some cumulus to develop. The low-level front 
that edged into our northern areas yesterday and teamed up with a 
middle level feature has moved farther to the north this afternoon. 
With little in the way of a trigger around, convection should be 
limited into this evening. We went with a 20 pop for the potential 
of isolated convection across the far western County Warning Area as the thinking 
is terrain should mainly be the trigger /a cell or two popped up 
north of York, Pennsylvania a little earlier/. The focus for some 
isolated convection should be confined to the higher terrain where 
the weak flow contributes to terrain induced circulations. 


Following any isolated convection, a mostly clear sky is expected 
tonight with some haze and perhaps patchy fog. The convective 
debris across the Great Lakes should generally remain to our north 
tonight. Any lower dew points will likely recover through the 
evening as we lose the daytime heating. There is some Theta-E 
advection up across the northwestern zones overnight, however 
moisture convergence appears to be limited. A few isolated storms 
popped early this morning, and while this cannot be ruled out, any 
true forcing to latch onto is difficult to find. As a result, we 
will go with a dry forecast for the overnight. 


&& 


Short term /6 am Saturday morning through Sunday night/... 
the hot and humid conditions will persist Saturday as we remain on 
the backside of a surface high pressure system and upper-level 
ridging. The overall flow is forecast to increase some from the 
southwest, which will pull in some more low-level moisture. The 
forecast soundings show once again decent mixing potential in the 
boundary layer, therefore some dew points may drop some during 
peak heating but this is expected to be fairly limited. The 12z 
radiosonde observation from Sterling, Virginia indicated an 850 mb temperature of 
+17c /63f/ and 925 mb temperature of +23c /73f/. These 
temperatures are forecast to not change a significant amount 
Saturday, therefore temperatures are expected to top out fairly 
close to this afternoon's. 


This heat event will continue this weekend and the heat indices that 
we are coming up with top out at about 100 degrees Saturday 
afternoon in the highly urbanized areas. Due to the length of this 
heat, we will maintain the excessive heat warning through Saturday 
for the highly urbanized areas. The hot conditions during the day 
will not back off a tremendous amount at night in the highly 
urbanized areas as overnight lows may not drop below 75 degrees. 
This, coupled with the buildings retaining the heat of the day at 
night, points to keeping the warning going. On Sunday, there are 
some uncertainties that become introduced as a frontal boundary 
will be lurking to our north and west. Some model guidance brings 
this boundary further south compared to some others. The GFS and 
WRF-nmm have a different scenario on the placement of a surface low 
on this boundary, with the WRF-nmm much more pronounced. The 15z 
sref mean sea level pressure mean is closer to the GFS. We do 
like the idea though of the low-level flow turning a bit more 
south to even southeasterly on Sunday as the low off the southeast 
coast may back the flow some into our area. Given that some clouds 
may be around and the potential southeasterly flow, high 
temperatures are forecast to be lower /still hot though/. Since it 
will still be hot and humid, adding another day to this event, and 
the uncertainty involved in the actual high temperatures, we felt 
it was best to keep the excessive heat watch for Sunday and not 
upgrade or cancel. This will allow later shifts to monitor 
observational and model guidance trends and adjust accordingly. 


Regarding convective possibilities, we cannot rule out some isolated 
convection across the northwestern zones Saturday afternoon as 
heating and perhaps terrain induced circulations combine to pop 
isolated activity. We believe it will be isolated as there is no 
solid trigger, therefore we went with a slight chance pop in the 
gridded database. Sunday may feature a slightly better chance for some 
convection, mainly in the afternoon and evening, as a frontal 
feature approaches from the west. We may even have a Lee side 
trough potentially develop and become activated to our west. For 
now, we placed low chance probability of precipitation in the afternoon from about Interstate 
95 on westward, with slight chance probability of precipitation to the east. We will need to 
watch the surface low near the southeastern U.S. Coast as some of 
that moisture could be transported northward, especially if the 
low-level flow around it becomes great enough from the south and 
southeast. The GFS wants to link up some of the moisture Sunday, 
however confidence in that at this time is low. 


A blended approach was mostly used for temperatures, however we 
leaned more toward the lower guidance numbers on Sunday given the 
potentially backed low-level flow off the ocean. 


&& 


Long term /Monday through Friday/... 
this time frame is expected to feature a pattern change as the 
overall upper-level flow GOES under an amplification phase. As 
nearly always the case, there are some uncertainties creeping in 
which mainly revolves around just how amplified the upper-level 
trough becomes. Also, the disturbance near the southeastern U.S. 
Coast and what comes of that and if any associated moisture can 
get pulled northward. First, the model guidance. 


HPC guidance used the 00z European model (ecmwf) for Monday and Tuesday, mostly 
because of its integrity with the potent impulse lifting up the 
eastern Seaboard. The 00z/12z GFS was sheared with this feature, 
which already looks compact on satellite. From Wednesday Onward, a 
switch to a blend of the 00z GFS ensemble mean and European model (ecmwf) ensemble 
mean was made, which are not divergent, but do differ on the depth 
and amplitude of the trough over southeastern Canada. The European model (ecmwf) 
mean is most concerning over the western Great Lakes, where it 
continues to imply a much more cooler air mass into the northern 
plains. The incorporation of the GFS ensemble mean, which is 
considerably flatter along the Canadian border by Thursday and 
Friday, basically removes some of the enhancement out of the European model (ecmwf) 
solution. We will see how this evolves and the European model (ecmwf) solution 
could pan out given the atmospheric fluctuations seen so far this 
Summer. However given the time range, our forecast used a 
compromise approach. Interestingly enough, the GFS continued to 
be an outlier among the models with the aforementioned trough, 
being much sharper and amplified with the wave. HPC mentioned that 
the instability of the deterministic GFS over the past several 
days is another strike against going solely with its solution 
at this time. We will have to see what comes of the surface low off the 
southeastern U.S. Coast, however indications are that this entity 
eventually tracks northeastward. We will monitor to see if it has 
any effects on our County Warning Area with time. 


Based on the above, a cold front is forecast to be entering our area 
on Monday with a moist southwesterly flow ahead of it. As the 
upper-level trough to our west continues to amplify, the flow should 
push the surface boundary to our south and east Monday night. At 
least based on the location of the axis of the upper trough attm, 
the surface front may stall not to far to our south and east 
Tuesday and Wednesday with ridging to its east. As short wave 
energy rotates through the base of the upper trough and ridging 
pokes into the plains, a disturbance may initiate Wednesday or 
Thursday in the plains along a warm front. Meanwhile, a weak area 
of high pressure should settle into our area with the low-level 
boundary lurking not to far away. Gradually, a wave of low 
pressure may form on the boundary and toss some added lift into 
our area Thursday and/or Friday. Timing the short wave energy and 
where the position of the boundary will be this far out is of 
lower confidence, therefore we will carry some chance probability of precipitation for 
several days with the best chance probably occurring in the afternoon 
and evening hours. 


Regarding temperatures, around 90 in the more urbanized areas Monday 
then all areas should see a step down as we get more into the 
influence of upper-level troughing. The humidity however may stay up 
for awhile though across most of the cwa, but even this may drop 
through time especially across the northern areas. Excessive heat is 
not anticipated attm, therefore no outlooks or watches will be 
issued. 


That is all for now. Thanks to the surrounding offices for the 
collaboration today. Have a good one! 


&& 


Aviation /20z Friday through Wednesday/... 
conditions were VFR at all eight of our taf sites at 2000z. There 
was generally a southwest wind at 8 to 10 knots. 


At 2000z, the radar was picking up the New Jersey sea breeze front 
hugging the beach from Sandy Hook down to Asbury Park, then drifting 
inland toward Lakewood and Bamber Lake, before returning back toward 
Tuckerton and eventually along the back bays of Atlantic County and 
Cape May County. The sea breeze is meeting some resistance from a 
general southwest to west flow near and just above the surface. As 
a result, it should not make much progress inland. We have 
indicated that it may reach kacy around 2100z. The Delaware Bay 
breeze is also meeting some resistance, and we have simply hinted 
that it could reach kilg for a brief time between 2300z and 0100z. 


At middle afternoon, numerous showers and thunderstorms were developing 
over central and eastern New York in response to a disturbance in 
the upper atmosphere. Most of that activity should remain well to 
the north of our taf sites. Meanwhile, isolated showers and 
thunderstorms had developed over central Pennsylvania, along with an 
isolated shower near Willow Grove. The Pennsylvania precipitation 
was mostly showing signs of weakening. However, we will continue to 
monitor it, as it could affect krdg, kabe, kttn or kpne. For now, 
it seems as though the chances are less than 30 percent, so we have 
left it out of those tafs. 


Once the sun sets, any precipitation should dissipate. Conditions 
at all eight of our taf sites should continue as VFR, but there may 
be some MVFR visibility restrictions tonight. 


After sunrise on Saturday, conditions should improve back to VFR, 
with a southwest wind redeveloping. 


Outlook... 
mainly VFR conditions are anticipated for Saturday afternoon through 
Wednesday. However, visibility restrictions are possible in the 
early morning hours each day, and there is the potential for mainly 
afternoon and evening showers and thunderstorms on Sunday, Monday, 
Tuesday and Wednesday. 


&& 


Marine... 
long period swells of 10 to 11 seconds continued on the coastal 
waters of New Jersey and Delaware today. Wave heights were mainly 
in the 3 to 4 foot range. As Bertha begins to move north of 40 
degrees north latitude late tonight, the swells should begin to 
become less of a factor for our coastal waters. Until that time, 
the swells will continue to cause rough conditions in and around the 
inlets along the coasts of New Jersey and Delaware. 


High pressure, located off the coast this afternoon, should continue 
to influence our region over the weekend. A slow moving area of 
low pressure, developing along the southeast coast, may pass off 
Cape Hatteras on Monday. Meanwhile, a cold front from the northwest 
is forecast to pass off the coast on Monday night before stalling 
for Tuesday and Wednesday. 


At this time, conditions are forecast to remain below Small Craft 
Advisory criteria through the period. 


Rip currents... 
the rip current risk remained in the moderate category for today, as 
swells from Bertha continued to reach our coastline. As Bertha 
begins to move north of 40 degrees north latitude late tonight, the 
swells should begin to become less of a factor for the coasts of New 
Jersey and Delaware. 


&& 


Phi watches/warnings/advisories... 
PA...excessive heat warning until 8 PM EDT Saturday for paz067>071. 
Excessive heat watch from Saturday evening through Sunday 
evening for paz067>071. 
New Jersey...excessive heat warning until 8 PM EDT Saturday for njz015- 
017>019. 
Excessive heat watch from Saturday evening through Sunday 
evening for njz015-017>019. 
Delaware...excessive heat warning until 8 PM EDT Saturday for dez001. 
Excessive heat watch from Saturday evening through Sunday 
evening for dez001. 
Maryland...none. 
Marine...none. 


&& 


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

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