Weather

425 
fxus61 kphi 291945 
afdphi 


Area forecast discussion 
National Weather Service Mount Holly New Jersey 
345 PM EDT Friday Aug 29 2008 


Synopsis... 
high pressure still anchored over northern New England will 
gradually move eastward tonight, giving way to another area of high 
pressure over the Great Lakes region Saturday into Sunday. However, 
a cold front is expected to move through our area later Saturday, 
followed by the aforementioned second area of high pressure settling 
in for Sunday and lingering through most of next week. 


&& 


Near term /until 6 am Saturday morning/... 
a surface analysis this afternoon places high pressure anchored over 
northern New England. This persistent high, although weaker, has 
continued to remain an entity all the way down into Delaware. West 
of this, a surface trough extended into southeastern Pennsylvania 
from Virginia. 


Surface observations along with radar imagery shows what looks to 
be a mesoscale-low near kdca this afternoon. Could this be the actual 
remnant circulation of fay? Some additional shower activity has 
developed with this. The model guidance suggests a short wave 
tracks across the area, and this appeared to have teamed up with 
the aforementioned surface trough and produced an organized area 
of rain currently moving across parts of eastern Pennsylvania. 
This surface trough may lag behind the short wave though, allowing 
the area of rain to perhaps gradually weaken with an eastward 
extent. The presence of the surface high wedging down the coast 
has yet again allowed a fair amount of clearing across Monmouth, 
ocean, Mercer and even into Burlington counties for awhile this 
afternoon. Surface observations also show some lower dew points in 
these areas, which even mixed out some further to the southwest. 
The 12z lwx radiosonde observation has a veering wind profile with height in the 
lower levels. Model data shows this with some warm air advection and Theta-E 
advection, however this gradually dwindles during the first of 
tonight as a weak short wave exits the region. This may be just 
enough to keep showers going, especially this evening. We will 
make a last minute decision on probability of precipitation given radar trends, but we 
will likely have to carry at least low chance probability of precipitation across a good part 
of the area through about midnight. We did bump up probability of precipitation to likely 
for portions of eastern Pennsylvania through the remainder of the 
afternoon based on radar trends. 


A good amount of moisture within the atmosphere will continue 
through tonight, with even the northeastern areas also seeing an 
increase. This coupled with a light southeasterly flow is expected 
to maintain a wealth of clouds tonight. Forecast soundings as well 
as MOS guidance suggest a good chance of stratus to develop. As a 
result, we will continue the idea of a cloudy/mostly cloudy sky. As 
the dew points come up some tonight coupled with the light winds, we 
may see patchy fog form particularly in areas that saw some rain 
last night and today. 


As for temperatures, MOS is mostly clustered for tonight and was 
generally followed. 


&& 


Short term /6 am Saturday morning through Sunday night/... 
Saturday may start out with low clouds and some fog for many as the 
low-level moisture pools. We are expecting some of this to mix out 
during the course of the morning as the sunshine dissipates some of 
it. The forecast soundings do show a good amount of moisture 
remaining with precipitable water values of 1.50 to 1.80 inches. 
This should allow for a fair amount of cumulus development as a cold 
front makes its way into our area. The overall timing of the front 
has not changed much, therefore it will be moving through during the 
afternoon and evening hours. Forecast soundings generate enough cape 
to maintain the mention of thunder, however midlevel lapse rates are 
not real impressive. A 250 mb jet is forecast to slide just to our 
north, and the northern zones especially are closer to the entrance 
region. The upper flow does show some diffluence, however the 
overall take on this front continues to be that it is fairly weak 
/lacking a pronounced thermal gradient/. There still is enough 
evidence that some scattered showers and thunderstorms will develop, 
therefore we will maintain chance probability of precipitation. Some model guidance still 
insist on perhaps a convective line forming ahead of the front, 
however the vertical shear looks weak enough to preclude an 
organized convective threat. 


The cold front will settle south and east of our County Warning Area Saturday night 
with the upper-level trough axis also passing overhead. There was 
some evidence that a weak wave develops along the front and perhaps 
slow it up some, however this looks scenario looks rather slim and 
rather weak even if it does, which should not have a noted impact on 
the frontal passage. As high pressure slides into the Great Lakes 
region Saturday night, much drier air is expected to infiltrate our 
entire region. Forecast soundings show this nicely as the 
atmospheric column dries from the top down. As a result, clearing is 
expected from northwest to southeast. High pressure then builds in 
from the northwest during Sunday as a upper-level ridge builds into 
the Great Lakes. Our weather will still be influenced to some extent 
as a closed low sits just east of New England. With heights 
gradually rising on Sunday though, we anticipate a bump up in 
temperatures. 


As for temperatures, went mainly with a compromise between the GFS 
and Eta model MOS as the GFS MOS looks to warm for many locations 
/especially on Saturday/. We utilized mostly the 12z mosguide data 
for dew points Sunday as the low-level moisture may mix out some 
during the afternoon. 


&& 


Long term /Monday through Friday/... 
this time frame is still expected to be dominated by a surface high 
sliding across the region in response to an amplifying upper-level 
ridge. It appears the uncertainty that was more pronounced regarding 
an upper-level low to our east has decreased due to additional model 
guidance agreement. The latest model guidance has not changed much 
from 24 hours ago, in that they continue to advertise a 
building/expanding ridge across the eastern United States. The only 
noted change is that the European model (ecmwf) has backed off the idea of the closed 
low to our east eventually retrograding. This is in response to the 
upper ridge sliding farther to the east, like the GFS had been 
advertising, which keeps the closed low farther away. The 500 mb GFS 
ensembles are nearly all in agreement with the upper ridge 
dominating our weather through the majority of this time period. HPC 
noted that the European model (ecmwf) ensemble mean is also in good agreement with 
the overall upper-level pattern. The sref 500 mb height mean looks 
rather similar with the building ridge and the closed low well to 
our east. We will be on the western side though of the closed low on 
Labor Day, however ridging is expected to dominate. HPC mainly 
leaned toward the European model (ecmwf) for their guidance, but did blend in a small 
percentage of the 12z GFS MOS. 


Based on the above, an expansive area of high pressure will be 
located over the eastern Great Lakes and sliding eastward on Labor 
Day. This will allow our entire area to be under a general northerly 
flow, and the surface flow may try and shift to the northeast or 
east, especially along the coast as the gradient looks fairly weak. 
This extensive area of high pressure at the surface is expected to 
be reinforced as the upper-level ridge amplifies through Wednesday. 
Given the consensus of the building upper-level ridge, this is 
expected to toss an increasingly warmer air mass into our region. 
The only possible fly in the ointment is perhaps a weak frontal 
boundary that may slide down the coast as a quick shot of some 
cooler air aloft moves southward on the east side of the upper 
ridge. This feature, if even eventually present, looks rather 
ill-defined. This feature is noted though in the thermal fields at 
about 850 mb with some weak cold air advection for a time Monday morning. Either 
way, that should be rather short lived as ridging quickly builds 
eastward. 850 mb temperatures are forecast to near +20c later Monday 
and last through most of the week. As a result, high temperatures 
were bumped up a bit more. If some models are correct, some areas 
may start September with a heat wave /highs 90 degrees or higher for 
3 or more consecutive days/. 


During Thursday and Friday, the surface high may start breaking down 
as the upper ridge could begin to be knocked down as some troughing 
pushes into the Midwest. Since high pressure is anticipated to be 
the main weather feature through the period, we opted to maintain a 
dry forecast. Overall, we did not deviate to much from HPC guidance, 
however some blending of 12z mosguide data was introduced in some 
fields mainly for collaborative purposes. The surface dew points may 
mix out during the afternoon's of Monday and Tuesday, and the 
mosguide data better reflected this. The overall surface flow looks 
weak, therefore a sea breeze may occur each afternoon. 


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


&& 


Aviation /20z Friday through Wednesday/... 
conditions are VFR across the taf sites with just some light 
precipitation falling at this time northwest of a kilg-kttn line. 
Of concern is the increase in the low level moisture that has been 
feeding north all day. Dew points are in the middle and upper 60s...up 
about 10 degrees over the past 24 hours. While dew points north of 
kttn are in the 50s I do expect to see dp's increase there too. This 
will present a problem with an onshore marine flow and lighter winds 
after dark. Upstream the ceilings and vsby's are falling into the 
MVFR/IFR category and see this happening in some of our sites late 
this evening for our Delaware/PA sites and then New Jersey taf sites after 
midnight... especially toward morning. A cold front is going to 
push through and finally carry the remnant moisture from Fay 
offshore as a super VFR high pressure spreads over the region. So 
at least 1/2 of the Holiday weekend will be excellent VFR weather. 


Outlook... 
fair weather expected sun into a good part of next week. 


&& 


Marine... 
sea state is half swell and half wind wave. Persistent onshore flow 
is strongest south of Little Egg Inlet and producing 4 feet waves at 
buoy 44009 that are in 5 and 11 second sets. Nearshore...waves at 
Avalon are 2.6 feet...1.9 feet at Ocean Grove...2.2 feet at Bethany and 
2.6 feet at Ocean City Maryland. These observations are consistent 
with the wave watch model. 


The gradient is tighter down over the southern waters/Delaware Bay 
as high pressure northern New Jersey into New England...interacts 
with an inverted trough extending south from Delaware to North 
Carolina. The trough will move east overnight with a low forming on 
it offshore. It becomes absorbed in the western Atlantic high on 
Saturday. 


A front will approach from the west Saturday afternoon and off the 
coast by evening. Following the front will be a stretch of nice 
weather as high pressure builds over the region Sunday into the end 
of next week. Great back to school weather. If you have a chance to 
fish or go boating the upcoming week will be excellent. 


Water temperatures are at their highest with averages of 70 to 75 
degrees from northern New Jersey to Delaware. 


Have a nice and safe Labor Day weekend and remember to wear your 
pfd!!!!! 


&& 


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


&& 


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

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