Weather

079 
fxus61 kphi 281942 
afdphi 


Area forecast discussion 
National Weather Service Mount Holly New Jersey 
342 PM EDT Thursday Aug 28 2008 


Synopsis... 
high pressure over northern New England will gradually move eastward 
tonight and Friday. Meanwhile, the remnant moisture of what once was 
Tropical Storm Fay will be mainly forced to our west. A cold front 
is then expected to move through our area on Saturday, with high 
pressure settling in for Sunday and lingering through most of next 
week. 


&& 


Near term /until 6 am Friday morning/... 
a surface analysis this afternoon shows high pressure parked over 
New England with a stationary front draped from North Carolina 
westward to southern Indiana. An upper air analysis shows the the 
best 850 mb warm air advection has shifted into the Great Lakes area. This has been 
translating northwestward with time as the southeasterly low-level 
jet slides northward as well. Water vapor imagery shows that the 
remnants of what once was Tropical Storm Fay has been pretty much 
absorbed into a midlevel trough. 


A rather interesting setup occurred today, which the majority of the 
models have been advertising, and that is the surface high in New 
England has maintained its present across our County Warning Area although it has 
been slowly giving way. Despite the onshore flow, a good amount of 
dry air has been maintained across our area with the exception of 
the Delaware-Maryland-Virginia where dew points have increased into the 60s. The 
low-level jet in conjunction with lower level convergence acting on 
an overrunning surface has produced areas of rain to our west and 
southwest all day. The radar trends however are continuing to show a 
weakening trend south of Pennsylvania, with the activity mostly 
translating to more of a shower nature. The precipitation shield has 
pretty much worked its way around our area today, with portions of 
the Delaware-Maryland-Virginia zones and our extreme western zones getting clipped 
with a few showers. For the folks in Middlesex, Monmouth and ocean 
counties, plenty of sunshine has occurred for the bulk of the day 
as drier air has held firm here. So, where do we go from here? 


The remnant moisture will continue to track northward with the main 
focus to our west where the best low-level forcing travels. The GFS 
is to far northeast with its measurable precipitation ending at 18z, 
while the NAM was to far west. The sref 6-hour total precipitation 
mean is much to far northeast with the measurable precipitation. 
Overall, we are not anticipating much in the way of showers tonight 
with the exception of our far western areas and near Chesapeake Bay 
as lingering moisture rotates northward through this evening. We 
tried to keep many locations dry through tonight, although with 
some short wave energy nearby and the moisture inflow plume still 
to our south, we cannot rule out a few showers. The northeastern 
zones should have the best chance of not seeing any showers as high 
pressure and drier air holds on the longest here. 


Forecast soundings do indicate deepening moisture through the night, 
and this will combine with a continued onshore flow. We believe that 
a lot of clouds will cover the region tonight with areas up north 
even clouding up eventually. If the lower level moisture deepens enough, 
we may see some stratus develop. It appears this moisture will 
not be deep enough though to produce some drizzle across the cwa, 
with the exception of perhaps the far northwestern zones. As of 
now, we believe any fog would be rather limited given the dryness 
the bulk of our County Warning Area continues to experience. 


As for temperatures, MOS numbers are closely clustered, therefore we 
generally utilized the GFS values. 


&& 


Short term /6 am Friday morning through Saturday night/... 
on Friday, the lingering moisture will continue to dissipate and be 
shunted northward as the upper-level flow becomes more zonal. We 
very well may start the day with a wealth of clouds as we await at 
least some mixing from daytime heating. We are expecting a fair 
amount of clouds though as the low-level moisture may increase a bit 
more leading to cumulus develop below middle to high level clouds. It 
appears that there will not be much across our area to focus showers. 
Given lingering moisture, via forecast soundings and a potential 
short wave sliding nearby, we carried a slight chance to low chance pop. 


Our next feature of interest will be an advancing cold front moving 
through the Great Lakes and Ohio Valley on Friday. The models are in 
good agreement with the short wave and associated surface cold front 
moving through the eastern Great Lakes and the possibility of a 
closed low near northern New England. The UKMET and Canadian show 
weaker solutions for this system, but the Canadian in particular has 
shown a wide range of solutions for New England over its past few 
days. For now we leaned more toward continuity with this feature 
which is still fairly close to the operational GFS and NAM. The 
cold front is expected to move through our area on Saturday with 
some showers and thunderstorms possible. This front still does not 
look to strong, although most of the model guidance suggesting 
that perhaps a line of convection develops ahead of the front. 
Hopefully we will be able to squeeze out some rain from this front 
as our County Warning Area has missed out on the beneficial rains from the 
remnants of Tropical Storm Fay. 


Any showers and thunderstorms should end early Saturday evening as 
the front slides to our east. High pressure will begin to arrive 
from the west with a northerly flow becoming established, allowing 
for drier air to infiltrate the region. As a result, a clearing sky 
is expected during Saturday night. As for temperatures especially 
Friday, with the anticipation of a fair amount of clouds and still 
an east to southeasterly flow Friday, we opted to go closer with 
the cooler Eta model MOS. 


&& 


Long term /Sunday through Thursday/... 
this time frame is expected to be dominated by a surface high 
sliding across the region and an amplifying upper-level ridge. As 
with any longer term forecast, there are some uncertainties that 
try and crop up mostly with the details and there is at least one 
during this time frame. More on this in a little bit. 


The latest model guidance has not changed much from 24 hours ago, in 
that they continue to advertise a building ridge across the eastern 
United States. The European model (ecmwf) supports recent model indications from the 
GFS and both the GFS/European model (ecmwf) ensemble means that the pattern to start 
out in this period will become increasingly amplified with troughs 
across the West Coast and over southeastern Canada, that are 
expected to eventually become closed off. Between the troughs will 
be a strong ridge. Model guidance differences become very apparent 
in the east by Tuesday and Wednesday, with the European model (ecmwf) more aggressive 
with the closed low remaining near the East Coast /actually 
retrograding some/, while most of the GFS ensembles advertise a 
ridge pretty much holding strong. Only a couple of the GFS ensemble 
members bring the upper-level low toward New England late Tuesday 
through Wednesday. HPC utilized a various blended approach of the 
European model (ecmwf) and GFS ensemble mean, which provides the best compromise of 
addressing the West Coast trough and upper-level low near the 
northeast. Some incorporation of the European model (ecmwf) mean instead of the GFS 
ensemble mean was made by HPC. 


Based on the above, the cold front from the short term is 
expected to be well off the coast Sunday morning, with high 
pressure building across the Great Lakes. This will place our 
entire area under a northerly flow, and the surface flow may try 
and shift to the northeast, especially along the coast as the 
gradient may tend to weaken. 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 airmass into our region. As a result, high 
temperatures were bumped up some. During Wednesday and Thursday, 
the surface high may start breaking down as the upper ridge 
starts to be knocked down. Also, we will have to watch the closed 
low to our east, however for now we will maintain mostly high 
pressure across the area. Given the uncertainty on any effects 
from the upper low to our east, we opted to go with a dry forecast 
for now Wednesday night and Thursday. Overall, we did not deviate 
to much from HPC guidance, however some blending of some mosguide 
data was introduced in some fields mainly for collaborative 
purposes. 


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


&& 


Aviation /20z Thursday through Tuesday/... 
the current radar trend is similar to that which has been occurring 
all day. There is a swath of precipitation mainly to the west of the County Warning Area that 
is moving very slowly nwd, and hardly eastward at all. Overall, the 
precipitation has shown a weakening trend both in intensity and areal 
coverage. With that said, the most likely locations for any precipitation 
would be krdg and kabe. However, there is another small swath to 
the south that could clip kmiv or kacy if it holds together, and I 
am not confident that it will. With that said, will continue the 
status quo in the tafs. Will keep a vcsh for krdg and kabe and not 
have any mention of precipitation elsewhere. Probability of precipitation are all too low at this time. 


Depending on which model you believe, if any, there could be some 
lingering chances tomorrow. The NAM continues to be lower with its 
probability of precipitation than the GFS, and this seemed to work well for today. 


Generally, VFR conditions are expected through the taf period. The 
only exception would be if there is a period of moderate or heavy 
rain. 


Outlook... 
a cold front will cross the region on Saturday, and this 
feature may bring a better chance of showers and even some 
thunderstorms. Again, there is some diff in the models, with the 
GFS being wetter (as is usually the case) than the NAM. VFR 
conditions are expected outside of any periods of shwrs/tstms. 


&& 


Marine... 
high pressure over the region will keep things relatively benign 
over the marine zones. Waves have been hovering right around 4 feet 
and are expected to continue in the 3 to 4 feet range. On Sat, winds 
will swing around to S as a cold front approaches from the west. The 
front will cross the waters late Sat or early sun and winds will 
then shift to the northwest. Attm, Small Craft Advisory conds are not expected through the 
forecast period. 


&& 


Climate... 
it has been a cooler than average August across the County Warning Area so far as 
all our climate sites are 2 to 3 degrees below average /except 
Georgetown/. Philadelphia has only reached 90 degrees or higher once 
this month /to date/. The last time this occurred was back in 1996. 
The last time no 90-degree days occurred in August in Philadelphia 
was back in 1984. Overall, this August in Philadelphia has a chance 
of being the coolest August since 2000 /average temperature was 
74.0 degrees/. It will also be the first time since 1994 that August 
will be cooler than June in Philadelphia. Prior to that, it was 
1976. 


&& 


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...nierenberg 
marine...nierenberg 
climate...gorse/gigi 

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