951 
fxus61 kphi 102336 
afdphi 


Area forecast discussion 
National Weather Service Mount Holly New Jersey 
636 PM EST Wednesday Mar 10 2010 


Synopsis... 
an area of low pressure will continue to organize across the central 
and Southern Plains over the next few days and slowly move eastward 
by this weekend. As the low moves toward the upper Ohio Valley 
Friday, a secondary low will develop near the middle Atlantic coast 
Saturday. This system will then slowly move eastward over the 
weekend, with it gradually moving father away from our area during 
the first part of next week. 


&& 


Near term /until 6 am Thursday morning/... 
a 19z surface analysis placed a warm front from the Delaware-Maryland-Virginia to the 
Northern Ohio valley then northwestward to low pressure in 
north-central Iowa. Another low was located in southwestern Oklahoma 
with a cold front trailing to the south. Meanwhile, high pressure 
was centered near Hudson Bay. An upper air analysis placed a 
midlevel ridge from the middle Atlantic up through the Great Lakes 
region. A compact and potent closed low embedded in a large trough 
was over the Southern Plains. A small 300 mb jet was from Kentucky 
to the Delmarva, with a strong jet from south Texas across Mexico. 


The water vapor imagery this afternoon shows a weakening midlevel 
low across the northern plains. Meanwhile, a potent and compact 
closed low was across the Southern Plains. This feature will be the 
one to watch as it gradually slides across the eastern half of the 
nation and expands over the next several days. Backing up a bit 
though, the initial midlevel low has sent a warm front toward the 
middle Atlantic region. There is warm air advection at 850 mb /albeit weak/ ongoing 
this afternoon and this is expected to weaken a bit more overnight. 
The weak warm air advection coupled with the flow turning to the southeast to 
southwest with height has produced a good amount of mostly middle to 
high level clouds across the region. There has been some returns 
showing up on the radar imagery today, although only a few ground 
reports indicated some sprinkles reaching the surface. This activity 
is so light, limited in areal coverage and not expected to last, 
therefore it was not included in the forecast /a short term 
forecast was issued earlier to cover it/. Overall, the synoptic lift 
is rather weak through tonight with only a modest increase in the 
wind field above the surface forecast. As a result, we believe 
that any lift will go into generating just clouds and not any 
precipitation through the night. 


As upper-level ridging slides across our area overnight, a surface 
high is forecast to strengthen some and build into northern New 
England. This feature will have some influence in the forward 
propagation of the large closed low to our west. Based on this, we 
have slowed down the arrival of the probability of precipitation for measurable 
precipitation. The visible satellite imagery shows some thinning of 
the cloud canopy to the west and southwest, and this should move 
over parts of our area for a time tonight. However, as the moisture 
continues to deepen and the surface dew points respond, clouds 
should increase and thicken once again. The moisture increase does 
not appear to be quick enough to produce problematic fog overnight, 
therefore we will not include at this time. 


As for temperatures, we generally took an evenly weighted blend of 
the NAM/GFS MOS as it should not be an ideal radiational cooling 
night. 


&& 


Short term /6 am Thursday morning through Friday/... 
an extended period of active weather will be gradually ramping up 
during this time frame. A massive closed low across the plains is 
forecast to gradually slide eastward as some blocking remains 
downstream. The model guidance is in pretty good agreement with the 
larger picture, however there are differences with respect to the 
surface low that develops near the middle Atlantic coast at the end of 
this time frame. Where this develops and how the moisture transport 
behaves as increasing frontogenetic forcing arrives will be key in 
determining where the heaviest quantitative precipitation forecast falls. However, the heaviest quantitative precipitation forecast 
event appears to be centered beyond this time frame. As the closed 
low really wraps itself up across the central part of the nation and 
an expansive surface high is positioned just east of northern New 
England, a slower solution seems to be the best way to go /the NAM 
appears to be to slow, which has been the case many times this 
winter season/. The warm air advection aloft is forecast to generally be on the 
weaker side through the majority of this time frame. While the 
isentropic lift gradually becomes more efficient, the depth of the 
moisture takes some time to deepen. This combined with the midlevel 
ridge hanging around in our vicinity longer, makes US lean toward 
lower quantitative precipitation forecast values through this entire time frame. 


Based on the above, we continued to paint in a good deal of 
cloudiness through this entire period as the moisture will continue 
to deepen with time. We went ahead and slowed the arrival of the 
probability of precipitation from west to east with slight chance probability of precipitation in parts of the area 
Thursday afternoon. As the warm air advection and stronger moisture advection/wind 
field begins to arrive Thursday night and Friday, we ramped up the 
probability of precipitation fairly quickly to categorical /fairly light qpf/. Given the 
slower solution, the true surface warm front is expected to stay 
south our area. The quantitative precipitation forecast amounts are anticipated to average up to one 
half of an inch through Friday day. We generally went with the 12z 
HPC guidance with just slight adjustments. Of note is the potential 
for some fog to develop. While the extent is not real certain, there 
is the potential that some develops Thursday night and persists 
through Friday. While confidence is not high on this development, 
some cooler air may get trapped across the western portion of the 
County Warning Area as moisture continues to advect in, therefore we went with 
patchy fog there for now. 


As for temperatures, they could be a bit tricky Thursday night and 
Friday especially. The NAM MOS is particularly cooler for many 
locales during this time frame as it is farther south and slower 
with the system. This brings in cooler air into our County Warning Area from the 
influence of the surface high well to our northeast. Attm, we 
believe the NAM values are to cold and we leaned much more heavily 
toward the milder GFS MOS. Otherwise, a more even blend of the 
GFS/NAM MOS was utilized. 


&& 


Long term /Friday night through Wednesday/... 
the consensus of the models, including the European model (ecmwf) since at least the 
00z/09 run, is that there is going to be a heavy rainfall somewhere 
in the Friday night through Saturday evening time period over our 
area. 800 mb dewpoints of 10 degree c clip our southern zones and the 800 mb 
Theta-E advection on a stiffening southeast flow looks quite strong. 
A very pronounced trowal has been progged, as has strong lower level 
frontogenesis, strong q-vector convergence aloft, negative or 
near-negative equivalent potential vorticity aloft, and even some 
elevated upright instability over the south. We are not adding 
thunder at this time, but we are adding the mention of heavy rain 
centered on Saturday. This system is slow to move out as it all 
works around a very slow moving middle level low pressure area /we 
don't drop all mention of precipitation until Monday night/, but it 
still appears that the worst of the precipitation ends by daybreak 
Sunday. Flooding remains a real concern, not just because some snow 
pack with high water content remains over the far northwest. Winds 
around Saturday, especially near the coast, may also be of some 
concern. There may be a changeover to snow or mixed precipitation 
over the far northwest before we are done with this system, but it 
does not look to be significant at this time. 


Temperatures in the period start off well above normal /there may be 
some rising temperatures on Friday night, but we won't forecast that 
for the time being as the low level flow is southeast rather than 
south/, then move toward normal but remain on the warm side of 
climatology. 


&& 


Aviation /00z Thursday through Monday/... 
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg, 
kmiv, kacy and surrounding areas. 


Because of an onshore flow that started around 15z/10, we are 
paying a little more credence to the development of lower clouds 
late overnight tonight. Since confidence in this is not high, we 
are only going with MVFR conditions lasting for about six hours. 


Layered VFR stratocumulus and altostratus are forecast Thursday, 
with rain now not expected for the afternoon in the 30 hour kphl 
taf, as the system has slowed down. 


Winds through the taf period should be mainly from an easterly 
direction, except southeast for a time through this afternoon. 
Speeds will be mostly less than 10 knots. 


Outlook... 
broad low pressure from the west and south is anticipated to bring 
periods of rain and low conditions from later Thursday night into 
the coming weekend. Easterly winds are expected to increase for the 
period from late Friday into late Saturday...and may be quite strong 
especially off the surface Saturday afternoon. 


&& 


Marine... 
residual effects from high pressure offshore will contribute to a 
continuation of good marine conditions tonight and Thursday. 
However, all good things must come to an end. Broad low pressure is 
forecast to build into our region from the west and south Friday and 
Saturday. The center of low pressure Saturday may be just south and 
east of our area, so strong easterly flow is expected to occur. How 
much of the winds aloft mix to the surface is problematic, but 
at the least Small Craft Advisory conditions are forecast. Waves on 
our ocean waters are also forecast to build above 5 feet on or 
during Friday, or perhaps by Friday evening. And, seas should remain 
high, possibly getting above 10 feet, over the weekend. 


&& 


Hydrology... 
the GFS/Euro solutions are most preferred this afternoon. The NAM is 
the least. Not because it doesn't show an axis of heavy rain, but 
because of where it puts the heavy rain, and that is further 
south than other solutions. 


Predicted rainfall, Friday night through Sunday, has increased the 
past 24 hours. A widespread 2 to 4 inches looks likely across the 
hsa. Higher amounts are possible. 


As of early this afternoon, HPC also feels that the axis of heaviest 
precipitation will lie across northern Delmarva, NJ, southeast PA, 
and southeast New York. 


With these numbers, flooding is likely this weekend into early next 
week. Both smaller streams and larger rivers could experience 
flooding. Streamflow is already running high across southeast PA. So they 
don't need the snowmelt to flood. On the other hand, rivers like the 
Delaware will need the snowmelt to flood, and in this case, it 
appears the heavy rain will fall across the headwater snowpack which 
is three times normal for this time of year. 


So in very general terms, 1 to 2 inches of widespread rainfall will 
cause minor flooding across the smaller streams, creeks and rivers 
across the Passaic, Raritan, and Delaware basins, with little or no 
flooding along the mainstem rivers /Lehigh, Schuylkill, and Delaware/. 
2.0 to 3.5 inches would cause moderate flooding along these same 
smaller creeks and streams, with some flooding possible along the 
mainstem rivers, especially the Schuylkill. 3.5 to 5.0 inches of 
rainfall, both locally and over the existing snowpack in the 
north, would cause widespread moderate/major flooding, with minor 
flooding along the mainstem Delaware. 


The Lehigh Valley has been a bit drier over the last 30, 60, and 
90 days. For this reason, this particular area can tolerate a 
little more water before flooding. This flood potential will 
continue to be highlighted in the hazardous weather outlook at 
this time. 


&& 


Tides/coastal flooding... 
an increasing onshore flow is expected from Friday into Sunday. We 
will continue to monitor the developing situation for the 
possibility of at least minor tidal flooding over the weekend. At 
this time, later Saturday afternoon and Saturday night appear to be 
the times which may have the highest storm surge. 


&& 


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...delisi 
aviation...delisi/ 
marine.../ 
hydrology...kruzdlo 
tides/coastal flooding.../ 

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