857 
fxus61 kphi 120147 
afdphi 


Area forecast discussion 
National Weather Service Mount Holly New Jersey 
847 PM EST Thursday Mar 11 2010 


Synopsis... 
an area of low pressure near the middle Mississippi Valley will meander 
around through Friday night before gradually moving more eastward. 
Meanwhile, a secondary low pressure system is forecast to develop 
along a frontal boundary near the middle Atlantic coast late Friday 
night and Saturday. This system will then meander in our area 
through the weekend before gradually moving out to sea later Sunday 
and Monday. High pressure will then gradually build toward our area 
Tuesday through Thursday, although a weak cold front may slide 
through late Tuesday. 


&& 


Near term /until 6 am Friday morning/... 
a 00z surface analysis placed a warm front from the Delaware-Maryland-Virginia region 
to Northern Ohio then westward to low pressure in Iowa. A cold 
front was draped from this low down through the Mississippi 
Valley. Meanwhile, high pressure was anchored just north of Maine. 
An upper air analysis showed a potent closed low west of the upper 
Mississippi Valley, which is clearly seen on water vapor imagery. 
A potent short wave was located in south central Texas. A 
cyclonically curved 300 mb jet was from The Four Corners region to 
the Gulf Coast states then into the Tennessee Valley. There was 
850 mb warm air advection analyzed from the Ohio Valley southward to the Gulf 
Coast states. 


A weak short wave is forecast to weaken the midlevel ridge tonight 
as it crosses our area. There will be at least some warm air advection and moisture 
advection continuing through the night, although the overall lift is 
not that strong or organized. The continued moistening of the low 
levels is expected to result in a wealth of cloudiness tonight. The 
moisture return/push has been somewhat weak today, but enough to 
keep a good amount of low clouds across most of the County Warning Area. This should 
tend to expand and lower some more as we go through the night. 
Coupled with a continuing onshore flow, this should develop some 
upslope precipitation across our County Warning Area. This looks rather light, 
therefore will go with patchy drizzle starting late this evening, 
then bring in the slight chance to chance probability of precipitation from south to north. Some 
stronger lift is expected to near the southwestern part of the County Warning Area 
toward daybreak, therefore will bring in likely probability of precipitation late overnight. 
Any quantitative precipitation forecast tonight is expected to be on the light side. Given the 
upslope flow and model soundings showing plenty of moisture below 
5,000 feet, we maintained the mention of patchy fog in the 
northwestern part of the County Warning Area later tonight. 


The fog across eastern New Jersey has remained somewhat patchy and 
light, but somewhat more dense fog formed over the southern 
Delaware-Maryland-Virginia near the warm front. This may affect some of our Delaware-Maryland-Virginia 
region, and it was added to the forecast prior to the onset of 
rain. 


As for temperatures, with the clouds and rising dew points the NAM 
MOS looks a bit too cool in some areas, therefore we went closer to 
the GFS MOS for most locales. 


&& 


Short term /6 am Friday morning through Sunday/... 
a rather active period is expected during the bulk of this time 
frame as a moderate to heavy rain event looks likely to move 
through. The heaviest rain looks to fall during Saturday. We 
continue to see a flood potential this weekend, therefore see the 
hydrology section for the complete details as a Flood Watch is being 
issued this afternoon. 


An expansive closed upper-level low meandering around the middle 
Mississippi Valley will gradually slide eastward Saturday through 
Sunday. Due to the slow eastward propagation of this feature, an 
abundance of moisture will be drawn northward into our area. This 
moisture advection will be enhanced by a strong east to 
southeasterly low-level jet and increasing precipitable water values. As the 
midlevel ridge axis slides to our east tonight, the warm air advection and 
isentropic lift will increase some. In addition, a deep 
southeasterly flow will be an upslope component which should tend to 
begin squeezing out the deepening moisture. The slower arrival of 
the more widespread precipitation continues with the majority of the 
model guidance, which does seem logical given the strength of the 
closed low and its slow eastward progression. As a result, the probability of precipitation 
will ramp up for Friday. The rain is expected to increase in 
intensity some through Friday, however the heaviest amounts are 
expected to wait until the strongest forcing arrives. 


Speaking of forcing, a rather robust low to middle level wind field 
within a warm air advection/moisture advection regime will allow for strong 
vertical velocities to overspread the entire area later Friday night 
and especially Saturday. This will be in conjunction with a surface 
low developing in the vicinity of the southern middle Atlantic and then 
moving northward during Saturday. As heights fall and there is some 
diffluence aloft, the vertical velocities will respond by 
increasing. In addition, a belt of strong frontogenetic forcing is 
forecast to move from south to north across our County Warning Area during mostly 
Saturday as strong positive vorticity advection arrives in conjunction with the strongest 
wind field and warm air advection. This all points to moderate to heavy rain 
marching across the County Warning Area. The frontogenesis from 850 mb to 700 mb 
suggests that there may be bands of heavier rain elements within a 
larger rain shield. This is where the highest rainfall rates may 
occur. The strengthening wind field will allow for breezy to windy 
conditions to develop later Friday night and Saturday, however given 
the warm air advection the strongest winds should remain above the surface. It 
should be noted that there are differences in the positioning and 
strength of the secondary surface low developing in the middle Atlantic 
Friday night through Saturday. In addition, the midlevel low has 
been meandering around in the guidance as it moves eastward, and 
this is probably due to the amount of short wave energy revolving 
around it. Keeping all of this in mind, introduces some uncertainty 
on where the heaviest rainfall axis sets up. Overall though, there 
is a heightened potential for excessive rainfall over our County Warning Area. 


There does not appear to be much in the way of instability being 
advected into our cwa, although our Delaware-Maryland-Virginia counties could be close 
Saturday /especially is some sort of a dry slot works in/. Given the 
lower probability/confidence of thunder attm, we decided to not 
include it for now. As the large closed low moves across our area 
during the weekend, the strongest lift and wind field is pushed to 
the north. This will take the plume of strong moisture 
advection/lift and shove it northward. However, there may be some 
comma head rain wrapping back around our County Warning Area Saturday night and even 
through Sunday. Overall, the trend of lighter rainfall is 
anticipated Saturday night and especially Sunday as the rain 
transitions to showers. 


As for temperatures, there could be challenges with this given the 
strong warm air advection that will be aimed into our area for especially Saturday. 
Meanwhile, a surface high up across northern New England will try 
and push some cooler air into our region /perhaps even some 
low-level drying/. The MOS guidance seems to have trended cooler 
across a good part of our cwa, but did not want to go fully toward 
this especially across central and southern areas. As a result, we 
generally took a blend of the GFS/NAM MOS as a compromise for most 
locales. 


&& 


Long term /Sunday night through Thursday/... 
HPC has gone with a forecast that depends heavily on the European model (ecmwf) early 
and that incorporates the European model (ecmwf) ensembles late. This has the 
stacking low pressure system that likely will cause US some problems 
through the short term slowly exiting our area in the Sunday night 
to Monday time frame. The precipitation type in the northwest as 
this system exits still does not look as if it will be a significant 
issue. 


After that, it appears that we will have weak high pressure at the 
surface punctuated by some dry troughs or cold fronts thanks to an 
eastern Canadian vortex at middle levels. If this comes to pass /and 
model consensus is fairly good/, we eventually will see some welcome 
drying with tolerable winds. Temperatures through the period are 
forecast to remain above normal, although not quite as mild as they 
have been recently. 


&& 


Aviation /02z Friday through Tuesday/... 
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg, 
kmiv, kacy and surrounding areas. 


A little piece of vorticity moved through Delaware earlier this 
morning...and the resultant showers there have dissipated. Also, the 
early morning stratus and fog...densest and lowest toward the New 
Jersey coast...lifted by middle morning. However, the lower atmosphere 
continues to Load up with moisture with the persistent onshore 
flow...and deterioration was already trying to happen early this 
afternoon as stratocumulus was forming around the area and lowering. 
Secondary low pressure developing off the southeast coast later 
tonight will Stem from the main low in the upper Mississippi Valley. 
However, a warm front appears to emanate from the main low and 
extend eastward north of the developing secondary during tonight, 
just south of our area. It appears that this will serve to stimulate 
mild upward vertical motion and enhance rain chances later tonight. 
The upshot is that ceilings and visibilities are forecast to lower 
to IFR conditions in stratus and developing fog and drizzle during 
this evening, or by 05z /midnight/. IFR weather is expected to 
continue Friday, with rain possibly becoming steadier and heavier 
for Friday afternoon, at least according to GFS Omega fields and 
precipitation output. 


Outlook... 
widespread IFR conditions with periods of rain and fog will continue 
across the area Friday night and on into the weekend. Low level wind 
shear may occur late Friday night and into Saturday with a decent 
low level jet expected. Slight improvement may develop Sunday but 
showers will be likely. The responsible low pressure area is 
forecast to move away during Monday. 


&& 


Marine... 
marine conditions will continue to be sub-advisory tonight. However, 
winds and seas are still expected to increase Friday with the 
approach of low pressure, which will contrast with a strong high in 
eastern Canada. Winds and seas will build further Friday night, 
Saturday, and into Saturday night. The Small Craft Advisory will be 
continued for Friday at this time. However, as the winds just off 
the surface are projected to increase to over 50 knots later Friday 
night and Saturday, and into Saturday evening north, concern grows 
about the need for a gale headline. NAM 1000 hpa winds are not quite 
as strong as the GFS, but there could be 35 to 40 knots gusts making 
it down to the surface /35 knots on the NAM/, especially north. 
Since winds will be increasing in our third period, a gale watch 
will be issued with this package for the ocean waters and lower 
Delaware Bay. The Upper Bay will be left in the Small Craft 
Advisory, which will be extended through Saturday evening. 


Wind speeds look to diminish for a time Sunday with the proximity of 
the low, but are likely to increase again for Sunday night and 
Monday as the low begins moving away. 


&& 


Hydrology... 
predicted rainfall, Friday through early Sunday, remains pretty much 
the same as 24 hours ago. A widespread 2 to 4 inches looks likely 
across the hsa. Higher amounts remain possible. A Flood Watch is being 
issued. 


With these numbers, both smaller streams and larger rivers will 
experience flooding this weekend into early next week. 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 a 
portion of 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, Delaware/. 


2.0 to 3.5 inches would cause moderate flooding along these same 
smaller creeks and streams, with some minor 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 flooding, 
with minor flooding along the mainstem Delaware. A smattering of 
major flooding cannot be ruled out either. 


The Lehigh Valley has been a bit drier over the last 30, 60, and 
90 days, and for this reason can tolerate a little more water than 
adjacent basins. 


&& 


Tides/coastal flooding... 
the approaching and intensifying with its persistent onshore flow, 
along with the approaching new moon, will contribute to higher than 
normal tides Friday and through the weekend. The best time for 
problems will be with the strongest onshore flow, which is Saturday 
night and into early Sunday. Current projections from the aviation 
storm surge guidance show tide heights just under moderate tidal 
flooding levels for our northern shore areas in New Jersey...north 
of Atlantic City. Since this will be in our fourth period, it was 
decided to refrain from issuing a coastal Flood Watch at this time 
due to the lack of confidence in achieving moderate flooding, as 
well as the luxury of having some time. Since it is too early for a 
coastal Flood Advisory, the wording in the hazardous weather outlook 
will be enhanced a bit. 


&& 


Phi watches/warnings/advisories... 
PA...Flood Watch from Friday evening through Sunday morning for 
paz054-055-060>062-067>071. 
New Jersey...Flood Watch from Friday evening through Sunday morning for 
njz001-007>010-012>027. 
Delaware...Flood Watch from Friday evening through Sunday morning for 
dez001>004. 
Maryland...Flood Watch from Friday evening through Sunday morning for 
mdz008-012-015-019-020. 
Marine...gale watch from Friday evening through Saturday evening for 
anz431-450>455. 
Small Craft Advisory from 10 am to 9 PM EST Friday for anz431- 
450>455. 
Small Craft Advisory from 3 PM Friday to midnight EST Saturday 
night for anz430. 


&& 


$$ 
Synopsis...gorse 
near term...gorse/delisi 
short term...gorse 
long term...delisi 
aviation.../ 
marine.../ 
hydrology...kruzdlo 
tides/coastal flooding.../ 

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