734 
fxus61 kphi 211505 
afdphi 


Area forecast discussion 
National Weather Service Mount Holly New Jersey 
1005 am EST Sat Nov 21 2009 


Synopsis... 
high pressure over the Ohio River valley this morning will 
strengthen and build toward New England through the weekend, then 
recede east slowly. Low pressure will develop off the southeast 
coast on Monday and move north slowly, passing by to the east on 
Tuesday. 


&& 


Near term /until 6 PM this evening/... 
high pressure, located over the Ohio River valley this morning, 
will continue to build slowly to the east and northeast this 
afternoon. 


At 10:00 am, high clouds covered our forecast area. The cirrus 
overhead is expected to thin a bit for this afternoon as the 
entire cloud shield drifts to the east. Meanwhile, some 
stratocumulus may nudge down from the northwest and over some of 
our counties in eastern Pennsylvania and northern New Jersey. All 
in all, a partly sunny forecast should cover the situation. 


It continues to appear as though afternoon temperatures in our 
region will be mainly in the 50s. 


&& 


Short term /6 PM this evening through Monday night/... 
the weak middle level low drifts towards US tonight, and again the 
model soundings hold onto some low level moisture, especially but 
not exclusively northwest, while some high level moisture 
approaches ahead of the next system that is advancing in the 
southern stream. We go with somewhat more cloudiness northwest and 
south than elsewhere, but we allow for the possibility of some 
decoupling and radiating over the normal radiating spots on the 
min temperatures. Elsewhere, we stay with the continuity/guidance 
combination. Winds are expected to be light. 


On Sunday, the middle level low over US and a middle level high to its north 
very much resemble a Rex block, and this actually seems to allow a 
little ridging upstream of US that slows down advancing energy 
from the southern stream. We have backed down a bit on our Sunday 
probability of precipitation in response, and now we only carry a slight chance far south. 
We do increase the clouds some down south, but we back off on that 
trend north. The gradient looks to begin to tighten a bit, 
especially south, between the strengthening surface high to the 
north and nascent surface troughing off the southeast coast. The 
cooler h925 temperatures off the NAM have good support from the 00z 
ECMWF, and so we have leaned toward the cooler met stat guidance 
with regard to maxes. 


On Sunday night, the models start to differ on whether the Rex 
block continues to slow the progress of southern stream energy or 
the energy gets pulled toward the low pressure part of said block. 
Even the models that favor the latter solution have slowed down 
the progress of moisture, i295 isentropic lift and lower level 
frontogenesis, so we continued the trend of backing off some on 
the probability of precipitation. We do bring in more cloudiness. We leaned toward the 
cooler met stat guidance again on the mins, basically for the same 
reason that we sited for Sunday. 


On Monday into Monday night, there is some agreement that 
isentropic lift moves across the area, frontogenesis strengthens 
/especially off the coast/ and q-vector convergence develops 
/again, especially off the coast/. The middle level block is prognosticated 
to lift northeast and open, and surface low pressure is prognosticated to 
develop and lift from the southeast coast on Monday to somewhere 
off the middle Atlantic coast late Monday night. How organized that 
low gets is up in the air, but even the weaker European model (ecmwf) has a decent 
pressure gradient between the low and the high to the north. We 
have gone with probability of precipitation that range from likely southeast to chance 
elsewhere as the indication seems to be that the bulk of the 
precipitation will fall off the coast. We do carry a fairly strong 
northeast flow in this regime, but hopefully nothing like we dealt 
with around Veteran's Day. We generally continue to lean toward 
the cooler guidance. 


&& 


Long term /Tuesday through Friday/... 
following trends from HPC which generally went with the European model (ecmwf) showed 
low pressure moving away from the area late Monday night with high 
pressure in eastern Canada extending south over the region. This 
should mean drying trends late Monday night with a dry day on 
Tuesday. Tuesday night and Wednesday will see another front from the 
west move east. This front should cross the region Wednesday afternoon. 
With the warm advection ahead of the front late Tuesday night could 
see some rain and showers are possible during the day Wednesday. 


For Thanksgiving, an upper trough digs into the western Great Lakes. 
We have warm advection over the area and due to this combination the 
forecast will include a chance of showers. 


Thursday night, a strong cold front moves through from the northwest and we 
get into a cold advection pattern for Friday with cyclonic flow 
aloft and very cool temperatures on Friday. Over the far northern 
zones, there is a chance of rain or snow showers on Friday. 


HPC temperatures are generally reasonable. 


&& 


Aviation /15z Saturday through Wednesday/... 
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg, 
kmiv, kacy and surrounding areas. 


VFR flying conditions will support aviation interests today and 
tonight, and probably Sunday morning as well. 


Surface high pressure over the eastern Ohio Valley this morning 
will strengthen slightly this afternoon and tonight as it drifts 
northeastward. This high will initially be underneath an upper 
trough over the Great Lakes and Ohio Valley. The southwest flow 
ahead of this trough was kicking some jet stream cirrus 
northeastward into our area early this morning. However, probably 
a good bit of this will be thin today. So, other than some 
cirrus, a few cumulus may form toward midday and linger into the 
afternoon. A dry atmosphere will mean no tendency for fog 
formation early this morning. West winds early this morning will 
become northwest later this morning and continue this afternoon, 
veering to north by dark. Sustained wind speeds should be mostly 
under 10 knots today. 


Outlook... 
dry weather is forecast to continue Sunday. A chance of rain is 
expected Sunday night as low pressure will move off the southeast 
U.S. Coast Sunday night and develop northward Monday...and perhaps 
remain just off the middle Atlantic coast Monday night and Tuesday to 
plague our area with an oceanic flow. 


&& 


Marine... 
wind gusts and seas were under Small Craft Advisory criteria this 
morning and should lessen somewhat during today as high pressure 
centered just to our west early this morning is forecast to build 
northeastward tonight and Sunday. Low pressure is then expected to 
move northward along or near the coast later Sunday and Monday and 
remain into Tuesday. Right now this low looks like it will not 
become too strong, but at least strong enough for advisory level 
winds and seas, with the Sunday night into Tuesday time frame 
being the prime time. However, the gradient may be tight enough 
over our southern waters later Sunday to warrant a Small Craft 
Advisory. Wavewatch has been consistent in building seas to 5 feet 
Sunday afternoon, so a Small Craft Advisory will be issued with 
this morning/S package from Little Egg Inlet south for Sunday 
afternoon (our third period) and continued through Sunday night 
for now (our fourth period). This will likely have to be extended 
in time as well as geographically. Our northern waters Don/T look 
to start before Sunday night (which is our fourth period and too 
far in advance to start an advisory); lower (or maybe even all of) 
Delaware Bay could need an advisory as well. Once it starts, 
wavewatch has advisory level seas through this coming week. 
Obviously, confidence is pretty low after Monday and Tuesday. 


&& 


Tides/coastal flooding... 
tidal anomalies are likely to increase again in response to an 
expected developing onshore flow Sunday and particularly Sunday 
night through Monday night. Low pressure forecast to develop to our 
southeast will contrast with high pressure over northern New England 
and the Canadian Maritimes. Aviation storm surge guidance is 
probably underdone at this early stage, and anomalies may rise to 
around 1 foot above normal. The time period of concern will be 
Sunday night into tuesday; this period is between new moon and full 
moon, so the astronomical tides are not as high as they were last 
week. Under normal circumstances, this wouldn't pose any problems, 
but with the severe beach erosion that occurred with last week's 
storm, it may be that lower tidal anomalies will have an effect when 
they ordinarily would not. It is still to be determined whether this 
might cause any level of tidal flooding, so the situation will be 
closely monitored during this weekend. 


&& 


Phi watches/warnings/advisories... 
PA...none. 
New Jersey...none. 
Delaware...none. 
Maryland...none. 
Marine...Small Craft Advisory from 1 PM Sunday to 6 am EST Monday for 
anz452>455. 


&& 


$$ 


Synopsis...delisi 
near term...iovino 
short term...delisi 
long term...stauber 

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