437 
fxus61 kctp 120145 
afdctp 


Area forecast discussion 
National Weather Service State College PA 
845 PM EST Thursday Mar 11 2010 


Synopsis... 
a large slow moving storm over the Central Plains will move east 
into the central Appalachians...inducing a strong low pressure 
system along the middle Atlantic coast this weekend. This system 
will likely be accompanied by heavy rainfall. The combination of 
heavy rain and snow melt may result in flooding. 


&& 


Near term /until 6 am Friday morning/... 
regional radar loop showing band of light rain working into 
southwest PA along axis of low level jet. Latest short range model 
data supports light rain spreading NE across central PA 
overnight...reaching the NE counties around 09z. Quantitative precipitation forecast amts should be 
low...as best lg scale forcing will remain near upper low over the 
Great Lakes and ahead of shortwave over the coastal Carolinas. 
However...have raised probability of precipitation to near 100 percent over most of central and 
southern PA based on upstream radar and 21z sref. 


Overcast skies and increasing low level moisture should result in a 
mild night. Based on current dewpoints...expect readings going no 
lower than the l40s across the north mountains...to u40s southern half of 
the state. 


&& 


Short term /6 am Friday morning through 6 PM Friday/... 
Friday will finally be the cloudy day we have been expecting from 
this plodding storm that continues to take its time getting here. 
Moist easterly flow should result in low clouds and occasional light 
rain/drizzle. However...lg scale forcing looks weak...suggesting 
precipitation amounts will remain on the light side with the bulk of the 
rains holding off until Friday night and Sat. 


Cloud cover and occasional light rain should hold temperatures from rising 
much from morning lows...resulting in high temperatures some 10 to 15 degree 
cooler than today. The recent string of fine weather will most 
assuredly have come to an end. 


&& 


Long term /Friday night through Thursday/... 
middle shift did up quantitative precipitation forecast amts for the storm. Was looking close to 
2 inches yesterday for most of the area...now lookiing at 
2 inches in most areas...but around 3 inches eastern part 
of the County Warning Area. Did not change much...did edge up here and there. 
Still a lot of spread in the quantitative precipitation forecast fields. Eastern areas near 
thermal ridge...a big concern. Storm total quantitative precipitation forecast grids in 3 
sections...one for Friday...another for Sat...and one for 
Sunday. 


Main change was to up probability of precipitation some Friday night across the north... 
and again in a few areas Saturday night into Sunday...given that 
storm will likely be slow to move out...even though quantitative precipitation forecast amts 
may not be that high by 12z Sunday. 


Support for heavy rain is strong dynamics...negative tilt 
to the system...secondary low...position of thermal ridge... 
low level jet...and abnormally strong easterly flow. 


Snow is not in the forecast anymore...not really cold enough at low 
levels. It is still possible that some wet snow could mix with 
the rain across The Laurels...but hard to see it accumulating... 
given such strong flow from the east and NE. 


Did not change temperatures much. 


Still looking at the upper level ridge expanding eastward 
early next week...thus looking at clearing skies and no precipitation 
after 00z Tuesday. 


Temperatures for the later periods typical of middle March. 


&& 


Aviation /01z Friday through Tuesday/... 
stratus deck between 2-3kft overspreading southeast PA this evening 
as increasingly moist southeast flow envelops the area. Latest ruc13 low 
level relative humidity suggests all central PA terminals will have at least MVFR 
ceilings by late this evening...with IFR ceilings to follow after 
midnight. Light rain will overspread the area from the south 
beginning around 03z across the southwest /jst/ and reaching the 
northeast /kipt/ around 09z. 


Moist easterly flow will produce widespread IFR ceilings with 
occasional -ra during Friday. A major storm is expected to form 
over the Carolinas Saturday before moving up the coast as a 
developing gale. It will create widespread IFR with rain and and 
possibly even embedded thunderstorms...from after midnight Friday 
through much of the weekend. Strong east-southeast flow will also 
support local areas of low level wind shear. 


Outlook... 
Sat-sun...generally IFR ceilings/visibility occasional rain. 
Monday...drying/improvement to VFR possible. 
Tuesday...no sig weather expected. 


&& 


Hydrology... 
observations show several inches of water still locked up in the 
snow pack over southwestern PA. Models continue to zero in on a large area 
of 2 to 4 inch rains starting after midnight Friday lasting into 
the day Saturday...and centered roughly over the area south of 
I-80. 


Combination of snow melt and storm runoff will lead to the increasing 
potential of flooding. 


At this time a Flood Watch is in effect for The Laurels where 
there is the most water remaining in the snow pack...and flooding 
could ensue simply due to snow melt even before the bulk of the 
rain arrives. 


The watch is still being considered for expansion...but with some 
36-48 hours remaining before the heaviest rains arrive...will 
likely hold off until later today or overnight when we can further 
zero in on the highest threat areas. 


With the amount of rain expected...and some areas still having 
considerable amounts of high water content snow on the 
ground...this has the potential to be a very dangerous flooding 
situation developing over the next couple of days. Persons near 
flood prone areas will want to keep a close watch on later 
forecasts and changing weather conditions. 


&& 


Ctp watches/warnings/advisories... 
Flood Watch from Friday evening through late Saturday night 
for paz025>028-034>036-056>059-063>066. 
Flood Watch through late Saturday night for paz024-033. 


&& 


$$ 
Synopsis...la corte 
near term...Fitzgerald 
short term...la corte/Fitzgerald 
long term...Martin 
aviation...la corte/Fitzgerald 
hydrology...la corte