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fxus61 kpbz 212041 
afdpbz 


Area forecast discussion 
National Weather Service Pittsburgh PA 
341 PM EST Sat Nov 21 2009 


Synopsis... 
high pressure will provide a dry weekend as it moves across the 
upper Ohio Valley. A low pressure system moving northeastward 
along the Atlantic coast may spread showers as far west as 
communities near the Appalachian Mountains Monday into Tuesday. A 
low pressure system coming eastward from the Canadian rockies will 
bring showers starting Tuesday night followed by colder temperatures 
by Thanksgiving night. 


&& 


Near term /through Sunday/... 
although recent satellite images continue to show a fairly 
extensive deck of stratocumulus clouds across the upper Ohio 
Valley. The clouds remain trapped beneath a strong subsidence 
inversion. Recent NAM model profiles show the clouds eroding 
overnight. With the low level hydrolapse /vertical humidity 
profile/ showing moisture increasing with height...subsequent 
nocturnal radiational cooling may lead to fog patches by early 
morning. 


During daytime Sunday...expect just scattered clouds and hence 
considerable sunshine. 


Forecasted temperatures to be milder than normals using close to 
recent blend of GFS and NAM MOS values along with sref model 
output. 


&& 


Short term /Sunday night through Tuesday/... 
continue to forecast a chance of showers early Monday into Tuesday near 
the Appalachian Mountains as recent sref model output continues to 
have a few members showing rain showers from a coastal low 
pressure system spreading into the Appalachian Mountains. 


Expect the remainder of the upper Ohio Valley to remain dry Monday 
and Monday night...but there is a chance that the initial showers 
with the low pressure system coming eastward from the Canadian 
rockies can begin spreading across the upper Ohio Valley Tuesday afternoon. 


Forecasted temperatures to continue warmer than normal as 
suggested by recent GFS and NAM MOS values. 


&& 


Long term /Tuesday night through Saturday/... 
large upper level trough will be the main player in the extended 
portion of the forecast. As the trough develops and moves over the 
East Coast...the dry weather and above normal temperatures that have 
persisted for much of the month will come to an end. 


Even though models are in very good agreement about the development 
of the upper level system...the associated surface low pressure 
solutions are quite varied in the models. Nevertheless...overcast 
conditions and a chance of precipitation can be expected from 
Wednesday all the way into Saturday morning before high pressure 
builds in. 


Temperatures should be cold enough that each night rain 
may mix with snow...but warm ground temperatures would likely 
prevent any accumulation. The one exception to this could be Friday 
night...when favorable flow off Lake Erie will allow for lake 
enhancement. After seasonable temperatures Wednesday...expect highs 
to drop to 5-10 degrees below normal...with seasonable low 
temperatures. 




&& 


Aviation /20z Saturday through Thursday/... 
ceilings continue to hover right around the 3kft mark at all taf 
sites. These clouds should eventually begin to scatter out by 
sunset...with advancing high pressure expected to bring in drier 
air. MVFR ceilings may still be possible Sunday morning at favored 
locations to the north. Patchy fog is possible mainly in the river 
valleys. High pressure should then allow for VFR conditions to 
continue for the rest of Sunday. 


A low pressure system moving up the East Coast Sunday night will 
bring the next chance for showers with reduced ceilings 
expected...especially southeast of a kmgw-klbe line. Reduced 
ceilings will be possible across the area on Tuesday through 
Thursday with an upper level system dominating the weather across 
the Great Lakes. 




&& 


Pbz watches/warnings/advisories... 
Maryland...none. 
Ohio...none. 
PA...none. 
WV...none. 


&& 


$$