Weather

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Area forecast discussion 
National Weather Service Sacramento California 
317 PM PDT Thursday Aug 21 2008 


Short term discussion /through Sunday/... 
ridging builds into Oregon as this Pacific northwest trough pushes east into 
the Great Basin. This has tightened the northern and northwestern pressure 
gradient with acv-rdd up to 7.0mb and mfr-rdd up to 4.3mb. This 
gradient has been strong enough to offset the diurnal upvalley 
flow...thus winds over the northern SAC valley and adjacent foothills are 
fairly light. However...a northerly wind will pickup tonight over 
much of the SAC valley and adjacent foothills and continue through 
Sunday with the strongest flow overnight and weakening and possibly 
reversing flow late in the afternoon due to the higher pressure offshore 
enhancing a Delta flow into the southern SAC valley. 12z NAM 850 mb temperatures 
rise to 24-25c by Sat from SAC to Redding respectively...but the 12z 
GFS is only rising to 22-23c. The h100-h50 thicknesses for both 
models are staying virtually the same. The 24-25c 850 mb temperatures 
supports from about 101 to 103f at the surface...however we do not see 
any increase in h100-h50 thickness so the forecasted middle to upper 
90s still looks good. A longwave trough over the eastern Pacific will stave 
off desert SW ridge with a stiffarm and keep northern cal from any 
significant warmup. Jclapp 


Long term discussion /Monday through Thursday/... 
GFS ensembles in decent agreement through Monday as Gulf of Alaska upper 
low moves into western Canada/Yukon territory on Monday. Associated cold 
front will move across the Pacific northwest and onto the northern California coast bringing 
chance of precipitation to those areas. The southern end of the front is forecast 
to undergo frontolysis as it moves inland with extended models 
generally keeping precipitation from moving into the interior of norcal. The 
cooling trend which began sun will likely continue into Monday as the 
weakening front moves inland...with near normal temperatures. From Tuesday through 
the rest of the week...the GFS ensemble members begin to diverge 
with several members maintaining a weak trough and seasonable weather over 
California...and other members suggesting a connection between high pressure 
cells over the eastern Pacific and the desert SW. With the European model (ecmwf) and 
extrapolation of the Canadian model after 12z Wednesday...we favor the 
connection of the high pressure cells and the warmer model 
solutions. 12z Thursday mex guidance followed this trend...and our 
afternoon grids show a warm up for the latter part of the efp...with 
maximum temperatures several degree /up to 6 or 7 degree or so/ above normal. GFS 
cross section of relative humidity shows the atmosphere drying after Tuesday with 
little cloud cover expected. Jhm 


&& 


Aviation... 
VFR conditions expected for the next 24 hours. Southwesterly/onshore 
flow will prevail over the interior of norcal. This will result in 
upvalley flow during the afternoon hours...becoming downvalley flow 
evening and overnight. Expect localized gusty winds up to 25 kts 
over the higher terrain through the early evening...decreasing this 
evening. Garcia 


&& 


Sto watches/warnings/advisories...none. 


&& 


$$