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Area forecast discussion 
National Weather Service Sacramento California 
930 PM PST Thursday Nov 20 2008 


Discussion... 
weak frontal system that brought light rainfall to the far northern 
County Warning Area has now moved into the northern Great Basin this evening with 
tail end of front dissipating over the central Sacramento Valley. 
Secondary shortwave trough now speeding through Pacific northwest 
and norcal providing just enough enhancement to the tail end of the 
frontal boundary for some upper and middle level clouds over Tehama 
County but nothing so far on radar so do not expect any more precipitation 
this evening. Will likely update current forecast package a bit to 
get rid of precipitation wording in western Plumas zone. Should see 
clearing this evening as upper ridge begins building over the West 
Coast. Current temperatures running a few degrees cooler most 
locations over the forecast area under clearer skies and should come 
in a few cooler overnight despite slight airmass warming. Main 
forecast issue tonight will be in the Central Valley where fog will 
become a real possibility. Dewpoints are currently in the middle 40 to 
lower 50 degree range. There is a slight and increasing northerly 
surface gradient however and this gradient is expected to increase 
slightly more overnight. Therefore there may be enough northerly 
breeze this evening to keep fog from becoming substantially dense. 
Some patchy fog is a pretty good bet however as skies clear. Should 
see a warming trend most areas Friday as airmass warms. Morning fog 
will determine maximum temperatures in the valley but areas that receive full 
sunshine all day Friday may come in a little warmer than currently 
forecast. Another weak shortwave through moves through Pacific 
northwest early Saturday. This system is visible on the infrared satellite 
as a mass of middle/hi clouds crossing 140 west at this time. It is not 
expected to bring any precipitation to our area. A split flow 
develops Sunday along 140w as high pressure builds over the West 
Coast and a low pressure system begins digging towards the Southern 
California coast. 


Extended discussion (monday through thursday)... 
ridging should remain over California on Monday as GFS and European model (ecmwf) 
models keep a closed low near 33n 135w. Latest 00z run of GFS more 
progressive with this low but still keeps it off the coast through 
Monday. 00z GFS run now tapping into some tropical moisture with 
showers spreading northeastward into County Warning Area on Tuesday. Confidence in 
this timeframe is low considering model flip flopping and model 
inconsistency so definitely best to leave extended alone until 
models become in better alignment with each other and run to run. 


&& 


Aviation... 
current observations show light valley winds. Overnight surface 
winds will be light northerly across the County Warning Area with light to moderate 
westerly flow aloft into Friday. Hazy conds could develop overnight 
in the valley which could lead to periods of MVFR. Best threat of 
MVFR/IFR conditions from about ove southward into the San Joaquin 
Valley. General VFR conds should return near 18-20z Friday. 


&& 


Sto watches/warnings/advisories...none. 


&& 


$$