063 
fxus66 kmtr 211147 
afdmtr 


Area forecast discussion 
National Weather Service San Francisco Bay area California 
345 am PST Friday Nov 21 2008 


Discussion...as of 3:40 am PST Friday...a weak upper low 
near 30n/130w is producing some high clouds which are drifting as 
far north as Monterey County. But for the most part a broad and flat 
upper ridge centered along the West Coast is producing a tranquil 
weather pattern across our region. Surface high pressure has been 
building into Oregon and far northern California overnight...resulting in 
light to moderate north to northeast winds in the hills of the sf Bay 
area. This weak offshore flow has cleared all low clouds from the 
sf Bay area and pushed dewpoints down into the middle to upper 30s 
across portions of the north and East Bay. Farther S...enough low 
level moisture remains for patches of low clouds and fog across 
Monterey and San Benito counties. North to northeast flow just above 
the boundary layer should develop farther south by sunrise and result 
in clearing of all low clouds and fog by middle morning. A warmer and 
drier airmass today...as well as more sun...will mean warmer high 
temperatures compared to yesterday. MOS forecast highs are likely too 
cool today...especially in the North Bay valleys. No major changes 
are expected through the weekend. A weak shortwave trough moving 
inland far to our north late tonight and Saturday morning won't have 
much impact. Temperatures should remain near normal through the 
weekend. Skies will be mostly clear except for a few passing high 
clouds. Patches of night and morning valley fog are likely in typical 
fog-prone areas. 


The forecast from Monday Onward becomes quite challenging. The 
system currently out near 160w is forecast to split as it approaches 
the West Coast on Sunday. The models indicate that the southern 
portion of the split will form a cutoff low somewhere off the 
California coast by Monday. The GFS forecasts this low to form off 
the central California coast. And according to the 06z GFS...the low will be 
close enough to the coast to produce widespread rain across our County Warning Area 
by Monday afternoon (the 00z GFS kept all rainfall just offshore 
through monday). Meanwhile...the 00z European model (ecmwf) develops the cutoff much 
farther to the SW...thereby keeping our area dry through at least 
Monday night. And then there's the Canadian model which moves an open 
wave inland across northern California late Sunday night into Monday. Beyond 
Monday the picture does not get any clearer as the models vary 
on location and timing of individual weather features. In any 
case...there appears to be a good chance that unsettled weather will 
continue through midweek and that a potentially stronger system will 
move in from the northwest on Thanksgiving day or shortly thereafter. 
Given the high level of uncertainty have elected to broad-brush 
chance/slight chance probability of precipitation across most of the County Warning Area from Monday right on 
through Thanksgiving day. 


&& 


Aviation...as of 3:45 am PST Friday...some patchy valley fog will 
affect sns and sts this morning. Otherwise it will be VFR conditions 
for the sfo approach zone and mry through Saturday. 


Terminal Aerodrome forecast (tafs) format change... 


As of 0000z (november 5 2008) the National Weather Service in agreement with the FAA has 
changed the taf format to conform to international standards outlined 
in icao annex 3. The change includes the initiating of 30-hour tafs 
for high impact U.S. Airports. To accommodate the extended taf 
period...the format of all tafs have changed. The change involves 
identifying the data associated with each time group. 


Ksfo and koak are now 30-hour tafs. 


For additional information regarding the change please visit the 
aviation weather centers website: 


Http://aviationweather.NOAA.Gov/notice/taf30.Php 


&& 


Mtr watches/warnings/advisories... 
today...sca...all coastal waters out to 60 nm 
Small Craft Advisory...rough bar conditions...sf bar && 


$$ 


Public forecast: dykema 
aviation/marine: west pi 


National Weather Service on the web at weather.Gov/sanfrancisco