136 
fxus66 kmtr 211020 
afdmtr 


Area forecast discussion 
National Weather Service San Francisco Bay area California 
320 am PDT sun Mar 21 2010 


Discussion...as of 3:20 am PDT Sunday...onshore flow is increasing 
early this morning as a weak weather system approaches the West 
Coast. Low clouds and fog have developed along the coast...and 
locally in the valleys. The low level inversion is pretty close to 
the surface...less than 1000 feet per The Fort Ord profiler. This has 
resulted in areas of fog near Monterey Bay...with local dense 
patches in the North Bay valleys and the northern Salinas valley. The 
moist boundary layer should deepen as the incoming system 
approaches...so widespread dense fog is not expected this morning. 


The weak cold front with the approaching system has begun to spread 
some rain onto the far northern California coast early this morning. Latest 
model output indicate that the southern extent of that front may 
produce some light rain across northern Sonoma County around midday 
today. Otherwise...this system's primary impact on our forecast area 
will be increased clouds and somewhat cooler temperatures. Low 
clouds and patchy fog will linger across the region tonight and 
into Monday morning. Skies will clear by midday Monday as a 
surface high builds to our north and generates a drying northerly 
flow. 


The flow is forecast to turn light offshore by late Monday night as 
the surface high builds into Nevada. Meanwhile...the airmass aloft 
will be warming as an upper ridge builds over California. The 
result will be a mostly sunny and warmer day on Tuesday. The warming 
trend won't last long. By Wednesday the flow will turn back onshore 
as another weak weather system approaches the Pacific northwest and northern 
California. The midweek system may generate some light rain across the far 
northern fringes of our forecast area by late Wednesday evening. 


The models diverge somewhat late in the week. The GFS develops a 
shortwave ridge across California on Friday...while the European model (ecmwf) slides 
another weak trough across the northern part of the state. 
Confidence in the dry GFS forecast is rather high given that all of 
the GFS ensemble members show a dry solution. But will keep a 
slight chance of rain in the North Bay for now. 


Looks as though rain chances will increase next weekend and early 
into the following week as a more substantial weather system 
approaches the West Coast. Both the GFS and European model (ecmwf) forecast decent 
rainfall across northern California by Monday the 29th. 


&& 


Aviation...as of 10:30 PM PDT Saturday...VFR continues at this hour 
and will likely extend for a few more hours tonight. By daybreak 
Sunday morning a few patches of stratus are possible in a moistening 
boundary layer. VFR is forecast by late Sunday morning. A shortwave 
trough moves through the area Sunday evening which should help keep 
stratus coverage to a minimum due to increased mixing and drying of 
the airmass. 


Vicinity of ksfo...tempo broken ceilings are forecast Sunday morning. 
VFR is forecast by 18z. 


Ksfo bridge approach...same as ksfo. 


Mry Bay area terminals...VFR but broken ceilings are possible by early 
Sunday morning. VFR is forecast by late Sunday morning. 


Proposed policy change...effective 15z Tuesday April 6th 2010 the 
San Francisco Bay area office at Monterey will begin 3-hourly taf 
issuances for ksfo. These intermediate tafs at 03z...09z...15z... 
and 21z will be issued as amendments to the tafs issued at 00z... 
06z...12z...and 18z. The intermediate tafs will be issued during a 
20-minute window between 10 minutes before the designated issuance 
hour and 10 minutes after the hour. 


&& 


Mtr watches/warnings/advisories... 
.None 


&& 


$$ 


Public forecast: dykema 
aviation/marine: canepa 


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