Weather

075 
fxus66 kmtr 070530 aaa 
afdmtr 


Area forecast discussion 
National Weather Service San Francisco Bay area California 
1030 PM PDT Sat Sep 6 2008 


Discussion...update as of 8:00 PM PDT Saturday...hot weather 
finally comes to an end. 


Everything appears to still be on track for cooler temperatures 
tomorrow and much cooler weather on Monday as the hot dome of air 
that has been overhead the past few days is displaced further to 
the southeast. Temperatures today soared in many spots with many 
spots near or above the century mark. Overall highs will be 
considerably cooler tomorrow which will be the start of more 
enjoyable weather for much of next week. Still looks like virtually 
all spots will remain fog free tonight with the high resolution WRF 
model showing only patchy conditions over Monterey Bay and along 
the coast from northern Marin County up through much of Sonoma 
County. Looking toward the end of next week the European model (ecmwf) still shows a 
low off the Southern California coast helping to direct monsoonal 
moisture toward US while the GFS offers a dramatically different 
solution which would keep any shower activity well away from US. At 
this time agree with the previous shift that it is far too soon to 
make a call on which way this will pan out. 




Previous discussion as of 1:50 PM PDT Saturday...conditions 
early this afternoon are nearly identical to yesterday afternoon with 
warm temperatures at the coast and hot temperatures inland. Record high 
temperatures are possible in a few spots...but unlikely in most areas as 
records are generally pretty high for this date. 


Tomorrow still looks to be the transition day as we finally begin 
to cool off. A weak upper level trough currently dropping south 
along the Washington coast is forecast to sag into northern California late 
tonight and Sunday. As it does so...there will be some slight 
airmass cooling. But more importantly for our forecast...the 
approaching trough will trigger a return to onshore flow. 
Temperatures should cool from 7 to 15 degrees near the coast and 
bays on Sunday...and in some of the coastal valleys as well. The 
marine layer will remain quite shallow through Sunday and so cooling 
in the hills and the valleys farthest inland will probably be 
limited to only a few degrees. 


The Big Sur coast is the only area that has seen fog today and recent 
satellite trends indicate the fog layer there is retreating to the 
south. Fog and low clouds will develop along our entire coastline 
sometime in the next 24-36 hours...but pinpointing the timing of 
this isn't easy. The 18z NAM indicates most coastal fog development 
will hold off until late Sunday afternoon/Sunday evening. The high 
resolution WRF indicates patchy fog developing on the North Bay coast 
late tonight...and then becoming more widespread through the day on 
Sunday. Will forecast only patchy coastal low clouds and fog 
tonight...with much more widespread coverage Sunday night into Monday 
morning. 


The cooling trend will spread farther inland Monday and Tuesday as 
the upper level trough deepens along the West Coast. By Tuesday... 
most areas should see temperatures return to near normal. No big 
changes are then anticipated through the remainder of next week as a 
week trough lingers near the West Coast. By late in the week the 
medium range models develop a closed low somewhere near the central 
or Southern California coast. Earlier model runs of both the GFS and European model (ecmwf) 
had indicated that circulation around this low would draw significant 
middle/upper level subtropical moisture up over our region late in the 
week and into next weekend. However...the 12z version of both models 
agree that nearly all moisture will remain well to our east. 
Therefore...have decided to leave probability of precipitation at zero through the extended. 


&& 


Aviation...as of 10:30 PM PDT Saturday...VFR continues across the 
area. A few patches of stratus/fog are possible very late tonight for 
ksns and kmry. Bay area terminals are likely to stay VFR tonight and 
Sunday. 00z WRF model shows a weak vorticity minimum moving over the area 
tonight suggesting additional airmass subsidence. This is followed by 
upper level troughing Sunday and the arrival of the trough should 
generate more onshore breezes. 00z WRF shows an increase in boundary 
layer humidity Sunday night. If this verifies a solid layer of 
stratus may be ready to advance inland Sunday night and Monday 
morning. 


&& 




Mtr watches/warnings/advisories... 
... Bay evening hours. 


$$ 


Public forecast: dykema/Bell 
aviation/marine: canepa 




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