Weather

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fxus66 kmtr 292334 
afdmtr 


Area forecast discussion 
National Weather Service San Francisco Bay area California 
435 PM PDT Friday Aug 29 2008 


Discussion...as of 3:00 PM PDT Friday...visible satellite imagery 
shows stratus thickening along the coast with the batch offshore 
growing and almost in contact with the closer-in stratus. The 
surface pressure gradients have weakened considerably in the 
offshore direction with wmc-sfo down to 3.8 mb as compared to 7.4 
mb yesterday at this time...while the onshore gradient has increased 
slightly (sfo-SAC 1.3 mb versus 1.0 mb yesterday at this time.) 
Winds have switched around to westerly most areas with 10-20 miles per hour 
speeds in the hills. Temperatures are running cooler than yesterday 
at most locations except for in the East Bay valleys...where 
temperatures in the 100-102 degree range are a couple degrees 
warmer. The heat advisory remains in effect across the East Bay 
interior valleys and the Santa Clara Valley through 6 PM this 
evening. 


Today is expected to be the last hot day with the increasing marine 
layer due to an approaching upper trough which will bring cooler 
temperatures to the area Saturday and Sunday with temperatures cooling back 
down towards normal most areas. As the trough pushes eastward Sunday 
night into Labor Day strong northerly winds are expected to develop 
along the coast and over the hills. The 18z nam12 puts 40 knot wind 
barbs over eastern Napa County at the 925 mb level for the Monday 
morning time frame...with stronger winds over the coastal waters. 
Will need to continue to monitor the forecast trends...but wind 
advisories might be needed by the Sunday night/Monday morning time 
frame. 


The trough is forecast to move eastward into the Great Basin on 
Monday with another upper level ridge edging eastward towards the 
coast. Medium range models mostly keep this ridge over the eastern 
Pacific as they keep the trough in place over the northern Great 
Basin resulting in dry and seasonable temperatures across our 
area next week. 


&& 


Fire weather...as of 4:30 am PDT Friday...the red flag warnings 
for the north and East Bay hills continue through 8 PM today. 


No changes to fire weather headlines. The current red flag looks 
good with little relief across the north and East Bay hills today. 
Another very hot day with no relative humidity recovery last night. Winds remain 
light but the change to onshore flow is a critical period later 
today and this evening. As stated in previous forecast it will take 
a little while for relative humidity to recover as the strong onshore winds pick up. 
End time of 8 PM looks good with much better relative humidity recovery tonight. 


Will have to continue to monitor the Sunday night wind event. This 
past event has mainly been a relative humidity red flag event. The Sunday night 
event will feature much cooler temperatures and higher relative humidity values. 
Latest NAM runs show 35-40 knot wind barbs in the hills Sunday 
night on the back side of the trough. If this were later in the fall 
after some rain we wouldn't need to monitor so close but given the 
state of the fuels after this last heat spell its precarious. 


The event is analogous to the may 15th heat wave earlier this year 
with The Summit fire following several days later on the back side 
of a cold but windy trough. Wind trumps everything in regards to 
extreme fire behavior and growth. 


&& 


Aviation...as of 4:35 PM PDT Friday...low clouds are increasing 
along the coast late this afternoon. But the marine layer is still 
shallow and VFR conditions should continue at all area terminals 
until at least 06z. An approaching upper level trough will result in 
increasing onshore flow and a deepening marine layer by late evening. 
IFR ceilings will develop at ksfo...koak...and kmry once the marine layer 
begins to surge inland...but probably not until after 08z. Onshore 
winds will increase through early evening with 20g27 kts expected at 
ksfo through at least 04z. Brisk onshore flow overnight and into 
Saturday morning will keep IFR/MVFR ceilings over ksfo until about 18z on 
Saturday morning...and probably later at kmry. 


&& 


Mtr watches/warnings/advisories... 
... Arena to Pigeon pt 20-60 nm...pt Arena 
to pt Reyes out 20 nm...and sf Bay. 
.Red flag warning for the North Bay and East Bay hills 
(zones 507 and 511) until 8 PM Friday. 
.Heat advisory for the East Bay interior valleys and the 
Santa Clara Valley through 6 PM Friday. 
$$ 


Public forecast: Anderson 
aviation/marine: dykema/Larry 
fire weather: r walbrun 


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