813 
fxus66 kmtr 210353 
afdmtr 


Area forecast discussion 
National Weather Service San Francisco Bay area California 
900 PM PDT Sat Mar 20 2010 


Discussion...as of 9:00 PM PDT Saturday...quiet through next week. 


Weak onshore flow today along with lower 500 mb heights and 850 mb 
temperatures helped to drop highs across the County Warning Area today. Currently... 
many spots are running close to 10 degrees cooler today compared to 
yesterday. Satellite loop shows an area of low pressure heading 
toward the British Columbia coast with an associated cold front moving to the 
West Coast. Models continue to show an outside chance that the 
southern extent of the moisture will make it down into the far 
northern portions of the County Warning Area. Timing has been relatively consistent 
and indicates the best shot of rainfall Sunday morning into the 
early afternoon hours. Again...this looks extremely limited so most 
people in the County Warning Area will probably only notice an increase in clouds 
along with some additional cooling. Other issue to deal with will 
be the possibility of low clouds and patchy fog tonight in advance 
of the front. Guidance continues to suggest that any fog that forms 
will be very limited in scope. However...with the marine layer 
forecast to be around 800 feet...fog can definitely not be ruled 
out. 


After the system sweeps by on Sunday...more dry weather can be 
expected on Monday and Tuesday as a ridge builds back overhead 
giving a weak Omega block signature. Another very light round of 
rain will return for the North Bay late Wednesday as a Pacific 
system moves to the Washington coast as the ridge flattens out. 
High temperatures will mostly be in the 60s and lower 70s all week 
with lows in the 40s. 
&& 


Aviation...as of 5:20 PM PDT Saturday...a thin stratus layer over 
the coastal waters was seen on visible imagery this morning but has 
since mixed out nearly completely with the passage of a shortwave 
trough. Plenty of cirrus clouds have accompanied the trough. The 
majority of them appear to be leaving the area and headed toward the 
Sierra Nevada. 


The latest WRF model boundary layer humidities did OK picking up on 
the areas of thin stratus earlier today but did not account for 
them mixing out this afternoon. The WRF model indicates an increase 
in boundary layer humidities later tonight mainly over the Bay area 
which suggests some stratus will redevelop tonight but at a later 
time since much of it has already mixed out. Some stratus patches 
could linger Sunday morning. VFR is generally forecast for Sunday. 


Otherwise 24 hour trends indicate the airmass is cooler and more 
humid compared to Friday. Low level cooling continues tonight which 
accompanied by the weak lift of a shortwave trough should help 
stratus redevelop overnight...mainly late at night. A wind shift to 
the northwest Sunday with better wind linkage indicated by the 
models Sunday afternoon and Sunday evening supports VFR extending 
Sunday night. 


Vicinity of ksfo...west winds under 20 knots due to a weak onshore 
component from Half Moon Bay to ksfo which is weak at less than 1/2 
mb. A lack of stratus nearby over the coastal waters and remaining 
cirrus clouds overhead moderate to high confidence VFR will last 
the entire evening...and probably well into the night. Steady low 
level cooling and increasing humidities do support some stratus by 
dawn Sunday. VFR is forecast to return by late morning. West to 
northwest winds are forecast Sunday afternoon/evening. 


Ksfo bridge approach...same as ksfo. 


Mry Bay area terminals...VFR forecast this evening. In general the 
WRF model indicates humidities will not be as high as they are 
expected to by over the Bay area by Sunday morning. Between midnight 
and daybreak Sunday forecast confidence lowers quickly. For now best 
approach appears to be to forecast a tempo group for broken ceilings 
Sunday morning for ksns and kmry. VFR is likely to return by late 
morning though sometimes when the surface winds turn more 
northwesterly low level clouds can linger much longer than originally 
expected. 


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: Bell 
aviation/marine: canepa 


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