FXUS66 KSTO 261659
AFDSTO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SACRAMENTO CA
900 AM PST SAT DEC 26 2009

.DISCUSSION...WATER VAPOR IMAGERY SHOWS A LARGE OMEGA BLOCK OVER THE 
WEST...WITH NORTHERN CALIFORNIA SITUATED UNDER THE RIDGE OF HIGH 
PRESSURE.  HOWEVER...THERE IS SOME ENERGY RETROGRADING INTO EASTERN 
OREGON AND NEVADA THIS MORNING...WHILE ANOTHER LOW APPROACHES THE 
WEST COAST.  THIS LOW APPROACHING THE WEST COAST HAS A VERY 
ELONGATED FRONTAL BAND ASSOCIATED WITH IT...WHICH IS SPREADING 
PLENTY OF MID AND HIGH LEVEL CLOUDS ACROSS CALIFORNIA THIS MORNING.  
THIS CLOUD COVER IS HELPING TO LIMIT THE FOG FORMATION...WITH MOST 
LOCATIONS SEEING VISIBILITIES AT 3 MILES OR BETTER.  WILL UPDATE THE 
FORECAST THIS MORNING TO REMOVE FOG WORDING AND INCREASE CLOUD COVER 
FOR THE DAY.  

OTHER MAIN CONCERN FOR THE SHORT TERM IS WITH THIS LOW AND FRONTAL 
BAND UNDERCUTTING THE RIDGE.  AT THIS TIME...COMPOSITE RADAR DOES 
SHOW SOME LIGHT SHOWERS OFF THE COAST.  HOWEVER...THE 12Z SOUNDINGS 
FROM OAKLAND AND MEDFORD SHOW A SIGNIFICANT DRY LAYER BELOW ABOUT 
450 MBS...WITH PRECIPITABLE WATER VALUES AT 0.46 AND 0.35 
INCHES...RESPECTIVELY.  THIS LIKELY MEANS THAT THE RADAR ECHOS ARE 
NOT REACHING THE SURFACE...AND IT WILL TAKE A BIT TO OVERCOME THIS 
DRY LAYER.  NEW 12Z MODELS ARE ALSO SUGGESTING THAT THE BRUNT OF THE 
PRECIPITATION FOR THE NORTHERN INTERIOR WILL START AFTER 00Z...AND 
WILL MOST LIKELY OCCUR LATE TONIGHT AND ON SUNDAY.  THEREFORE...WILL 
ALSO UPDATE THE POPS ALONG THE COASTAL RANGE FOR THIS 
AFTERNOON...DECREASING THEM A BIT.  REST OF THE FORECAST LOOKS ON 
TRACK...AND NO OTHER CHANGES ARE PLANNED AT THIS TIME.  PALMER 

.PREVIOUS DISCUSSION (ISSUED AT 330 AM PST)...SOUTHERN BRANCH OF A 
SPLIT PACIFIC STORM TRACK IS FORECAST TO DIG SOUTHWARD ON MONDAY 
FORMING A TROUGH ALONG THE WEST COAST. ALTHOUGH MOST OF THE MOISTURE 
AND INSTABILITY ASSOCIATED WITH THIS TROUGH IS EXPECTED TO REMAIN 
WELL SOUTH OF THE CWA...INSTABILITY GENERATED BY THE TROUGH COULD BE 
ENOUGH TO PRODUCE A FEW MOUNTAIN SHOWERS OVER NORCAL AND HAVE KEPT 
SLIGHT CHANCE POPS OVER THE MOUNTAINS MONDAY AND MONDAY NIGHT. 
ANOTHER WEAK PACIFIC STORM SYSTEM IS FORECAST TO TRACK THROUGH THE 
PACIFIC NORTHWEST AND NORCAL LATE TUESDAY AND WEDNESDAY BRINGING AND 
INCREASED THREAT OF LIGHT RAIN AND MOUNTAIN SNOW. 

RATHER FLAT RIDGE FORMS OVER THE WEST COAST BY THE END OF NEXT WEEK. 
THIS SHOULD KEEP THE SOUTHERN CWA DRY BUT ALLOW FOR A CONTINUED 
THREAT OF PRECIP OVER THE NORTHERN FORECAST AREA. SMITH 

&&

.AVIATION...HIGH CLOUDS HAVE SPREAD IN AHEAD OF A WEAKENING FRONTAL 
BOUNDARY. AREAS OF MVFR FROM BR HAVE DEVELOPED OVER THE CENTRAL AND 
SOUTHERN SACRAMENTO VALLEY...THE DELTA...AND THE NORTHERN SAN 
JOAQUIN VALLEY THIS MORNING. THE CLOUDS ARE LIMITING THE DEVELOPMENT 
OF DENSE FOG...WITH LOCAL IFR VSBYS AROUND 1SM EXPECTED THROUGH 
AROUND 20Z. MID LEVEL CLOUDS WILL MOVE IN THIS AFTERNOON AND THEN 
LOWER THIS EVENING. THE POSSIBILITY OF GENERALLY LIGHT RAIN WILL 
SPREAD OVER THE COASTAL MOUNTAINS THIS AFTERNOON AND THEN INTO THE 
VALLEY AFTER 00Z...CONTINUING THROUGH THE NIGHT INTO SUNDAY. SNOW 
LEVELS OVER THE SIERRA NEVADA WILL DROP TO AROUND 3500-4500 FEET 
TONIGHT BRINGING LOCAL IFR/LIFR THERE. 

&&

.STO WATCHES/WARNINGS/ADVISORIES...NONE.

&&

$$