FXUS66 KSTO 262250
AFDSTO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SACRAMENTO CA
250 PM PST SAT DEC 26 2009

.DISCUSSION...WATER VAPOR IMAGERY SHOWS A TROUGH OF LOW PRESSURE 
APPROACHING THE WEST COAST THIS AFTERNOON...SPREADING PLENTY OF HIGH 
LEVEL CLOUDS ACROSS CALIFORNIA.  COMPOSITE RADAR IMAGERY IS SHOWING 
SOME LIGHT ECHOS ACROSS NORTHERN CALIFORNIA...WITH SOME STRONGER 
ECHOS OFF THE NORTHWEST COAST.  IT IS DOUBTFUL THAT MUCH IS 
REACHING THE SURFACE THOUGH...AS PRECIPITATION GAGES HAVE NOT 
RECORDED ANYTHING JUST YET.  THESE SHOWERS ALSO HAVE TO OVERCOME A 
DEEP DRY LAYER BELOW ABOUT 450 MBS AS SEEN ON THE 12Z OAKLAND AND 
MEDFORD SOUNDINGS.  THEREFORE...BELIEVE IT WILL MOST LIKELY BE SOME 
TIME THIS EVENING BEFORE THE NORTHERN INTERIOR REALLY STARTS TO SEE 
ANY PRECIPITATION...WITH THE BRUNT OF THE MOISTURE MOVING THROUGH 
LATE TONIGHT AND EARLY TOMORROW MORNING.  THE MODELS HAVE BEEN 
TRENDING WETTER WITH THIS SYSTEM...BUT ALL IN ALL...IT STILL DOES 
NOT LOOK LIKE THAT IMPRESSIVE OF A SYSTEM.  THE VALLEY MAY SEE 
ANYWHERE FROM A FEW HUNDREDTHS UP TO ABOUT A QUARTER OF AN INCH IF 
LUCKY.  IN THE SIERRA...QPF VALUES ARE NOT MUCH BETTER...RANGING 
BETWEEN A QUARTER AND A THIRD OF AN INCH...AND MAYBE A FEW LOCATIONS 
SEEING UP TO A HALF INCH.  DO TO THE LIMITED MOISTURE CONTENT...THIS 
SYSTEM WILL NOT BE A BIG SNOW PRODUCER.  HOWEVER...THE SIERRA MAY 
SEE AN UP TO 3 INCHES NEAR PASS LEVEL...WITH LOCAL AMOUNTS UP TO 5 
INCHES OVER THE HIGHER PEAKS.  SINCE SNOW LEVELS WILL BE AROUND 4000 
FEET...THIS WILL LIKELY CAUSE SOME MINOR TRAVEL DIFFICULTIES FOR 
THOSE TRAVELING TO THE SIERRA LATE TONIGHT AND DURING THE SUNDAY 
MORNING TIMEFRAME.

A SECONDARY WAVE WILL MOVE ACROSS THE REGION ON SUNDAY NIGHT AND 
MONDAY...BRINGING THE MAIN TROUGH AXIS ACROSS NORTHERN CALIFORNIA.  
THIS SECONDARY PUSH ALSO LOOKS RELATIVELY DRY...BUT A FEW SHOWERS 
MAY CONTINUE TO BE POSSIBLE OVER THE HIGHER TERRAIN.  THIS SYSTEM 
WILL ALSO HELP OPEN THE DOOR FOR ANOTHER SYSTEM TO APPROACH THE 
REGION LATE TUESDAY.  OTHERWISE...EXPECT TEMPERATURES TO REMAIN NEAR 
OR SLIGHT BELOW NORMAL THROUGH THE PERIOD.  PALMER

.EXTENDED DISCUSSION (TUESDAY NIGHT THROUGH SUNDAY)...WEAK RIDGING 
GIVES WAY TO AN UPPER LOW WITH A COLD FRONT MOVING IN TUESDAY NIGHT 
INTO WEDNESDAY. MODELS CONTINUE TO DISAGREE ON THE AMOUNT OF 
MOISTURE WITH THIS SYSTEM. THE LATEST GFS RUNS HAVE THE SHORTWAVE 
MOVING THROUGH FAIRLY QUICKLY WITH LIMITED PRECIPITATION MAINLY OVER 
THE MOUNTAINS RIDGING REBUILDING IN THE AFTERNOON...WITH JUST LIGHT 
MOUNTAIN SHOWERS INTO THURSDAY. THE ECMWF BRINGS IN ANOTHER WAVE 
WITH QUITE A BIT MORE PRECIPITATION WEDNESDAY NIGHT INTO 
THURSDAY...EVEN AS THE UPPER RIDGE IS REBOUNDING. AM SKEPTICAL OF 
THIS AND WILL GO WITH CHANCES OF LIGHT VALLEY RAIN MOUNTAIN SNOW FOR 
NOW AND WAIT TO SEE HOW FUTURE MODEL RUNS TREND. SIMILAR PROBLEMS 
FOR FRIDAY WITH ECMWF BRINGING A TROUGH THROUGH...WHILE THE GFS 
KEEPS DRIER RIDGING IN...SO CONFIDENCE TOWARD THE END OF THE WEEK IS 
NOT HIGH.

&&

.AVIATION...HIGH CLOUDS HAVE SPREAD IN AHEAD OF A WEAKENING FRONTAL 
BOUNDARY. A FEW PATCHES OF MVFR HZ REMAIN FROM THIS MORNING...THOUGH 
MOST AREAS ARE PRIMARILY VFR THIS AFTERNOON. EXPECT LOWERING CIGS 
AND INCREASING COVERAGE OF -RA THIS EVENING AS WEATHER FRONT 
CONTINUES TO MOVE ONSHORE. MOST AREAS SHOULD SEE MVFR TO IFR 
PRECIPITATION BY 12Z WITH OFF AND ON SHOWERS CONTINUING THROUGH 
SUNDAY AFTERNOON. SNOW LEVELS OVER THE SIERRA NEVADA WILL DROP TO 
AROUND 3500-4500 FEET AND LOCALLY LOWER IN MOUNTAIN BASINS TONIGHT 
BRINGING LOCAL IFR/LIFR CONDITIONS. 

&&

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

&&

$$