FXUS66 KSTO 251211
AFDSTO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SACRAMENTO CA
400 PM PST THU DEC 24 2009

.DISCUSSION...
UPPER LEVEL HIGH PRESSURE RIDGE AXIS CENTERED ALONG THE WEST COAST 
THIS MORNING WITH ASSOCIATED SUBSIDENCE BRINGING MAINLY CLEAR SKIES. 
INCREASED SUBSIDENCE AND HIGHER DEW POINTS HAVE ALLOWED SOME FOG TO 
FORM IN THE CENTRAL VALLEY THIS MORNING FROM ABOUT MARYSVILLE 
SOUTHWARD BUT SO FAR NONE HAS BECOME SIGNIFICANTLY DENSE. 
TEMPERATURES ARE RUNNING SIMILAR TO 24 HOURS AGO EXCEPT IN THE 
REDDING AREA WHERE LIGHTER WINDS ARE ALLOWING FOR SOMEWHAT COOLER 
TEMPERATURES. UPPER RIDGE WILL BRING FAIR SKIES TO THE FORECAST AREA 
FOR THE REMAINDER OF CHRISTMAS DAY WITH DAYTIME HIGHS NEAR OR A 
LITTLE ABOVE THURSDAYS. FRONTAL SYSTEM NOW APPROACHING 135 WEST OFF 
THE NORCAL COAST FORECAST TO NUDGE UPPER RIDGE EASTWARD ON SATURDAY. 
INCREASED INSTABILITY AND POSSIBLE HIGH CLOUDINESS SHOULD PRECLUDE 
MUCH IN THE WAY OF FOG OVERNIGHT. CLOUDINESS WILL BE ON THE INCREASE 
DURING THE DAY ON SATURDAY BUT LATEST FORECAST MODELS KEEP MOST OF 
FORECAST AREA DRY ON SATURDAY. ENERGY WITH THIS SYSTEM GETS SPLIT AS 
PACIFIC WESTERLIES ENTER THE COAST THEN GET SPLIT NORTH AND SOUTH 
AROUND UPPER RIDGE. THEREFORE...NOT EXPECTING A SIGNIFICANT AMOUNT 
OF PRECIPITATION FROM THIS SYSTEM AS IN MOVES ACROSS THE FORECAST 
AREA SATURDAY NIGHT. ISOLATED SHOWERS REMAIN A THREAT ON SUNDAY IN 
UNSTABLE AIR BEHIND THE FRONT BUT MODELS BEGIN TO DIVERGE HERE ON 
AMOUNT OF RESIDUAL MOISTURE LEFT OVER FOR SHOWERS. 

MID RANGE MODELS FAIRLY CONSISTENT ON IDEA OF DIGGING A PACIFIC 
TROUGH SOUTHWARD AND FORMING A LOW OFF THE SOCAL COAST BY SUNDAY 
AFTERNOON. THIS ENHANCES THE SPLIT IN THE PACIFIC STORM TRACK WHICH 
SHOULD KEEP CWA MOSTLY DRY THROUGH THE EARLY PORTION OF NEXT WEEK. 
ECMWF AND GFS DO LEAVE A WEAK TROUGH OVER NORCAL HOWEVER SO CAN NOT 
RULE OUT MOUNTAIN SHOWER THREAT COMPLETELY AND HAVE LEFT SLIGHT 
THREAT OF MOUNTAIN SHOWERS IN THE FORECAST MONDAY AND TUESDAY. BOTH 
ECMWF AND GFS BRING A WEAK TROUGH AND THE SOUTHERN BRANCH OF THE 
WESTERLIES INTO NORCAL NEXT WEDNESDAY FOR A SLIGHTLY INCREASED 
THREAT OF PRECIP CWA WIDE. AGAIN ANY PRECIP WITH THIS SYSTEM WOULD 
LIKELY BE FAIRLY LIGHT. 

ECMWF...GFS AND GEM MODELS ALL BUILD A RIDGE OVER THE EASTERN 
PACIFIC AND WEST COAST NEXT THURSDAY AND FRIDAY BUT DIFFER ON 
STRENGTH. GFS FLATTER RIDGE SOLUTION WOULD ALLOW PRECIP THREAT TO 
CONTINUE WHILE MORE AMPLIFIED RIDGE SOLUTION OF GEM AND ECMWF WOULD 
KEEP CWA DRY. HAVE SETTLED FOR SOMEWHAT LESS THAN CLIMO POPS FOR 
THIS TIME PERIOD UNTIL MODELS BECOME MORE CONSOLIDATED.   

&&

.AVIATION...
GENERALLY VFR SKC CONDITIONS ACROSS THE INTERIOR OF NORCAL TODAY 
EXCEPT FOR AREAS OF MVFR CONDITIONS DUE TO BR AND ISOLD LIFR 
CONDITIONS DUE TO FG UNTIL AROUND 18Z TODAY IN THE SACRAMENTO AREA 
AND NORTHERN SAN JOAQUIN VALLEY. LOCAL NORTHEAST WIND GUSTS UP TO 
40-45 KTS OVER SIERRA THROUGH FRIDAY MORNING. LIGHT NLY SURFACE FLOW 
OVER VALLEY RTES TODAY. 

&&

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

&&

$$