FXUS66 KSTO 251702
AFDSTO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SACRAMENTO CA
900 AM PST FRI DEC 25 2009

.DISCUSSION...
UPPER LEVEL RIDGE CENTERED VERY NEARLY OVERHEAD THIS MORNING 
BRINGING MAINLY CLEAR SKIES ACROSS INTERIOR NORTHERN CALIFORNIA. THE 
ASSOCIATED SUBSIDENCE AND GENERALLY LIGHT WINDS BROUGHT SLIGHTLY 
MORE WIDESPREAD FOG THAN THE PRIOR FEW MORNINGS...WITH A FEW PATCHES 
OF DENSER FOG IN LOW LYING AREAS AND NEAR WATER SOURCES IN THE 
SOUTHERN SACRAMENTO AND NORTHERN SAN JOAQUIN VALLEYS. LOWS THIS 
MORNING WERE AROUND THE SAME AS THEY WERE YESTERDAY...THE EXCEPTION 
BEING TOWARDS THE FAR NORTHERN SACRAMENTO VALLEY WHERE NORTHERLY 
WINDS WEAKENED AND ALLOWED THE BOUNDARY LAYER TO DECOUPLE. 

ANOTHER BEAUTIFUL DAY IS IN STORE TODAY AS HIGH PRESSURE REMAINS IN 
CONTROL. EXPECT TEMPERATURES THIS AFTERNOON TO REACH COMPARABLE 
LEVELS TO PERHAPS A DEGREE OR TWO WARMER THAN YESTERDAY. TOWARDS THE 
NORTHERN SAC VALLEY...THE LACK OF NORTHERLY WINDS MAY COOL 
TEMPERATURES A FEW DEGREES. THE CURRENT FORECAST PACKAGE REFLECTS 
THESE TRENDS WELL AND NOT EXPECTING ANY UPDATES AT THIS TIME. FOG 
MAY BE A BIT MORE PREVALENT LATE TONIGHT INTO SATURDAY MORNING AS 
LOW LEVEL FLOW REVERSES AND BECOMES MORE SOUTHERLY...ALLOWING BETTER 
RH RECOVERIES. 

THE FOCUS THEN SHIFTS TOWARDS THE UPSTREAM WEATHER 
SYSTEM...CURRENTLY APPROACHING 132W. STILL SEEING QUITE A BIT OF 
MODEL UNCERTAINTY WITH THIS WEATHER SYSTEM...WITH THE GFS NAM AND 
CANADIAN SPLITTING THE SYSTEMS ENERGY AS IT APPROACHES THE COAST AND 
LEAVING MOST OF THE INTERIOR DRY. THE ECMWF HAS STUCK TO ITS GUNS 
AND BRINGS A SOGGY SOLUTION TO THE FORECAST AREA SATURDAY NIGHT INTO 
SUNDAY. THE FORECAST LEANS MORE TOWARDS THE DRIER SOLUTION...THOUGH 
COULD STILL SEE A FEW LIGHT SHOWERS HIT THE VALLEY FLOOR LATE 
SATURDAY INTO SUNDAY. DANG

.PREVIOUS DISCUSSION...
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...
ANOTHER DAY OF WIDESPREAD VFR CONDITIONS ACROSS THE REGION. LOCAL 
SHALLOW FOG IN THE LOWER SAC AND SAN JOAQUIN VALLEYS WILL DISSIPATE 
BY 18Z.  STRONG GUSTY E AND NE WINDS THROUGH CHANNELED AREAS OF 
SIERRA WILL CONTINUE THROUGH TODAY AS OFFSHORE GRADIENT REMAINS.
OVERNIGHT PRES GRADIENTS WILL SLACKEN AND EXPECT MORE FOG ON SAT 
MORNING...BUT THIS WILL BURN OFF BY 18Z. HIGHER LAYERS OF CLOUDS 
WILL SPREAD IN THE NORTH PART OF THE AREA LATE TONIGHT AHEAD OF 
APPROACHING WEAK SYSTEM. DK2

&&

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

&&

$$