FXUS66 KSTO 022344
AFDSTO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SACRAMENTO CA
300 PM PST WED DEC 2 2009

.DISCUSSION...
LITTLE CHANGE IN WEATHER CONDITIONS TODAY WITH THE RIDGE OF HIGH 
PRESSURE REMAINING OVER THE AREA. ANOTHER CHILLY NIGHT IN THE 
VALLEYS IS EXPECTED WITH MAINLY CLEAR SKIES ACROSS THE REGION. 
PATCHY FOG WILL PROBABLY DEVELOP AFTER MIDNIGHT FROM AROUND 
MARYSVILLE AND SOUTH INTO THE NORTHERN SAN JOAQUIN VALLEY AND NEAR 
THE DELTA UNDER MAINLY CLEAR SKIES AND LIGHT WINDS. THE FOG IS 
EXPECTED TO DISSIPATE BY 10 AM AS WE BEGIN TO WARM UP UNDER DURING 
THE DAY. HIGH TEMPERATURES ARE EXPECTED TO BE SIMILAR TO A BIT 
WARMER TOMORROW WITH THE RIDGE AXIS MOVING TO THE WEST COAST. 

THE RIDGE WEAKENS OVER THE AREA AS AN UPPER LEVEL LOW DROPS DOWN 
FROM CANADA INTO THE PACIFIC NORTHWEST FRIDAY INTO FRIDAY NIGHT THIS 
WILL START THE COOLING TREND.  THE MODELS STILL VARY WITH THE 
TIMING...TRACK AND STRENGTH OF THIS LOW WITH THE ECMWF BEING FURTHER 
WEST AND DEEPER. EITHER WAY THIS WILL BRING SOME COLD CONTINENTAL 
AIR DOWN OVER NORTHERN CALIFORNIA. HIGH TEMPERATURES WILL DROP 
AROUND 10 TO 20 DEGREES OVER THE WEEKEND COMPARED TO WHAT WE SAW 
TODAY. THE GFS REMAINS DRY WITH A MORE EASTERLY TRACK AND THE ECMWF 
BRINGS SOME PRECIPITATION OVER THE SIERRA SATURDAY INTO EARLY 
SUNDAY. THE FORECAST LEANS TOWARD A FURTHER WESTERLY TRACK WITH THE 
LOW FOR A SLIGHT CHANCE TO CHANCE OF SHOWERS OVER THE SIERRA AND 
SHASTA COUNTY MOUNTAINS SATURDAY INTO EARLY SUNDAY. COLD AIR WILL BE 
COMING DOWN WITH THIS SYSTEM DROPPING SNOW LEVELS DOWN TO AROUND 
3000 FEET IN THE NORTH TO AROUND 4500 FEET SOUTH OF TAHOE. THIS 
COULD BRING WINTER DRIVING CONDITIONS TO THE MOUNTAIN PASSES 
SATURDAY NIGHT INTO SUNDAY. ALTHOUGH THE SYSTEM IS FAIRLY DRY SO 
SNOWFALL ACCUMULATIONS ARE EXPECTED TO BE LESS THAN 6 INCHES AT THE 
HIGHEST ELEVATIONS AT THIS TIME. AS THE SYSTEM DIGS OUT INTO THE 
PACIFIC IT WILL BRING A SLIGHT CHANCE OF RAIN AND SNOW OVER THE AREA 
ON SUNDAY. HSO

.EXTENDED DISCUSSION (SUNDAY THROUGH WEDNESDAY)...
COLD CANADIAN LOW PRESSURE SYSTEM BACKS INTO NORTHERN CALIFORNIA AS 
A HIGH AMPLITUDE RIDGE ALONG 135W BEGINS TO PINCH OFF. 18Z GFS AND 
12Z ECMWF HAVE A 527-529DM UPPER LOW CENTERED NEAR MEDFORD OREGON 
SUNDAY AFTERNOON WITH A 110KT JET SLICING ACROSS CENTRAL CA. NEARLY 
ALL OF THE MODELS ROTATE A SHORTWAVE INTO CENTRAL CA SUNDAY 
NIGHT/MONDAY BEFORE THE LOW FILLS AND SHIFTS INTO THE NORTHERN GREAT 
BASIN. THE SUNDAY-MONDAY STORM APPEARS TO BE NOTHING EXTRAORDINARY 
FOR THIS TIME YEAR EXCEPT SOME SNOW LEVELS DOWN TO 2500-3000 FEET AS 
700MB TEMPERATURES PLUNGE TO -12C BEHIND THE FRONT. THERE IS A 
PERIOD OF MODEST OROGRAPHICS IN WSW WINDS 700-850MB 40KTS EARLY 
MONDAY WHICH MIGHT PROVIDE SOME ENHANCEMENT ALONG THE FRONT. 
CONVECTIVE SHOWERS ARE ALSO POSSIBLE AT THE 500MB COLD POOL OF -34C 
SETTLES OVER THE MT. SHASTA AREA. INITIAL GUESS-TIMATES WOULD BE 1 
TO 3 INCHES OF LIQUID ACROSS THE REGION AND CROSS SECTIONS SUGGEST 
THIS WILL BE A FAST MOVING SYSTEM. 

WE ARE ALL KEEPING A WARY EYE ON THE RETROGRADING UPPER RIDGE AND 
THE POSSIBILITY THAT THE WESTERLIES WILL BREAK THROUGH AND BRING A 
PROLONGED PERIOD OF WET WEATHER LATER IN THE WEEK. THE 18Z/12Z GFS 
HAVE A SHORTWAVE RIDGE OVER THE REGION TUESDAY AND THE 18Z RUN 
ESPECIALLY BACKED AWAY FROM A VERY WET STORM MOVING INTO THE REGION 
BY THURSDAY (IN FACT THE 18Z GFS HAS A MUCH WEAKER SURFACE LOW OF 
999MB OFF THE OREGON COAST 06Z THU). THE GEM HAS MUCH LESS RIDGING 
TUESDAY AND IT AND THE ECMWF HAVE A 985MB SURFACE LOW AT THIS TIME.  
THE 12Z ECMWF KEEPS A STEADY STREAM OF SHORTWAVES PROGRESSING INTO 
CA FROM THE PACIFIC THROUGH DAY 8. SEVERAL OF THE GFS ENSEMBLE 
MEMBERS SHOW THIS AS WELL. THE FOLKS AT HPC PREFER TO TAKE BLENDS OF 
THESE SOLUTIONS FOR NOW...NOT COMMITTING TO ANY PARTICULAR SOLUTION. 
ESRL REANALYSIS DATA HAS A BLOCK DEVELOP NE OF HAWAII SUGGESTING 
PERHAPS MORE AMPLIFICATION PER THE GFS THROUGH AT LEAST 
TUESDAY...AFTER WHICH UNCERTAINTY BECOMES LARGE...WILL CONTINUE TO 
MONITOR...SG

&&

.AVIATION...
UPR RDG CONTS OVR NORCAL WITH NW-N FLOW ALF. VFR CONDS OVR INTR 
NORCAL EXC AREAS OF MVFR IN BR FM VCNTY KMYV SWD AFT 08Z TIL ARND 
18-20Z WITH LCL IFR/LIFR CONDS IN FOG POSS BTWN 10Z-17Z. ISOLD NE-E 
SFC WND GSTS 25-35 KTS OVR SIERNEV FTHLS/W SLPS AFT 06Z TIL ARND 18Z 
THU.

&& 

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

&&

$$