FXUS66 KSTO 030530
AFDSTO

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

.DISCUSSION...
HIGH AMPLITUDE RIDGE OVER THE EASTERN PACIFIC AND WEST COAST REMAINS 
DOMINANT WEATHER FACTOR THIS EVENING BRINGING CLEAR SKIES TO THE 
FORECAST AREA. WEDNESDAY DAYTIME HIGHS RAN SIMILAR TO TUESDAYS AND 
CONDITIONS REMAIN SIMILAR TO 24 HOURS AGO. ONE EXCEPTION IS THE 
REDDING AREA WHERE TEMPERATURES ARE RUNNING 10 DEGREES ABOVE TUESDAY 
EVENINGS THANKS TO A LIGHT NORTHERLY WIND. SURFACE HIGH PRESSURE 
MAKING ITS WAY THROUGH THE PACIFIC NORTHWEST AND INTO THE NORTHERN 
GREAT BASIN IS RESPONSIBLE FOR THE INCREASED NORTHERLY PRESSURE 
GRADIENT. SOME FAVORED SOUTH AND WEST FACING CANYONS OF THE NORTHERN 
MOUNTAINS AND SOUTHERN CASCADES WILL LIKELY SEE SOME INCREASED 
BREEZES TONIGHT. IN THE CENTRAL VALLEY FARTHER SOUTH...SIMILAR 
MOISTURE CHARACTERISTICS WILL BRING SIMILAR FOG PATTERN TO THIS 
MORNINGS WITH AREAS OF LIGHT FOG REFORMING AROUND MIDNIGHT AND 
LASTING THROUGH MID MORNING FROM ABOUT MYV SOUTH TO THE NORTHERN SAN 
JOAQUIN VALLEY. FOR TONIGHT...PERSISTENCE FORECAST A GOOD BET AND 
HANDLED IN CURRENT FORECAST PACKAGE. UPPER RIDGE NUDGES INLAND A BIT 
ON THURSDAY FOR A SLIGHT WARMING. LIGHT WINDS WILL CONTINUE UNDER 
WEAK SURFACE AND UPPER LEVEL PRESSURE GRADIENTS. 

SIGNS OF WEATHER CHANGE BEGIN ON FRIDAY AS UPPER LOW NOW OVER 
WESTERN ALASKA MOVES OVER THE WEST COAST RIDGE...THROUGH WESTERN 
CANADA AND INTO THE PACIFIC NORTHWEST. A WEAK LOW IN THE EASTERN 
PACIFIC NOW BETWEEN 130 AND 140 WEST IS FORECAST TO BREAK APART AS 
IN ENCOUNTERS THE WESTERN U.S RIDGE. SOME HIGH CLOUDINESS FROM THIS 
SYSTEM IS FORECAST TO SPILL OVER NORCAL ON FRIDAY AND THIS 
CLOUDINESS COMBINED WITH A SLIGHT AIRMASS COOLING FROM LOW TO THE 
NORTH WILL BRING A SLIGHT COOLING TO MOST AREAS. PREVIOUS RUNS OF 
THE GFS DUG A SHORTWAVE DOWN THE BACKSIDE OF THE GREAT BASIN LOW ON 
SATURDAY AND INDICATED A THREAT OF PRECIP OVER THE SIERRA. 00Z KEEPS 
THIS FEATURE FARTHER TO THE EAST. IF THIS TREND HOLDS IN FUTURE RUNS 
THEN FORECASTS WILL NEED TO BE ADJUSTED TO REMOVE THIS THREAT. 
EITHER WAY...COOLER AIR CONTINUES TO FILTER IN FROM THE NORTH FOR 
COOLER TEMPERATURES SATURDAY BRINGING DAYTIME HIGHS DOWN TO NEAR 
NORMAL FOR THIS TIME OF YEAR. 

.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. ALL MID RANGE 
MODELS HAVE SOME SORT OF ELONGATED LOW ACROSS SOUTHERN OREGON BY 
00Z MONDAY. ALTHOUGH THIS LOW MAY DESTABILIZE THE ATMOSPHERE ENOUGH 
FOR A SLIGHT SHOWER THREAT SUNDAY ACROSS THE NORTHERN 
MOUNTAINS...ITS VERY DRY NATURE...00Z GFS PRECIPITABLE WATER VALUES 
OF AROUND .1 INCH...ANY PRECIP SUNDAY WOULD BE ON THE VERY LIGHT 
SIDE. 

ECMWF...GFS AND GEM ALL ROTATE A SHORTWAVE INTO CENTRAL CA SUNDAY 
NIGHT/MONDAY BEFORE THE LOW FILLS AND SHIFTS INTO THE NORTHERN GREAT 
BASIN. THIS MONDAY STORM APPEARS TO BE NOTHING EXTRAORDINARY FOR 
THIS TIME OF YEAR AND MODELS DIFFER ON QPF BUT SNOW LEVELS WILL BE 
QUITE LOW DOWN TO 2500-3000 FEET AS 700MB TEMPERATURES PLUNGE TO 
-12C BEHIND THE FRONT ACCORDING TO THE ECMWF MODEL WITH GFS 700 MB 
TEMPS PREDICTED TO GO EVEN LOWER. 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 AS THE 500MB COLD POOL SETTLES OVER THE MT. SHASTA 
AREA. . 

MODELS VARY GREATLY NEXT TUESDAY AS ECMWF CONTINUES TO SHOW BREAK 
THROUGH OF WESTERLIES TO THE WEST COAST AND SIGNIFICANT PRECIP EVENT 
WITH FRONTAL SYSTEM TAPPING INTO SUB TROPICAL MOISTURE. 
MEANWHILE...GFS KEEPS UPPER RIDGING OVER WEST COAST TUESDAY FOR A 
DRY DAY FORECAST. ECMWF KEEPS A PROLONGED PERIOD OF HEAVY PRECIP 
OVER THE AREA INTO MID WEEK AS SUBSEQUENT MOIST SYSTEMS ARE ROTATED 
OUT OF THE PACIFIC AND INTO NORCAL. GFS FINALLY BREAKS PACIFIC 
WESTERLIES INTO NORCAL BY WEDNESDAY FOR A MUCH LIGHTER PRECIP EVENT 
WITHOUT A TROPICAL MOISTURE TAP. GEM HAS SIMILAR SOLUTION TO THE 
GFS. WITH GREAT VARIANCE IN EXTEND MODEL RUNS...FORECAST FOR END OF 
FORECAST PERIOD REMAINS VERY UNCERTAIN. SMITH

&&

.AVIATION...
UPR RDG CONTS OVR NORCAL WITH NW-N FLOW ALF. VFR CONDS OVR INTR 
NORCAL EXC AREAS OF MVFR VSBY IN BR FM VCNTY KMYV SWD 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 PTNS OF NRN INTR MTNS AND SIERNEV FTHLS/W 
SLPS OVNGT INTO THU.

&& 

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

&&

$$