FXUS66 KSTO 040528
AFDSTO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SACRAMENTO CA
900 PM PST THU DEC 3 2009

.DISCUSSION...
WEAK FRONTAL SYSTEM MOVING INTO PACIFIC NORTHWEST SPREADING SOME 
HIGH CLOUDINESS OVER THE NORTHERN CWA THIS EVENING. 
OTHERWISE...UPPER LEVEL HIGH PRESSURE RIDGE OVER THE SOUTHWEST U.S. 
CONTINUES TO BRING FAIR SKIES TO THE FORECAST AREA THIS EVENING. 
MAIN FORECAST ISSUE THIS EVENING AND TONIGHT WILL BE REFORMATION OF 
FOG IN THE CENTRAL VALLEY. SEVERAL AREAS SAW LOCALLY DENSE FOG 
FORMATION THIS MORNING. AT THIS TIME...TEMPERATURE DEW POINT SPREADS 
ARE SIMILAR TO A LITTLE LESS THAN 24 HOURS AGO AND FOG HAS BEEN 
SETTING IN EARLIER THIS EVENING IN THE SACRAMENTO AREA. HIGH 
CLOUDINESS WILL LIKELY DO LITTLE TO CURTAIL THE FOG FORMATION SO IT 
IS LIKELY THAT AREAS OF LOCALLY DENSE FOG WILL FORM AGAIN TONIGHT. 
WILL TWEAK CURRENT FORECAST PACKAGE TO REFLECT THIS POSSIBILITY.  
OVERNIGHT TEMPERATURES SHOULD COME IN SIMILAR TO THIS MORNINGS. ONE 
EXCEPTION WILL BE THE FAR NORTHERN SACRAMENTO VALLEY WHERE BREEZY 
NORTH WINDS KEPT OVERNIGHT LOWS HIGHER. LIGHTER WINDS THERE TONIGHT 
WILL ALLOW A RETURN TO OVERNIGHT LOWS CLOSE TO FREEZING. AIRMASS 
COOLS A BIT ON FRIDAY AS UPPER LOW NOW OVER WESTERN CANADA DROPS 
SOUTHEASTWARD INTO THE PACIFIC NORTHWEST. MOST LOCATIONS WILL SEE A 
LITTLE COOLING. FOR THE SOUTHERN SACRAMENTO AND NORTHERN SAN JOAQUIN 
VALLEYS FOG DISSIPATION WILL PLAY MORE OF A FACTOR AND COULD SEE 
TEMPERATURES A LITTLE WARMER FRIDAY IF FOG BREAKS OUT EARLIER. 
PATCHY FOG LIKELY TO BE AN ISSUE AGAIN FRIDAY NIGHT BUT A LITTLE 
MORE CLOUD COVER AND SLIGHTLY MORE NORTHERLY SURFACE WIND SHOULD 
KEEP IT A LITTLE MORE PATCHY. AIRMASS COOLS CONSIDERABLY ON SATURDAY 
AS UPPER LOW DIGS INTO NORTHERN GREAT BASIN AND COLDER AIR FILTERS 
IN FROM THE NORTH. SHORTWAVE PROGGED TO ROTATE AROUND BACKSIDE OF 
LOW AND DOWN SIERRA CASCADE RANGE WILL BRING SOME CLOUDINESS THERE 
SATURDAY AFTERNOON. NAM A LITTLE DEEPER WITH THIS FEATURE AND EVEN 
GENERATES A LITTLE PRECIP OVER THE NORTHERN SIERRA SATURDAY 
AFTERNOON WHILE GFS REMAINS DRY CWA WIDE. 

A SECOND LOW DROPPING OUT OF CANADA ON SUNDAY WILL BRING MUCH COLDER 
AIRMASS ON SUNDAY FOR MUCH COOLER TEMPERATURES. CENTRAL VALLEY HIGH 
TEMPERATURES WILL STRUGGLE TO MAKE IT INTO THE 50S WITH CURRENT MAX 
TEMPERATURES PROGGED ONLY IN THE UPPER 40S FOR THE NORTHERN 
SACRAMENTO VALLEY. BOTH EVENING GFS AND MORNING ECMWF KEEP PRECIP 
FROM THIS SYSTEM NORTH OF THE FORECAST AREA SUNDAY SO CHANCE AND 
SLIGHT CHANCE POPS OVER THE NORTHERN MOUNTAINS MAY BE A BIT 
OVERDONE. 

.EXTENDED DISCUSSION (MONDAY THROUGH THURSDAY)...
MID RANGE MODELS CONSISTENT IN DROPPING THE COLD CANADIAN LOW 
PRESSURE SYSTEM INTO NORCAL ON MONDAY FOR A THREAT OF PRECIP CWA 
WIDE SUNDAY NIGHT THROUGH MONDAY. GFS SOMEWHAT DRIER WITH THIS 
SYSTEM THAN ECMWF BUT NEITHER PROJECT EXTRAORDINARY AMOUNTS OF 
PRECIP. TEMPERATURES WILL BE QUITE COLD HOWEVER AT AROUND 10 DEGREES 
BELOW NORMAL IN THE VALLEY. EVEN THE WARMER ECMWF WOULD DROP SNOW 
LEVELS DOWN TO THE 3000 FOOT LEVEL OVER THE NORTHERN MOUNTAINS AND 
CASCADES WITH THE GFS PREDICTING LEVELS SUBSTANTIALLY LOWER THAN 
THAT. UNDER CLEARING SKIES AND DECREASING WINDS MONDAY NIGHT THERE 
COULD BE SOME FROST / FREEZING ISSUES. TUESDAY A SHORT WAVE RIDGE 
MOVES OVER CALIFORNIA AS COOL TEMPERATURES PERSIST AND STRONG 
INVERSIONS ARE LIKELY IN THE MOUNTAINS. MODEL DIFFERENCES CONTINUE 
BEYOND WEDNESDAY. THE ECMWF AND ITS ENSEMBLES CONTINUE TO DEVELOP A 
STRONG ZONAL PACIFIC MID LEVEL FLOW UNDERCUTTING THE WEST COAST 
RIDGE AND BRINGING A SERIES OF WAVES INTO THE WEST COAST FOR THE 
POTENTIAL OF HEAVY RAINFALL WEDNESDAY AND THURSDAY. THE OPERATIONAL 
GFS HAS BEEN LESS CONSISTENT BUT LAST COUPLE OF RUNS CONTINUE TO 
SPLIT THE JET STREAM ENERGY BRINGING MUCH DRIER AND WEAKER SYSTEMS 
INTO NORCAL LATE WEDNESDAY AND THURSDAY. WE HAVE ADVERTISED THE 
POSSIBILITY OF A PROLONGED WET PERIOD TOWARDS THE MIDDLE AND END OF 
NEXT WEEK BASED ON MORE CONSISTENT ECMWF SOLUTIONS BUT UNCERTAINTY 
CONTINUES AS MODELS REMAIN DISCREPANT. SMITH

&&

.AVIATION...
UPPER LEVEL RIDGE OF HIGH PRESSURE WEAKENS OVER NORTHERN CALIFORNIA 
TONIGHT INTO FRIDAY AS A LOW PRESSURE SYSTEM DROPS INTO WASHINGTON 
IDAHO AND MONTANA. GENERALLY VFR/MVFR CONDITIONS ARE EXPECTED ACROSS 
THE AREA THROUGH FRIDAY EXCEPT AREAS OF FOG BRINGING IFR/LIFR 
CONDITIONS FROM AROUND KMYV AND SOUTH TO NEAR KMOD AND IN THE DELTA 
NEAR VALLEJO AND EAST TO AROUND KSUU AFT 06Z UNTIL AROUND 19Z. SOME 
HIGH CLOUDS ARE EXPECTED TO SPREAD OVER THE AREA ON FRIDAY AS THE 
RIDGE WEAKENS OVER THE AREA.

&& 

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

&&

$$