FXUS66 KSTO 050536
AFDSTO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SACRAMENTO CA
900 PM PST FRI DEC 4 2009

.DISCUSSION...
WEAK LOW OFF THE SOCAL COAST GENERATING SOME HIGH AND MID CLOUDINESS 
OVER THE FORECAST AREA. SOME HIGH CLOUDINESS ALSO STREAMING INTO 
NORCAL OFF A FRONTAL BAND NOW AT ABOUT 147 WEST. ONE EFFECT OF THIS 
CLOUDINESS SHOULD BE TO KEEP FOG AND HAZE WHICH HAS ALREADY BEGUN TO 
DEVELOP FROM BECOMING TOO DENSE TONIGHT.  OTHER MAIN AFFECT TONIGHT 
WILL BE TO KEEP MINIMUM TEMPERATURES UP A FEW DEGREES OVER THIS 
MORNINGS DESPITE THE FACT THAT COOLER AIR HAS ALREADY BEGUN TO 
FILTER INTO THE AREA. THIS COOLER AIR IS BEING PULLED IN AROUND A 
COLD LOW PRESSURE SYSTEM NOW DROPPING OUT OF CANADA INTO THE 
NORTHERN ROCKIES. 

THIS LOW IS FORECAST TO MOVE SOUTH SOUTHEAST OVER THE NEXT 24 HOURS. 
AT THIS TIME...MODELS ARE INDICATING A SHORTWAVE ROTATING AROUND THE 
LOW WILL TRACK TOO FAR TO THE EAST TO BRING PRECIP THREAT TO THE 
WEST SIDE OF THE SIERRA BUT CLOUDINESS WILL LIKELY FORM OVER THE 
SIERRA WITH SOME SHOWERS POSSIBLE NEAR AND EAST OF THE CREST. 
NORTHERLY FLOW AROUND THE LOW WILL BRING COOLER AIR INTO THE AREA 
WITH A SIGNIFICANT DROP IN DAYTIME HIGHS EXPECTED. AIRMASS COOLING 
WOULD INDICATE FROM 5 TO 10 DEGREES OF COOLING ON SATURDAY DESPITE 
RH PROG INDICATIONS THAT SKIES WILL BE MOSTLY SUNNY ON SATURDAY WEST 
OF THE SIERRA CREST. LESSOR COOLING WILL OCCUR FARTHER TO THE SOUTH  
AND FARTHER FROM THE COLD AIR SOURCE.  

A SECOND...MUCH COLDER CLOSED LOW IS FORECAST TO DROP SOUTHWARD 
DIRECTLY INTO NORCAL LATE SUNDAY AND MONDAY. THIS SYSTEM IS FORECAST 
TO TAP DIRECTLY INTO AIR FROM THE ARCTIC.  AIRFLOW AROUND THIS LOW 
WILL ALSO HAVE MORE OF AN OVER WATER TRAJECTORY. THIS SYSTEM DOES 
NOT APPEAR WILL BE A LARGE PRECIP PRODUCER WITH THIS SOUTHWARD 
MOVING SYSTEM SHUNTING THE HIGHER PRECIPITABLE WATER SOUTHWARD. THIS 
COLD SYSTEM IS FORECAST TO HAVE 850 MB TEMPERATURES BELOW ZERO 
ACROSS THE ENTIRE FORECAST AREA. THEREFORE SNOW LEVELS WILL BE QUITE 
LOW AND 18Z BUFKIT EVEN PREDICTS SOME BRIEF SNOWFALL IN THE REDDING 
AREA. CURRENT FORECAST PACKAGE MENTIONS SNOW IN THE SURROUNDING 
FOOTHILLS OF THE NORTHERN SAC VALLEY AND LOOKS OKAY FOR NOW. THIS 
SYSTEM SHOULD CLEAR SOUTH OF THE AREA BY TUESDAY MORNING WITH 
SUBSIDENCE BRINGING CLEARING. COLD AIRMASS IN PLACE COMBINED WITH 
CLEARING SKIES WILL MEAN VERY COLD TEMPERATURES EARLY TUESDAY. 
OVERNIGHT LOWS IN THE CENTRAL VALLEY ARE LIKELY TO DROP INTO THE 20S 
UNDER THIS SCENARIO. 850 MB TEMPS TUESDAY AFTERNOON ARE PROGGED 
AROUND -5 WHICH WOULD TRANSLATE TO DAYTIME HIGHS ONLY INTO THE 40S 
AT THE LOWEST ELEVATIONS. 

.EXTENDED DISCUSSION (WEDNESDAY THROUGH SATURDAY)...THE MODELS STILL 
APPEAR TO BE ON TRACK WITH A WET WEATHER PATTERN DEVELOPING AS A 
STRONG PACIFIC JETSTREAM UNDERCUTS THE UPPER RIDGE OVER ALASKA. THE 
ECMWF HAS RETURNED TO BEING THE FASTEST AND WETTEST MODEL...THE GEM 
AND GFS ARE ABOUT 12 HOURS SLOWER. EVEN MORE SOUTHERLY TRACK OF 
MOISTURE OF THE GFS WOULD MAKE FOR A WET PATTERN WEDNESDAY THROUGH 
THURSDAY WITH NEWEST GFS KEEPING SOUTHERLY BRANCH OF WESTERLIES OVER 
CWA FOR CONTINUED WET PATTERN VERSUS RIDGING FORECAST BY 18Z RUN. 
WARMER PACIFIC INFLUENCE WILL MEAN MUCH HIGHER SNOW LEVELS FOR THIS 
ROUND OF PRECIP. 

&&

.AVIATION...HIGH CLOUDS CONTINUE TO SPREAD ACROSS NORTHERN 
CALIFORNIA THIS EVENING...WITH PREDOMINATELY VFR CONDITIONS 
PREVAILING.  HOWEVER...AREAS OF MVFR VISIBILITIES IN HAZY OR MIST 
ARE POSSIBLE FROM KRBL SOUTHWARD TO KMOD.  THESE AREAS OF MVFR 
VISIBILITIES WILL BECOME MORE WIDESPREAD OVERNIGHT TONIGHT...WITH 
LOCAL IFR CONDITIONS DEVELOPING AFTER 11Z IN THE SOUTHERN SACRAMENTO 
VALLEY.  CONDITIONS WILL GRADUALLY IMPROVE AFTER 17Z...AS NORTHERLY 
WINDS BEGIN TO DEVELOP DOWN THE VALLEY.

&& 

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

&&

$$