FXUS66 KSTO 051418
AFDSTO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SACRAMENTO CA
620 AM PST SAT DEC 5 2009

UPDATED AVIATION DISCUSSION FOR STRATUS DEVELOPMENT

.DISCUSSION...
COLD UPPER TROUGH CONTINUES TO DEEPEN OVER THE WESTERN US. INITIAL 
BAROCLINIC BAND OF MID AND HIGH CLOUDS HAS PUSHED SOUTHWARD 
OVERNIGHT AND WILL EXIT THE SOUTHERN PORTION OF THE FORECAST AREA 
EARLY THIS MORNING. STRONGER SHORT WAVE MOVING AROUND THE UPPER LOW 
WILL DROP SOUTHWARD FROM OREGON TODAY PUSHING A COLD FRONT THROUGH 
THE REGION BY THIS AFTERNOON. MOISTURE WITH THIS FEATURE IS VERY 
LIMITED AS EVIDENT BY A LACK OF MUCH CLOUDINESS ON SATELLITE IMAGERY 
AND MODEL FORECAST PRECIPITABLE WATER VALUES OF UNDER A QUARTER 
INCH. NEVERTHELESS...ENOUGH SHALLOW MOISTURE AND INSTABILITY WITH 
THIS FEATURE MOVING SOUTH FOR A FEW FLURRIES THIS AFTERNOON ALONG 
THE CREST OF THE NORTHERN SIERRA.

THIS SETS THE STAGE FOR A STRONGER AND MUCH COLDER UPPER LOW TO DROP 
SOUTHWARD SUNDAY...MOVING INTO FAR NORTHERN CALIFORNIA SUNDAY NIGHT 
AND MONDAY. STRONG COLD ADVECTION INTERACTS WITH PACIFIC MOISTURE 
AND MODERATELY STRONG UPPER FORCING TO GENERATE PRECIPITATION ACROSS 
THE REGION BEGINNING SUNDAY EVENING AND CONTINUING THROUGH MUCH OF 
MONDAY. PRECIPITABLE WATER VALUES INCREASE TO AROUND A HALF INCH AS 
SOME PACIFIC MOISTURE IS DRAWN NORTHWARD INTO NORTHERN CALIFORNIA. A 
COLD AIRMASS ORIGINATING FROM WESTERN CANADA WILL MOVE SOUTHWARD 
INTO THE REGION WITH 850 MB TEMPS FORECAST TO LOWER INTO THE -4 TO 
-6 C RANGE AND 1000 TO 500 MB THICKNESSES LOWER TO 522 TO 525 DM 
MIDDAY ON MONDAY. THIS WILL RESULT IN VERY LOW SNOW LEVELS ACROSS 
THE REGION (1000 TO 2000 FEET OR LOWER). OROGRAPHIC SETUP DOESN/T 
APPEAR TOO STRONG...BUT WILL ENHANCE SNOWFALL SOMEWHAT ON THE WEST 
SLOPES OF THE NORTHERN SIERRA. QPF/SNOW RATIOS WILL BE VERY HIGH 
OVER THE NORTHERN SIERRA WITH SEVERAL FEET OF SNOW POSSIBLE ALONG 
THE CREST OF THE NORTHERN SIERRA WITH UP TO NEARLY A FOOT POSSIBLE 
IN SOME OF THE FOOTHILLS.

THIS SYSTEM EXITS TO THE SOUTH AND EAST MONDAY NIGHT WITH CLEARING 
SKIES EXPECTED OVER THE CENTRAL VALLEY. VERY COLD MINIMUM 
TEMPERATURES WILL BE POSSIBLE EARLY TUESDAY WITH SOME VALLEY TEMPS 
POSSIBLY DIPPING INTO THE LOWER 20S. AMOUNT OF CLOUD COVER WITH 
TIMING OF THE SYSTEM EXITING TO THE EAST WILL BE KEY...SO FORECAST 
WILL CONTINUE TO BE REFINED THE NEXT DAY OR TWO...BUT A FREEZE WATCH 
MAY BE ISSUED IN THE NEXT 24 HOURS IF THE PATTERN HOLDS.

AT THIS TIME...TUESDAY LOOKS DRY AHEAD OF ANOTHER SERIES OF SYSTEMS 
FOR THE LATTER HALF OF THE WEEK.

&&

.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 BRINGS A WARMER AIRMASS...RAISING SNOW LEVELS. THE ECMWF 
CONTINUES TO BE THE FASTEST AND WETTEST MODEL WITH THE GEM AND GFS 
ABOUT 12-18 HOURS SLOWER. OUR FORECAST TIMING TRENDS MORE TOWARD THE 
ECMWF. GREAT OROGRAPHIC FLOW MAY RESULT IN SIGNIFICANT PRECIP 
AMOUNTS OR AT LEAST CONTINUOUS WIDESPREAD PRECIP THROUGH THURSDAY.  
THEN THE JET STREAM SHIFTS SOUTH OF OUR AREA INTO CENTRAL AND SRN 
CAL.  JMC

&&

.UPDATED AVIATION...
SWATH OF IFR STRATUS FORMED FROM SUTTER BUTTES DOWN INTO DELTA AND 
EXTREME NRN SJ VALLEY AS HIGH CLOUDS CLEARED.  AREAS OF MVFR VIS IN 
FOG OVER SAME AREAS WITH LOCAL IFR CONDITIONS TIL 18Z IN THE 
SOUTHERN SACRAMENTO VALLEY. CONDITIONS WILL GRADUALLY IMPROVE AFTER 
18Z...AS NORTHERLY WINDS BEGIN TO DEVELOP DOWN THE VALLEY.    JMC

&& 

.STO WATCHES/WARNINGS/ADVISORIES...WINTER STORM WATCH SUNDAY NIGHT 
AND MONDAY FOR THE WEST SLOPES OF THE NORTHERN SIERRA NEVADA... 
WESTERN PLUMAS COUNTY AND THE LASSEN PARK AREA...NORTHERN SIERRA 
NEVADA FOOTHILLS AND MOTHERLODE. 

&&

$$