FXUS66 KSTO 110456
AFDSTO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SACRAMENTO CA
855 PM PST THU DEC 10 2009

.DISCUSSION...WATER VAPOR IMAGERY SHOWS NEGATIVELY TILTED TROUGH 
APPROACHING THE CALIFORNIA COAST THIS EVENING...WHICH WILL MOVE 
INLAND LATE TONIGHT AND ON FRIDAY.  COMPOSITE RADAR IMAGERY SHOWS 
MULTIPLE BANDS OF SHOWERS SPREADING INLAND AT THIS TIME...WITH ONE 
BAND EXTENDING FROM NEAR JACKSON BACK THROUGH SACRAMENTO AND 
LAKEPORT.  THESE SHOWERS ARE LIGHT IN NATURE...WITH ONLY TRACE 
AMOUNTS TO A FEW COUPLE OF HUNDREDTHS REPORTED IN THE VALLEY THUS 
FAR.  A SECOND MORE POTENT BAND EXTENDS FROM BAKERSFIELD NORTHWARD 
THROUGH MERCED AND THEN BACK INTO SAN JOSE....AND IS LIFTING 
NORTHWARD TOWARDS MODESTO AND STOCKTON.  

TEMPERATURES THIS EVENING ARE MUCH MORE MILD...BUT STILL COLD 
COMPARED TO THE PAST FEW NIGHTS...RUNNING 5 TO 15 DEGREES WARMER 
THAN THE TEMPERATURES AT THIS TIME LAST NIGHT.  THIS IS IN PART DUE 
TO THE CLOUD COVER AND WARM AIR ADVECTION ALOFT...BUT ALSO DUE TO 
THE CHANGE IN THE AIR MASS FROM AN ARCTIC AIR MASS TO A MOIST 
MARITIME AIR MASS.  HOWEVER...SOME OF THE DRIER AND COLD AIR REMAIN 
TRAPPED IN THE HIGHER MOUNTAIN VALLEYS AND FOOTHILLS...WHICH MAY 
CAUSE SOME LOCALLY LOWER SNOW LEVELS.  IN FACT...ONE OF OUR SPOTTERS 
IN COBB REPORTED LIGHT SNOW TRANSITIONING FROM LIGHT RAIN AND A 
TEMPERATURE OF 32 DEGREES.  EXPECT THIS SITUATION MAY ALSO BE 
POSSIBLE IN THE MOTHERLODE REGION LATE TONIGHT.  THIS IS ONE OF 
THOSE BORDERLINE SITUATIONS WHERE THEY MAY STAY LIGHT RAIN...WITH 
THE WARM AIR MOVING IN ALOFT...OR TRANSITION TO SNOW WITH THE COLD 
AIR TRAP...WITH WET BULB COOLING.  AT THIS POINT IN TIME...BELIEVE 
THAT IT WILL MOST LIKELY TRANSITION TO SNOW FOR SOME TIME.  ALSO 
BELIEVE THAT A SNOW ADVISORY MAY BE WARRANTED FOR LAKE COUNTY.  
THEREFORE...WILL DO A QUICK UPDATE TO THE FORECAST THIS EVENING.  
OTHERWISE...FORECAST LOOKS ON TRACK...WITH RELATIVELY WET WEATHER 
CONTINUING THROUGH THE WEEKEND...AS A STRONG SECOND SYSTEM TAKES AIM 
AT NORTHERN CALIFORNIA ON SATURDAY.  PALMER

&&

.AVIATION...WEAK FRONT IS CURRENTLY APPROACHING THE CALIFORNIA COAST 
SPREADING MID TO HIGH CLOUDS ACROSS THE AREA. A FEW SHOWERS HAVE 
POPPED UP EARLY IN THE EVENING...BUT SHOULD INCREASE IN INTENSITY 
AND COVERAGE BY 06Z AND SPREAD SOUTH TO NORTH. MVFR TO LOCAL IFR 
CEILINGS AND VIS CAN BE EXPECTED IN RAIN WITH IFR TO LIFR CONDITIONS 
IN SNOW OVER THE HIGHER ELEVATIONS. PRECIPITATION MAY START OFF AS 
SNOW TOWARDS THE NORTHERN END OF THE SACRAMENTO VALLEY TONIGHT 
BEFORE TRANSITIONING OVER TO RAIN BY EARLY FRIDAY MORNING. SNOW 
LEVELS SHOULD RISE TO THE 4500 TO 6000 FT RANGE BY FRIDAY MORNING. 
PRECIPITATION WILL BECOME MORE SHOWERY IN THE VALLEY BY LATE FRIDAY 
MORNING THOUGH SNOW SHOULD CONTINUE OVER THE HIGHER ELEVATIONS. 

&& 

.STO WATCHES/WARNINGS/ADVISORIES...
...WINTER WEATHER ADVISORY FROM 8 PM THIS EVENING TO 10 PM PST 
   FRIDAY FOR WEST SLOPE NORTHERN SIERRA NEVADA-WESTERN PLUMAS 
   COUNTY/LASSEN PARK.
...WINTER STORM WATCH FROM FRIDAY EVENING THROUGH SUNDAY MORNING FOR 
   WEST SLOPE NORTHERN SIERRA NEVADA-WESTERN PLUMAS COUNTY/LASSEN 
   PARK.
...WINTER WEATHER ADVISORY FROM 8 PM THIS EVENING TO 10 AM PST    
   FRIDAY FOR MOTHERLODE AND SOUTHERN LAKE COUNTY.

&&

$$