FXUS66 KMFR 092309
AFDMFR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MEDFORD OR
309 PM PST WED DEC 9 2009

.SHORT TERM...TONIGHT THROUGH SATURDAY NIGHT...CLEAR SKIES WILL
CONTINUE OVER MOST OF THE CWA TONIGHT AS WE REMAIN UNDER THE
INFLUENCE OF THE DRY NORTHWEST FLOW ALOFT AND A RIDGE AXIS LOCATED
JUST OFFSHORE. THE FRIGID AIR MASS WILL SLOWLY START TO EASE ITS
GRIP OVER THE NEXT FEW NIGHTS...AND LOW TEMPERATURES TONIGHT WILL
BE A FEW DEGREES HIGHER THAN LAST NIGHTS...LOW 20S AT THE COAST TO
SEVERAL DEGREES BELOW ZERO ON THE EAST SIDE.

THE CURRENT VIS SHOWS CLOUDS EXTENDING FROM JUST WEST OF SAN
FRANCISCO NORTHWESTWARD TO NEAR 45N 140W...AND THIS SYSTEM IS THE
START OF AN UNSETTLED WEATHER PATTERN FOR LATER THIS WEEK INTO THE
WEEKEND. THE SPLIT FLOW REGIME THAT EXISTED SOUTHWEST OF THE CWA
NEAR 40N 130W WAS SHEARING THE SYSTEM APART LAST NIGHT WITH PIECES
OF IT GETTING PULLED EITHER NORTH OR EAST. THE MORE RECENT IR
IMAGES TODAY NOW SHOW IT SLIGHTLY MORE ORGANIZED. THE MODELS ARE
HAVING DIFFICULTY RESOLVING THE STRENGTH AND TIMING OF ITS ARRIVAL
INTO THE CWA. THE GFS BRINGS THE SYSTEM INTO NORTHERN CALIFORNIA
THURSDAY NIGHT AND CARRIES IT NORTH AND EAST THROUGH FRIDAY
NIGHT...WITH A STRONGER LOW ARRIVING QUICKLY ON ITS HEELS SATURDAY
INTO SUNDAY. THIS SOLUTION IS ALSO SUPPORTED BY THE CANADIAN. THE
ECMWF IS SLOWER WITH THE INITIAL WAVE AND IT DOES NOT REACH THE
CWA UNTIL DURING THE DAY FRIDAY. THE SECOND LOW SATURDAY INTO
SUNDAY IS NOT AS STRONG ON THE ECMWF AS IT IS ON THE GFS. THE NAM
WAS DRIER...SLOWER...AND WEAKER THAN THE OTHER TWO WITH
PRECIPITATION NOT ARRIVING UNTIL LATE FRIDAY.

WITH THE COLD DRY AIR MASS ALREADY IN PLACE...THE MAIN CONCERNS
ARE WHEN THE ATMOSPHERE MOISTENS UP ENOUGH FOR PRECIPITATION TO
REACH THE GROUND AND WHEN THE COLD AIR ERODES FROM THE LOW
ELEVATIONS TO ALLOW SNOW LEVELS TO RISE. IF THE PRECIPITATION
ARRIVES EARLIER AS THE GFS INDICATES...COLD AIR STILL TRAPPED IN
THE VALLEYS WILL RESULT IN SNOW LEVELS TO THE VALLEY FLOORS. AT
THIS TIME...WE HAVE LEANED TOWARD THE ECMWF IN THE SHORT TERM WITH
PRECIPITATION REACHING SISKIYOU AND MODOC COUNTIES EARLY FRIDAY
AND SPREADING NORTHWARD DURING THE DAY INTO SOUTHERN OREGON. WE
LOWERED SNOW LEVELS EARLY FRIDAY MORNING WITH THE COLD AIR STILL IN
CONTROL. PRECIPITATION WILL INITIALLY BE LIGHT WITH THIS FIRST
WAVE BEFORE INCREASING WITH THE SECOND LOW SATURDAY INTO SUNDAY.
LATER SHIFTS WILL NEED TO EVALUATE THE SNOW POTENTIAL IN OUR
NORTHERN CALIFORNIA ZONES FRIDAY NIGHT AND SATURDAY IF THE COLD
AIR IS SLOW TO LEAVE BUT CONFIDENCE IS LOW AT THIS TIME.

.LONG TERM...VALLEY RAIN AND MOUNTAIN SNOW CONTINUE OVER SOUTHERN 
OREGON AND NORTHERN CALIFORNIA SUNDAY.  SNOW LEVELS WILL AVERAGE 
ABOUT 4000 FEET SUNDAY AND MONDAY.  GFS AND ECMWF AGREE ON A BRIEF 
BREAK IN PRECIPITATION SUNDAY NIGHT INTO MONDAY MORNING WITH A 
SHORTWAVE RIDGE OVERHEAD. A MORE POTENT WEATHER SYSTEM WILL BRING 
WARM ADVECTION TO THE REGION ON TUESDAY WITH STRONG WINDS POSSIBLE 
ALONG THE COAST.  SNOW LEVELS WILL IN TURN RISE TO ABOUT 5000 FEET 
ON THE WEST SIDE WITH COLDER AIR POSSIBLY HANGING ON OVER THE EAST 
SIDE.  MODELS DIVERGE AT THIS POINT WITH GFS SUGGESTING AN UPPER 
RIDGE AND DRIER WEATHER OVER THE NORTHWEST AND ECMWF PROMOTING A 
CONTINUED WET AND WARMER PATTERN.  IN GENERAL...SUNDAY THROUGH 
TUESDAY LOOKS UNSETTLED AS A TRANSITION FROM THE PERSISTENT RIDGE TO 
A MORE PROGRESSIVE PATTERN BEGINS...WITH MORE UNCERTAINTY TOWARDS 
THE MIDDLE OF NEXT WEEK.

&&

.AVIATION...VFR CONDITIONS WILL CONTINUE TONIGHT AND TOMORROW WITH
GOOD VISIBILITY AND ONLY SOME HIGH CLOUDS AROUND.

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...FREEZE WARNING FROM 11 PM THIS EVENING TO 9 AM PST THURSDAY FOR 
     ORZ021-ORZ022.
     FREEZE WARNING FROM 11 PM THURSDAY TO 9 AM PST FRIDAY FOR ORZ021-
     ORZ022.

CA...NONE.

PACIFIC COASTAL WATERS...NONE.

$$

SHORT TERM: MW
LONG TERM: NSK
MARINE: DW