FXUS66 KMFR 102255
AFDMFR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MEDFORD OR
255 PM PST THU DEC 10 2009

.SHORT TERM...CHALLENGING FORECAST TODAY WITH REGARDS TO
PRECIPITATION TYPE...AMOUNT...AND TIMING FOR SOUTHWESTERN OREGON AND
NORTHERN CALIFORNIA. GENERAL THINKING IS A WARM FRONT WILL MOVE
NORTH AND BRING MAINLY LIGHT SNOW TO NORTHERN CALIFORNIA AND
SOUTHWESTERN OREGON FRIDAY INTO FRIDAY NIGHT WITH A CHANGEOVER TO
MIXED RAIN AND SNOW THEN RAIN LATE FRIDAY NIGHT INTO SATURDAY.
GIVEN SPLIT-FLOW REGIME AND VERY DRY AIR IN PLACE...PRECIPITATION
AMOUNTS WILL REMAIN FAIRLY LOW WITH THIS SYSTEM. SUBTLETIES IN
THE TIMING...TEMPERATURE FIELD AND PROJECTED SNOW LEVELS YIELD
VERY DIFFERENT OUTCOMES...SO THERE IS SOME UNCERTAINTY IN THE
DETAILS OF THE FORECAST. ISSUED WINTER WEATHER ADVISORY FOR 1 TO 3
INCHES EXPECTED IN LOWER TERRAIN OF CENTRAL AND WESTERN SISKIYOU
COUNTY. NEXT AREA OF FOCUS IS THE ROGUE AND ILLINOIS VALLEYS AS
PRECIPITATION NEARS THE AREA FRIDAY MORNING. MODELS SUGGEST MOSTLY
RAIN WITH WARM ADVECTION SCOURING OUT COLD AIRMASS.
HOWEVER...REALITY WILL LIKELY BE DIFFERENT AS CLOUD COVER TONIGHT
MAY KEEP THE VALLEYS COLDER THAN MODELS INDICATE. NAM MOS IS THE
FAVORED SOLUTION WITH COLDER TEMPERATURES FRIDAY THAN THE GFS
GUIDANCE. THIS WILL LIKELY RESULT IN SOME LIGHT
SNOW...ACCUMULATIONS UP TO AN INCH...IN THE ROGUE AND ILLINOIS
VALLEYS DURING THE DAY AS PRECIPITATION FALLING COOLS THE AIR
ALOFT SOME. FREEZING RAIN IS A POSSIBILITY BUT CURRENT THINKING IS
THAT THIS WILL BE MORE OF AN ISOLATED OCCURRENCE.  BY FRIDAY
NIGHT...A COLD RAIN IS LIKELY IN THE VALLEYS AS SNOW LEVEL RISES
TO NEAR 2000 FEET WITH SATURATED AIRMASS AND REDUCED EVAPORATIONAL
COOLING. PRECIPITATION WILL BE ALL SNOW ON THE EAST SIDE AND THE
CASCADES AS COLD AIR IS MORE STUBBORN TO EXIT THAT AREA. ON
SATURDAY AND SUNDAY A MORE POTENT LOW WILL IMPACT THE CWA WITH
MORE PRECIPITATION. SNOW WILL FALL ON THE EAST SIDE AND SNOW
AMOUNTS WILL NEED TO BE LOOKED AT CLOSELY FOR ANY POTENTIAL
ADVISORY OR WARNING AMOUNTS. SNOW LEVELS WILL RANGE BETWEEN 2500
TO 3000 FEET IN MOST AREAS...SO SISKIYOU SUMMIT WILL LIKELY BE
AFFECTED BY ACCUMULATING SNOWFALL...BUT LOWER ELEVATIONS WILL
LIKELY RECEIVE ALL RAIN.

.LONG TERM...
MONDAY:
12Z GFS/ECMWF/CANADIAN INDICATE 500 HPA SHORTWAVE RIDGING MONDAY 
AHEAD OF THE NEXT WEATHER SYSTEM. HAVE FOLLOWED NEIGHBORS AND 
DECREASED POPS JUST A BIT AS MONDAY IS EXPECTED TO BE MAINLY DRY 
ACCORDING TO THE NEW AND SLOWER MODEL SOLUTIONS.

TUESDAY:
AN UPPER LEVEL TROF WILL PUSH A SURFACE COLD FRONT THROUGH THE 
REGION TUESDAY. THE 10.12Z GFS IS THE SLOWEST WITH THE FRONT 
COMPARED TO THE 10.12Z ECMWF AND 10.12Z CANADIAN. REGARDLESS OF THE 
TIMING...FRONTAL PASSAGE STILL LOOKS VERY LIKELY FOR THE AREA. WARM
AIR ADVECTION AHEAD OF THIS SYSTEM WILL INCREASE SNOW LEVELS
BETWEEN 4500 TO 5500 FEET SO AM EXPECTING SNOW ACCUMULATIONS TO BE
MAINLY ON THE RIDGES AND HIGHER TERRAIN.

WEDNESDAY AND THURSDAY:
THE FLOW BECOMES A BIT MORE ZONAL WITH A FLAT RIDGE BUILDING IN FROM 
THE SOUTHEAST. HAVE RETAINED HIGHEST POPS ALONG THE COAST WITH LOWER 
CHANCES ACROSS THE DESERTS ALTHOUGH THERE IS UNCERTAINTY IN THIS 
PERIOD AS THE GFS CONTINUES THE ZONAL PATTERN WITH WEAK SHORTWAVES 
MOVING THROUGH WHILE THE ECMWF SHOWS A LARGE AMPLITUDE 500 HPA RIDGE 
BUILDING OVER THE WEST COAST.

&&

.AVIATION...VFR CONDITIONS WILL LIKELY CONTINUE THROUGH FRIDAY MORNING.
LIGHT SNOW MAY REDUCE VISIBILITIES OVER NORTHERN CALIFORNIA BY
FRIDAY MORNING...AND THEN ROGUE VALLEY AND KLAMATH FALLS AREA BY
FRIDAY EVENING. CEILINGS WILL LIKELY DROP TO MVFR BY 18Z FRIDAY
WITH MOUNTAINS OBSCURED AS WELL.

&&

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

CA...NONE.

PACIFIC COASTAL WATERS...NONE.

$$

99/99/99

NK/KS