Blizzard and Heavy Snow in California, December 20 to 23, 2012

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MEDFORD OR
849 PM PST WED DEC 19 2012

.UPDATE... CURRENT FORECAST AND HEADLINES APPEAR TO BE ON
TRACK...SO HAVE MADE FEW CHANGES TO THE FORECAST. SATELLITE
IMAGERY AND BUOY OBSERVATIONS SUGGEST A WAVE FORMING ALONG THE
OFFSHORE FRONTAL BOUNDARY THAT SHOULD THEN PUSH INLAND ALONG WITH
THE FRONT OVER THE NEXT 12 HOURS. HAVE MADE A FEW ADJUSTMENTS TO
ACCOUNT FOR THIS.

MAJOR QUESTION OF THE NIGHT CONTINUES TO BE THE
CHANCES FOR SNOW WITHIN THE ROGUE VALLEY...WHICH WILL DEPEND
GREATLY ON THE TIMING AND STRENGTH OF THE DOWNSLOPE WINDS THE AREA
IS CURRENTLY EXPERIENCING. TAKING INTO ACCOUNT THE CURRENT 24
DEGREE DEWPOINT DEPRESSION AT MFR AND THE LATEST MODEL
SOUNDINGS...CURRENT THINKING IS THAT THE WINDS WILL DECOUPLE
AROUND 12Z OR SLIGHTLY AFTER...ALLOWING PRECIP TO BEGIN ENTERING THE
AREA. WETBULB EFFECT WOULD THEN ALLOW TEMPERATURES AND SNOW LEVELS
TO DROP ENOUGH TO ALLOW RAIN AND OR SNOW TO BEGIN. A TRANSITION TO
ALL SNOW WOULD THEN BEGIN AROUND 15Z...WITH LIGHT ACCUMULATIONS
POSSIBLE. THIS WOULD BE A RARE CHAIN OF EVENTS...BUT MODELS AND
BUFR SOUNDINGS CONTINUE TO PUSH FOR JUST SUCH A SCENARIO. WILL
LET THE NIGHT SHIFT MONITOR THE SITUATION AND ADJUST THE FORECAST
AS NEEDED IN TIME FOR THE MORNING FORECAST PACKAGE.

&&

.AVIATION...A POTENT COLD FRONT WILL MOVE ONSHORE TONIGHT. THE MAIN
IMPACT WILL BE THE STRONG SOUTH TO SOUTHEAST WINDS ALOFT IN ADVANCE
OF THE FRONT. WIND GUSTS TO 35 KT ARE POSSIBLE AT KOTH, KMFR AND
KLMT. STRONGER GUSTS WILL BE FELT ALONG THE SOUTH COAST. THERE IS
STILL THE POTENTIAL FOR LOW LEVEL WIND SHEAR THROUGH EARLY
TONIGHT...ESPECIALLY AT KRBG, WHERE THE WINDS ALOFT ARE LESS
LIKELY TO SURFACE. SHOULD STILL BE PLENTY OF TURBULENCE THOUGH
REGIONWIDE. MODERATE TO LOCALLY HEAVY RAINFALL ALONG THE COAST WILL
SPREAD INLAND OVERNIGHT. MODELS SHOWING STRONG DOWNSLOPE FLOW AT
KMFR UNTIL EARLY MORNING. THIS SHOULD KEEP MOST OF THE PRECIPITATION
AT BAY UNTIL ABOUT 12Z. IT IS INTERESTING TO NOTE THAT THE MODELS
ALSO RAPIDLY PLUNGE FREEZING LEVELS BY THURSDAY MORNING TO ABOUT
1500 FEET. IT WOULD BE A HIGHLY UNUSUAL EVENT AT KMFR TO HAVE STRONG
DOWNSLOPE FLOW FOLLOWED BY SNOW. BUT BUFR SOUNDINGS ATTEMPTING TO
DRIVE SNOW LEVELS DOWN TO VALLEY FLOORS WHEN HEAVIER PRECIPITATION
ARRIVES. HAVE OPTED FOR A RAIN/SNOW MIX FOR AWHILE IN THE TAF TO
ACCOUNT FOR THIS POTENTIAL, BUT MOST SNOW ACCUMULATIONS SHOULD BE
NEAR AND ABOVE 2000 FEET ON THURSDAY. CIGS HAVE BEEN MOSTLY VFR THIS
EVENING, BUT THESE SHOULD DETERIORATE TO MVFR OVERNIGHT INTO
THURSDAY MORNING AS THE FRONT MOVES EAST. EXPECT WIDESPREAD MOUNTAIN
OBSCURATION AS WELL.

&&

.PREV DISCUSSION... /ISSUED 300 PM PST WED DEC 19 2012/

DISCUSSION...19/12Z NAM/GFS IN.

A WEAK RIDGE HAS BROKEN TO THE EAST OF THE AREA...AND THE STORM
DOOR HAS SWUNG OPEN ONCE AGAIN. WINDS ARE GUSTING OVER 70 MPH AT
CAPE BLANCO...AND UP TO 58 MPH AT OTHER COASTAL LOCATIONS.
PRECIPITATION IS BECOMING MORE WIDESPREAD FROM THE CASCADES
WEST...BUT AMOUNTS ARE STILL LIGHT. MUCH HEAVIER PRECIPITATION
WILL DEVELOP TONIGHT INTO THURSDAY.

AN OFFSHORE LONGWAVE TROUGH WILL DIG ALONG 140W THROUGH FRIDAY
MORNING...AND THEN PROGRESS EAST AND WEAKEN FRIDAY THROUGH THE
WEEKEND. DURING THIS INTERVAL IT WILL EJECT SEVERAL STRONG SHORT
WAVES TOWARD THE PACIFIC NORTHWEST. THE STRONGEST WAVES WILL MOVE
ONSHORE THURSDAY AFTERNOON AND EVENING...AROUND MIDDAY
FRIDAY...FRIDAY NIGHT...AND SATURDAY NIGHT. AS CAN BE INFERRED
FROM THIS TIMETABLE...A BUNCH OF WET AND WINDY SYSTEMS WILL MOVE
ONSHORE DURING THIS INTERVAL...AND IT MAY BE DIFFICULT IN SOME
AREAS...ESPECIALLY NORTHERN CALIFORNIA...TO TELL WHEN ONE STORM
ENDS AND THE NEXT BEGINS.

THE FIRST OF THE STORMS IS NOW DEVELOPING AS A WARM FRONT MOVES
THROUGH THE AREA. WINDS ARE INCREASING BEHIND THE FRONT AND
WILL CONTINUE UNTIL THE TRAILING COLD FRONT MOVES ONSHORE THURSDAY
MORNING. HIGH WIND WARNINGS HAVE BEEN ADJUSTED TO REFLECT THE
SLOWER TIMING. THIS WILL BE THE WETTEST AND WINDIEST OF THE
COMING SPATE OF SYSTEMS. MORE STORMS WILL FOLLOW WITH THE FOCUS OF
PRECIPITATION OVER A CRESCENT EXTENDING FROM THE KALMIOPSIS
WILDERNESS SOUTH THROUGH WESTERN SISKIYOU COUNTY...THEN EAST TO
THE MOUNT SHASTA AREA. THE MOUNT SHASTA AREA STILL LOOK LIKE IT
WILL BE THE HARDEST HIT WITH SNOWFALL MEASURED IN FEET TONIGHT
THROUGH SATURDAY MORNING.

THE REMNANTS OF THE TROUGH WILL MOVE ONSHORE OVER VANCOUVER
ISLAND SUNDAY NIGHT. ONE MORE SHORT WAVE SLIDING DOWN THE BACKSIDE
OF THE TROUGH WILL MOVE ONSHORE MONDAY...THEN A LONG WAVE RIDGE
WILL SET UP OFFSHORE...BRINGING A BREAK IN THE STORM ACTION FOR A
DAY OR SO. THAT RIDGE WILL BREAK TO THE EAST TUESDAY EVENING AND
THE NEXT STORM WILL COME IN HOT ON ITS HEELS TUESDAY NIGHT INTO
WEDNESDAY.

LONG TERM...SATURDAY MORNING, DECEMBER 22ND THROUGH WEDNESDAY
NIGHT, DECEMBER 26TH.

IN SUMMARY...SNOW LEVELS WILL BE LOW ACROSS THE AREA THROUGH ABOUT
CHRISTMAS DAY AND THEN WILL BEGIN TO RISE. THE STORM TRACK WILL
REMAIN ACTIVE ACROSS THE AREA THROUGH MUCH OF THIS TIME PERIOD.
THERE MAY BE A BREAK IN STORM ACTIVITY ON OR ABOUT CHRISTMAS DAY,
WITH SOMEWHAT WARMER STORM SYSTEMS LIKELY ARRIVING THEREAFTER
THROUGH ABOUT THE NEW YEAR. THOSE PLANNING TO TRAVEL ON OR BEFORE
CHRISTMAS DAY, ESPECIALLY ACROSS THE MOUNTAIN PASSES AND EAST SIDE,
SHOULD BE PREPARED FOR WINTER WEATHER DRIVING CONDITIONS. MORE
SPECIFICS FOLLOW BELOW.

ON SATURDAY MORNING MODERATE TO HEAVY PRECIPITATION, IN THE FORM OF
SNOW ABOVE 2000 TO 3000 FOOT ELEVATIONS, WILL BE OCCURRING IN SOUTH
AND SOUTHWEST FLOW FAVORED AREAS. TRAVELERS ARE LIKELY TO EXPERIENCE
TRAVEL PROBLEMS DUE TO THIS SNOW IN THE MOUNT SHASTA AREA AND EAST
SIDE, SISKIYOU SUMMIT ON INTERSTATE FIVE, AND POSSIBLY IN THE SEXTON
SUMMIT AREA BETWEEN GRANTS PASS AND ROSEBURG. THE BRIGHT SIDE IS
THAT THERE SHOULD BE A RELATIVE LULL IN THE PRECIPITATION AS IT
LIGHTENS UP SATURDAY AFTERNOON.

SUNDAY THROUGH MONDAY THE OPERATIONAL GFS AND EUROPEAN MODELS HAVE
COME INTO BETTER AGREEMENT ON TRACKING A LOW PRESSURE SYSTEM
FARTHER NORTH THAN THEY DID PREVIOUSLY, YIELDING MORE SNOWFALL
IMPACTS FOR ELEVATIONS ABOVE 2000 TO 3000 FEET. THIS WILL AFFECT THE
SAME AREAS AS MENTIONED FOR SATURDAY.

TUESDAY THROUGH WEDNESDAY MODELS INDICATE A BIT OF A BREAK IN THE
STORM ACTION. IT APPEARS SNOW LEVELS WILL RISE DURING THIS TIME
PERIOD MORE INTO THE 3500 TO 5000 FOOT RANGE. THIS SHOULD EASE
TRAVEL DIFFICULTIES ACROSS THE AREA. BTL

AVIATION...A STRONG FRONTAL SYSTEM FEATURING HIGH WINDS WILL
CONTINUE TO IMPACT THE AREA TONIGHT THROUGH THURSDAY. THE PRIMARY
IMPACT AT THE TAF SITES THROUGH THIS EVENING WILL BE INCREASING
SOUTH AND SOUTHEAST WINDS AS A WARM FRONT PASSES THROUGH AND A COLD
FRONT APPROACHES, YIELDING THE POTENTIAL FOR LOW LEVEL WIND SHEAR.
RAINFALL WILL CONTINUE TO INCREASE THROUGH THIS EVENING AT THE
COAST, EVENTUALLY PROGRESSING INLAND TONIGHT THROUGH THURSDAY. THERE
SHOULD BE CONSIDERABLE MOUNTAIN WAVE TURBULENCE ON THE WEST SIDE
THIS AFTERNOON, TRANSLATING TO THE EAST SIDE BY EVENING. EXPECT
VALLEY VFR CONDITIONS TO BECOME MVFR AND THEN IFR ALONG AND NEAR THE
FRONT AS IT MOVES INLAND LATE THIS AFTERNOON THROUGH THURSDAY. IN
THE MOUNTAINS A COMBINATION OF HIGH WINDS, AMPLE MOISTURE, AND LOW
SNOW LEVELS WILL CAUSE PARTIAL MOUNTAIN OBSCURATIONS AHEAD OF THE
FRONT TO BECOME TOTAL AS THE FRONT MOVES INLAND TONIGHT THROUGH
THURSDAY. ADDITIONALLY...FREEZING LEVELS WILL DROP SHARPLY TONIGHT
THROUGH THURSDAY. SNOW LEVELS BEGINNING 2500 TO 3500 FEET WILL FALL
TO BETWEEN 2500 AND 1500 FEET ON THURSDAY MORNING. IT IS POSSIBLE
THAT SNOW LEVELS COULD FALL FURTHER AT TIMES...TO NEAR 1000 FEET,
BUT MOST ACCUMULATIONS SHOULD BE NEAR AND ABOVE 2000 FOOT ELEVATIONS
ON THURSDAY. BTL

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...HIGH WIND WARNING UNTIL 2 PM PST THURSDAY FOR ORZ030-031.
     WINTER WEATHER ADVISORY FROM 4 AM THURSDAY TO 4 AM PST FRIDAY
     FOR ORZ030-031.
     WINTER WEATHER ADVISORY FROM 4 AM THURSDAY TO 4 AM PST FRIDAY
     FOR ORZ027-028.
     HIGH WIND WARNING UNTIL 4 AM PST THURSDAY FOR ORZ021-022.
     WINTER WEATHER ADVISORY FROM 4 AM THURSDAY TO 4 AM PST FRIDAY
     BELOW 2000 FEET FOR FOR ORZ024.
     WINTER STORM WARNING UNTIL 3 PM PST FRIDAY ABOVE 2000 FEET FOR
     FOR ORZ024.
     HIGH SURF WARNING UNTIL 4 AM PST THURSDAY FOR ORZ021-022.
     WINTER WEATHER ADVISORY FROM 4 AM THURSDAY TO 4 AM PST FRIDAY
     ABOVE 1500 FEET FOR FOR ORZ023.
     HIGH WIND WARNING UNTIL 1 PM PST THURSDAY FOR ORZ026.
     WINTER WEATHER ADVISORY FROM 4 AM THURSDAY TO 4 AM PST FRIDAY
     FOR ORZ025-026.

CA...WINTER STORM WARNING UNTIL 3 PM PST FRIDAY ABOVE 2000 FEET FOR
     FOR CAZ080.
     HIGH WIND WARNING UNTIL 1 PM PST THURSDAY FOR CAZ081.
     WINTER WEATHER ADVISORY FROM 4 AM THURSDAY TO 4 AM PST FRIDAY
     FOR CAZ081.
     WINTER STORM WARNING UNTIL 10 AM PST SATURDAY FOR CAZ082-083.

PACIFIC COASTAL WATERS...STORM WARNING UNTIL 4 AM PST THURSDAY FOR PZZ350-356-370-376.
     HAZARDOUS SEAS WARNING UNTIL 4 PM PST FRIDAY FOR
     PZZ350-356-370-376.

BPN/MAS/JRS