Heavy Snow in Oregon, December 16 to 17, 2015

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MEDFORD OR
857 PM PST WED DEC 16 2015

.UPDATE...A WARM FRONT IS MOVING OVER THE AREA TONIGHT AND
THURSDAY. THIS WILL BRING LIGHT TO MODERATE PRECIPITATION INTO THE
AREA TONIGHT AND THURSDAY. HEAVIEST PRECIPITATION WILL BE ALONG
THE COAST AND IN THE OREGON CASCADES. SNOW LEVELS WILL RISE WEST
OF THE CASCADES TONIGHT INTO THURSDAY MORNING. EAST OF THE
CASCADES AND IN THE MOUNT SHASTA AREA, THE PRECIPITATION TYPE IS
A SIGNIFICANT CONCERN WITH SNOW LEVELS REMAINING LOW THIS EVENING
INTO TONIGHT, THEN AS A WARM LAYER MOVES OVERHEAD EXPECT A CHANCE
OF LIGHT FREEZING RAIN TO DEVELOP, ESPECIALLY IN VALLEYS, TONIGHT
AND THURSDAY MORNING. THIS WOULD CAUSE  DANGEROUS TRAVEL ON AREA
ROADS. BY THURSDAY AFTERNOON EXPECT A MIX OF RAIN AND SNOW OVER
THE AREA.

BASED ON THE LATEST 00Z NAM, FREEZING RAIN CHANCES ARE HIGHEST
TONIGHT AND THURSDAY MORNING IN THE MOUNT SHASTA AREA, EASTERN
SISKIYOU COUNTY AND WESTERN MODOC COUNTY IN CALIFORNIA AND HIGHEST
IN THE KLAMATH BASIN AND PORTIONS OF SOUTHERN LAKE COUNTY IN
SOUTHERN OREGON. IN THE SOUTHERN OREGON CASCADES, THERE IS ALSO A
CHANCE OF FREEZING RAIN TONIGHT FOR AREAS NEAR LAKE OF THE WOODS
SOUTHWARD. HAVE ADDED A WINTER WEATHER ADVISORY FOR LIGHT SNOW
AND FREEZING RAIN TO SOUTH CENTRAL AND EASTERN SISKIYOU COUNTY AS
WELL AS WESTERN MODOC COUNTY. WINTER WEATHER ADVISORIES REMAIN IN
EFFECT FOR THE SOUTHERN OREGON CASCADES EAST WITH WINTER STORM
CONDITIONS OVER THE HIGHER ELEVATIONS IN THE SOUTHERN OREGON
CASCADES AND NEAR CHEMULT WHERE SNOW LEVELS ARE EXPECTED TO REMAIN
LOW.

&&

.AVIATION...FOR THE 17/00Z TAF CYCLE...VFR CONDITIONS WILL
DETERIORATE THIS EVENING THROUGH THURSDAY MORNING AS A STRONG WARM
FRONT PUSHES INTO THE FORECAST AREA FROM THE WEST. EXPECT MVFR TO
DEVELOP ACROSS THE AREA THIS EVENING, WITH SNOW LEVELS NEAR 3500
FEET. WINDS SHEAR IS EXPECTED ALONG AND NEAR THE COAST, TO INCLUDE
KOTH, BEGINNING AROUND 06Z AND CARRYING THROUGH THE TAF PERIOD TO
18Z. OVERNIGHT IFR WILL DEVELOP AND PRECIPITATION WILL INTENSIFY.
SNOW IS EXPECTED AT KLAMATH FALLS, ACCUMULATING 1-3 INCHES AT KLMT
BETWEEN 09Z AND 18Z. AROUND DAYBREAK THROUGH ABOUT 18Z SNOW COULD
MIX WITH OR CHANGE TO FREEZING RAIN OVER PORTIONS OF THE EAST SIDE,
TO INCLUDE KLMT. AN AIRPORT WEATHER WARNING IS IN EFFECT AT KLMT FOR
THIS ACCUMULATING FROZEN AND FREEZING PRECIPITATION FROM 09Z-18Z. AT
MEDFORD AIRPORT WE EXPECT LIFR CONDITIONS AROUND 15Z THURSDAY
MORNING. BTL/SPILDE

&&

.MARINE...UPDATED 345 PM PST WEDNESDAY 16 DEC 2015...MODERATE SWELL
WILL BECOME WESTERLY TONIGHT...AND SEAS WILL BECOME QUITE HAZARDOUS
AS SOUTH WINDS INCREASE TO AROUND GALE FORCE. WINDS AND SEAS REMAIN
HAZARDOUS TO SMALL CRAFT LATE TONIGHT INTO THURSDAY. A STRONG COLD
FRONT WILL MOVE THROUGH LATE THURSDAY AFTERNOON AND THURSDAY NIGHT.
EXPECT ANOTHER ROUND OF STRONG SOUTH WINDS...PERHAPS REACHING STORM
FORCE IN SOME AREAS. HAVE ISSUED A STORM WATCH FROM ABOUT 5 TO 30
NAUTICAL MILES FROM SHORE AND ALSO NEAR ALL OF THE CAPES DURING THIS
TIME FRAME WHERE GUIDANCE INDICATES BOUNDARY LAYER WINDS SOLIDLY IN
THE 50-55KT RANGE. SEA HEIGHTS PROBABLY REACH OR PERHAPS EVEN EXCEED
20 FT IN THE CORE OF THE STRONGEST WINDS. HEAVY WEST TO NORTHWEST
SWELL WILL DEVELOP THROUGH THE WEEKEND. YET ANOTHER FRONT WILL MOVE
THROUGH THE AREA SATURDAY NIGHT INTO SUNDAY. SPILDE

&&

.PREV DISCUSSION... /ISSUED 350 PM PST WED DEC 16 2015/

SHORT TERM...THROUGH SATURDAY NIGHT.
THE FRONT IS SPREADING LIGHT RAIN INTO SW OREGON THIS AFTERNOON.
SNOW LEVEL IS AROUND 4000 FEET THIS AFTERNOON SO PRECIPITATION IS
FALLING AS SNOW AT THE HIGHER MOUNTAINS. THIS FRONT WILL GRADUALLY
SPREADING MORE PRECIPITATION INTO THE CASCADES AND EAST OF THE
CASCADES THIS EVENING. WE HAVE CONTINUED THE WINTER WEATHER
ADVISORY FOR THE CASCADES. ONE MAJOR CONCERN FOR TONIGHT AND
THURSDAY IS THE TYPE OF PRECIPITATION FOR AREAS EAST OF THE
CASCADES. NORTH OF DIAMOND LAKE JUNCTION THE COLD AIR IS DEEP
ENOUGH THAT PRECIPITATION WILL BE ALL SNOW AND SNOWFALL COULD BE
HEAVY ENOUGH TO WARRANT A HEAVY SNOW WARNING OF 4 TO 7 INCHES.
ELSEWHERE IN THE KLAMATH AND TULELAKE BASINS, WE ARE CONCERNED
WITH A WARM LAYER COMING IN BETWEEN 850-700MB LAYER. THIS WOULD
LEAD TO PERIOD OF FREEZING RAIN ACROSS THE EAST SIDE. WHILE ICE
ACCUMULATION WILL NOT BE THAT GREAT, WE ARE CONCERNED THAT THE
COMBINATION OF SNOW AND FREEZING RAIN COULD LEAD TO TRAVEL
PROBLEMS, HENCE THE BASIS FOR THE WINTER WEATHER ADVISORY FOR
TONIGHT THROUGH THURSDAY. PRECIPITATION IS EXPECTED TO TURN TO ALL
RAIN BY THURSDAY AFTERNOON.

WEST OF THE CASCADES PRECIPITATION WILL BE ALL RAIN AS THE SNOW
LEVELS WILL BE ABOVE THE LOWER PASSES. SNOW LEVELS WILL RISE TO
AROUND 6000 FEET THURSDAY AFTERNOON AND STAY ABOVE THE MAJOR PASSES
THROUGH THURSDAY NIGHT. HOWEVER THE COLD FRONT IS EXPECTED TO MOVE
THROUGH BY FRIDAY AFTERNOON AND SNOW LEVELS WILL START TO COME DOWN
AGAIN...LOWERING TO 4000 FEET OVER THE SISKIYOUS AND 5000 FEET OVER
LAKE/MODOC COUNTIES. PRECIPITATION WILL BE MAINLY SHOWERY BUT THERE
WILL BE ENOUGH OF THEM THAT WE WILL NOT GO COMPLETELY DRY DURING THE
WEEKEND.

WITH CONTINUED RAIN INTO THE COASTAL MOUNTAINS...THE COQUILLE RIVER
AT COQUILLE WILL STAY ABOVE FLOOD STAGE TONIGHT...RISING TO MAJOR
FLOOD BY FRIDAY. THE SOUTH FORK OF THE COQUILLE RIVER NEAR MYRTLE
POINT IS ALSO ABOVE FLOOD STAGE BUT THE NORTH FORK OF THE COQUILLE
RIVER NEAR MYRTLE POINT IS CLOSE AND ONLY THE FLOOD WATCH IS IN
EFFECT. /FB

LONG TERM...SUNDAY THROUGH WEDNESDAY NIGHT...
THERE IS GOOD AGREEMENT IN THE MODELS ON ANOTHER HEALTHY FRONTAL
SYSTEM, IN THE FORM OF A COLD OCCLUSION, MOVING THROUGH ON
SUNDAY. SNOW LEVEL INITIALLY IN THE 2500-3500 FOOT RANGE ARE
EXPECTED TO RISE ONLY TO 3000-4000 FEET. PRECIPITATION AMOUNTS
ALONG THE COAST LOOK AS IF THEY WILL BE IN THE 0.50-1.5" RANGE,
WITH 6 TO 12 INCHES OF SNOW ABOVE 6KFT FROM THE CASCADES WESTWARD.
EXPECT 3 TO 6 INCHES BETWEEN 3000 AND 6000 FEET ON THE WEST SIDE.
WINDS MAY KICK UP TO ADVISORY LEVELS IN SOME AREAS SUCH AS THE
SHASTA VALLEY AT WEED AND IN THE SUMMER LAKE AREA.

MONDAY THROUGH WEDNESDAY THERE IS GOOD AGREEMENT ON A VERY WET
FRONTAL SYSTEM MOVING INTO THE FORECAST AREA. WHAT THERE IS NOT
AGREEMENT ON IS SNOW LEVELS AND EXACTLY HOW WET. THE GFS BASED
GUIDANCE SURGES SNOW LEVELS UP TO 8KFT DURING THIS TIME PERIOD,
WHILE THE ECMWF PUSHES THEM ONLY TO ABOUT 6KFT, HIGHEST TUESDAY
MORNING. THE DECISION FOR TODAY'S FORECAST PACKAGE WAS TO SIDE
WITH MODEL CONSENSUS, AS BOTH MODELS INDICATE A STRONG JET STREAM
ACROSS THE GULF OF ALASKA FEEDING INTO THE PACIFIC NORTHWEST. THE
GFS INDICATES SLIGHT RIDGING OFFSHORE INTO THE COAST, WHEREAS THE
ECMWF SHOWS SLIGHT TROUGHING. WITH SUCH STRONG ZONAL FLOW, AND
CLIMATOLOGY AND EL NINO COMPOSITES FAVORING MORE TROUGHING IN THE
GULF OF ALASKA, A LEAN TOWARD THE ECMWF WAS TAKEN, ESPECIALLY FOR
DAY 7-8. THIS IS ALSO WHAT THE PREVIOUS FORECAST REFLECTED. THE
MADDEN JULIAN OSCILLATION IS ALSO ACTIVE, IN PHASE 4, WHICH
FAVORS NEAR TO COOLER THAN AVERAGE TEMPERATURES WITH ABOVE AVERAGE
PRECIPITATION FOR OUR FORECAST AREA. WE WILL NEED TO BE WARY OF
ANY TREND TOWARD THE GFS, AS IT WOULD PROBABLY MEAN LOTS OF WEST
SIDE FLOODING DUE TO RAIN ON SNOW WITH MODERATE TO HEAVY
PRECIPITATION IF IT WERE TO VERIFY. ENSEMBLES FAVOR MORE OF A
CONSENSUS SOLUTION WHICH WOULD EFFECTIVELY KEEP SNOW LEVELS NEAR
CLIMATOLOGY. PRELIMINARY CONSENSUS INDICATES AN ADDITIONAL 5-10
INCHES OF RAIN IN AND NEAR THE COAST RANGE DURING THIS TIME PERIOD
WITH 3-6 FEET OF ADDITIONAL SNOW IN THE MOUNTAINS ABOVE 6KFT.
TRAVELERS SHOULD PAY CLOSE ATTENTION TO FORECAST UPDATES IF THEY
WILL BE GOING ON ROADWAYS ABOVE ABOUT 3500 FEET DURING THIS TIME
PERIOD, AS WE EXPECT WINTER WEATHER IMPACTS, AT TIMES.

AN EARLY LOOK AT THE 24TH AND 25TH INDICATES A MORE SHOWERY
NORTHWEST FLOW PATTERN WITH SNOW LEVELS BETWEEN 2500 AND 3500
FEET. BTL

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...WINTER WEATHER ADVISORY UNTIL NOON PST THURSDAY FOR ORZ030.
     WINTER STORM WARNING UNTIL 10 AM PST THURSDAY FOR ORZ029-030.
     WINTER WEATHER ADVISORY UNTIL NOON PST THURSDAY FOR ORZ029-031.
     WINTER WEATHER ADVISORY UNTIL 10 AM PST THURSDAY FOR ORZ027-028.

CA...WINTER WEATHER ADVISORY UNTIL NOON PST THURSDAY FOR CAZ084-085.
     WINTER WEATHER ADVISORY FROM 1 AM TO NOON PST THURSDAY FOR
     CAZ082.
     WINTER WEATHER ADVISORY FROM 1 AM TO NOON PST THURSDAY FOR
     CAZ083.

PACIFIC COASTAL WATERS...GALE WATCH FROM THURSDAY AFTERNOON THROUGH LATE THURSDAY NIGHT
     FOR PZZ350-356-370-376.
     STORM WATCH FROM THURSDAY AFTERNOON THROUGH LATE THURSDAY NIGHT
     FOR PZZ350-356-370-376.
     SMALL CRAFT ADVISORY UNTIL 4 PM PST THURSDAY FOR
     PZZ350-356-370-376.
     GALE WARNING UNTIL 2 AM PST THURSDAY FOR PZZ350-356-370-376.

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