Heavy Snow in Oregon, January 4 to 5, 2015

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PENDLETON OR
300 AM PST SUN JAN 4 2015

.SHORT TERM...TODAY THROUGH MONDAY NIGHT...A COMPLEX WINTER STORM
WILL BE AFFECTING THE AREA THROUGH MUCH OF THE SHORT TERM
PERIOD...BRINGING WINTER PRECIPITATION AND A PERIOD OF STRONG WINDS.
WE HAVE MULTIPLE WINTER HEADLINES IN EFFECT THROUGH 06/12Z MONDAY.
THE STORM SYSTEM IS CURRENTLY MOVING ONSHORE OVER WESTERN WASHINGTON
AND OREGON...AND IS ALREADY BEGINNING TO SPREAD MOISTURE OVER THE
AREA. SNOW IS CURRENTLY FALLING ALONG THE EAST SLOPES OF THE
WASHINGTON CASCADES IN EASTON AND AT SNOQUALMIE PASS. DECIDED TO
UPGRADE THE EAST SLOPES OF THE WASHINGTON CASCADES TO A WINTER STORM
WARNING WITH THIS PACKAGE AS PERIODS OF HEAVY SNOW WILL LIKELY
AFFECT THIS REGION FOR A 12 TO 18 HOUR PERIOD TODAY. CONTEMPLATED
UPGRADING THE NORTHERN BLUE MOUNTAINS OF OREGON TO A WINTER STORM
WARNING...BUT THERE WAS TOO MUCH UNCERTAINTY...ESPECIALLY SINCE SNOW
LEVELS ARE FORECAST TO RAPIDLY RISE THIS EVENING IN THE INTERSTATE
84 CORRIDOR AND MEACHAM AREA. LEFT ALL OTHER WINTER HEADLINES AS IS.
WITH MANY LOCATIONS IN NORTHERN OREGON AND SOUTHEASTERN WASHINGTON
BELOW FREEZING AT THE SURFACE AND WITH VARYING DEPTHS OF
SUB-FREEZING AIR IN PLACE OVER THESE REGIONS EXPECT A MIXTURE OF
SNOW...SLEET...FREEZING RAIN AND PLAIN RAIN TO AFFECT THE AREA
THROUGH THIS EVENING BEFORE SNOW LEVELS RISE CHANGING MOST LOCATIONS
OVER TO RAIN. THERE IS A CONCERN FOR LIGHT FREEZING RAIN OR SLEET TO
LINGER IN THE YAKIMA VALLEY AND THE COLUMBIA BASIN INTO THE EVENING
HOURS...BUT THIS WAS ALREADY WELL COVERED IN THE WINTER WEATHER
ADVISORY. NEXT CONCERN WILL BE THE INCREASING WINDS TONIGHT INTO
MONDAY. WE WILL BE ISSUING SEVERAL WIND ADVISORIES WITH THIS UPDATE
FOR LOCATIONS IN NORTHERN OREGON AND MUCH OF SOUTHEAST WASHINGTON.
THE HIGHEST WINDS WILL LIKELY OCCUR FROM LATE THIS EVENING INTO
MONDAY AFTERNOON...WITH PEAK GUSTS OF 40 TO 55 MPH POSSIBLE.
GUIDANCE IS INDICATING VERY STRONG WINDS JUST ABOVE THE
SURFACE...BUT ALSO INDICATES THAT THESE WINDS WILL HAVE A TOUGH TIME
MIXING ALL THE WAY DOWN TO THE SURFACE DUE TO A STUBBORN STABLE
LAYER.WILL NEED TO WATCH THIS CAREFULLY TO SEE IF ANY SUBSTANTIAL
DOWNWARD MOTION CAN IN FACT DRAG MORE INTENSE WINDS DOWN TO THE
SURFACE. 77

.LONG TERM...TUESDAY THROUGH SUNDAY...MODELS ARE STILL IN GOOD
AGREEMENT THAT AN UPPER RIDGE WILL DOMINATE THE WEATHER FOR AT LEAST
THE BEGINNING OF THE EXTENDED FORECAST PERIOD. HIGH PRESSURE ALOFT
WILL AMPLIFY WITH THE RIDGE AXIS OVER THE CWA. THIS WILL BRING MILD
AND DRY CONDITIONS WITH MODERATELY HIGH SNOW LEVELS TO THE CWA ON
TUESDAY THROUGH THURSDAY. THERE WILL LIKELY BE SURFACE BASED
INVERSIONS IN THE LOWER COLUMBIA BASIN AND SURROUNDING VALLEYS WHICH
MAY CAUSE FOG AND LOW CLOUDS TO DEVELOP AND PERSIST. THIS WILL TEND
TO KEEP DAYTIME TEMPERATURES DOWN AND NIGHT TIME TEMPERATURES UP.
ELSEWHERE...WHERE IT IS FOG/CLOUD FREE TEMPERATURE SWINGS BETWEEN
DAY AND NIGHT WILL BE GREATER. IN FACT DAYTIME HIGHS IN THE
MOUNTAINS MAY ACTUALLY BE WARMER THAN IN THE AREAS WITH LOW CLOUDS
AND FOG (I.E. THE BASIN AND SURROUNDING VALLEYS). SNOW LEVELS WILL
RANGE FROM AROUND 4500-5500 FEET NORTH AND 6000-7000 FEET SOUTH...
LOWERING ABOUT 1000 FEET AT EACH NIGHT. THE GFS IS HINTING AT A WEAK
BACK DOOR COLD FRONT FROM THE NORTHEAST. THIS WILL ALSO TEND TO KEEP
TEMPERATURES COOLER THAN WHAT THEY COULD NORMALLY GET TO UNDER A
HIGH PRESSURE RIDGE. THE NEXT WEATHER SYSTEM TO AFFECT THE CWA WILL
ARRIVE THURSDAY NIGHT INTO FRIDAY WITH AN UPPER PACIFIC TROUGH
SWINGING ACROSS THE PACIFIC NORTHWEST IN A NEGATIVE TILT. THIS WILL
BRING MOSTLY RAIN TO THE FORECAST AREA...WITH SNOW CONFINED TO THE
HIGHEST PEAKS. THIS SYSTEM WILL LINGER THROUGH SATURDAY WITH PERIODS
OF VALLEY RAIN AND HIGHER MOUNTAIN SNOW. HIGH PRESSURE WILL REBUILD
OVER THE REGION SATURDAY NIGHT INTO SUNDAY. HOWEVER MOISTURE MOVING
OVER THE TOP OF THE RIDGE WILL KEEP A SLIGHT CHANCE TO CHANCE OF
VALLEY RAIN AND HIGHER MOUNTAIN SNOW SHOWERS IN THE FORECAST. DUE TO
INCREASING MODEL DIFFERENCES THIS FAR OUT HAVE NOT MADE TOO MANY
CHANGES TO THE EXTENDED FORECAST PERIOD THAT FAR OUT. 88

&&

.AVIATION...12Z TAFS...MVFR OR IFR CONDITIONS WILL PERSIST AT KPDT
AND KALW DUE TO LOW CIGS AS WELL AS REDUCED VISIBILITY IN AREAS OF
FOG. PRECIPITATION IN THE FORM OF SNOW...SLEET OR FREEZING RAIN WILL
DEVELOP ACROSS THE AREA THIS MORNING WHICH MAY CAUSE OTHER TAF SITES
TO DROP TO MVFR OR IFR CONDITIONS AS WELL. THESE CONDITIONS WILL
PREVAIL THROUGH THE DAY INTO THIS EVENING WITH IMPROVEMENT AFTER 06Z
TONIGHT. WINDS WILL REMAIN LIGHT AND VARIABLE TODAY BUT WILL BEGIN
TO INCREASE FROM THE SOUTHWEST TONIGHT WITH WIND SPEEDS OF AT LEAST
15 TO 25 KTS BECOMING LIKELY AT MOST TAF SITES. 88

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  38  39  49  40 /  80  90  60  30
ALW  38  40  51  43 /  90  90  60  40
PSC  32  32  51  40 /  70  70  40  30
YKM  30  30  47  37 /  60  60  50  30
HRI  35  36  52  40 /  70  70  40  20
ELN  29  29  46  38 /  80  80  50  30
RDM  38  36  48  34 /  30  40  20  10
LGD  33  32  45  38 /  90  90  70  40
GCD  37  34  48  39 /  70  70  50  20
DLS  41  34  51  41 /  60  70  50  20

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...WIND ADVISORY FROM 10 PM THIS EVENING TO 3 PM PST MONDAY
     ORZ044-507-510.

     WINTER STORM WARNING UNTIL 4 AM PST MONDAY ORZ050.

     WINTER WEATHER ADVISORY UNTIL 7 PM PST THIS EVENING ORZ044-507.

     WINTER WEATHER ADVISORY UNTIL 10 PM PST THIS EVENING ORZ502.

     WINTER WEATHER ADVISORY UNTIL 10 PM PST THIS EVENING ORZ049.

     WIND ADVISORY FROM 10 PM THIS EVENING TO 3 PM PST MONDAY ORZ508.

WA...WIND ADVISORY FROM 10 PM THIS EVENING TO 3 PM PST MONDAY
     WAZ028-029-521.

     WINTER STORM WARNING UNTIL 10 PM PST THIS EVENING WAZ030.

     WINTER WEATHER ADVISORY UNTIL 7 PM PST THIS EVENING WAZ521.

     WINTER WEATHER ADVISORY UNTIL 7 PM PST THIS EVENING WAZ029.

     WINTER WEATHER ADVISORY UNTIL 10 PM PST THIS EVENING WAZ028.

     WINTER STORM WARNING UNTIL 10 PM PST THIS EVENING WAZ520.

     WINTER STORM WARNING UNTIL 10 PM PST THIS EVENING WAZ026.

     WINTER WEATHER ADVISORY UNTIL 10 PM PST THIS EVENING WAZ027.

     WIND ADVISORY FROM 10 PM THIS EVENING TO 3 PM PST MONDAY WAZ520.

&&

FOR ADDITIONAL WEATHER INFORMATION, CHECK OUR WEB SITE AT...
WWW.WEATHER.GOV/PENDLETON

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