Area Forecast Discussion
Issued by NWS Pendleton, OR

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FXUS66 KPDT 240058 AAA
AFDPDT

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE PENDLETON OR
455 PM PST TUE DEC 23 2014

UPDATED AVIATION DISCUSSION

.SHORT TERM...TONIGHT THROUGH WEDNESDAY NIGHT...A COMPLEX WINTER
WEATHER STORM WILL IMPACT OUR AREA TOMORROW. A STORM SYSTEM IS
FORECAST TO WORK ITS WAY INLAND TONIGHT AND SPREAD SHOWERS TO THE
AREA FROM WEST TO EAST TOMORROW MORNING. THIS RAPIDLY MOVING SYSTEM
WILL STRIKE THE AREA MOUNTAINS WITH ACCUMULATING SNOW WEDNESDAY AND
WEDNESDAY NIGHT AND THE GREATEST IMPACT WILL BE OVER THE
BLUE/WALLOWA MOUNTAINS...SOUTHERN BLUE MOUNTAINS...JOHN DAY AREAS AS
WELL AS THE GRANDE RONDE VALLEY WHERE WINTER WEATHER
WARNINGS/ADVISORIES ARE IN PLACE.

ALTHOUGH THE SNOW MAY FIRST BEGIN AS RAIN IT SHOULD QUICKLY SWITCH
OVER TO SNOW IN THE SOUTHERN BLUES AS COLDER AIR INVADES THE AREA
LOWERING SNOW LEVELS. THE SNOW LEVELS SHOULD FALL IN THE BLUE
MOUNTAINS TO 3500 FEET ALLOWING FOR THE SNOW TO ACCUMULATE ALONG
ROAD AND HIGHWAYS SUCH AS INTERSTATE 84 IN THE MEACHAM AREA.

ANOTHER CHALLENGE COULD BE WITH WINDS AND THEY MAY BECOME BREEZY
OVER THE KITTITAS VALLEY AND SURROUNDING AREAS. ACROSS THE FOOTHILLS
OF THE BLUES A SW DOWNSLOPE WIND COULD CHALLENGE OVERNIGHT AND
MORNING TEMPERATURES KEEPING THEM FROM FALLING. MEANWHILE WINDS
SHOULD SHIFT BY MIDDAY ALLOWING FOR COOLER AIR TO FILTER INTO THE
ENTIRE AREA WEDNESDAY. DRIER AIR SHOULD DECREASE CLOUDS AND PCPN
CHANCES WEDNESDAY NIGHT. HOWEVER AN UPSLOPE WIND OVER THE AREA
MOUNTAINS WILL KEEP SNOW SHOWERS CONTINUING IN THESE AREAS INTO
THURSDAY MORNING.

.LONG TERM...CHRISTMAS THROUGH SUNDAY AFTERNOON...A FAST MOVING
UPPER LEVEL DISTURBANCE WILL HAVE EXITED THE REGION INTO IDAHO
BEFORE DAYBREAK ON CHRISTMAS FOR A CHANCE OF SNOW CHRISTMAS MORNING
IN THE MOUNTAINS WITH DECREASING POPS IN THE CENTRAL AND EASTERN
OREGON MOUNTAINS BY THE AFTERNOON OF CHRISTMAS DAY. THURSDAY NIGHT
AN UPSTREAM UPPER LEVEL RIDGE BUILDS OFFSHORE WITH THE RIDGE AXIS
NEAR 135W FOR A DRY NORTHERLY FLOW ALOFT ACROSS FORECAST AREA. THIS
PATTERN PERSISTS THROUGH FRIDAY AFTERNOON RESULTING IN DRY
CONDITIONS CONTINUING THROUGH FRIDAY AFTERNOON. DURING THE DAY ON
FRIDAY THE MODELS BEGIN TO EXHIBIT DIFFERENCES IN UPSTREAM FEATURES
AS THE ECMWF IS MORE PROGRESSIVE THAN THE GFS IN MOVING AN UPPER
LEVEL SHORTWAVE ACROSS THE GULF OF ALASKA. BOTH MODELS KEEP THE
UPPER RIDGE MORE OR LESS OFFSHORE WITH RIDGE AXIS IN GENERAL AREA OF
130-140W, WHICH KEEPS INTERIOR PACIFIC NORTHWEST IN A NORTHERLY FLOW
ALOFT AND BASICALLY DRY CONDITIONS AT LOWER ELEVATIONS AND JUST A
SLIGHT CHANCE OF SNOW SHOWERS IN THE MOUNTAINS THROUGH FRIDAY NIGHT.
WENT WITH THE NEW 12Z ECMWF RUN, WHICH SHOWS AFOREMENTIONED UPPER
LEVEL SHORTWAVE MOVING OVER THE RIDGE AND THEN DIGGING SOUTHEAST
ACROSS BRITISH COLUMBIA FRIDAY NIGHT AND THEN MOVING INTO THE
FORECAST AREA DURING THE DAY ON SATURDAY AND THEN CONTINUING TO
MOVE ACROSS FORECAST AREA SATURDAY NIGHT. RAIN AND SNOW WILL BE
LIKELY SATURDAY NIGHT, ALONG THE EAST SLOPES OF THE CENTRAL
WASHINGTON CASCADES AND IN SOUTHEAST WASHINGTON SOUTH AND EAST OF
THE TRI-CITIES AREA AND IN NORTHEAST OREGON. THE RIDGE AMPLIFIES
OFFSHORE ON SUNDAY THUS RETURNING THE REGION TO A NORTHERLY FLOW
ALOFT AND DRIER CONDITIONS IN THE LOWER COLUMBIA BASIN AND ADJACENT
VALLEYS WITH DRY CONDITIONS RETURNING IN CENTRAL AND NORTH CENTRAL
OREGON. HOWEVER, THE NORTHERLY FLOW WILL KEEP A CHANCE OF OROGRAPHIC
SNOW SHOWERS IN THE EASTERN MOUNTAINS OF OREGON AND IN THE BLUE
MOUNTAINS IN SOUTHEAST WASHINGTON THROUGH SUNDAY AFTERNOON.  POLAN

SUNDAY NIGHT THROUGH TUESDAY...A RIDGE OF HIGH PRESSURE ALONG THE
COAST WILL CAUSE NORTHERLY FLOW OVER THE AREA. ON SUNDAY NIGHT...
THERE MAY BE A FEW LINGERING SNOW SHOWERS OVER THE EASTERN
MOUNTAINS...BUT IT WILL OTHERWISE BE COLD AND DRY GOING INTO MONDAY.
TEMPERATURES WILL BE BELOW NORMAL ON MONDAY WITH HIGHS IN THE MID
20S TO MID 30S. BOTH THE GFS AND ECMWF THEN BRING A WEAK DISTURBANCE
OVER THE RIDGE ON MONDAY NIGHT AND TUESDAY...BUT DETAILS VARY
GREATLY. HAVE THEREFORE ADDED A SLIGHT CHANCE OF LIGHT SNOW IN THE
MOUNTAINS TO ACKNOWLEDGE THIS FEATURE...ALTHOUGH CONFIDENCE IS LOW.
TEMPERATURE FORECASTS WERE A STRUGGLE DURING THIS PERIOD WITH MODELS
WANTING TO WARM THE AREA WELL ABOVE THE PREVIOUS FORECAST. HAVE
THEREFORE ACCOUNTED FOR SOME WARMING ON TUESDAY...BUT NOT AS MUCH AS
THE GFS AND ECMWF JUST YET. IT IS MORE LIKELY THAT DUE TO HIGH
PRESSURE AND NORTHERLY FLOW...COLD AIR WILL BECOME TRAPPED IN THE
VALLEYS AND IT WILL TAKE A MUCH STRONGER DISTURBANCE TO SCOUR IT
OUT.  COBB

&&

.AVIATION...00Z TAFS...VFR CONDITIONS WILL OCCUR THROUGH LATE
TONIGHT. FEW-SCT MID LEVELS CLOUDS AROUND 5000-6000 FEET AGL AT KRDM
AND KBDN AND BKN CLOUDS AT 6000-8000 FT AGL INITIALLY AT OTHER TAF
SITES. LATE TONIGHT AND WEDNESDAY A FAST MOVING COLD FRONT WILL
SWEEP ACROSS THE FORECAST AREA. INCREASING CLOUDS AND LOWERING CIGS
WILL OCCUR TONIGHT AHEAD OF THE COLD FRONT. KDLS WILL DEVELOP A MVFR
CEILING AND VISIBILITY BY 12Z/24TH. SHOWERS SHOULD BEGIN AT KYKM AND
KDLS BY 12Z/24TH. KYKM MAY HAVE IFR CIGS AND VISIBILITY FROM 05Z-09Z
TONIGHT. AT OTHER TAF SITES SHOWERS SHOULD HOLD OFF OFF UNTIL AFTER
13Z/24TH. WINDS WILL REMAIN BELOW 10 KTS FOR NEXT 24 HOURS, EXCEPT
THAT SUSTAINED WINDS AT KPDT WILL INCREASE TO 12 KTS WITH GUSTS TO
20 KTS AT THE TIME OF FROPA AROUND 16Z/24TH WITH THESE WIND
CONDITIONS CONTINUING UNTIL 20Z/24TH, WHEN WINDS WILL DIMINISH
SOMEWHAT IN THE AFTERNOON.  POLAN

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  38  45  30  37 /  20  90  70  10
ALW  38  46  30  36 /  20  90  70  10
PSC  36  43  29  39 /  20  80  50  10
YKM  34  40  26  37 /  30  70  30  10
HRI  37  44  30  41 /  20  80  60  10
ELN  33  36  26  40 /  30  80  30  10
RDM  34  42  24  35 /  10  90  60  10
LGD  31  37  23  35 /  10  90  80  30
GCD  30  35  27  38 /  10  90  80  30
DLS  40  43  32  43 /  30  80  50  10

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...WINTER STORM WARNING FROM 10 AM WEDNESDAY TO 4 AM PST THURSDAY
     ORZ506.

     WINTER STORM WARNING FROM 4 AM WEDNESDAY TO 4 AM PST THURSDAY
     ORZ502.

     WINTER WEATHER ADVISORY FROM 10 AM WEDNESDAY TO 4 AM PST
     THURSDAY ORZ505.

     WINTER STORM WARNING FROM 10 AM WEDNESDAY TO 4 AM PST THURSDAY
     ORZ503.

     WINTER WEATHER ADVISORY FROM 10 AM WEDNESDAY TO 4 AM PST
     THURSDAY ORZ049.

     WINTER WEATHER ADVISORY FROM 10 AM WEDNESDAY TO 4 AM PST
     THURSDAY ORZ508.

     WINTER WEATHER ADVISORY FROM 10 AM WEDNESDAY TO 4 AM PST
     THURSDAY ORZ050.

WA...WINTER STORM WARNING FROM 4 AM WEDNESDAY TO 4 AM PST THURSDAY
     WAZ030.

&&

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

$$

97/99/82/99





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