FXUS66 KPDT 052354 AAA
AFDPDT

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE PENDLETON OR
355 PM PST SAT DEC 5 2009

UPDATED AVIATION DISCUSSION

.SHORT TERM...TONIGHT THROUGH MONDAY NIGHT...AN UPPER LOW OVER 
BRITISH COLUMBIA CONTINUES TO MOVE SOUTH THIS AFTERNOON. THIS LOW 
WILL DROP INTO THE PACIFIC NORTHWEST TO A POSITION OVER WESTERN 
OREGON BY DAYBREAK SUNDAY. AS IT MOVES SOUTH IT WILL PULL AN AREA OF 
MOISTURE OUT OF SOUTHWESTERN CANADA AND WRAP IT WEST AND SOUTH 
TOWARDS THE CASCADES. THIS SYSTEM WILL ALSO TAP SOME MOISTURE 
LINGERING OVER IDAHO AND MONTANA. THUS EXPECT VARIOUS CHANCES OF 
PRECIP AS THIS MOISTURE MOVES ACROSS THE AREA TONIGHT AND SUNDAY. SO 
FAR PRECIP WITH THE MOISTURE IN CANADA APPEARS TO BE LIMITED BASED 
ON OBSERVATIONS...WHILE SOME LIGHT SNOW HAS BEEN FALLING OVER 
PORTIONS OF WESTERN MONTANA. THUS DO NOT EXPECT THIS SYSTEM TO BE 
PARTICULARLY EFFICIENT IN CREATING PRECIP AMOUNTS. YET WITH LOW 
LEVEL WINDS BECOMING EASTERLY TONIGHT INTO EARLY SUNDAY...DECENT 
CHANCES OF SNOW WILL BE POSSIBLE OVER THE WASHINGTON CASCADE EAST 
SLOPES...THE YAKIMA AND KITTITAS VALLEYS... AND ALONG THE EAST 
FACING SLOPES OF THE BLUE AND WALLOWA MOUNTAINS. LOCAL NORTH TO 
NORTHEAST FLOW DURING THIS PERIOD WILL ALSO ALLOW FOR THE BETTER 
CHANCES TO SPREAD INTO PORTIONS OF NORTH-CENTRAL AND CENTRAL OREGON 
LATE TONIGHT AND ON SUNDAY. OVERALL SNOW AMOUNTS IN ALL THESE AREAS 
ARE EXPECTED TO REMAIN BELOW ADVISORY LEVELS THROUGH SUNDAY 
AFTERNOON. THE UPPER LOW WILL THEN SWING THROUGH NORTHERN CALIFORNIA 
AND THE GREAT BASIN SUNDAY NIGHT THROUGH MONDAY NIGHT. AS A RESULT 
WILL SEE PRECIP END FROM THE NORTH...THOUGH SOME SNOW SHOWERS WILL 
PERSIST OVER CENTRAL OREGON UNTIL TUESDAY DUE TO CONTINUED UPSLOPE 
FLOW. THIS SYSTEM WILL ALSO PULL EVEN COOLER AIR INTO THE REGION 
FROM CENTRAL CANADA ON SUNDAY. AS A RESULT WILL SEE SOME RATHER COLD 
TEMPS SUNDAY NIGHT THROUGH MONDAY NIGHT. 90 

.LONG TERM...TUESDAY THROUGH SATURDAY...AN ELONGATED UPPER LEVEL 
RIDGE OFFSHORE WILL RESULT IN A COLD NORTHERLY FLOW ALOFT ACROSS 
EASTERN WA/OR THROUGH MOST OF THE EXTENDED PERIOD. BELIEVE THE AIR 
MASS WILL REMAIN COLD AND FAIRLY DRY...THUS HAVE REMOVED OR LOWERED 
POPS AS WELL AS LOWERED TEMPERATURES TUES THROUGH THURS NIGHT. WILL 
KEEP 20 PERCENT POPS IN THE CENTRAL AND EASTERN OREGON MOUNTAINS 
WHERE THERE IS SOME OROGRAPHIC LIFT AND LIMITED MOISTURE. THE COLD 
AIR ALOFT WILL INHIBIT RADIATIONAL INVERSION AT NIGHT...THUS I 
EXPECT VERY LITTLE...IF ANY...FOG OR STRATUS DEVELOPMENT. THE GFS 
DIFFERS FROM THE ECMWF SHOWING MOISTURE ASSOCIATED WITH A PACIFIC 
LOW NEAR 40N/135W SPREADING INTO SW OREGON THURS NIGHT AND A WARM 
FRONT BRINGING WINTER PRECIPITATION ACROSS OREGON AND SOUTHERN 
WASHINGTON ON FRI. THE ECMWF IS MUCH SLOWER AND DOES NOT ADVERTISE 
PRECIPITATION UNTIL SAT. FOR NOW...WILL KEEP SLIGHT CHANCE POPS FOR 
FRI BUT WILL INCREASE TO AROUND 20-40 PERCENT SAT AND SUN. JOHNSON 

&&

.AVIATION...00Z TAFS. THE COLD FRONT THAT SWEPT ACROSS THE REGION 
LAST NIGHT HAS HELPED TO STIR THE AIR MASS WITH SOME IMPROVEMENT IN 
THE STRATUS CLOUDS AND FOG.  CIGS WILL VARY FROM VFR TO MVFR AT KALW 
AND KPDT FOR THE NEXT COUPLE OF HOURS. THE NEXT SYSTEM TO ARRIVE 
TONIGHT WILL CONTINUE TO PROVIDE MIXING ACROSS THE REGION...AND I 
WOULD EXPECT CIGS TO LIFT TO VFR AT ALL TAF SITES WITH THE EXCEPTION 
OF KRDM WHERE LOW LEVEL OROGRAPHIC LIFT WILL RESULT IN CIGS/VSBY 
FALLING BELOW VFR ALONG WITH LIGHT -SHSN AFTER MIDNIGHT. WINDS WILL 
BE FROM N-NE 5-15 KT THROUGH 18Z SUNDAY. JOHNSON 

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  18  27  11  20 /  40  30  10   0 
ALW  21  28  14  22 /  40  20  10   0 
PSC  15  31   9  26 /  20  10   0   0 
YKM  16  29   8  25 /  60  20   0   0 
HRI  16  30   7  25 /  20  20  10   0 
ELN  15  28   7  23 /  60  20  10   0 
RDM  13  24   4  19 /  50  40  40  20 
LGD  18  27   9  23 /  40  30  20  10 
GCD  15  27   7  23 /  30  50  30  20 
DLS  23  31  15  26 /  60  30  10   0 

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...NONE.
WA...NONE.
&&

THREAT INDEX
SUNDAY : GREEN
MONDAY : GREEN
TUESDAY : GREEN

GREEN: NONE OR LIMITED NEED FOR WATCHES, WARNINGS, OR ADVISORIES.
YELLOW: A FEW WATCHES, WARNINGS, OR ADVISORIES POSSIBLE OR IN EFFECT.
RED: NUMEROUS WATCHES, WARNINGS, OR ADVISORIES POSSIBLE OR IN EFFECT.

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

$$

90/85/85