286 
FXUS66 KPDT 231803 AAA
AFDPDT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PENDLETON OR
1058 AM PDT THU MAY 23 2013

UPDATED THE AVIATION DISCUSSION

.SHORT TERM...TODAY AND TONIGHT...A LARGE UPPER LEVEL LOW PRESSURE 
SYSTEM WILL REMAIN OVER THE REGION WITH UNUSUALLY COLD AIR ALOFT 
ASSOCIATED WITH IT. THIS COLD AIR ALOFT COMBINED WITH THE SURFACE 
HEATING WILL CREATE AN UNSTABLE ATMOSPHERE DURING THE AFTERNOON AND 
EVENING RESULTING IN SHOWERS AND SOME ISOLATED THUNDERSTORMS. THIS 
HAS BEEN THE PATTERN THE PAST COUPLE OF DAYS AND WILL BE THE CASE 
AGAIN TODAY. THE INSTABILITY SHOWERS WILL DECREASE AROUND SUNSET. 
MODELS ARE ALSO SUGGESTING AN AREA OF MORE STEADY PRECIPITATION 
WRAPPING AROUND THE LOW PRESSURE INTO CENTRAL OREGON LATE TODAY AND 
OVERNIGHT. THIS COULD PROVIDE SOME OVERNIGHT STEADY PRECIPITATION TO 
THIS PORTION OF THE FORECAST AREA NOT RELIANT ON THE UNSTABLE 
CONDITIONS DISCUSSED PREVIOUSLY. TEMPERATURES WILL ALSO REMAIN COOL 
AND BELOW NORMAL FOR THIS TIME OF YEAR WITH SNOW LEVELS BETWEEN 4000 
AND 5000 FEET. CONVECTIVE BURSTS OF SNOW ASSOCIATED WITH TODAYS 
CONVECTION ARE POSSIBLE WHICH COULD RESULT IN SNOW LEVELS BELOW 4000 
FEET UNDER CONVECTIVE CELLS. NOT EXPECTING THE NEED FOR ANY FREEZE 
WARNINGS MAINLY DUE TO ANTICIPATED CLOUD COVER OVERNIGHT. 
HOWEVER...IF SOME PROLONGED CLEARING TAKES PLACE OVERNIGHT IT IS 
POSSIBLE FOR TEMPERATURES TO SPIKE DOWN RAPIDLY UNDER THIS WEATHER 
PATTERN. 

.AVIATION...A LOW PRESSURE SYSTEM IS FORECAST TO MOVE INLAND AND 
INCREASE CLOUDS AND SHOWERS ACROSS THE AREA INTO TONIGHT. MVFR 
CEILINGS WILL CONTINUE AT TAF SITE KDLS IN AND NEAR SHOWERS. 
OTHERWISE DETERIORATING CEILINGS CAN BE EXPECTED AT TAF SITES KYKM 
AND KRDM THAT WILL LOWER TO AREAS OF MVFR CONDITIONS AFTER 22Z. 
AFTER 02Z THE REMAINDER OF THE TAF SITES SHOULD EXPERIENCE MVFR 
CEILINGS IN SHOWERS INTO FRIDAY MORNING. 

&&

.PREV DISCUSSION... /ISSUED 155 AM PDT THU MAY 23 2013/ 

SHORT TERM...TODAY THROUGH FRIDAY NIGHT...THE MODELS ARE IN VERY 
GOOD AGREEMENT WITH THE POSITION AND STRENGTH OF A LARGE UPPER LOW 
PRESSURE SYSTEM OVER THE REGION THAT HAS BEEN PROVIDING SIGNIFICANT 
PRECIPITATION. THE FORECAST MODELS REMAIN IN GOOD AGREEMENT THROUGH 
TONIGHT AND THEN THEY BEGIN TO DIVERGE FROM EACH OTHER ON FRIDAY. IN 
ANY EVENT THEY ALL KEEP AN UPPER LOW OR AN UPPER TROUGH OVER THE 
PACIFIC NORTHWEST THROUGH THE SHORT TERM PERIOD. THIS WILL KEEP A 
COOL AND UNSETTLED WEATHER REGIME OVER THE FORECAST AREA FOR THE 
SHORT TERM FORECAST PERIOD. TODAY THE CENTER OF THE UPPER LOW IS 
EXPECTED TO PIVOT BACK AROUND TO THE WEST AND THEN DROP SOUTHWARD 
WEST OF THE CASCADES TONIGHT AND THEN EASTWARD AGAIN. THIS WILL 
BRING A SLUG OF MOISTURE BACK WESTWARD AND THEN SOUTHWARD AND 
EASTWARD ACROSS SOUTHERN PORTIONS OF THE FORECAST AREA. THE SOUTHERN 
FORECAST AREA HAS SEEN THE LEAST AMOUNT OF PRECIPITATION SO FAR WITH 
THIS SYSTEM. THERE IS A POTENTIAL THAT THESE AREAS (CENTRAL OREGON 
AND THE OCHOCO-JOHN DAY HIGHLANDS) COULD SEE MORE SIGNIFICANT 
PRECIPITATION BEGINNING THIS AFTERNOON AND CONTINUING THROUGH 
TONIGHT INTO FRIDAY MORNING. THE ATMOSPHERE IS FORECAST TO BECOME 
UNSTABLE THROUGH MUCH OF THE FORECAST AREA FROM THE EASTERN 
MOUNTAINS ACROSS THROUGH CENTRAL OREGON AND UP ALONG THE CASCADE 
EAST SLOPES INTO SOUTH CENTRAL AND CENTRAL WASHINGTON. HAVE DECIDED 
TO INCLUDE ISOLATED THUNDERSTORMS IN MOST AREAS OF THE CWA FOR THIS 
AFTERNOON AND THIS EVENING EXCEPT THE LOWER COLUMBIA BASIN AND THE 
BLUE MOUNTAIN FOOTHILLS. THE ATMOSPHERE BECOMES UNSTABLE AGAIN 
FRIDAY AFTERNOON AND ALTHOUGH NOT AS WIDESPREAD HAVE ALSO INCLUDED A 
SLIGHT CHANCE OF THUNDERSTORMS AGAIN IN MUCH OF THE SAME AREAS 
FRIDAY AFTERNOON AND EVENING. THE UPPER LOW BECOMES ELONGATED FROM 
SOUTHWEST TO NORTHEAST LATE FRIDAY NIGHT AND EARLY SATURDAY 
RESULTING IN LESS ORGANIZATION OF THE OVERALL SYSTEM. THE RESULT 
WILL BE DECREASING COVERAGE OF PRECIPITATION ACROSS THE FORECAST 
AREA FRIDAY NIGHT. TEMPERATURES WILL REMAIN COOLER THAN NORMAL OF 
ABOUT 8 TO 12 DEGREES ACROSS THE AREA...ESPECIALLY DAYTIME HIGHS. 
FREEZE WARNINGS ARE IN EFFECT THIS MORNING FOR THE SOUTHERN BLUE 
MOUNTAIN FOOTHILLS AND THE GRANDE RONDE VALLEY. THERE MAY BE SOME 
SNOW IN THE SIMCOE HIGHLANDS AND IN THE UPPER CASCADE EAST SLOPES 
BUT AT THIS TIME SNOW AMOUNTS WILL BE UNDER ADVISORY CRITERIA IN 
THESE AREAS. 88

LONG TERM...SATURDAY THROUGH THURSDAY...THE UPPER LOW OVER THE AREA 
WILL LIFT INTO SOUTHERN CANADA OVER THE WEEKEND WITH SLIGHTLY LOWER 
CHANCES FOR SHOWERS. HOWEVER...ANOTHER LOW WILL DROP SEWD INTO THE 
REGION FOR MUCH OF NEXT WEEK WITH INCREASING CHANCES FOR 
SHOWERS...WITH THE BEST CHANCE LOOKING TO BE LATE MONDAY THROUGH 
TUESDAY. THERE COULD BE AN ISOLATED THUNDERSTORM OR TWO THROUGH THE 
PERIOD BUT AS OF NOW IT DOES NOT LOOK LIKE THE ACTIVITY WILL BE TOO 
SIGNIFICANT. TEMPERATURES WILL BE AROUND NORMAL FOR THIS TIME OF 
YEAR WHICH EQUATES TO 70S FOR HIGHS LOWER ELEVATIONS AND 50S AND 60S 
MOUNTAINS. 

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  61  42  62  41 /  30  30  50  20 
ALW  64  42  63  44 /  30  30  40  20 
PSC  67  43  67  42 /  20  20  40  20 
YKM  60  42  63  43 /  20  20  40  30 
HRI  66  44  66  42 /  20  20  50  20 
ELN  58  38  60  42 /  30  30  40  30 
RDM  56  38  56  32 /  30  60  40  20 
LGD  58  35  55  36 /  30  40  50  20 
GCD  56  34  54  32 /  30  40  50  20 
DLS  61  44  64  45 /  20  60  50  20 

&&

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

THREAT INDEX
TODAY : GREEN
FRIDAY : GREEN
SATURDAY : 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

$$

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