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000
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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