000
FXUS66 KPDT 221529
AFDPDT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PENDLETON OR
830 AM PDT WED MAY 22 2013

.SHORT TERM...TODAY THROUGH THURSDAY NIGHT...DEEP CLOSED UPPER
LEVEL LOW PRESSURE AREA OVER SOUTH CENTRAL WASHINGTON THIS MORNING
WILL GRADUALLY FILL THROUGH THURSDAY NIGHT AS IT MEANDERS AROUND THE
PACIFIC NORTHWEST.  HAVE LOWERED TEMPERATURES 2-4 DEGREES IN
WASHINGTON AND NEAR THE COLUMBIA RIVER IN OREGON DUE TO THE
PROXIMITY OF THE LOW. TODAYS TEMPERATURES WILL BE A RECORD LOW
MAXIMUM IN AREAS UNDERNEATH THE UPPER LOW.

UNSTABLE CONDITIONS WITH DAYTIME HEATING UNDER THE LOW WILL CAUSE
CLOUDS TO DEVELOP WITH SCATTERED SHOWERS AND ISOLATED THUNDERSTORMS.
SHOWERS WILL BE MORE PREVALENT IN UPSLOPE AREA NEAR THE CASCADES.
UNSTABLE CONDITIONS ON THURSDAY WILL GIVE ANOTHER ROUND OF SHOWERS
AND TEMPERATURES SHOULD BE WARMER WITH THE UPPER LOW OVER NW
WASHINGTON.

.LONG TERM...


&&

.AVIATION...


&&

.PREV DISCUSSION... /ISSUED 315 AM PDT WED MAY 22 2013/

SHORT TERM...TODAY THROUGH THURSDAY NIGHT...A LARGE MID/UPPER LEVEL
LOW WILL LINGER OVER THE PACIFIC NORTHWEST THROUGH THURSDAY NIGHT
THUS KEEPING UNSETTLED CONDITIONS. SNOW LEVELS THIS MORNING ARE
AROUND 2700-3500 FEET. SNOW ACCUMULATIONS THROUGH THIS MORNING OF
2-4 INCHES ARE EXPECTED IN THE BLUE MOUNTAINS ABOVE 4000 FEET. AS
SUCH A WINTER WEATHER ADVISORY FOR SNOW ACCUMULATIONS OF 2-4 INCHES
IS IN EFFECT FOR THE BLUE MOUNTAINS UNTIL 1000 AM PDT THIS MORNING.
THE UPPER LOW WILL WOBBLE AND REMAIN QUASI-STATIONARY OVER THE
SOUTHERN WASHINGTON CASCADES TODAY AND KEEP PERIODS OF RAIN SHOWERS
IN KITTITAS, YAKIMA, KLICKITAT, AND BENTON COUNTIES UNTIL LATE THIS
AFTERNOON. THE CENTER OF THE LOW RETROGRADES SLIGHTLY WESTWARD
TONIGHT WITH THE CENTER NEAR OLYMPIA, WASHINGTON BY AROUND DAYBREAK
THURSDAY. THIS WILL RESULT IN LESS DYNAMIC LIFT EAST OF THE CASCADES
AND THUS A REDUCTION IN POPS THROUGH THURSDAY MORNING. THEN DAYTIME
SURFACE HEATING WILL HELP TO DESTABILIZE THE BOUNDARY LAYER
THURSDAY AFTERNOON YIELDING INCREASING POPS OVER THE MOUNTAINS.
HOWEVER, THE LOWER COLUMBIA BASIN AND THE YAKIMA VALLEY WILL LIKELY
REMAIN DRY DUE TO SINKING AIR ASSOCIATED WITH WESTERLY FLOW ALOFT
EAST OF THE CASCADES. THURSDAY NIGHT SNOW LEVELS WILL BE AROUND
4000-5000 FEET RESULTING IN SNOW SHOWERS IN THE HIGHER MOUNTAINS
WITH RAIN SHOWERS AT LOWER ELEVATIONS.

STEEP TEMPERATURE LAPSE RATES AND DYNAMIC FORCING ASSOCIATE WITH THE
UPPER LOW THIS AFTERNOON AND THURSDAY AFTERNOON WILL YIELD MARGINAL
INSTABILITY FOR A SLIGHT CHANCE OF THUNDERSTORMS IN PORTIONS OF THE
FORECAST AREA IN THE AFTERNOON AND EARLY EVENING TODAY AND THURSDAY.
POLAN

LONG TERM...FRIDAY THROUGH WEDNESDAY...A LARGE UPPER LOW PRESSURE
SYSTEM WILL KEEP UNSETTLED SHOWERY AND COOL CONDITIONS OVER THE
FORECAST AREA THROUGH THE WEEKEND INTO EARLY NEXT WEEK. HOWEVER THE
LONGER RANGE MODELS SUCH AS THE GFS AND ECMWF SHOW INCREASING
DIVERGENCE WITH EACH OTHER AS TIME GOES ON. NEVERTHELESS THEY BOTH
SHOW AN OVERALL TROUGH PATTERN OVER THE PACIFIC NORTHWEST WITH THE
ECMWF SHOWING A CLOSED LOW THROUGH AT LEAST MONDAY AND THEN IT OPENS
UP TO A TROUGH ON TUESDAY AND WEDNESDAY ALONG WITH THE GFS. THIS
PATTERN WILL KEEP RELATIVE COOL AND SHOWERY CONDITIONS OVER THE
FORECAST AREA. HOWEVER AS THE UPPER LOW/TROUGH WEAKENS OVER THE
WEEKEND TEMPERATURES WILL BEGIN TO MODERATE BACK INTO THE 70S OR
LEVELS SLIGHTLY ABOVE NORMAL. IN THE MEAN TIME THEY WILL BE SLIGHTLY
BELOW NORMAL...ESPECIALLY FRIDAY AND SATURDAY AND THEN A WARMING
TREND WILL CONTINUE ON SUNDAY THROUGH MONDAY. THEN THEY LEVEL OFF IN
THE LOWER TO MID 70S ON TUESDAY AND WEDNESDAY IN THE LOWER
ELEVATIONS. TEMPERATURES WILL BE COOLER AND MOSTLY IN THE 40S AND
50S TO BEGIN WITH IN THE MOUNTAINS...WARMING TO THE MID 50S TO MID
60S BY EARLY NEXT WEEK. THE ATMOSPHERE WILL HAVE SOME INSTABILITY
EACH AFTERNOON AND IT WILL NOT TAKE MUCH SUNSHINE AND HEATING TO
PRODUCE A SLIGHT CHANCE OF THUNDERSTORMS MOST AFTERNOONS THROUGH AT
LEAST SUNDAY. THEN THE ATMOSPHERE BEGINS TO BECOME MORE STABLE AS
THE UPPER TROUGHS/LOW CONTINUES TO WEAKEN AND BECOME LESS ORGANIZED.
88

AVIATION...12Z TAFS...VFR CONDITIONS AND OCCASIONALLY MVFR
CONDITIONS WILL PREVAIL TODAY AND TONIGHT WITH AN UPPER LOW PRESSURE
SYSTEM THAT WILL CAUSE NUMEROUS RAIN SHOWERS AND POSSIBLE AFTERNOON
THUNDERSTORMS. VISIBILITIES COULD BE REDUCED TO BELOW 6 MILES AT
TIMES DURING SOME OF THE HEAVIER RAIN SHOWERS AND CIGS BELOW 030 AT
TIMES AT ALL TAF SITES. WINDS WILL ALSO BE BREEZY AND GENERALLY 10
TO 20 KTS AT MOST TAF SITES THIS AFTERNOON AND THEN THEY WILL BECOME
LIGHT TONIGHT AND GENERALLY LESS THAN 10 KTS BY 06Z-12Z. 88

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  51  34  61  41 /  40  20  20  20
ALW  53  37  65  45 /  50  20  20  20
PSC  55  37  68  44 /  50  20  20  20
YKM  53  37  65  43 /  90  60  20  20
HRI  55  34  66  44 /  40  20  20  20
ELN  50  35  63  41 /  90  60  30  40
RDM  52  26  57  34 /  30  20  20  30
LGD  51  32  61  39 /  30  30  30  30
GCD  52  33  61  38 /  30  20  30  30
DLS  56  38  64  47 /  60  20  20  30

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...WINTER WEATHER ADVISORY UNTIL 10 AM PDT THIS MORNING ABOVE 4000
     FEET FOR THE ORZ502-503.

WA...WINTER WEATHER ADVISORY UNTIL 10 AM PDT THIS MORNING ABOVE 4000
     FEET FOR THE WAZ030.

&&

THREAT INDEX
TODAY : GREEN
THURSDAY : GREEN
FRIDAY : 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

$$

79/89/89