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000
FXUS66 KPDT 171122 AAA
AFDPDT

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE PENDLETON OR
421 AM PDT FRI MAY 17 2013

UPDATED AVIATION AND LONG TERM DISCUSSIONS

.SHORT TERM...TODAY THROUGH SATURDAY NIGHT...A BROAD UPPER LEVEL
TROUGH WILL LINGER OVER EASTERN WASHINGTON AND OREGON TODAY THROUGH
SATURDAY MORNING. THE TROUGH WILL SHIFT EAST SATURDAY AFTERNOON. THE
TROUGH WILL KEEP CLOUDS AND VARYING CHANCES OF SHOWERS OVER THE AREA
TODAY THROUGH SATURDAY EVENING. DAYTIME HEATING WILL ENHANCE THE
INSTABILITY ASSOCIATED WITH THE TROUGH THIS AFTERNOON...ESPECIALLY
OVER GRANT COUNTY TO THE BLUES AND WALLOWA COUNTY. THIS MAY TRIGGER
A FEW THUNDERSTORMS DURING THE AFTERNOON AND EVENING IN THESE AREAS.
THERE WILL BE A BRIEF BREAK LATE TONIGHT AS THE ATMOSPHERE BRIEFLY
STABILIZES WITH THE LOSS OF DAYTIME HEATING. AS THE TROUGH SHIFTS
EAST SATURDAY EVENING SHOWERS WILL END. SHOWERS WILL LINGER
OVERNIGHT IN WALLOWA COUNTY. WINDS WILL INCREASE SATURDAY AFTERNOON
AND EVENING BEHIND THE TROUGH...PARTICULARLY THROUGH THE COLUMBIA
RIVER GORGE AND KITTITAS VALLEY WHERE GAPS IN THE CASCADES FUNNEL
THE WINDS.  COONFIELD

.LONG TERM...SUNDAY THROUGH MONDAY NIGHT...A MID/UPPER LEVEL TROUGH
WILL EXIT THE FORECAST AREA SUNDAY FOR A DRYING TREND KICKING IN FOR
MOST OF THE FORECAST AREA. HOWEVER, SOME WRAP AROUND MOISTURE WILL
KEEP A SLIGHT CHANCE OF SHOWERS OVER THE BLUE MOUNTAINS AND FAR
NORTHEAST OREGON THROUGH SUNDAY AFTERNOON. SUNDAY NIGHT A LARGE
UPPER LEVEL LOW PRESSURE SYSTEM MOVES SOUTHEAST ALONG THE SE ALASKAN
ARCHIPELAGO AND COMES INTO PHASE WITH A FRONTAL ZONE MOVING EAST
NEAR 45N/137W IN THE SOUTHERN JETSTREAM. MONDAY THESE TWO SYSTEMS
BEGIN MERGING WITH THE SOUTHERN SYSTEM PULLING MOISTURE NORTHWARD
INTO THE EASTERN FLANK OF THE MID/UPPER LEVEL CIRCULATION ASSOCIATED
WITH THE UPPER LOW MOVING SOUTHEAST NEAR QUEEN CHARLOTTE ISLAND.

TUESDAY THROUGH THURSDAY...ON TUESDAY THE LARGE UPPER LOW CONTINUES
MOVING SOUTHEAST WITH THE CENTER OF THE LOW ARRIVING ON THE COAST
JUST NORTH OF THE COLUMBIA RIVER BY TUESDAY AFTERNOON. THIS WILL
CAUSE A SOUTHWEST FLOW ALOFT TO BACK TO THE SOUTH OVER THE INTERIOR
PACIFIC NORTHWEST AND TRANSPORT MOISTURE AND INCREASING INSTABILITY
INTO THE REGION. THE APPROACHING COLD UPPER LOW WILL USHER IN A
COOLING TREND, AS THE UPPER LOW CENTER COMES NEARER TO THE FORECAST
AREA. THE UPPER LOW WILL PERSIST OVER THE PACIFIC NORTHWEST THROUGH
THURSDAY. HOWEVER, FROM ONE RUN TO THE NEXT, THE GFS AND ECMWF
SOLUTIONS ARE EXHIBITING SIGNIFICANT DIFFERENCES IN THE PLACEMENT OF
THE CENTER OF THE UPPER LOW, AND A COMPARISON OF THE PLACEMENT OF
THE CENTER OF THE UPPER LOW BETWEEN THE GFS AND ECMWF MODELS FOR A
GIVEN FORECAST BASE TIME LIKEWISE SHOW SIGNIFICANT DIFFERENCES. AS
SUCH UNCERTAINTY IN THE FORECAST IS GREATER THAN NORMAL FOR TUESDAY
THROUGH THURSDAY IN TERMS OF LARGE STANDARD DEVIATIONS IN THE
OVERALL 500MB HEIGHT PATTERN IN BOTH MODELS. HOWEVER, WITH EACH
ADDITIONAL MODEL RUN, THE GFS AND ECMWF MODELS ARE DISPLAYING BETTER
AGREEMENT FOR TUESDAY THROUGH THURSDAY. EXPECT COOL AND UNSETTLED
CONDITIONS WITH PERIODS OF SHOWERS EACH DAY FROM TUESDAY THROUGH
THURSDAY. DAYTIME HIGH TEMPERATURES WILL BE SLIGHTLY COOLER THAN
NORMAL.  POLAN

&&

.AVIATION...12Z TAFS...VFR CONDITIONS WILL OCCUR OVER THE NEXT 24
HOURS, EXCEPT AT KBDN WHERE MVFR CIGS ARE LIKELY THIS MORNING.
HOWEVER, SOME TAF SITES MAY EXPERIENCE BRIEF MVFR CONDITIONS DUE TO
LOWERED CIGS FROM SHOWERS. AN APPROACHING MID/UPPER LEVEL TROUGH
JUST WEST OF THE CASCADES AT 4 AM THIS MORNING IS STEERING SHOWERS
ALONG A TRACK DUE NORTHWARD FROM THE EASTERN COLUMBIA RIVER GORGE
SOUTH ACROSS NORTH CENTRAL OREGON AND TO THE SOUTHEAST OVER THE JOHN
DAY RIVER VALLEY. SHOWERS WILL INCREASE IN AREAL COVERAGE AS THE
TROUGH OVERSPREADS CENTRAL AND NE OREGON LATE THIS MORNING THROUGH
THIS EVENING. SHOWERS WILL THEN TAPER OFF AFTER 03Z/18TH. WINDS WILL
BE BREEZY AFTER 21Z AT JUST ABOUT ALL TAF SITES OTHER THAN KPSC AND
KYKM. EXPECT WINDS 5 TO 15 KTS WITH GUSTS OF 20 TO 25 KTS.  POLAN

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  69  45  68  43 /  20  20  20  20
ALW  70  51  70  48 /  20  20  20  20
PSC  76  51  75  48 /  10  10  10  10
YKM  72  47  70  45 /  20  10  10  10
HRI  73  49  72  48 /  10  10  20  10
ELN  72  48  68  46 /  20  10  20  10
RDM  63  40  63  33 /  20  10  30  20
LGD  63  43  64  43 /  40  40  20  20
GCD  63  44  61  39 /  50  30  20  20
DLS  70  51  67  48 /  20  10  20  20

&&

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

THREAT INDEX
TODAY : GREEN
SATURDAY : GREEN
SUNDAY : 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

$$

76/99/99