Area Forecast Discussion
Issued by NWS Pendleton, OR

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FXUS66 KPDT 180959
AFDPDT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PENDLETON OR
255 AM PDT THU SEP 18 2014

.SHORT TERM...TODAY THROUGH SATURDAY NIGHT...AN UPPER LEVEL TROUGH
IS NEAR THE COAST AS IT SLOWLY SHIFTS EASTWARD. THE TROUGH IS
SPLITTING AS IT MOVES INLAND WITH THE NORTHERN SECTION IN WASHINGTON
AND THE SOUTHERN SECTION IN CALIFORNIA. THERE ARE TWO BANDS OF
PRECIPITATION ON RADAR AND MODELS. ONE IS NEAR THE CASCADES ON THE
EAST SIDE OF THE TROUGH AXIS AND THE SECOND ONE IS IN EASTERN GRANT
COUNTY...WALLOWA COUNTY...AND BAKER COUNTY. HRRR SHOWS THE FIRST
BAND REMAINING IN CENTRAL/NORTH CENTRAL OREGON AND CENTRAL
WASHINGTON THIS MORNING. THE SECOND BAND IS ASSOCIATED WITH A JET
STREAK. HRRR SHOWS THIS BAND REMAINING SEPARATE AND MOVING NORTHEAST
OUT OF THE OREGON BY 16Z. THE MORNING FORECAST HAS THE TWO SEPARATE
BANDS...THEN THE FIRST BAND WILL SPREAD ACROSS CENTRAL TO NORTHEAST
OREGON AS THE TROUGH AXIS MOVES ACROSS EASTERN WASHINGTON AND OREGON
THIS AFTERNOON. THE BEST INSTABILITY WILL MOVE FROM CENTRAL TO
NORTHEAST OREGON WHERE THE DIFFLUENCE FROM THE SPLITTING TROUGH IS
ADDED TO THE INSTABILITY AHEAD OF THE TROUGH. ISOLATED THUNDERSTORMS
MAY DEVELOP IN THIS REGION DURING THE AFTERNOON AND EARLY EVENING.
THE TROUGH AXIS WILL BE IN IDAHO BY 6Z/2300 PDT WITH RAIN SHOWERS
ENDING EXCEPT FOR OROGRAPHIC SHOWERS NEAR THE CASCADE CREST. FRIDAY
AND SATURDAY THE UPPER LEVEL RIDGE WILL BUILD OVER THE PACIFIC
NORTHWEST WITH CLEARING SKIES AND WARMING DAYTIME TEMPERATURES.
COONFIELD

.LONG TERM...SUNDAY THROUGH WEDNESDAY...00Z MODEL RUNS INDICATE VERY
SIMILAR SOLUTIONS OVER FORECAST AREA AND NORTHEAST PACIFIC WITH A
MID/UPPER LEVEL RIDGE OF HIGH PRESSURE CONTINUING OVER THE PACIFIC
NORTHWEST SUNDAY AND MONDAY. MODELS SHOW CONSISTENT SOLUTIONS FOR
TUESDAY AS WELL WITH THE RIDGE MOVING EAST INTO THE NORTHERN PLAINS
BY DAYBREAK TUESDAY AS THE NEXT LONGWAVE TROUGH SPREADS EASTWARD TO
130-140W. THE GFS IS MORE PROGRESSIVE IN TAKING THE AXIS OF AN OPEN
LONGWAVE TROUGH TO 132W BY TUESDAY AFTERNOON WHEREAS THE ECMWF KEEPS
THE LOWEST HEIGHTS IN THE OVERALL TROUGH NEAR 140W WHERE A CLOSED
LOW WITH MUCH LOWER HEIGHTS DEVELOPS IN THE ECMWF SOLUTION. BOTH GFS
AND ECMWF SHOW A VERY WEAK DISTURBANCE MOVING THROUGH FORECAST AREA
MONDAY NIGHT AND TUESDAY SO I HAVE INCREASED POPS A TAD TO SLIGHT
CHANCE FOR POSSIBLE ISOLATED LIGHT SHOWERS IN THE BLUE MOUNTAINS
MONDAY NIGHT AND TUESDAY. THE TROUGH APPROACHES THE COAST TUESDAY
NIGHT AND SPREADS INCREASING CLOUDS, MOISTURE, AND RAIN TO THE
CASCADES JUST BEFORE DAYBREAK ON WEDNESDAY. THE PRECIPITATION
SPREADS ACROSS FORECAST AREA WEDNESDAY MORNING. AS ALREADY
MENTIONED, THE GFS IS FASTER IN TERMS OF BRINGING THE TROUGH AXIS TO
THE COAST BY LATE WEDNESDAY MORNING WHILE THE ECMWF KEEPS THE TROUGH
AXIS WELL OFFSHORE. THIS PHASING DIFFERENCE BETWEEN THE TWO MODELS
ON WEDNESDAY WITH RESPECT TO THE PLACEMENT OF THE TROUGH AXIS
RESULTS IN THE ECMWF TAKING LONGER TO SPREAD RAIN TO THE BLUE
MOUNTAINS UNTIL LATE WEDNESDAY NIGHT OR THURSDAY MORNING. AS SUCH
HAVE LOWER CONFIDENCE IN POP FORECAST FOR WEDNESDAY IN THE EASTERN
SECTIONS OF FORECAST AREA.  POLAN

&&

.AVIATION...12Z TAFS...VFR CONDITIONS ARE EXPECTED TO PERSIST
THROUGH THE NEXT 24 HOURS. THERE MAY BE SOME LIGHT SHOWERS AT
KDLS...KRDM...KBDN...KPDT AND KALW TODAY WITH POSSIBLY A
THUNDERSTORM AT KRDM AND KBDN IN THE AFTERNOON AND EVENING. WINDS
WILL BE GENERALLY LIGHT ALTHOUGH GUSTY AND ERRATIC WINDS ARE LIKELY
NEAR ANY THUNDERSTORMS. CIGS AND VISIBILITIES WILL REMAIN ABOVE VFR
DUE TO THE DRYNESS OF THE AIR MASS IN THE LOWER LEVELS THROUGH THIS
EVENING.  POLAN

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  82  53  80  52 /  20  10   0   0
ALW  83  59  80  57 /  20  10   0   0
PSC  86  57  85  51 /  10  10   0   0
YKM  82  56  84  54 /  30  10   0   0
HRI  84  54  85  48 /  10  10   0   0
ELN  79  57  83  52 /  30  10   0   0
RDM  76  41  79  40 /  20  20   0   0
LGD  82  50  79  44 /  20  20   0   0
GCD  80  49  81  47 /  20  20   0   0
DLS  79  55  83  52 /  30  10   0   0

&&

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

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

$$

76/99/99







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