520
FXUS66 KPDT 040618
AFDPDT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PENDLETON OR
1018 PM PST WED DEC 3 2025
UPDATE AVIATION...06Z TAFS
THANKS TO THE LOW STRATUS LAYER,
THIS WILL DROP KDLS/KPSC TO MVFR WITH KYKM AT IFR AND KPDT/KALW TO
LIFR CONDITIONS. KDLS/KPSC MAY BRIEFLY SEE PATCHY FOG OVERNIGHT
INTO EARLY THURSDAY MORNING (35% CONFIDENCE), BUT DENSE FOG COULD
REDEVELOP FOR KYKM/KALW WHILE CONTINUING FOR KPDT THUS DROPPING
VSBYS TO 1/2SM OR LESS. THE FOG SHOULD THEN DISSIPATE AROUND 12Z
FOR KDLS/KPSC AND LATER IN THE AFTERNOON FOR KYKM/KALW. KPDT MAY
HAVE LINGERING FOG/MIST THROUGH THE DAY THURSDAY (40-50% CONFIDENCE).
IN ADDITION TO THE FOG AND LOW CLOUD DECKS, A WEAK SYSTEM WILL
BEGIN APPROACHING EARLY THURSDAY MORNING WITH LIGHT RAIN STARTING
KDLS FIRST BEFORE OTHER SITES. KRDM/KBDN WILL BE VFR WITH LIGHT
RAIN THURSDAY MORNING UNTIL MVFR CONDITIONS RETURN EARLY THURSDAY
EVENING WHEN LOW CLOUDS REDEVELOP. WINDS WILL REMAIN LESS THAN
10KTS. FEASTER/97
PREV DISCUSSION
/ISSUED 229 PM PST WED DEC 3 2025/
DISCUSSION
REST OF TODAY THROUGH THURSDAY MORNING: A WEAK
SHORTWAVE MOVING OVER THE REGION HAS HELPED TO LIFT FOG IMPACTING
AREAS OF THE COLUMBIA BASIN AND ADJACENT VALLEYS/FOOTHILLS THIS
AFTERNOON, THOUGH CLOUD DECKS REMAIN FAIRLY LOW ACROSS THESE
AREAS. THIS REPRIEVE IS EXPECTED TO BE BRIEF, AS THERE IS MOD-HIGH
CONFIDENCE (55-85%) THAT DENSE FOG WILL REDEVELOP IN THESE AREAS
BY THIS EVENING. THAT SAID, CONFIDENCE IS LOW(25-40%) IN THE
EXACT TIMING AND AREAL EXTENT OF THE FOG REDEVELOPING. REGARDLESS,
OPTED TO EXTEND THE CURRENT FOG ADVISORIES THROUGH 12PM PST
TOMORROW FOR THE KITTITAS/YAKIMA VALLEYS AND THE NORTHERN BLUE
MOUNTAIN FOOTHILLS OF OR/WA.
BY THURSDAY MORNING, THE FIRST IN A SERIES OF COUPLED SHORTWAVES
AND SURFACE LOWS WILL ARRIVE TO THE PACNW, WHILE A PLUME OF
MOISTURE ORIGINATING FROM THE HAWAII REGION WILL AID PRECIPITATION
AMOUNTS WITH EACH INCOMING SYSTEM. AS FOR IMPACTS THURSDAY
MORNING, DENSE FOG IS EXPECTED TO PERSIST ACROSS PORTIONS OF THE
COLUMBIA BASIN AND ADJACENT FOOTHILLS/VALLEYS, WITH RAIN AND
HIGH MOUNTAIN SNOW OR RAIN/SNOW MIX STARTING DEVELOPING ALONG THE
CASCADE CREST, THEN SPREADING SOUTHEAST TO THE REMAINDER OF THE
FORECAST AREA. ONE CONCERN WITH INCOMING PRECIPITATION WILL BE THE
POTENTIAL FOR FREEZING RAIN TO DEVELOP TOMORROW MORNING IF THE
COLD POOL STRENGTHENS AND DROPS TO OR BELOW FREEZING TONIGHT.
AREAS THAT WOULD BE MOST LIKELY IMPACTED WOULD BE PORTIONS OF THE
KITTITAS/YAKIMA VALLEYS, AND ALONG THE NORTHERN BLUE MOUNTAIN
FOOTHILLS. THAT SAID, CONFIDENCE IS VERY LOW (<15%) IN THIS
SCENARIO PLAYING OUT WITH ONSET OF PRECIPITATION TOMORROW.
THURSDAY AFTERNOON THROUGH SATURDAY: ENSEMBLE GUIDANCE IS IN
GREAT AGREEMENT THROUGH SATURDAY NIGHT THAT A PERSISTENT UPPER
LEVEL RIDGE IN THE NORTHEAST PACIFIC WILL FORCE SEVERAL SURFACE
LOWS AND ATTENDANT SHORTWAVE TROUGHS TO DIVE SOUTHEAST ACROSS THE
FORECAST AREA. WHILE EACH SYSTEM WILL BE APPROACHING FROM THE
NORTHWEST, WHICH WOULD GENERALLY MEAN THEIR AIRMASSES WOULD BE
FAIRLY COOL/COLD, A PLUME OF TROPICAL MOISTURE WITH IVTS IN THE
150 TO 250 KG/M/S RANGE WILL RIDE UP AND OVER THE RIDGE INTO THE
PACNW, KEEPING THE AIRMASS GENERALLY SATURATED WITH WELL ABOVE
NORMAL TEMPERATURES THAN NORMAL THIS TIME OF YEAR (CONFIDENCE
70-90%). AS THE AIRMASS INFILTRATES THE PACNW, SNOW LEVELS WILL
SLOWLY RISE ABOVE PASS LEVEL, KEEPING ANY SNOW OR WINTRY MIXES TO
THE HIGHER MOUNTAIN RIDGES AND PEAKS THROUGH THIS PERIOD.
BY THURSDAY AFTERNOON, THE FIRST IN THE SERIES OF SYSTEMS WILL
GLIDE OVER THE FORECAST AREA, RESULTING IN WIDESPREAD RAIN AND
HIGH MOUNTAIN SNOW CHANCES THE REMAINDER OF THURSDAY. BY FRIDAY, A
MORE POTENT SURFACE LOW WITH A STRONG COLD FRONT WILL APPROACH THE
PACNW, RESULTING IN A TIGHTENING OF PRESSURE GRADIENTS AT THE
SURFACE. WESTERLY WINDS WILL INCREASE AS EARLY AS FRIDAY MORNING,
WITH WINDS FORECAST TO PEAK FRIDAY EVENING AS THE SURFACE LOW AND
COLD FRONT MOVE ACROSS THE FORECAST AREA. WIND GUSTS WILL
INCREASE TO 25-40MPH ACROSS MUCH OF THE AREA(CONFIDENCE 60-80%),
WITH A 50-80% CHANCE THAT WIND GUSTS WILL EXCEED 45MPH ACROSS
PORTIONS OF THE OR COLUMBIA BASIN, BLUE MOUNTAIN FOOTHILLS, NORTH
CENTRAL OR, SIMCOE HIGHLANDS, AS WELL AS RIDGES IN THE WA COLUMBIA
BASIN TO THE WA CASCADE EAST SLOPES. WINDS WILL GRADUALLY DECREASE
THROUGH SATURDAY AS THE LOW DEPARTS THE REGION TO THE EAST.
OTHERWISE, DURING THIS PERIOD, THERE IS A 60-85% CHANCE OF THE
CASCADE CREST AND EAST SLOPES RECEIVING 1.5 INCHES OF QPF WHILE
THE NORTHERN BLUES WILL ONLY SEE A 30-55% CHANCE. LOOKING AT 0.5
INCHES OF QPF, THE NORTHERN BLUE MOUNTAIN FOOTHILLS AND THE
REMAINDER OF THE EASTERN MOUNTAINS WILL SEE A 35-70%. FOR THE
REMAINDER OF THE AREAS, THERE IS GENERALLY A 20-60% CHANCE FOR
0.25 INCHES OF PRECIPITATION, WITH THE LOWEST CHANCES IN THE WA
COLUMBIA BASIN AND ACROSS PORTIONS OF CENTRAL OR.
SUNDAY THROUGH TUESDAY: GUIDANCE CONTINUES TO BE IN GOOD
AGREEMENT SUNDAY THROUGH TUESDAY AS ALL BUT ABOUT 10-13% OF
ENSEMBLE CLUSTER MEMBERS SHOW THE RIDGE AXIS MOVING INLAND AND
FLATTENING SUNDAY AND MONDAY, WITH ZONAL FLOW DEVELOPING OVER THE
PACNW. MEANWHILE, ENSEMBLE MEMBERS OF THE GFS AND ECMWF CONTINUE
TO DEPICT A 60-80% CHANCE OF AT LEAST A WEAK ATMOSPHERIC RIVER
(AR) WITH IVT OF 150 KG/M/S FILTERING INTO THE AREA. UNDER THE
ZONAL FLOW, WEAK SHORTWAVES WITH SURFACE FRONTS WILL CONTINUE TO
IMPACT THE FORECAST AREA THROUGH TUESDAY, AND WITH THE
ACCOMPANYING AR, THIS WILL RESULT IN PERIODS OF MODERATE TO HEAVY
PRECIPITATION IN THE MOUNTAINS WITH LIGHTER AMOUNTS FURTHER DOWN
ALONG THE CASCADE EAST SLOPES AND THE NORTHERN BLUE MOUNTAIN
FOOTHILLS. OF THE ENSEMBLE CLUSTER MEMBERS, THE SOLUTIONS MADE UP
OF THE EARLIER MENTIONED 10-13% FAVOR LOWER 500MB HEIGHTS OVER
THE PACNW, WHICH WOULD TRANSLATE TO LOWER SNOW LEVELS AND
POTENTIALLY SNOW AT OR JUST ABOVE PASS LEVEL IN THE MOUNTAINS.
OTHERWISE, THE REMAINING MEMBERS FAVOR MOSTLY RAIN WITH HIGH
MOUNTAIN SNOW DURING THIS PERIOD. LAWHORN/82
PRELIMINARY POINT TEMPS/POPS
PDT 28 43 37 53 / 0 80 70 90
ALW 31 42 37 49 / 10 80 80 90
PSC 28 42 32 51 / 10 70 50 60
YKM 27 42 31 52 / 10 40 40 70
HRI 29 43 36 53 / 0 80 60 70
ELN 27 43 31 46 / 20 50 50 80
RDM 25 47 35 55 / 0 50 40 60
LGD 26 40 35 48 / 0 90 90 100
GCD 26 42 37 49 / 0 80 90 90
DLS 36 48 42 56 / 10 70 70 90
PDT WATCHES/WARNINGS/ADVISORIES
OR...DENSE FOG ADVISORY UNTIL 11 AM PST THURSDAY FOR ORZ507.
WA...DENSE FOG ADVISORY UNTIL 11 AM PST THURSDAY FOR WAZ029.
FREEZING FOG ADVISORY UNTIL 11 AM PST THURSDAY FOR WAZ026-027.
DISCUSSION...82
AVIATION...97
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