977
FXUS66 KPDT 141311
AFDPDT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PENDLETON OR
511 AM PST SUN DEC 14 2025
UPDATE
FOG HAS EXPANDED ACROSS THE LOWER COLUMBIA BASIN OF WASHINGTON AND
THE YAKIMA VALLEY AS OBSERVATIONS AND AREA WEBCAMS INDICATE
VISIBILITIES OF ONE QUARTER MILE OR LESS. THUS, A DENSE FOG
ADVISORY HAS BEEN ISSUED UNTIL 8 AM FOR THESE AREAS. 75
PREV DISCUSSION
/ISSUED 338 AM PST SUN DEC 14 2025/
DISCUSSION...
KEY MESSAGES:
1. HYDRO CONCERNS PERSIST, WITH RISES EXPECTED LATER THIS WEEK.
*FLOOD WARNINGS ACTIVE*
2. WINDY CONDITIONS MONDAY, RETURNING TUESDAY NIGHT TO WEDNESDAY.
3. SIGNIFICANT MOUNTAIN SNOW POTENTIAL WEDNESDAY ONWARD.
THE UPPER LEVEL RIDGE OF HIGH PRESSURE THAT BROUGHT DRIER
CONDITIONS YESTERDAY, CONTINUES TO LINGER THROUGH THE WEEKEND
BEFORE SHIFTING EAST AHEAD OF ANOTHER ACTIVE WEATHER PATTERN. THE
PRESENT RIDGE AND ACCOMPANYING LOW WINDS AND LOW LEVEL MOISTURE
HAS ATTRIBUTED TO THE DEVELOPMENT OF FOG THIS MORNING ACROSS THE
KITTITAS VALLEY, WHICH HAS WARRANTED A DENSE FOG ADVISORY UNTIL 8
AM DUE TO REDUCED VISIBILITIES OF 1/4 MILE OR LESS. THERE ARE
ALSO POCKETS OF ISOLATED FOG IN THE TRI-CITIES AREA, WHICH WILL
CONTINUE TO BE MONITORED THROUGH THE EARLY MORNING HOURS. THE
RIDGE HAS ALSO BROUGHT A MUCH NEEDED DRY PERIOD, WHICH HAS ALLOWED
MOST AREA RIVER REACHES TO RECEDE BELOW FLOOD STAGE. HOWEVER, THE
YAKIMA RIVER AT KIONA WILL CONTINUE TO BE IN MODERATE FLOOD STAGE
THROUGH THIS AFTERNOON BEFORE DROPPING INTO MINOR FLOOD STAGE
THROUGH MONDAY MORNING. AN ACTIVE RIVER FLOOD WARNING CONTINUES
FOR THE YAKIMA RIVER AT KIONA UNTIL FURTHER NOTICE.
AS THE UPPER LEVEL RIDGE SHIFTS EAST MONDAY, ENSEMBLES ARE IN
AGREEMENT THAT THE FIRST OF A PAIR OF UPPER LEVEL SHORTWAVES WILL
BE PASSING THROUGH THE AREA. AN UPPER LEVEL LOW PRESSURE SYSTEM,
LOCATED IN THE GULF OF ALASKA, WILL BE DRIVING THEM THROUGH THE
PACIFIC NORTHWEST AND ACCOMPANYING THE SECOND LATE TUESDAY INTO
WEDNESDAY. BOTH SYSTEMS WILL BE SUPPLIED MY A MODERATE ATMOSPHERIC
RIVER (AR) AS ECMWF AND GFS ENSEMBLES SUGGEST PRECIPITABLE WATER
(PW) VALUES OF 0.80-1.00", WHICH IS 200-250% ABOVE NORMAL. AS A
RESULT, 1-2 INCHES OF RAINFALL IS FORECAST AT HIGHER TERRAIN
ACROSS THE CASCADE AND NORTHERN BLUE MOUNTAINS. THIS RAINFALL WILL
LEAD TO PORTIONS OF THE NACHES AND YAKIMA RIVERS TO REBOUND INTO
MINOR FLOOD STAGE EARLY TUESDAY TO WEDNESDAY.
THE ARRIVAL OF THE FIRST SHORTWAVE WILL BE ACCOMPANIED BY A COLD
FRONT THAT WILL CAUSE A SLIGHT PRESSURE GRADIENT TO DEVELOP ACROSS
THE COLUMBIA BASIN. THIS WILL BRING SUSTAINED SOUTHWEST WINDS OF
20-30 MPH WITH GUSTS UP TO 40 MPH ACROSS THE BASIN, WITH HIGHEST
WINDS EXPECTED TO OCCUR OVER THE SIMCOE HIGHLANDS. A MORE
SUBSTANTIAL COLD FRONT, COUPLED WITH MID-LEVEL JET SUPPORT, WILL
ALLOW SUSTAINED WEST WINDS OF 30-40 MPH WITH GUSTS UP TO 60 MPH
ACROSS THE SIMCOE HIGHLANDS, NORTH-CENTRAL OREGON, BLUE MOUNTAIN
FOOTHILLS, AND THE LOWER COLUMBIA BASIN. CONFIDENCE IN THESE WIND
VALUES IS MODERATE (50-70%) AS THE NBM ADVERTISES A 60-90% CHANCE
ACROSS THE SIMCOE HIGHLANDS, AND A 50-80% CHANCE OVER THE
REMAINING AREAS OF GUSTS REACHING 60 MPH OR GREATER WEDNESDAY.
WINDS ARE EXPECTED TO PEAK DURING THE EARLY MORNING HOURS ON
WEDNESDAY, AS THESE VALUES MEET WARNING CRITERIA. IF THE TREND
PERSISTS, A WIND WATCH MAY BE NECESSARY IN SUBSEQUENT FORECASTS.
THE SECONDARY SYSTEM ON WEDNESDAY WILL ADVECT A COLDER AIR MASS
OVER THE AREA, EFFECTIVELY DROPPING SNOW LEVELS FROM THE 5000-6000
FOOT RANGE TUESDAY NIGHT TO BETWEEN 2500-3500 FEET LATE WEDNESDAY
MORNING. PRECIPITATION WILL BE CHANGING OVER FROM RAIN TO SNOW
OVER MOUNTAIN ZONES INTO WEDNESDAY MORNING AS 5-10 INCHES OF
PASS-LEVEL SNOWFALL IS FORECAST OVER THE CASCADES AND 2-5 INCHES
OVER THE NORTHERN BLUE MOUNTAINS THROUGH THE DAY ON WEDNESDAY.
ENSEMBLES ALSO SUGGEST A SECOND UPPER LEVEL TROUGH DROPPING DOWN
THE BRITISH COLUMBIA COAST TO BRING ANOTHER, MORE SUBSTANTIAL
ROUND OF MOUNTAIN SNOWFALL THURSDAY WITH ANOTHER 8-18 INCHES OVER
THE CASCADE PASSES AND AT HIGHER TERRAIN OVER THE NORTHERN BLUE
MOUNTAINS. HOWEVER, CONFIDENCE IS LACKING DUE TO TIMING AND
LOCATION OF THE DROPPING SYSTEM WITH 20% OF MEMBERS SUGGESTING
HIGHER AMOUNTS AND 15% OF MEMBERS HINTING AT LOWER SNOW AMOUNTS ON
WEDNESDAY. LOW CONFIDENCE (20-50%) IN SNOW AMOUNTS EXTENDS INTO
THURSDAY AND FRIDAY, BUT GUIDANCE DOES ALIGN WITH DROPPING SYSTEM
OFF THE GULF OF ALASKA BEING ABLE TO TAP INTO SUBTROPICAL MOISTURE
THAT ELEVATES MOUNTAIN SNOW CHANCES AND AMOUNTS. IT IS WORTH
NOTING THAT TRENDS OVER THE LAST 48 HOURS HAVE SEEN THESE SNOW
TOTALS DECREASE AND THE TIMING DELAY, BUT THIS ACTIVE WEATHER
PATTERN THROUGH THE WEEK DOES SEEM TO BRING IN THE ARRIVAL OF
WINTER TO OUR MOUNTAIN ZONES. 75
AVIATION...12Z TAFS...LOCALIZED DENSE FOG HAS IMPACTED PSC AND
DLS OVERNIGHT, AND WITH LIGHT WINDS CONTINUING, THE FOG IS LIKELY
(80%) TO STILL BE ONGOING AFT LEAST THOUGH SUNRISE AND BOUNCING
BETWEEN HIGHER IFR AND BACK SOWN UNTIL SOME BOUNDARY LAYER WARMING
TAKES PLACE. WINDS WILL REMAIN MOSTLY LIGHT AND VARIABLES ACROSS
THE LOCAL TERMINALS, AND BY LATE EVENING, RAIN SHOWERS COULD BE IN
THE VICINITY OF YKM AND DLS.
PRELIMINARY POINT TEMPS/POPS
PDT 56 42 64 45 / 0 10 60 40
ALW 56 46 62 46 / 10 10 80 60
PSC 53 41 62 43 / 10 10 60 20
YKM 50 40 61 39 / 10 30 70 30
HRI 54 41 62 42 / 10 10 60 30
ELN 49 38 54 35 / 10 50 90 50
RDM 58 38 63 39 / 10 10 40 30
LGD 52 39 51 40 / 0 10 70 60
GCD 56 37 57 42 / 0 10 50 40
DLS 52 44 59 45 / 10 40 90 70
PDT WATCHES/WARNINGS/ADVISORIES
OR...NONE.
WA...DENSE FOG ADVISORY UNTIL 8 AM PST THIS MORNING FOR WAZ026>028.
DISCUSSION...75AVIATION...71
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