369
FXUS66 KPDT 201123
AFDPDT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PENDLETON OR
323 AM PST SAT DEC 20 2025
DISCUSSION
KEY MESSAGES:
1. SIGNIFICANT MOUNTAIN SNOWFALL THROUGH TODAY.
*ACTIVE WINTER STORM WARNINGS*
2. HYDRO CONCERNS LINGER.
*ACTIVE FLOOD WARNING*
3. MOUNTAIN SNOW AND LOWER ELEVATION RAIN CHANCES SUNDAY.
4. WIDESPREAD PRECIPITATION TUESDAY, BASIN SNOW CHANCES MIDWEEK.
CURRENT RADAR AND INFRARED SATELLITE IMAGERY SHOWS LIGHT TO MODERATE
SNOWFALL ACROSS THE CASCADES WITH DRY CONDITIONS ACROSS THE COLUMBIA
BASIN AND CENTRAL OREGON UNDER MOSTLY CLEAR SKIES. THIS IS IN
RESPONSE TO A DEPARTING UPPER LEVEL SHORTWAVE EMBEDDED IN ZONAL FLOW
ACROSS THE PACIFIC NORTHWEST. FLOW ALOFT WILL SHIFT SLIGHTLY FROM
THE WEST TO MORE FROM THE SOUTHWEST THROUGH THE DAY TODAY, KEEPING
UPSLOPE SNOWFALL OCCURRING ACROSS THE WASHINGTON AND OREGON
CASCADES THAT SLOWLY TAPERS OFF THROUGH THE AFTERNOON AND EVENING.
AN ADDITIONAL 5-10 INCHES OF SNOW IS EXPECTED TO FALL THROUGH THE
DAY TODAY, WHICH IS ON TOP OF ABOUT 8 INCHES WITHIN THE PREVIOUS
24 HOURS AS REPORTED BY THE NORTHWEST AVALANCHE CENTER (NWAC) OVER
SNOQUALMIE PASS. ALSO, AN ADDITIONAL 2-3 INCHES OF SNOWFALL IS
ALSO ANTICIPATED ALONG THE OREGON CASCADES ABOVE 3500 FEET ON TOP
OF 4-6 INCHES OVER THE LAST 24 HOURS (MCKENZIE/HOGG PASS SNOTEL).
AS A RESULT, WINTER STORM WARNINGS CONTINUE TO BE ACTIVE FOR THE
EAST SLOPES OF THE OREGON CASCADES UNTIL 10 AM THIS MORNING AND
THE UPPER EAST SLOPES OF THE WASHINGTON CASCADES UNTIL 4 PM TODAY.
HYDRO CONCERNS ARE STILL PRESENT FOR SOME AREA RIVER REACHES,
PRIMARILY ACROSS THE YAKIMA VALLEY AND THE LOWER COLUMBIA BASIN OF
WASHINGTON. CURRENTLY, THE ONLY RIVER IN MINOR FLOOD STAGE IS THE
NACHES RIVER AT NACHES, WHICH CRESTED EARLY FRIDAY MORNING AND IS
FORECAST TO DROP OUT OF FLOOD STAGE LATER THIS MORNING. THUS, A
RIVER FLOOD WARNING IS ACTIVE UNTIL NOON TODAY. THERE ARE ALSO
SEVERAL RIVERS IN ACTION STAGE INCLUDING THE YAKIMA RIVER AT KIONA
AND THE WALLA WALLA RIVER AT TOUCHET, WHICH ARE EXPECTED TO DROP
BELOW ACTION STAGE IN THE NEXT 48 HOURS. THE NACHES RIVER AT
CLIFFDELL, WHICH IS ALSO IN ACTION STAGE, IS CURRENTLY FORECAST TO
LINGER IN ACTION STAGE THROUGH FRIDAY, DECEMBER 26TH. EVEN WITH
WIDESPREAD PRECIPITATION RETURNING SUNDAY AND AGAIN TUESDAY ONWARD,
RIVER LEVELS ARE EXPECTED TO CONTINUE TO DECLINE.
AN UPPER LEVEL TROUGH WILL CLOSE INTO A LOW PRESSURE OFF THE COAST
OF BRITISH COLUMBIA THIS EVENING INTO SUNDAY AS A SHARP SHORTWAVE
SPINS OFF AND MOVES ONSHORE AND ACROSS THE REGION SUNDAY. THIS
SYNOPTIC FEATURE PROVIDES ADDITIONAL MOUNTAIN SNOWFALL AND LOWER
ELEVATIONS RAIN ON SUNDAY. PRECIPITATION INTENSITY PEAKS THROUGH THE
AFTERNOON AND EVENING AS THE ASSOCIATED WEAK COLD FRONT FROM THE
NORTHWEST MEETS A LIFTING SURFACE LOW FROM NORTHERN NEVADA. CASCADE
PASSES CAN EXPECTED 5-10 INCHES OF SNOWFALL, WITH 1-3 INCHES OVER
THE NORTHERN BLUE MOUNTAINS ABOVE 4000 FEET. THE AMOUNTS OVER THE
CASCADES MAY WARRANT A WINTER WEATHER ADVISORY AS THE NBM SUGGESTS A
50-70% CHANCE OF 5 INCHES OR GREATER ACROSS SNOQUALMIE, WHITE, AND
SANTIAM PASSES THROUGH MONDAY MORNING. PERIODS OF LIGHT RAINFALL IS
ALSO LIKELY ACROSS LOWER ELEVATIONS SUNDAY, WITH BEST CHANCES (40-
70%) AFTER 4 PM. RAIN AMOUNTS WILL STAY BELOW 0.10", WITH LOWER
ELEVATIONS OF THE YAKIMA VALLEY AND THE LOWER COLUMBIA BASIN OF
WASHINGTON POTENTIALLY STAYING DRY.
ENSEMBLES ARE IN FAIRLY GOOD AGREEMENT WITH THE OVERALL PATTERN OF
AN UPPER LEVEL TROUGH THAT DROPS FROM THE BRITISH COLUMBIA COAST
SOUTH OVER THE EASTERN PACIFIC TUESDAY AND EXTENDING THROUGH
MIDWEEK. SEVERAL SHORTWAVE TROUGHS LOOK TO SPIN OFF THE PARENT
SYSTEM THROUGH THIS TIMEFRAME, INTRODUCING MOISTURE ACROSS THE
REGION WHILE COOLER AIR IS ADVECTED WEDNESDAY THROUGH FRIDAY
ASSOCIATED WITH NORTHWEST FLOW ALOFT. THIS MAY LEAD TO LOW
ELEVATION SNOW ACCUMULATIONS OVER PORTIONS OF THE YAKIMA VALLEY,
LOWER COLUMBIA BASIN, AND BLUE MOUNTAIN FOOTHILLS OVERNIGHT
WEDNESDAY THROUGH FRIDAY. CURRENTLY, THERE IS A 10-20% CHANCE OF
MEASURABLE SNOWFALL (0.01" OR GREATER) OVER THE AFOREMENTIONED
AREAS. 59% OF ENSEMBLE MEMBERS ARE CURRENTLY LEANING TOWARD LESS
THAN 10% CHANCE FOR MEASURABLE SNOWFALL OVERNIGHT INTO THURSDAY
MORNING. HOWEVER, 80% OF MEMBERS SUGGEST A BETTER CHANCE FOR THE
FOOTHILLS, 58% OF MEMBERS SUGGEST A BETTER CHANCE OVER TRI-CITIES,
AND 49% OF MEMBERS ADVERTISE A BETTER CHANCE OVER YAKIMA THURSDAY
NIGHT INTO FRIDAY MORNING. THIS BETTER CHANCE WOULD BE ON THE
HIGHER END OF THE EARLIER MENTIONED RANGE FROM 10-20%, WHICH
RELATES TO ENSEMBLES STRUGGLING WITH TIMING AND STRENGTH OF THE
PASSING SHORTWAVES TUESDAY ONWARD AND THE OVERALL LOW PRESSURE
SYSTEM OFF THE WEST COAST. 75
AVIATION...12Z TAFS
VFR CONDITIONS ARE MAINLY EXPECTED THROUGH THE TAF
PERIOD. HOWEVER, THERE ARE A FEW MINOR EXCEPTIONS. AT DLS, THERE
COULD BE SOME LIGHT RAIN THIS MORNING (30 PERCENT CHANCE) THAT MAY
REDUCE VSBYS TO MVFR.
OTHERWISE, TOWARD THE END OF THE TAF PERIOD...LATE TONIGHT/EARLY
SUNDAY MORNING, LOW CLOUDS ARE EXPECTED TO AFFECT BDN AND RDM AND
PRECIPITATION WILL MOVE IN WHICH COULD BE RAIN, SNOW OR A MIXTURE,
DEPENDING ON WHAT TEMPERATURES ULTIMATE TURN OUT TO BE. THE LOW
CLOUDS WILL BRING MVFR CONDITIONS. IF THERE IS SNOW OR A MIXTURE, IFR
CONDITIONS CAN NOT BE RULED OUT.
WINDS WILL MAINLY BE 10 KTS OR LESS, THOUGH AT PSC, THERE COULD BE
SOME GUSTS TO AROUND 25 KTS THROUGH THE MORNING BEFORE THEY DECREASE
TO 10 KTS OR LESS AS WELL.
PRELIMINARY POINT TEMPS/POPS
PDT 48 32 45 34 / 10 10 60 60
ALW 48 35 45 36 / 20 10 60 70
PSC 50 32 44 33 / 10 10 40 40
YKM 45 28 40 26 / 20 10 30 40
HRI 50 32 44 33 / 10 10 50 50
ELN 38 24 36 24 / 40 30 30 60
RDM 43 25 47 28 / 0 20 70 60
LGD 39 27 42 31 / 10 20 80 80
GCD 42 30 46 31 / 0 50 90 70
DLS 46 35 43 34 / 60 20 60 90
PDT WATCHES/WARNINGS/ADVISORIES
OR...WINTER STORM WARNING UNTIL 10 AM PST THIS MORNING FOR ORZ509.
WA...WINTER STORM WARNING UNTIL 4 PM PST THIS AFTERNOON FOR WAZ522.
DISCUSSION...75
AVIATION...77
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