724
FXUS66 KPDT 211131
AFDPDT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PENDLETON OR
331 AM PST SUN DEC 21 2025
DISCUSSION
KEY MESSAGES:
1. MOUNTAIN SNOWFALL PERSISTS ACROSS THE CASCADES.
*ACTIVE WINTER WEATHER ADVISORIES*
2. ELEVATED RIVER LEVELS LINGER.
*HYDROLOGIC STATEMENT ISSUED*
3. WIDESPREAD PRECIPITATION MIDWEEK, BASIN SNOW POSSIBLE.
CURRENT RADAR AND INFRARED SATELLITE IMAGERY SHOWS LIGHT RETURNS
EXTENDING ACROSS THE OREGON CASCADES AND THROUGH CENTRAL OREGON
UNDER CLOUDY TO MOSTLY CLOUDY SKIES. THIS IS IN RESPONSE TO AN
APPROACHING UPPER LEVEL SHORTWAVE AND ATTENDANT COLD FRONT THAT
WILL BE SLOWLY PUSHING ONSHORE THIS MORNING BEFORE BREAKING UP
INTO THE EVENING. THIS FEATURE WILL KEEP MOUNTAIN SNOW SHOWERS
OCCURRING ACROSS THE CASCADES THROUGH THE DAY AS AN ADDITIONAL 4
TO 8 INCHES OF SNOW ACROSS THE OREGON CASCADES AND 5 TO 10 INCHES
OF SNOW ACROSS THE WASHINGTON CASCADES IS EXPECTED. AS A RESULT,
WINTER WEATHER ADVISORIES HAVE BEEN ISSUED ACROSS BOTH THE
WASHINGTON AND OREGON CASCADES THROUGH 4 AM MONDAY MORNING.
CONFIDENCE IN THESE SNOW AMOUNTS IS HIGH (70%) AS THE NBM SUGGESTS
A 75-90% CHANCE OF 4 INCHES, 60-80% OF 6 INCHES, AND 35-60%
CHANCE OF 8 INCHES OF PASS-LEVEL SNOW OVER THE NEXT 24 HOURS. SNOW
RATES WILL BE INCREASING THROUGH THE AFTERNOON HOUR BEFORE
TAPERING OFF OVERNIGHT INTO THE EARLY MORNING HOURS ON MONDAY. A
SECOND SHORTWAVE IS EXPECTED TO SPIN OFF THE UPPER LEVEL LOW
PRESSURE LOCATED OFF THE BRITISH COLUMBIA COAST MONDAY, PROVIDING
ANOTHER 2-5 INCHES OVER THE WASHINGTON CASCADES DURING THE
AFTERNOON AND EVENING. CONFIDENCE IN MONDAY'S SNOW AMOUNTS IS HIGH
(80%) AS THE NBM ADVERTISES AN 80-90% CHANCE OF 3 INCHES AND A
40-60% CHANCE OF 5 INCHES OF PASS-LEVEL SNOWFALL.
THE BREAK IN WIDESPREAD PRECIPITATION AND LOWER SNOW LEVELS HAVE
ALLOWED RIVERS TIME TO RECEDE, BUT SOME RIVER REACHES ARE STILL IN
ACTION/BANKFULL STAGE. THESE INCLUDE THE NACHES RIVER AT CLIFFDELL
AND NACHES, AND THE YAKIMA RIVER AT KIONA. THE LATTER REACH IS
EXPECTED TO DROP BELOW ACTION STAGE THIS EVENING. HOWEVER, THE
NACHES RIVER AT BOTH CLIFFDELL AND NACHES ARE FORECAST TO STAY IN
ACTION STAGE UNTIL LATE IN THE WEEK AS LEVELS SLOWLY DROP. EVEN
WITH PERIODS OF WIDESPREAD PRECIPITATION TUESDAY THROUGH THURSDAY,
ENSEMBLES HIGHLIGHT PRECIPITABLE WATER (PW) AMOUNTS OF 0.2-0.7"
AND 80-110% OF NORMAL. THUS, RAIN AMOUNTS THIS WEEK WILL NOT
CONTRIBUTE TO AREA RIVER RISES AND WILL ALLOW RIVERS TO CONTINUE
TO RECEDE. WIDESPREAD PRECIPITATION IS EXPECTED TUESDAY, WITH
LOWER ELEVATIONS OF THE COLUMBIA BASIN, YAKIMA VALLEY, AND THE
BLUE MOUNTAIN FOOTHILLS PICKING UP 0.10-0.20", WITH 0.10" OR LESS
LIKELY OVER CENTRAL OREGON. CONFIDENCE IN THESE RAIN AMOUNTS IS
MODERATE TO HIGH (50-70%) AS ENSEMBLES SHOW AN OVERALL SPREAD OF
0.01-0.05" OF 24 HOUR RAINFALL ACROSS THE COLUMBIA BASIN.
ENSEMBLES ARE STILL HINTING AT A LOW CHANCE (10-30%) OF
MEASURABLE SNOWFALL (0.01" OR GREATER) ACROSS LOWER ELEVATIONS OF
CENTRAL OREGON, YAKIMA VALLEY, COLUMBIA BASIN, AND THE BLUE
MOUNTAIN FOOTHILLS DURING THE MORNING HOURS WEDNESDAY ONWARD, WITH
WEDNESDAY MORNING BEING THE BEST CHANCE. GUIDANCE STILL STRUGGLES
WITH THE OVERALL STRENGTH AND LOCATION OF THE UPPER LEVEL LOW
PRESSURE THAT WILL BE DROPPING SOUTH FROM THE BRITISH COLUMBIA
COAST AND ALONG THE EAST PACIFIC THROUGH THE WEEK. THIS RELATES TO
THE AMOUNT OF MOISTURE AVAILABLE FOR THE SYSTEM TO SEND INTO THE
PACIFIC NORTHWEST AND HOW EFFICIENT COLD AIR ADVECTION WILL BE VIA
THE INITIATION OF NORTHWEST FLOW ALOFT. CURRENTLY, 26% OF
ENSEMBLE MEMBERS SUGGEST A WEAKER AND CLOSER UPPER LOW FEATURE
WHICH WOULD PROVIDE AN 80-90% CHANCE ACROSS CENTRAL OREGON AND THE
YAKIMA VALLEY AND A 50-60% CHANCE OVER THE NORTHERN BLUE
MOUNTAINS AND THE TRI-CITIES AREA OF MEASURABLE SNOWFALL
OVERNIGHT TUESDAY NIGHT INTO WEDNESDAY MORNING. CHANCES CURRENTLY
LOOK MUCH LOWER WEDNESDAY NIGHT INTO THURSDAY MORNING WITH ONLY
30% OF ENSEMBLE MEMBERS PROVIDING A 25-40% CHANCE OF MEASURABLE
SNOWFALL OVER THE NORTHERN BLUE MOUNTAIN AND LOWER COLUMBIA BASIN,
WHICH SLIGHTLY IMPROVES TO 30% OF ENSEMBLES PROVIDING A 40-60%
CHANCE OF MEASURABLE SNOWFALL THURSDAY NIGHT INTO FRIDAY MORNING.
EVEN IF LIGHT SNOWFALL OCCURS, MINIMAL IMPACTS ARE LIKELY DUE TO
AFTERNOON HIGHS REACHING INTO THE MID-40S.
CONFIDENCE IN MIDWEEK RAIN AMOUNTS ARE ALSO LOW TO MODERATE
(30-60%) AS DISCREPANCIES BETWEEN SYSTEM LOCATION AND STRENGTH
DIRECTLY RELATE TO THE AMOUNT OF AVAILABLE MOISTURE. ENSEMBLES
SHOWCASE THIS AS THE SPREAD BETWEEN THE 25TH-75TH PERCENTILE IN
24 HOUR PRECIPITATION WEDNESDAY IS BETWEEN 0.20-0.30" ACROSS THE
COLUMBIA BASIN. THIS IMPROVES SLIGHTLY TO AROUND 0.10" THURSDAY
AND FRIDAY, BUT CURRENT AMOUNTS ARE FORECAST TO BE MINIMAL (0.04"
OR LESS) BOTH DAYS. MIDWEEK HIGH TEMPERATURES CHANGE VERY LITTLE
ACROSS THE REGION, BUT OVERNIGHT LOWS ARE EXPECTED TO SLOWLY
DECREASE THROUGH THE WEEK. 75
AVIATION...12Z TAFS
LATEST RADAR SHOWED PRECIPITATION ACROSS SOUTHWESTERN
OREGON MOVING NORTHEAST. THIS PRECIPITATION WILL BEGIN TO
OVERSPREAD BDN/RDM SHORTLY AND THEN WORK NORTHEASTWARD THROUGHOUT
THE DAY.
VFR CONDITIONS ARE EXPECTED TO DETERIORATE TO MVFR AND IFR AS THE
PRECIPITATION MOVES IN. BDN AND RDM COULD SEE SN OR RASN AT THE
START BEFORE CHANGING TO RA. ALL OTHER SITES WILL SEE RA AS THE
PRECIPITATION WILL MOVE IN LATER AND TEMPERATURES WILL BE WARMER.
THERE IS A BIT OF UNCERTAINTY AS TO THE TIMING OF PRECIPITATION
MOVING IN AT ALL SITES, BUT LATEST FORECASTS SHOULD BE WITHIN A
COUPLE OF HOURS.
THERE WILL LIKELY BE A BIT OF A BREAK IN THE EARLY EVENING BEFORE
MORE RA MOVES IN OVERNIGHT.
WINDS WILL GENERALLY BE 10 KTS OR LESS.
PRELIMINARY POINT TEMPS/POPS
PDT 43 33 50 33 / 80 70 10 0
ALW 44 36 50 37 / 80 80 20 10
PSC 43 33 49 32 / 40 60 0 0
YKM 39 26 42 27 / 20 40 0 30
HRI 42 33 49 32 / 70 70 0 0
ELN 35 23 37 24 / 20 60 10 50
RDM 44 27 48 27 / 90 70 10 10
LGD 40 31 43 30 / 100 80 30 10
GCD 43 30 45 30 / 100 80 20 10
DLS 43 35 45 34 / 40 80 30 70
PDT WATCHES/WARNINGS/ADVISORIES
OR...WINTER WEATHER ADVISORY UNTIL 4 AM PST MONDAY FOR ORZ509.
WA...WINTER WEATHER ADVISORY UNTIL 4 AM PST MONDAY FOR WAZ522.
DISCUSSION...75
AVIATION...77
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