255
FXUS66 KPDT 212057
AFDPDT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PENDLETON OR
157 PM PDT TUE APR 21 2026
KEY MESSAGES
- CHANCE OF THUNDERSTORMS THIS AFTERNOON THROUGH OVERNIGHT
TONIGHT.
- BREEZY TO LOCALLY WINDY CONDITIONS EXPECTED ACROSS THE
COLUMBIA BASIN/NORTH CENTRAL OREGON THROUGH THE KITTITAS
VALLEY.
- MODERATE TO HEAVY SNOW EXPECTED IN ELEVATED AREAS IN THE
NORTHERN BLUE MOUNTAINS ON WEDNESDAY.
DISCUSSION
CURRENT SATELLITE IMAGERY SHOWS A CLOSED OFF LOW PRESSURE
SYSTEM JUST OFF-SHORE OF OREGON AND CALIFORNIA BRINGING A
MARITIME AIRMASS INTO THE PACNW REGION. HIGH CLOUD COVER TOPS
ARE CURRENTLY SITUATED OVER CENTRAL OREGON THAT WILL CONTINUE
TO BUILD INTO THE AREA AS THE LOW MOVES TO THE EAST. RADAR
CURRENTLY SHOWS VERY LIGHT RETURNS NEAR THE CASCADE MOUNTAINS
WITH A LIGHT THUNDERSTORM DEVELOPING IN UMATILLA COUNTY.
THE SYNOPTIC PATTERN SHOWS THAT THE PREVIOUS MENTIONED LOW
PRESSURE OFF-SHORE WILL CONTINUE PUSHING EAST, JUMPING RAIN
CHANCES UP TO 80-100 PERCENT BY THE LATER AFTERNOON HOURS TODAY
ACROSS THE BLUE MOUNTAINS AND FOOTHILLS, WITH MORE MODERATE
40-60 PERCENT CHANCES ACROSS THE REST OF THE CWA. OUR GREATEST
THUNDERSTORM WINDOW WILL BE THIS AFTERNOON THROUGH THE EVENING
HOURS TODAY AS 72% OF CLUSTERS DEVELOP 500-750 J/KG OF CAPE
THROUGH THE BLUE MOUNTAINS INTO THE FOOTHILLS & KITTITAS
VALLEY. HREF ENSEMBLES HAVE A CLUSTER OF >40 DBZ & GREATER THAN
50 MUCAPE PAINTBALLS RAPIDLY DEVELOPING IN THE BLUE MOUNTAINS
AND FOOTHILLS BY THE 3-4 PM WINDOW THAT TRAJECT THROUGH THE EAST
GOING INTO THE EVENING HOURS. BY THE LATE AFTERNOON/EARLY
EVENING HOURS, THE GREATEST THUNDERSTORM CHANCES WILL BE IN THE
WALLOWAS/NORTHERN BLUE MOUNTAINS AS THE LOW PRESSURE SYSTEM JOGS
ACROSS THE EAST.
GUIDANCE IS IN VERY GOOD AGREEMENT THAT AS THE CLOSED-LOW MOVES
TO THE IDAHO/MONTANA REGION, OUR WEATHER WILL BE MORE
INFLUENCED BY A SECONDARY TROUGH LOCATED IN BRITISH COLUMBIA
JOGGING SOUTH, THAT WILL BRING HEAVIER PRECIPITATION RATES ON
WEDNESDAY MORNING THROUGH THE EVENING HOURS. GUIDANCE POINTS
THAT THE 6-HOUR PRECIPITATION RATE THROUGH WEDNESDAY MORNING
WILL BE BETWEEN 0.15 TO 0.35 INCHES, WITH THE MORE HEAVIER RAIN
SITUATED ALONG THE FOOTHILLS AND THE BLUE MOUNTAINS. NBM AND
CLUSTER ANALYSIS ARE IN GOOD AGREEMENT THAT THE HEAVIEST RAIN
WILL OCCUR WEDNESDAY MORNING THROUGH THE LATE AFTERNOON WITH THE
MAXIMA QPF SITUATED ACROSS THE NORTHERN BLUE MOUNTAINS WITH AT
LEAST 2 TO 3 INCHES EXPECTED (70-80% CONFIDENCE). THE HIGH
CONFIDENCE IS BACKED BY THE EFI SHOWING 0.9 TO 1.0 VALUES ACROSS
THE NORTHERN BLUES INTO THE EASTERN MOST PARTS OF THE COLUMBIA
BASIN WITH A SHIFT OF TAILS VALUE OF 2. ONE LAST THING TO NOTE
ABOUT THE PRECIPITATION VALUES IS THAT CLUSTER ANALYSIS MOSTLY
SHOW 0.3 TO 0.5 INCH 6-HOUR RAIN RATES IN THE MORE IMPACTED
AREAS, BUT ABOUT 9% OF MEMBERS SUGGESTS STRONGER OROGRAPHIC LIFT
FROM THE NORTH/NORTHWEST WINDS TO BRING UP TO 1.25 INCHES OF
RAIN IN A 6-HOUR WINDOW ALONG THE FOOTHILLS AND SMALL PARTS OF
THE OREGON COLUMBIA BASIN. ALTHOUGH CHANCES ARE SMALL, THE
POSSIBILITY TO SEE SOME LOCALIZED FLOODING IN EXISTS. TIGHT
PRESSURE GRADIENTS FROM THE SECOND TROUGH WILL ENABLE BREEZY TO
LOCALLY WINDY CONDITIONS EMBEDDED WITH THE RAIN SHOWERS ACROSS
PARTS OF THE COLUMBIA BASIN THROUGH THE SIMCOE HIGHLANDS &
KITTITAS VALLEY. WIND GUSTS WILL BE AS HIGH AS 50 MPH (60-80%
CONFIDENCE) THAT WILL BRING REDUCED VISIBILITY ALONG ROADS,
ESPECIALLY FOR HIGHER PROFILE VEHICLES.
COOLER NORTHERN AIR FROM THE SECOND TROUGH WILL BRING SNOW
LEVELS DOWN TO ABOUT 4500-5500 FEET ACROSS THE CWA, BRINGING
HIGH LEVEL MOUNTAIN SNOW INTO THE REGION. ALTHOUGH THE NBM
WANTS TO BRING 20-25 INCHES IN CERTAIN PARTS OF THE NORTHERN
BLUE MOUNTAINS (MAINLY ABOVE 4500 FEET), THERE'S GREAT
UNCERTAINTY THAT WILL MANIFEST WITH THE 10 TO 1 SNOW RATIO AND
BEING IN A LATE SEASON SYSTEM. TEMPERATURES ARE JUST BELOW
FREEZING WEDNESDAY WITH THE HIGHS AROUND 30-35 THAT COULD
REGRESS SOME OF THE SNOW ACCUMULATION. GIVEN THAT, WE DON'T HAVE
MUCH CONFIDENCE TO WARRANT A WARNING FOR THIS SYSTEM, HOWEVER
THE RANGE INDICATED IN THE ADVISORY DEPICTS THAT OF A HIGH-END
ADVISORY.
THURSDAY ONWARDS WILL BRING A DRIER AND WARMER PERIOD THANKS TO
A REX-STYLE HIGH PRESSURE PATTERN. MOSTLY DRY CONDITIONS WITH A
FEW SHOWERS DEVELOPING IN THE WALLOWAS AND NORTHERN BLUES BUT NO
MAJOR IMPACTS EXPECTED THROUGH THE WEEKEND.
AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/
VFR CONDITIONS WILL PREVAIL THROUGH THE DAYLIGHT HOURS AS RAIN
BEGINS TO MOVE IN LATER TODAY AND INTO TONIGHT. RAIN WILL BECOME
STEADIER AND POSSIBLY HEAVIER OVERNIGHT WHICH WILL LEAD TO MVFR
CIGS (AND POSSIBLY VSBYS) BY MORNING.
THERE IS A LOW (<20 PERCENT) CHANCE OF TSRA THIS AFTERNOON INTO
THE EVENING HOURS, WITH RDM BDN, PDT AND ALW HAVING THE BEST OF
THE LOW CHANCES. WILL WAIT AND SEE HOW THINGS DEVELOP BEFORE
ADDING ANY CHANCES TO THE INDIVIDUAL TERMINALS.
WINDS WILL INCREASE AND BECOME GUSTY THIS AFTERNOON WITH GUSTS
AROUND 25 KTS AND WILL INCREASE FURTHER ON WEDNESDAY, WITH GUSTS
POSSIBLY AS HIGH AS 30 TO 35 KTS.
HYDROLOGY
RAINFALL FROM THE TWO SYSTEMS THROUGH WEDNESDAY WILL ACCUMULATE
TO BRING A FEW RIVERS TO EXCEED ACTION STAGE WEDNESDAY IN THE
NACHES. FURTHER DOWNSTREAM RIVERS IN TOUCHET, TROY, AND GIBBON
ARE CURRENTLY FORECASTED TO EXCEED ACTION A DAY LATER ON
THURSDAY.
PRELIMINARY POINT TEMPS/POPS
PDT 43 50 39 57 / 90 100 50 40
ALW 44 51 41 57 / 90 100 70 50
PSC 48 59 42 65 / 90 90 30 30
YKM 48 63 41 66 / 60 70 10 10
HRI 45 55 40 62 / 90 90 30 20
ELN 45 57 39 59 / 60 70 10 10
RDM 36 52 27 59 / 50 60 0 0
LGD 40 46 37 51 / 100 100 90 50
GCD 38 46 33 52 / 100 90 60 20
DLS 48 58 43 64 / 50 70 10 0
PDT WATCHES/WARNINGS/ADVISORIES
WA...WIND ADVISORY FROM 5 AM TO 11 PM PDT WEDNESDAY FOR WAZ024-029-
521.
WINTER WEATHER ADVISORY FROM 5 AM TO 5 PM PDT WEDNESDAY FOR
WAZ030.
WIND ADVISORY FROM 8 PM THIS EVENING TO 11 PM PDT WEDNESDAY
FOR WAZ026.
OR...WIND ADVISORY FROM 5 AM TO 11 PM PDT WEDNESDAY FOR ORZ041-044-
507-508-510.
WINTER WEATHER ADVISORY FROM 5 AM TO 5 PM PDT WEDNESDAY FOR
ORZ502.
DISCUSSION...95
AVIATION...77
HYDROLOGY...95
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