036
FXUS66 KPDT 201718
AFDPDT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PENDLETON OR
918 AM PST THU FEB 20 2025
UPDATED AVIATION...18Z TAFS
ALL TAF SITES MINUS YKM ARE VFR AND
ARE EXPECTED TO STAY AS SUCH THROUGH THE PERIOD. MOSTLY HIGH
CLOUDS EXPECTED THROUGH THE PERIOD. YKM WILL REMAIN IFR/LIFR
THROUGH AT LEAST 21Z WITH MODERATE CONFIDENCE (40-50%) IN THE
TIMING OF THE FOG LIFTING OUT BY THEN. YKM WILL SEE BRIEF VFR
CONDITIONS BEFORE THE FOG ROLLS BACK IN AROUND 5Z, AGAIN,
CONFIDENCE IN TIMING OF ONSET IS MODERATE (40-50%). WINDS ACROSS
ALL TAF SITES WILL BE LIGHT BELOW 12 KTS. BENNESE/90
PREV DISCUSSION
/ISSUED 345 AM PST THU FEB 20 2025/
SHORT TERM...TODAY THROUGH SATURDAY...KEY MESSAGES...
- PATCHY FOG THIS MORNING AND LATE TONIGHT/TOMORROW MORNING WITH
VISIBILITIES SUB-1SM ACROSS WA VALLEYS AND THE LOWER BASIN
- HIGH RAIN CHANCES IN THE CASCADES LATE TOMORROW AND SATURDAY
WITH LIMITED IMPACTS AND MINOR HIGH MOUNTAIN SNOW
- WARMING TREND WITH HIGHS PEAKING 4-8 DEGREES ABOVE NORMAL BY
SATURDAY ACROSS CENTRAL OR AND THE LOWER COLUMBIA BASIN.
LATEST WATER VAPOR SHOWS NORTHERLY FLOW ALOFT AS AN UPPER TROUGH
EXITS INTO THE CENTRAL GREAT BASIN WITH AN UPPER RIDGE BUILDING IN
ACROSS THE PACNW COAST AND SOUTHERN BRITISH COLUMBIA. LATEST
NTMICRO RGB IMAGERY SHOWS STRATUS FOG ACROSS PARTS OF THE YAKIMA
VALLEY INTO THE WA LOWER BASIN WITH VISIBILITIES VARYING
PERIODICALLY 1/2SM TO 5SM OVER THE LAST THREE HOURS WITH SCATTERED
HIGH CLOUDS DRIFTING SOUTH. MODEST 500 MB HEIGHT RISES OF 5-8 DAM
WILL BE SPREADING OVER THE REGION TODAY AS THE UPPER LEVEL RIDGE
BUILDS OVER THE INTERIOR NW WITH BROAD SURFACE HIGH PRESSURE
DOMINATING WITH MINOR SYNOPTIC IMPOSED PRESSURE GRADIENTS. THIS
SHOULD LEND TO FAIR WEATHER WITH STRATUS/FOG DECREASING BY MID-
LATE MORNING WITH MIXING. UNTIL THEN, PATCHY FOG WITH VISIBILITIES
AS LOW AS 1/2SM AT TIMES WILL BE LIKELY (50-80%) ACROSS I-82
CORRIDOR, WA ROUTE 240, AND US 395. DECENT SIGNAL APPARENT IN
HREF GUIDANCE AND THIS FURTHER INCREASES CONFIDENCE IN TANDEM
WITH OBS/TRENDS SEEN SO FAR. OTHERWISE, HIGHS WILL RISE TODAY TO
BE NEAR NORMAL OUTSIDE THE EASTERN MOUNTAINS. GENERALLY 6-12
DEGREES WARMER THAN YESTERDAY OUTSIDE THE CASCADES AND CENTRAL OR;
SIMILAR HIGHS ACROSS THE EASTERN MOUNTAINS AND CENTRAL OR
COMPARED TO YESTERDAY. THAT SAID, ANOTHER WINDOW FOR FOG IS
EXPECTED ACROSS THE SOUTH CENTRAL WA VALLEYS AND THE LOWER
COLUMBIA BASIN LATE TONIGHT-FRIDAY MORNING. CURRENT THINKING
THERE IS AT LEAST 40-70% CHANCE FOR LOW VISIBILITIES NEAR AND
BELOW 1SM WITH GREATEST CONFIDENCE ACROSS THE HANFORD AREA, YAKIMA
VALLEY, AND BENTON COUNTY.
A QUICK MOVING UPPER LOW WILL BE DEVELOPING WELL WEST OF VANCOUVER
ISLAND FRIDAY BEFORE WEAKENING INTO AN OPEN WAVE AND ROTATING
NORTHWARD INTO SOUTHEAST AK FRIDAY NIGHT. THE WEAK FRONTAL SYSTEM
IN TANDEM WITH A MODEST IVT PLUME ORIENTED SOUTHWEST-NORTHEAST
OVER WESTERN WA AND THE OLYMPICS LATE FRIDAY-FRIDAY NIGHT WILL
BRING HIGH CHANCES FOR PRECIP (> 75%) ACROSS THE WA CASCADES WITH
LOWER CHANCES ELSEWHERE. SCANT IMPACTS EXPECTED OWING TO
INCREASING SNOW LEVELS OF ~3.5 TO 5 KFT ACROSS THE WA CASCADES.
THIS DISTURBANCE WILL THEN BE REINFORCED BY ANOTHER DISTURBANCE
WITH A STRONG IVT SURGE MOVING IN SATURDAY. THE LATTER’S AXIS
FOCUSED MORE ACROSS THE OR/WA COAST WITH A SOUTHWESTERLY
TRAJECTORY. EPS MEAN IVT FORECASTS EXCEED THE 99.5TH PERCENTILE
BY SATURDAY AFTERNOON INCREASING CONFIDENCE IN CASCADE CREST/UPPER
EASTERN SLOPES PRECIP. WHILE CHANCES FOR MOUNTAIN RAIN RAMP UP IN
THE CASCADES 60-90%, LIMITED MOISTURE TRANSPORT INLAND THROUGH
EARLY AFTERNOON WILL BOUND CHANCES, PEAKING 15-30%, ACROSS THE
LOWER ELEVATIONS SATURDAY.
LONG TERM...SATURDAY EVENING THROUGH THURSDAY...AN ACTIVE WEATHER
PATTERN IS IN STORE FOR THE REGION THIS WEEKEND THROUGH EARLY NEXT
WEEK, WITH A REPRIEVE FROM PRECIPITATION MIDDLE OF NEXT WEEK.
THROUGH THE EXTENDED FORECAST, MAIN FORECAST CONCERNS WILL REVOLVE
AROUND AN ATMOSPHERIC RIVER(AR) EVENT OVER THE WEEKEND THAT WILL
INCREASE HYDROLOGICAL CONCERNS INTO EARLY NEXT WEEK.
ENSEMBLE CLUSTER GUIDANCE IS IN GREAT AGREEMENT SATURDAY EVENING
THAT A BROAD SOUTHWEST FLOW ALOFT WITH A DECENT JET STREAK WILL
BE SETUP OVER THE PACNW AS AN UPPER LEVEL RIDGE AXIS CENTERS
OVER THE NORTHERN ROCKIES WITH TROUGHING CENTERED NEAR THE
ALEUTIAN ISLAND CHAIN. THE FLOW REGIME ALOFT WILL ALLOW A SERIES
OF SHORTWAVE TROUGHS TO LIFT ACROSS THE PACNW FROM THROUGH EARLY
TUESDAY, THOUGH DISAGREEMENT ON TIMING GROWS LATE MONDAY INTO
TUESDAY.
THE FIRST, AND ARGUABLY WETTEST, IN A SERIES OF SHORTWAVE TROUGH
SYSTEMS WILL ARRIVE TO THE PACNW BY SATURDAY EVENING, AND WILL BE
ACCOMPANIED BY A WEAK TO MODERATE AR AND SURFACE WARM FRONT. SNOW
LEVELS WILL START OFF BETWEEN 5KFT TO 6.5KFT SATURDAY EVENING AND
WILL RISE TO 6.5KFT TO 8KFT AS A WARM, MOIST AIRMASS FILTERS INTO
THE REGION BEHIND THE WARM FRONT. THIS WILL FACILITATE MOSTLY
RAIN BELOW THE HIGHEST MOUNTAIN PEAKS ALONG THE CASCADE CREST AND
EASTERN MOUNTAINS THROUGH SUNDAY. THERE WILL BE PLENTY OF MOISTURE
TO WORK WITH AS THE AR WITH THIS SYSTEM WILL BE POINTED RIGHT
INTO THE PACNW. BOTH THE GFS AND ECMWF IVT ENSEMBLE SUITES SHOW AN
80-100% CHANCE OF IVT VALUES GREATER THAN 250 KG/M/S DURING A 24
HOUR WINDOW MAKING IT EAST OF THE CASCADE CREST, PROVIDING ENOUGH
MOISTURE FOR MODERATE TO HEAVY PRECIPITATION ACCUMULATIONS ALONG
THE CASCADE CREST AND THE INTERIOR NORTHERN BLUES. AS FOR THE
REMAINDER OF THE FORECAST AREA, THE WEST TO SOUTHWEST ORIENTATION
OF THE FLOW ALOFT WILL RESULT IN A MARKED RAINSHADOWING EFFECT
ACROSS THE LOWER ELEVATIONS AND PORTIONS OF THE EASTERN MOUNTAINS.
LATE SUNDAY, THE SECOND SHORTWAVE TROUGH IS EXPECTED TO QUICKLY
PASS THROUGH THE PACNW AND EXIT BY MONDAY MORNING, WITH THE AR
EFFECTIVELY ENDING SUNDAY NIGHT INTO MONDAY MORNING. LOOKING AT
NBM 48-HR PROBABILITIES ENDING MONDAY MORNING FOR PRECIPITATION
ACCUMULATIONS SHOWS A 90-100% CHANCE ALONG THE CASCADE CREST/EST
SLOPES AND A 55-70% CHANCE ALONG THE INTERIOR NORTHERN BLUES FOR 1
INCH OF RAIN ACCUMULATIONS. LOOKING AT 2 INCHES OR GREATER,
CHANCES ARE BETWEEN 60-90% ALONG THE CASCADE CREST, WITH LESS THAN
25% CHANCE ALONG THE INTERIOR BLUES. IN THE LOWER ELEVATION,
THERE IS GENERALLY A 60-80% CHANCE OF 0.25 INCHES OF RAIN ACROSS
CENTRAL/NORTH CENTRAL OR, THE YAKIMA/KITTITAS/GRANDE RONDE
VALLEYS, AND ALONG THE NORTHERN BLUE MOUNTAIN FOOTHILLS; A LESS
THAN 40% CHANCE ACROSS THE REMAINDER OF THE LOWER ELEVATION ZONES.
THE LAST OF THE SHORTWAVE TROUGH SYSTEMS TO IMPACT THE REGION
THROUGH EARLY NEXT WEEK IS EXPECTED TO ARRIVE THE SECOND HALF OF
MONDAY, THOUGH DISAGREEMENT GROWS AMONGST ENSEMBLE CLUSTER
SOLUTIONS IN TIMING OF THE SYSTEM ARRIVAL/PASSAGE. ABOUT 50% OF
INDIVIDUAL ENSEMBLE MEMBERS HAVE THE SYSTEM ARRIVE EARLY EVENING
MONDAY AND EXIT EARLY TUESDAY, WHILE THE OTHER 50% HAVE THE SYSTEM
ARRIVE LATE MONDAY NIGHT AND EXIT TUESDAY AFTERNOON. OTHERWISE,
CONFIDENCE IS MODERATE (50-65%) THAT THE CASCADE CREST AND
NORTHERN BLUES WILL SEE ANOTHER 0.25-0.5 INCHES IN RAIN, THOUGH
SNOW LEVELS WILL BE LOWERING TO 5KFT TO 6KFT, ALLOWING MORE SNOW
PRODUCTION ABOVE THESE ELEVATIONS. IN THE LOWER ELEVATIONS,
CONFIDENCE IS LOW-MODERATE(25-40%) IN UP TO ANOTHER 0.1 INCHES OF
RAIN ACCUMULATIONS. AFTER TUESDAY, ENSEMBLE GUIDANCE COMES BACK
INTO GREAT AGREEMENT IN AN UPPER LEVEL RIDGE BUILDING OVER THE
PACNW WITH DRY AND CALMER CONDITIONS ACROSS THE REGION.
WITH THE EXPECTATION OF RAIN AND WARMING CONDITIONS IN THE
MOUNTAINS THIS WEEKEND THROUGH EARLY NEXT WEEK, INCREASED RUNOFF
AND SNOWMELT WILL BE A CONCERN. THERE IS HIGH CONFIDENCE (75-90%)
THAT AREA RIVERS AND STREAMS WILL SEE INCREASED FLOWS AND LEVELS
INTO EARLY NEXT WEEK DUE TO THE RAIN ON SNOW, HOWEVER, CONFIDENCE
IS LOW (20-30%) ON WHETHER AREA RIVERS WILL REACH MINOR FLOOD
CATEGORIES. LAWHORN/82
PRELIMINARY POINT TEMPS/POPS
PDT 46 29 47 36 / 0 0 10 30
ALW 44 30 44 36 / 0 0 10 30
PSC 47 30 44 33 / 0 0 10 20
YKM 45 29 41 32 / 0 0 10 20
HRI 49 29 46 34 / 0 0 10 20
ELN 45 30 41 32 / 0 10 20 30
RDM 47 26 51 32 / 0 0 10 20
LGD 41 22 40 32 / 0 0 10 30
GCD 40 24 46 31 / 0 0 0 20
DLS 50 34 46 38 / 10 10 10 30
PDT WATCHES/WARNINGS/ADVISORIES
OR...NONE.
WA...FREEZING FOG ADVISORY UNTIL 11 AM PST THIS MORNING FOR WAZ027.
SHORT TERM...80
LONG TERM....82
AVIATION...90
CLICK HERE TO GO TO PREVIOUS BULLETINS.
Main Text Page