699
FXUS66 KPDT 211148
AFDPDT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PENDLETON OR
350 AM PST SAT DEC 21 2024
UPDATED AVIATION DISCUSSION
SHORT TERM...TODAY THROUGH MONDAY
KEY MESSAGES:
1. WIDESPREAD RAIN AND MOUNTAIN SNOW TODAY, RETURNING SUNDAY.
2. BREEZY WINDS THIS MORNING AND AFTERNOON, RETURNING SUNDAY.
*WIND ADVISORY ISSUED*
3. WIDESPREAD RAIN, MOUNTAIN SNOW, AND BREEZY WINDS LATE MONDAY.
CURRENT RADAR AND INFRARED SATELLITE IMAGERY SHOWING POCKETS OF
LIGHT RETURNS ALONG THE EAST SLOPES OF THE CASCADES, CENTRAL OREGON,
SOUTHERN BLUE MOUNTAINS, GORGE, AND THE SIMCOE HIGHLANDS UNDER
PARTLY TO MOSTLY CLOUDY SKIES. SATELLITE AND AREA WEBCAMS ARE ALSO
PICKING UP ON SOME AREAS OF DENSE FOG ACROSS THE NORTHERN BLUE
MOUNTAIN FOOTHILLS, LOWER COLUMBIA BASIN OF OREGON, AND THE KITTITAS
VALLEY, WHICH LOOKS TO PERSIST OVERNIGHT AND INTO THE MORNING ON
SATURDAY. THUS, A DENSE FOG ADVISORY HAS BEEN ISSUED ACROSS THE
NORTHERN BLUE MOUNTAIN FOOTHILLS AND THE LOWER COLUMBIA BASIN OF
OREGON, AND A FREEZING FOG ADVISORY HAS BEEN ISSUED ACROSS THE
KITTITAS VALLEY UNTIL 10 AM SATURDAY MORNING - SO USE CAUTION IF
TRAVELING THIS MORNING AS VISIBILITIES WILL BE AS LOW AS A
QUARTER MILE AND ROADWAYS MAY BE SLICK.
THE SPOTTY RAIN CURRENTLY OBSERVED ON RADAR IMAGERY IS A RESULT OF
AN APPROACHING UPPER LEVEL TROUGH AND ATTENDANT FRONTAL SYSTEM,
WHICH WILL ALLOW FOR WIDESPREAD RAINFALL AND MOUNTAIN SNOW THROUGH
THE DAY TODAY. THESE INITIAL LIGHT SHOWERS ARE ASSOCIATED WITH THE
WARM FRONT THAT WILL PASS FROM SOUTH TO NORTH THIS MORNING, AHEAD OF
THE COLD FRONT THAT WILL PASS FROM WEST TO EAST LATER THIS MORNING
THROUGH THE EARLY AFTERNOON. THE WARM FRONT AND PERSISTENT
SOUTHWEST FLOW ALOFT WILL ATTRIBUTE TO SNOW LEVELS STAYING IN THE
5000-6000 FOOT RANGE THROUGH THE AFTERNOON BEFORE DROPPING TO
4000-5000 FEET LATER THIS EVENING BEHIND THE COLD FRONT. THESE
RATHER HIGH SNOW LEVELS WILL KEEP SNOW CONFINED TO HIGHER
ELEVATIONS OF THE CASCADE AND BLUE MOUNTAINS AS ONLY AROUND 1 INCH
OF SNOWFALL IS ANTICIPATED ACROSS OUR CASCADE PASSES TODAY.
CONFIDENCE IN THESE SNOW AMOUNTS IS HIGH (80%) AS THE NBM SUGGESTS
A 40-50% CHANCE OF 1 INCH OR MORE SNOWFALL ACROSS SANTIAM AND
WHITE PASSES THROUGH 10 AM SUNDAY. MEASURABLE RAINFALL (0.01" OR
GREATER) IS EXPECTED FOR ALL LOCATIONS WITH 0.10-0.20" LIKELY
ALONG THE NORTHERN BLUE MOUNTAIN FOOTHILLS AND 0.01-0.10" ACROSS
LOWER ELEVATIONS OF THE COLUMBIA BASIN, CENTRAL OREGON, JOHN DAY-
OCHOCO BASIN, AND THE YAKIMA/KITTITAS VALLEYS. CONFIDENCE IN THESE
RAIN AMOUNTS IS MODERATE TO HIGH (70-80%) AS THE NBM HIGHLIGHTS A
60-90% CHANCE OF 0.10" OR MORE RAINFALL ACROSS THE NORTHERN BLUE
MOUNTAINS AND A 10-45% CHANCE ACROSS CENTRAL OREGON, LOWER
ELEVATIONS OF THE COLUMBIA BASIN, AND THE YAKIMA/KITTITAS VALLEYS.
ANOTHER UPPER LEVEL TROUGH AND ATTENDANT FRONTAL SYSTEM WILL BE
APPROACHING THE COAST ON SUNDAY, PASSING ACROSS THE AREA LATE
SUNDAY INTO MONDAY. THIS SECOND SYSTEM WILL AGAIN BRING WIDESPREAD
RAIN AS ANOTHER 0.10-0.20" OF RAINFALL IS LIKELY ALONG THE
NORTHERN BLUE MOUNTAIN FOOTHILLS AND 0.05-0.15" OF RAINFALL ACROSS
LOWER ELEVATIONS OF THE COLUMBIA BASIN, CENTRAL OREGON, AND THE
YAKIMA/KITTITAS VALLEYS. CONFIDENCE IN THESE RAIN AMOUNTS IS ALSO
MODERATE TO HIGH (70-80%) AS THE NBM SUGGESTS A 70-90% CHANCE OF
0.10" OR MORE RAIN ALONG THE NORTHERN BLUE MOUNTAIN FOOTHILLS AND
A 25-65% CHANCE OF 0.10" OR MORE RAIN OVER CENTRAL OREGON, LOWER
ELEVATIONS OF THE COLUMBIA BASIN, AND THE YAKIMA/KITTITAS VALLEYS.
SIMILAR TO TODAY'S SYSTEM, SUNDAY EVENING'S SYSTEM WILL ALSO LEAD
WITH A WARM FRONT ARRIVING FROM OUR SOUTH WHICH WILL INCREASE
SNOW LEVELS AGAIN TO THE 5500-6500 FOOT RANGE. SNOW LEVELS WILL BE
SLOWLY DROPPING THROUGH THE EARLY MORNING HOURS ON MONDAY AS THE
COLD FRONT ADVECTS COOLER AIR INTO THE REGION. THIS WILL RELATE TO
VERY LITTLE MOUNTAIN SNOW ACCUMULATIONS ON SUNDAY AND MONDAY,
WITH THE MAJORITY OF SNOWFALL EXPECTED ACROSS THE WASHINGTON
CASCADES IN THE FORM OF 1- 4 INCHES ABOVE 4500 FEET.
TODAY'S PASSING FRONTAL SYSTEM WILL ALSO BRING BREEZY WINDS ACROSS
THE AREA AS A PRESSURE GRADIENT IS EXPECTED TO DEVELOP ALONG THE
BLUE MOUNTAINS. THIS WILL ATTRIBUTE TO BREEZY WINDS ALONG THE BLUE
MOUNTAIN FOOTHILLS AND THE GRANDE RONDE VALLEY. WINDS ARE
ANTICIPATED TO BE HIGHEST ACROSS THE GRANDE RONDE VALLEY AS THE GFS,
NAM, AND SREF ADVERTISE A PRESSURE GRADIENT OF 5-7 MB BETWEEN BAKER
CITY (KBKE) AND MEACHAM (KMEH) AND 7-10 MB BETWEEN ONTARIO (KONO)
AND MEACHAM (KMEH). BOTH CROSS-SECTIONS MEET MINOR ADVISORY
THRESHOLDS (KBKE-KMEH OF 5 MB AND KONO-KMEH OF 10 MB), THUS A WIND
ADVISORY HAS BEEN ISSUED THROUGH 3 PM SATURDAY AS SUSTAINED SOUTH
WINDS OF 25-35 MPH AND GUSTS UP TO 50 MPH ARE EXPECTED. CONFIDENCE
IN THESE WIND VALUES IS HIGH (80-90%) AS THE HREF SUGGESTS AN 80%
CHANCE OF WIND GUSTS OF 45 MPH OR GREATER THROUGH THE MORNING AND
AFTERNOON HOURS. WINDS DO LOOK TO RETURN ON SUNDAY WITH THE SECOND
SYSTEM, BUT GUIDANCE IS HINTING AT A MUCH WEAKER PRESSURE GRADIENT
OF 3-5 MB BETWEEN BAKER CITY (KBKE) AND MEACHAM (KMEH) AS ADVERTISED
BY THE GFS, NAM, AND SREF. CONFIDENCE IN SUB-ADVISORY WINDS ACROSS
THE GRANDE RONDE VALLEY LATE SATURDAY INTO SUNDAY IS HIGH (90%) AS
THE HREF INDICATES A 0% CHANCE OF REACHING ADVISORY-LEVEL WIND GUSTS
(45 MPH OR GREATER) AND THE NBM SUGGESTS ONLY A 40% CHANCE.
THE ACTIVE PATTERN CONTINUES THROUGH THE END OF THE SHORT-TERM
PERIOD AS YET ANOTHER UPPER LEVEL SYSTEM APPROACHES THE COAST AS THE
FRONTAL SYSTEM NUDGES ONSHORE LATE MONDAY. THIS WILL LEAD TO ANOTHER
BOUT OF WIDESPREAD RAINFALL, MOUNTAIN SNOWFALL, AND BREEZY WINDS
ACROSS THE NORTHERN BLUE MOUNTAIN FOOTHILLS AND THE GRANDE RONDE
VALLEY. SNOW LEVELS WILL AGAIN INCREASE FROM THE SOUTH AS A WARM
FRONT PUSHES THROUGH THE AREA SOUTH-TO-NORTH, LEADING TO SNOW LEVELS
BETWEEN 6000-7000 FEET LATE MONDAY INTO TUESDAY MORNING. THIS WILL
KEEP SNOWFALL ABOVE PASS LEVEL AS LITTLE TO NO SNOW ACCUMULATIONS
ARE EXPECTED. 0.15-0.35" OF RAINFALL IS ANTICIPATED ACROSS THE
EAST SLOPES OF THE CASCADES, CENTRAL OREGON, AND THE
YAKIMA/KITTITAS VALLEYS, WITH 0.05-0.15" OF RAINFALL LIKELY OVER
THE NORTHERN BLUE MOUNTAIN FOOTHILLS AND LOWER ELEVATIONS OF THE
COLUMBIA BASIN. ANOTHER PRESSURE GRADIENT OF 4.5-7 MB WILL DEVELOP
ALONG THE BLUE MOUNTAINS BETWEEN BAKER CITY (KBKE) AND MEACHAM
(KMEH) AS ADVERTISED BY THE NAM, GFS, AND SREF, WHICH WOULD
CORRELATE TO LOW END WIND ADVISORY. CONFIDENCE IS MODERATE
(40-60%) IN WIND GUSTS REACHING ADVISORY CRITERIA AS THE NBM
SUGGESTS A 40-50% CHANCE OF WIND GUSTS REACHING 45 MPH OR GREATER
ACROSS THE GRANDE RONDE VALLEY LATE MONDAY INTO TUESDAY MORNING -
SO FURTHER ANALYSIS IS NECESSARY. 75
LONG TERM...TUESDAY THROUGH SATURDAY
ACTIVE WEATHER WILL BE
OBSERVED DURING THE WEEK, AND MODELS ARE SURPRISINGLY IN GOOD
AGREEMENT WITH THE PROGRESSIVE FLOW PATTERN AND TIMING OF INDIVIDUAL
UPPER LEVEL TROUGHS. CONFIDENCE IS HIGH THAT NEXT WEEK WILL BE WET
AND OCCASIONALLY WINDY.
AN ELONGATED UPPER TROUGH ON TUESDAY MOVES ONSHORE IN THE MORNING
THEN ACROSS THE WESTERN INTERIOR OF THE US DURING THE DAY. THE ECMWF
EFI HIGHLIGHTS THE FORECAST AREA WITH HIGHER THAN AVERAGE QPF BUT
NOTHING EXTREME. THIS IS LIKELY DUE TO THE 300-500 IVT AHEAD OF THE
TROUGH TUESDAY MORNING, AS ADVERTISED BY THE ECMWF AND GFS AR PROGS.
THE HEAVIEST PRECIPITATION WILL OCCUR EARLY IN THE DAY WHEN SNOW
LEVELS ARE RELATIVELY HIGH (4.5-5.5 KFT). HOWEVER, COLD AIR
ADVECTION WITH THE PASSING TROUGH WILL LOWER SNOW LEVELS DOWN TO 3-4
KFT IN THE EVENING. SKI RESORTS SHOULD BE QUITE PLEASED WITH THE
ACCUMULATING SNOW AROUND 3-5" BUT THE MOUNTAIN PASSES ARE NOT
EXPECTED TO RECEIVE MUCH ACCUMULATION. THE NBM SHOWS < 20% OF 2+
INCHES ALONG THE MAIN MOUNTAIN PASSES EXCEPT SANTIAM PASS WHERE
THERE IS A CHANCE (30-50%) FOR 2-4" OF SNOW. ANOTHER CONCERN WITH
TUESDAY'S SYSTEM ARE THE WINDS IN THE GRANDE RONDE VALLEY AND BLUE
MTN FOOTHILLS EARLY TUESDAY. THERE IS A STRONG LIKELIHOOD THAT
THESE AREAS WILL OBSERVE 45-55 MPH GUSTS, BUT EVEN A CHANCE FOR 55-
65 MPH GUSTS. THE ECMWF EFI EVEN HIGHLIGHTS THIS AREA FOR STRONG
GUSTS, WHICH BOOSTS CONFIDENCE IN THE FORECAST.
CHRISTMAS MORNING WILL BE MET WITH MOSTLY DRY CONDITIONS, AS THE
FORECAST AREA WAITS FOR THE NEXT PACIFIC TROUGH. PRECIPITATION IN A
SOUTHWEST FLOW ALOFT WILL ARRIVE TO THE CASCADES BY LATE MORNING
THEN EAST OF THE CASCADES DURING THE AFTERNOON. THERE ARE SOME
MINOR DIFFERENCES ON THE TIMING OF PRECIPITATION, PRIMARILY FROM THE
FASTER CANADIAN ENSEMBLES. BUT, NEARLY ALL SHOW THE HIGH POPS AND
WIDESPREAD QPF FOR CHRISTMAS NIGHT AND THURSDAY. FORECASTERS WILL
KEEP A CLOSE EYE ON THIS SYSTEM, AS HOLIDAY TRAVELERS MAY FACE SNOW
ACCUMULATIONS AT PASS LEVELS, ESPECIALLY ALONG THE WA CASCADES. SNOW
LEVELS ARE FORECAST TO RANGE FROM 3.5-4.5 KFT. THE EMCWF AND
CANADIAN ARE PARTICULARLY HEAVY ON THE QPF AND SNOW ALONG THE WA
CASCADES CHRISTMAS NIGHT WHERE BOTH PLACE THIS AREA UNDER THE LEFT
EXIT REGION OF THE POLAR JET. THE GFS? NOT SO MUCH. HOWEVER, MODELS
PUSH THE TROUGH ACROSS THE PACNW ON THURSDAY. IT'S A MOSTLY FLAT
TROUGH RESULTING IN A STRONG AND MOIST WESTERLY FLOW. CONFIDENCE IN
HEAVY MOUNTAIN SNOW IS ABOUT 30%. CONFIDENCE WOULD BE HIGHER IF NOT
FOR THE MINOR MODEL DIFFERENCES. IT WILL BE ANOTHER TIME PERIOD OF
BREEZY TO LOCALLY WINDY CONDITIONS.
THE WESTERLY FLOW ALOFT ADVERTISED BY THE MODELS FRIDAY THROUGH
SATURDAY WILL ALSO MEAN THE OPPORTUNITY FOR A FEW EMBEDDED WAVES TO
QUICKLY TRAVEL ACROSS WA/OR AND BRING ADDITIONAL MOUNTAIN SNOW.
THE WESTERLY FLOW IS NOT FAVORABLE FOR MUCH PRECIPITATION IN THE
LOWER ELEVATIONS, AND IT LOOKS LIKE HIGH POPS AND LOW QPF ACROSS THE
COLUMBIA BASIN AND COLUMBIA DESCHUTES PLATEAU AS WELL AS THE VALLEYS
EAST-SOUTHEAST OF THE BLUE MTNS. THE CHANCE OF SNOW AT ELEVATIONS
BELOW 3000 FEET IS VERY LOW (20% OR LESS), AS THERE ARE NO SIGNS OF
STRONG SURFACE COLD FRONTS TO BRING SNOW LEVELS DOWN TO THE
LOWLAND ELEVATIONS. WISTER/85
AVIATION...12Z TAFS
DENSE FOG HAS BEEN OBSERVED AT PDT AND ALW,
BUT CONDITIONS HAVE ALSO FLUCTUATED FROM VLIFR TO MVFR, AS FOG HAS
SURGED IN AND OUT AHEAD OF THE APPROACHING TROUGH. CONDITIONS WILL
IMPROVE BUT WILL STILL REMAIN BELOW VFR ONCE RAIN SPREADS ACROSS THE
FOOTHILLS EARLY THIS AFTERNOON. THE WINDS ALOFT HELPED TO BRING PSC
FROM LIFR TO VFR, BUT FEEL THIS WILL ONLY BE FOR A BRIEF PERIOD OF
TIME DUE TO THE STRATUS AND FOG THAT HAVE BEEN OBSERVED IN THE TRI-
CITIES AREA. SIMILAR TO PDT AND ALW, RAIN WILL CAUSE CONDITIONS TO
FALL TO MVFR OR LESS THIS AFTERNOON.
THE OTHER TAF SITES ARE NOT OBSERVING THE WIDELY VARIABLE
CONDITIONS. THE CIGS AT DLS LIFTED SLIGHTLY, AND MVFR CONDITIONS
ARE FORECAST WITH LIGHT RAIN AROUND 17Z. YKM IS CURRENTLY VFR AND
THERE IS A CHANCE FOR MVFR AROUND NOON. WILL LEAVE RDM AND BDN AS
VFR FOR THE NEXT 24 HOURS BUT WILL KEEP CIGS AROUND 3500-4000 FEET.
WINDS WILL BE THE STRONGEST AT RDM AND BDN--SW 10-15 GUSTING 20-25
KTS THIS AFTERNOON AND EVENING. WISTER/85
PRELIMINARY POINT TEMPS/POPS
PDT 43 34 48 36 / 90 40 30 70
ALW 45 37 47 40 / 90 50 20 80
PSC 42 35 43 38 / 90 20 30 80
YKM 41 31 40 31 / 80 10 50 70
HRI 41 35 43 37 / 90 30 40 70
ELN 39 30 38 31 / 80 10 50 70
RDM 48 33 47 30 / 60 10 40 50
LGD 45 31 43 36 / 80 60 30 80
GCD 47 31 45 35 / 80 40 50 90
DLS 45 38 45 38 / 80 30 70 80
PDT WATCHES/WARNINGS/ADVISORIES
OR...DENSE FOG ADVISORY UNTIL 10 AM PST THIS MORNING FOR ORZ044-507.
WIND ADVISORY UNTIL 3 PM PST THIS AFTERNOON FOR ORZ049.
WA...DENSE FOG ADVISORY UNTIL 10 AM PST THIS MORNING FOR WAZ029.
FREEZING FOG ADVISORY UNTIL 10 AM PST THIS MORNING FOR WAZ026.
SHORT TERM...75
LONG TERM....85
AVIATION...85
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