Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 191225
AFDPQR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PORTLAND OR
424 AM PST FRI DEC 19 2014

.SYNOPSIS...AN UPPER LEVEL TROUGH WILL MOVE ACROSS THE FORECAST AREA
TODAY...WHICH WILL LOWER SNOW LEVELS BELOW THE CASCADE PASSES. THERE
MAY BE ENOUGH INSTABILITY TODAY OVER THE COASTAL AREAS FOR A THREAT
OF THUNDERSTORMS. MILD AND VERY WET WEATHER IS EXPECTED THIS WEEKEND
WITH AN ATMOSPHERIC RIVER PATTERN RESULTING IN HEAVY RAIN ACROSS MUCH
OF THE REGION...ESPECIALLY MOUNTAINOUS TERRAIN. THE EXACT LOCATION OF
THE JET STREAM AND ASSOCIATED MOISTURE PLUME IS STILL IN
QUESTION...BUT THERE IS INCREASING CONFIDENCE THAT IT WILL TAKE AIM
AT SW WASHINGTON AND NW OREGON. THERE IS THE POTENTIAL FOR SOME
FLOODING LATE THIS WEEKEND INTO EARLY NEXT WEEK. LONGER RANGE MODEL
OUTPUT IS SUGGESTING A COLDER AIR MASS WILL SETTLE OVER THE PACIFIC
NORTHWEST LATE NEXT WEEK.

&&

.SHORT TERM....TODAY THROUGH SUNDAY...NWS KRTX DUAL-POL DOPPLER RADAR
AS OF 10Z SHOWS THE BACK EDGE OF THE FRONT MOVING ACROSS THE
CASCADES. SIX-HOUR RAINFALL AMOUNTS THROUGH 10Z WERE NOT ALL THAT
IMPRESSIVE...GENERALLY A QUARTER TO TWO-THIRDS OF AN INCH IN MOST
AREAS. RADAR AND INFRA-RED SATELLITE IMAGERY SHOW SHOWERS WITHIN THE
POST-FRONTAL COLDER AIR MASS ALREADY MOVING ONSHORE AND INTO THE
OREGON COAST RANGE AND SW WA WILLAPA HILLS. LIGHTNING DETECTION JUST
BEFORE 10Z INDICATED A POSITIVE STRIKE ALONG THE CENTRAL WA COAST.
JUST BEFORE 11Z A COUPLE STRIKES WERE NOTED NEAR FLORENCE.

THE MAIN WEATHER FEATURE TODAY WILL BE THE COLD UPPER TROUGH. LATEST
MODEL GUIDANCE SHOWS THE BULK OF THE TROUGH THROUGH THE AREA BY
MIDDAY. SNOW LEVELS DROP TO BELOW THE CASCADES PASSES TODAY...BUT THE
MAJORITY OF THE QPF HAS ALREADY OCCURRED. OROGRAPHIC ENHANCEMENT WILL
GIVE SOME SNOW TO THE CASCADES BUT UNDER ADVISORY CRITERIA. NAM MODEL
SOUNDING FOR KAST WOULD INDICATE THE BEST POTENTIAL FOR THUNDERSTORMS
TO BE THROUGH THE MORNING HOURS. NAM SOUNDING VALID 17Z TODAY SHOWS
AN LI OF 0 WITH A PRETTY HEFTY SWEAT INDEX OF 335...WELL ABOVE THE
GENERALIZED 250 SWEAT INDEX BENCHMARK USED FOR ELEVATED CONVECTIVE
CONFIDENCE. THE CONVECTIVE POTENTIAL THEN DIMINISHES IN THE AFTERNOON
AS THE AIR MASS ALOFT STARTS TO STABILIZE.

THE BIGGER STORY CONTINUES TO BE THE ATMOSPHERIC RIVER EVENT OVER THE
WEEKEND. LATEST MODEL RUNS ARE GENERALLY CONSISTENT WITH THEIR
RESPECTIVE PAST RUNS WITH FAIRLY DECENT CONTINUITY AMONGST THE
INDIVIDUAL MODELS. THE MAIN FORECAST ISSUES WILL BE THE FOCUS OF THE
HEAVIEST PRECIP AND THE DURATION. GFS HAS BEEN PERSISTENT IN
DIRECTING THE BAROCLINIC BAND OVER EXTREME SW WA AND NRN OREGON. THE
NAM AND ECMWF ARE FAIRLY SIMILAR...BUT THE ECMWF SEEMS TO FOCUS THE
HEAVIEST QPF OVER THE CENTRAL PART OF THE FORECAST AREA 12Z SUN
THROUGH 00Z MON. THE 00Z WRF-GFS 48-HR QPF TOTALS VALID 12Z SAT
THROUGH 12Z MON SHOW AT LEAST 3-6 INCHES FOR THE OREGON CASCADES AND
2-4 INCHES FOR THE OREGON COAST RANGE. IT HAS LOWER AMOUNTS IN THE S
WA CASCADES AND WILLAPA HILLS. GFS AND NAM HAVE STRONG ISENTROPIC
LIFT ALONG THE 290K SURFACE STARTING 12Z SAT AND CONTINUING THROUGH
SUN. 850 MB WLY FLOW OF 40-50 KT INTO THE OREGON COAST RANGE AND
CASCADES SAT THROUGH SUN WILL RESULT IN A HIGHLY EFFECIENT OROGRAPHIC
RAINFALL COMPONENT. LOOKING AT THE ENSEMBLE SITUATIONAL AWARENESS
TABLE...SPECIFICALLY THE ENSEMBLE MODEL CLIMATE QPF (OR M CLIMATE
QPF)...IT INDICATES A 99TH PERCENTILE OR MAX QPF EVENT FOR A GOOD
PORTION OF THE OREGON CASCADES SAT AFTERNOON THROUGH SUN MORNING.
THIS MEANS THAT THE GEFS ENSEMBLE IS FORECASTING AN INORDINATE AMOUNT
OF PRECIP COMPARED TO THE 1985-2012 WINTER ENSEMBLE MODEL CLIMATE.
WOULD NOT BE SURPRISED TO SEE 1.5 TO 2.0 INCHES PER SIX HOURS IN THE
CASCADES FOR A 12-18 PERIOD. FORTUNATELY...THERE IS NO LOW-ELEVATION
SNOW AND MINIMAL SNOW AT PASS LEVELS. HOWEVER...LATEST HYDRO GUIDANCE
SUGGESTS A FEW RIVERS WILL REACH FLOOD STAGE BY SUN OR MON. REFER TO
THE HYDROLOGY SECTION FOR MORE DETAILS.

DO NOT ANTICIPATE HIGH WIND HIGHLIGHTS FOR THE COASTAL AREAS BUT IT
WILL BE RATHER BLUSTERY. THE NAM INDICATES A KAST-KOTH GRADIENT OF
AROUND 10 MB BY LATE SAT AFTERNOON AND IN THE EVENING. THE BENCHMARK
GRADIENT IS AROUND 12 MB. EXPECT 45-55 MPH GUSTS ALONG THE BEACHES
AND HEADLANDS SAT THROUGH SAT NIGHT.

IN ADDITION...THOSE PLANNING TO BE ALONG THE COAST AT AREA BEACHES
SHOULD CLOSELY MONITOR THE CONDITIONS THIS WEEKEND. RAPIDLY BUILDING
SEAS WILL BRING THE POTENTIAL FOR DANGEROUS HIGH SURF ALONG THE
BEACHES AS EARLY AS TODAY. LONG PERIOD SWELLS MAY REACH SHORE AND
SURPRISE INDIVIDUALS WHO ARE UNAWARE. ANYONE ON THE BEACHES SHOULD
EXERCISE EXTREME CAUTION AND AVOID THE SURF ZONE AS LARGE WAVES MAY
WASH OVER BEACHES AND JETTIES WITH LITTLE WARNING. SOME POTENTIAL FOR
MINOR TIDAL OVERFLOW IS POSSIBLE FOR SATURDAY AND SUNDAY...ESPECIALLY
AROUND HIGH TIDES. THIS THREAT COULD BE ENHANCED BY HIGH FLOWS ON
RIVERS IN RESPONSE TO THE RAINFALL THIS WEEKEND.  WEISHAAR

&&

.LONG TERM...SUNDAY NIGHT THROUGH FRIDAY...MODELS AGREE THE WEEKEND
ATMOSPHERIC RIVER EVENT WILL WIND DOWN SUNDAY NIGHT INTO MONDAY.
UPPER RIDGING AMPLIFIES MON...BUT THIS WILL LIKELY BE A DIRTY RIDGE
AS THE GFS AND ECMWF HOLD ON TO SOME QPF...ESPECIALLY ACROSS THE NRN
HALF OF THE FORCAST AREA. HAVE ADJUSTED POPS TO REFLECT THIS
THINKING. A WEAK WARM FRONT MAY BRUSH THE NRN ZONES MON NIGHT AND TUE
FOR SOME ADDITIONAL PRECIP. THE GFS AND ECMWF THEN BRING A COLDER
UPPER LEVEL TROUGH ACROSS THE AREA WED. THE ECMWF DROPS 1000-500 MB
THICKNESS VALUES TO NEAR 530 DM 00Z THU WITH A DECENT AMOUNT OF QPF.
THUS...THE CASCADES AND SKI AREAS MAY FINALLY GET A DECENT SHOT OF
SNOW. COLDER NORTH FLOW WILL FOLLOW AND ALLOW TEMPERATURES TO COOL
WELL BELOW NORMAL AS CONDITIONS DRY OUT LATE WEEK. WEISHAAR

&&

.HYDROLOGY...SITUATION SETTING UP FOR A MEMORABLE HYDROLOGIC EVENT
MAINLY FOR LATER THIS WEEKEND AND HEADING INTO NEXT WEEK. STRONG AND
DEEP MOIST WESTERLY FLOW WILL BEGIN CROSSING THE PACNW COAST COME
SATURDAY MORNING. A VERY LONG FETCH OF MOISTURE IS STRETCHING ACROSS
THE PACIFIC WITH ORIGINS WEST TO THE SOUTH CHINA SEA. THIS MOIST AIR
MASS WILL BE PUSHED EAST BY A 150 TO 170 KT JET AND WESTERLY 25 TO
50 KT LOWER LEVEL WINDS. AS SUCH...THIS CONTINUES TO GIVE THE
APPEARANCE OF A CLASSIC HIGH PRECIPITATION ATMOSPHERIC RIVER EVENT
WHERE THE DEEP MOIST FLOW WILL SPEND A SIGNIFICANT AMOUNT OF TIME
ON A NEARLY PERPENDICULAR COURSE AGAINST THE COAST RANGE WITH THE
CASCADES TAKING A BIT HIGHER OF THE IMPACT. AS OF THE MORNING MODEL
RUNS AND RECENT NWRFC UPDATES...IT APPEARS MULTIPLE COAST RANGE
RIVERS HAVE A MODERATE POTENTIAL OF EXCEEDING FLOOD STAGE.
ADDITIONALLY...THIS EVENT HAS SUFFICIENT DEPTH TO BRING SIGNIFICANT
RAINS TO THE NORTH OREGON AND SOUTH WASHINGTON CASCADES WHERE RIVERS
WILL ALSO SEE POTENTIAL FOR NOTABLE FLOODING. WILL CONTINUE THE
HYDROLOGIC OUTLOOK (PDXESFPQR) PRODUCT HIGHLIGHTING CONCERNS. KEEP
IN MIND THAT DETAILS WILL ALWAYS BE IN FLUX UNDER THESE SCENARIOS AS
SUBTLE NORTH/SOUTH SHIFTS COLD MAKE DRAMATIC DIFFERENCES IN WHICH
BASINS RECEIVE EXTENDED PERIODS OF MODERATE TO HEAVY RAIN. PLEASE
CONTINUE TO FOLLOW THE LATEST FORECASTS AND IMPACTS AS CONDITIONS
WARRANT. RIVER FORECASTS ARE AVAILABLE ON OUR WEB PAGE AT:
HTTP://WATER.WEATHER.GOV/AHPS2/INDEX.PHP?WFO=PQR (ALL LOWER CASE)

/JBONK

&&
&&


.AVIATION...VFR CONDITIONS WILL PERSIST AT MOST TAF SITES
THROUGH THIS EVENING. HOWEVER...BRIEF PERIODS OF MVFR CIGS AND
VIS ARE POSSIBLE UNDER SHOWERY CONDITIONS. DUE TO AN UNSTABLE AIR
MASS OVER THE WATERS THERE REMAINS A CHANCE OF VCTS THROUGH AROUND
18Z TODAY FOR THE COASTAL TAF SITES. CONDITIONS WILL LIKELY BE A
MIX OF IFR TO VFR TONIGHT AND SAT AS A MUCH STRONGER FRONT PUSHES
ACROSS THE REGION. EXPECT THE FRONT TO APPROACH THE COAST AROUND
02Z...AND INLAND AFTER 05Z.

KPDX AND APPROACHES...VFR CONDITIONS TODAY AND TONIGHT.
HOWEVER...BRIEF PERIODS OF MVFR CIGS ARE POSSIBLE AS A FRONT
APPROACHES THE TERMINAL AFTER 05Z. /64

&&

.MARINE...THE SMALL CRAFT ADVISORY FOR WINDS AND HAZARDOUS SEAS
WAS EXTENDED TO LATE TONIGHT. EXPECT TO SEE CONTINUED GUSTS OF 25
KT OVER THE WATERS. SEAS ARE CURRENTLY RUNNING AROUND 12 FT AND
IS EXPECTED TO BUILD INTO THE MID TEENS THIS AFTERNOON. A STRONG
FRONT WILL MOVE ACROSS THE WATERS THIS EVENING AND TONIGHT. AS THE
FRONT APPROACHES THE WATERS WE CAN EXPECT THE SEAS TO CLIMB ABOVE
20 FT THIS EVENING WITH SOLID GALE GUSTS TO 45 KT EXPECTED AFTER
MIDNIGHT. AS SUCH...ISSUED A GALE WATCH STARTING LATE TONIGHT
THROUGH SATURDAY AFTERNOON. SEAS WILL SLOWLY SUBSIDE ON SATURDAY
AFTERNOON AND EVENING WITH WINDS RELAXING TO AROUND 25 KT
OVERNIGHT. THE ACTIVE WEATHER PATTERN CONTINUES INTO MONDAY.
HOWEVER...CONDITIONS WILL NOT BE AS INTENSE. /64

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...HIGH SURF ADVISORY FROM 10 AM THIS MORNING TO 4 PM PST SATURDAY
     FOR CENTRAL OREGON COAST-NORTH OREGON COAST.

WA...HIGH SURF ADVISORY FROM 10 AM THIS MORNING TO 4 PM PST SATURDAY
     FOR SOUTH WASHINGTON COAST.

PZ...GALE WATCH FROM LATE TONIGHT THROUGH SATURDAY AFTERNOON FOR
     COASTAL WATERS FROM CAPE SHOALWATER WA TO FLORENCE OR OUT 60 NM.

     SMALL CRAFT ADVISORY FOR WINDS UNTIL 1 AM PST SATURDAY FOR
     COASTAL WATERS FROM CAPE SHOALWATER WA TO FLORENCE OR OUT 60 NM.

     SMALL CRAFT ADVISORY FOR HAZARDOUS SEAS UNTIL 1 AM PST SATURDAY
     FOR COASTAL WATERS FROM CAPE SHOALWATER WA TO FLORENCE OR OUT
     60 NM.

     SMALL CRAFT ADVISORY FOR ROUGH COLUMBIA RIVER BAR UNTIL 6 PM
     PST SATURDAY.

&&

$$

INTERACT WITH US VIA SOCIAL MEDIA
WWW.FACEBOOK.COM/NWSPORTLAND
WWW.TWITTER.COM/NWSPORTLAND

THIS DISCUSSION IS FOR NORTHWEST OREGON AND SOUTHWEST WASHINGTON
FROM THE CASCADE CREST TO 60 NAUTICAL MILES OFFSHORE. THE AREA IS
COMMONLY REFERRED TO AS THE FORECAST AREA.



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