Area Forecast Discussion
Issued by NWS Portland, OR

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000
FXUS66 KPQR 181816
AFDPQR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PORTLAND OR
1016 AM PST THU DEC 18 2014

.SYNOPSIS...RAIN WILL INCREASE TONIGHT AS A MORE ORGANIZED FRONTAL
SYSTEM PUSHES ONSHORE. A COLDER UPPER TROUGH MOVES ACROSS THE AREA
FRIDAY...WHICH WILL LOWER SNOW LEVELS BELOW THE CASCADE PASSES. MILD
AND VERY WET WEATHER IS EXPECTED THIS WEEKEND. THERE IS POTENTIAL FOR
AN ATMOSPHERIC RIVER AND ASSOCIATED HEAVY RAIN TO DEVELOP OVER THE
AREA. THE EXACT LOCATION OF THE JET STREAM AND ASSOCIATED MOISTURE
PLUME ARE STILL IN DOUBT. THERE IS THE POTENTIAL FOR SOME FLOODING
LATE THIS WEEKEND INTO EARLY NEXT WEEK.

&&

.UPDATE...SOME LIGHT RAIN CONTINUES TO SPREAD ACROSS SOUTHWEST
WASHINGTON AND NORTHWEST OREGON THIS MORNING AHEAD OF AN APPROACHING
FRONTAL SYSTEM. MODELS INDICATE THAT THIS RAIN WILL CONTINUE THROUGH
THE DAY WITH INCREASING COVERAGE...PARTICULARLY ALONG THE COAST AND
ACROSS THE NORTHERN PORTIONS OF THE FORECAST AREA. THEREFORE
INCREASED POPS FOR THESE LOCATIONS WITH THE MORNING UPDATE. EXPECT
THESE RAINFALL ACCUMULATIONS TO REMAIN FAIRLY LIGHT FOR MOST
LOCATIONS DURING THE DAY...THOUGH HEAVIER RAIN WILL MOVE ONSHORE WITH
THE COLD FRONT THIS EVENING. OTHERWISE...FEW CHANGES TO THE FORECAST.
12Z MODELS REMAIN CONSISTENT WITH SIGNIFICANT RAINFALL FOR THE
WEEKEND WITH A DEVELOPING ATMOSPHERIC RIVER EVENT.  CULLEN

&&

.PREV DISC... /ISSUED 414 AM PST THU DEC 18 2014/

SHORT TERM...THURSDAY THROUGH SATURDAY...WATER VAPOR IMAGERY AS OF
1030Z SHOWS A DEEP UPPER LOW IN THE GULF OF ALASKA WITH A FRONTAL
SYSTEM TAKING SHAPE NEAR 135W. THE 06Z NAM12 RUN SHOWS A WEAK SURFACE
LOW OFF THE S OREGON COAST AT 09Z. THIS FEATURE IS VERY DIFFICULT TO
PICK OUT IN THE WATER VAPOR LOOPER. THE NAM SHOWS THIS LOW FILLING
OVERNIGHT AS IT DRIFTS TO THE NE. NWS PORTLAND DUAL-POL DOPPLER RADAR
SHOWING DECREASING RADAR RETURNS...WITH THE STRONGEST ECHOES IN THE
WILLAPA HILLS AND S WA CASCADES.

NAM...GFS AND ECMWF ALL INDICATE A WEAK 500 MB RIDGE TO MOVE ACROSS
THE AREA TODAY. HOWEVER...MODEL CROSS-SECTIONS AND TIME-HEIGHT
FORECASTS MAINTAIN MOIST LOW-LEVELS THROUGH THE DAY. THUS...STILL
EXPECT LIGHT QPF ACROSS THE FORECAST AREA TODAY...AHEAD OF THE NEXT
FRONTAL SYSTEM. MODELS APPEAR TO BE IN REASONABLE AGREEMENT WITH THIS
FRONT...WITH LANDFALL SOMETIME AROUND 00Z FRI OR SHORTLY THEREAFTER.
ONE CONCERN WAS THE POTENTIAL FOR HIGH WIND ALONG THE COASTAL
HEADLANDS. PREVIOUS SHIFT HAD ISSUED A HIGH WIND WATCH FOR LATE THIS
AFTERNOON THROUGH 08Z FRI. UPON FURTHER REVIEW...NOT SEEING ENOUGH IN
THE 850 MB TO 950 MB WIND FIELDS TO UPGRADE TO A WARNING. THE 00Z
WRF-GFS VALID 00Z FRI HAS A COASTAL JET SIGNATURE OVER THE OREGON
COASTAL WATERS...WITH FORECAST 10M WIND GUSTS OF 55 KT. HOWEVER...BY
03Z FRI THE MODEL HAS JUST A SMALL SLIVER OF 55 KT GUSTS JUST OFF THE
CENTRAL COAST. WOULD NOT BE SURPRISED TO SEE ISOLATED 55 TO 60 MPH
GUSTS ON EXPOSED HEADLAND AREAS LATE THIS AFTERNOON THROUGH THE
MID-EVENING HOURS...BUT DO NOT THINK THESE WILL BE WIDESPREAD AND
FREQUENT ENOUGH TO WARRANT A WARNING.

THE SYSTEM WILL BRING A 6-9 HR SHOT OF RAIN TO THE AREA...BUT SNOW
LEVELS ARE EXPECTED TO SLOWLY RISE AHEAD OF THE FRONT. THE COLDER
UPPER TROUGH SLIDES ACROSS THE REGION FRI...WITH 1000-500 MB
THICKNESS VALUES DROPPING INTO THE LOW TO MID 530S BY 18Z FRI. THIS
PUTS SNOW LEVELS WELL BELOW THE CASCADE PASSES...BUT BY THIS TIME THE
BULK OF THE PRECIPITATION HAS ALREADY OCCURRED. ALTHOUGH NOT IN THE
FORECAST GRIDS AS OF YET...NEED TO CONSIDER THE POSSIBILITY OF
THUNDERSTORMS FOR FRI. GFS SOUNDING FOR KAST VALID 22Z FRI SHOWS LI
VALUES NEAR 0 BUT A SWEAT INDEX OF 272. THE 1000-750 MB LAPSE RATES
ARE AROUND 7.5C/KM.

MODELS STILL ADVERTISING A CHANGE TO A MUCH WETTER PATTERN STARTING
SAT. NAM AND GFS CONTINUE TO SHOW AN ATMOSPHERIC RIVER SET UP
DIRECTED AT SW WASHINGTON AND NW OREGON SAT. THE GEM MAY BE A BIT
SLOWER...BUT STILL HAS SUBSTANTIAL QPF DIRECTED AT THE FORECAST AREA
BY SAT NIGHT. THE 00Z WRFGFS SHOWS VERY HEAVY QPF WITH UP TO 8-12
INCHES OF RAIN IN THE HIGHER TERRAIN AND 2-4 INCHES IN THE WILLAMETTE
VALLEY SAT THROUGH MONDAY. WILL NEED TO MONITOR THIS VERY CLOSELY
OVER THE NEXT FEW DAYS AS SIMILAR SITUATIONS HAVE PRODUCED FLOODING
ON AREA MAINSTEM RIVERS.  WEISHAAR

LONG TERM...SATURDAY NIGHT THROUGH WEDNESDAY...MODELS MAINTAIN THE
MILD AND MOIST ATMOSPHERIC RIVER PATTERN THROUGH AT LEAST SUN. THE
ECMWF AND GEM GRADUALLY SHIFT THE FOCUS OF THE HEAVIEST PRECIP TO THE
N SUN AFTERNOON BUT THE GFS DOES NOT. EXPECT PLENTY OF RAINFALL TO
CONTINUE THROUGH SUNDAY. ON MONDAY AND TUESDAY...SOME WEAK RIDGING
MOVES OVER THE REGION BUT SHOWERS LIKELY CONTINUE IN THE MOIST
ONSHORE FLOW. THE ECMWF THEN INDICATES ANOTHER MAJOR PATTERN CHANGE
AROUND THE MIDDLE OF NEXT WEEK WITH A MUCH COLDER AIR MASS SETTLING
OVER THE PAC NW. HOWEVER...MOISTURE DIMINISHES DUE TO THE DRIER N-NE
FLOW. THUS...AFTER WHATEVER SNOW THE CASCADES GET LATE THIS WEEK...IT
WILL BE LOST OVER THE WEEKEND. THE GFS ALSO HAS A CHANGE TO COLDER
NW-N FLOW ALOFT BUT IT MAINTAINS AN OPEN TROUGH INSTEAD OF THE CLOSED
UPPER LOW ECMWF SOLUTION. WHILE STILL SOME SIGNIFICANT UNCERTAINTY
EXISTS WITH RESPECT TO THE TIMING OF THE COLD AIR AND JUST HOW COLD
THE AIR WILL BE...IT DOES APPEAR THAT A CHANGE TO MUCH COOLER
CONDITIONS WILL MOVE INTO THE REGION. SNOW LEVELS SHOULD BE QUITE LOW
OVER THE HOLIDAY...BUT DROPPED POPS SUBSTANTIALLY NEXT THU AND THU
NIGHT. WEISHAAR

&&

.HYDROLOGY...WHILE RIVER FORECASTS OF CURRENT MODEL PRECIPITATION
INDICATE FLOODING IS POSSIBLE ON A RIVER OR TWO IN THE FORECAST AREA
SUNDAY INTO MONDAY...A LOOK AT THE MOST CORRELATED ANALOGS TO THE 96
HOUR GFS MODEL FORECAST SHED LIGHT ON FLOODING CHANCES ACROSS OUR
FORECAST AREA LATE IN THE WEEKEND AND EARLY NEXT WEEK. THE THREE
MOST CORRELATED ANALOG WEATHER PATTERNS TO THE CURRENT 96 HOUR GFS
MODEL FORECAST ARE LATE JANUARY 2003...EARLY JANUARY 1983 AND LATE
NOVEMBER 1999. ALL THREE OF THESE EVENTS PRODUCED ISOLATED TO
SCATTERED RIVER FLOODING WITHIN NORTHWEST OREGON AND/OR SOUTHWEST
WASHINGTON. WHILE THERE ARE CERTAINLY NOTABLE DIFFERENCES BETWEEN
THE CURRENT PATTERN AND EACH OF THESE HISTORICAL ANALOGS...SOME
CRITICAL WEATHER PARAMETERS APPEAR EVEN MORE FAVORABLE FOR PRODUCING
HEAVIER RAINS THAN THESE PAST EVENTS. THIS CERTAINLY HELPS BUILD
CONFIDENCE THERE WILL BE HYDROLOGICAL IMPACTS WITHIN OUR FORECAST
AREA OF RESPONSIBILITY SO LONG AS THE PATTERN HOLDS.
NONETHELESS...THERE IS STILL CONSIDERABLE UNCERTAINTY IN WHERE THE
HEAVIEST RAIN WILL SET UP...AND EXACTLY WHICH RIVERS WILL FLOOD AND
WHEN. PLEASE CONTINUE TO MONITOR THE FORECAST IF YOU LIVE NEAR ANY
RIVERS FED BY THE THE CASCADES OR COAST RANGE...AND PLEASE SEE
WATER.WEATHER.GOV/AHPS2/INDEX.PHP?WFO=PQR FOR THE LATEST RIVER
FORECASTS. /NEUMAN

&&

.AVIATION...MIX OF VFR MVFR WILL CONTINUE THROUGH THE DAY TODAY AS
RAIN MOVES IN ACROSS THE AREA...BRINGING DOWN CIGS AND VIS. FRONT
MOVES ONSHORE AROUND 03Z...REACHING INLAND AROUND 05Z. THIS WILL
BRING PERIODS OF MVFR/IFR CIGS AND VIS WITH LIGHT TO MODERATE
RAIN. CIGS AND VIS TO STAY LOW MVFR/IFR THROUGH EARLY FRIDAY AS
CIGS WILL LIKELY LOWER BEHIND THE COLD FRONT.

KPDX AND APPROACHES...MVFR CIGS LIKELY TO PERSIST THROUGH THIS
MORNING AS RAIN MOVES THROUGH THE AREA. THERE MAY BE A PERIOD
WHERE CIGS LIFT TO VFR AFTER THIS INITIAL BAND...BUT CIGS SHOULD
DROP BACK TO MVFR WITH OCCASIONAL IFR AROUND 05Z AS THE FRONT
BRINGS RAIN THROUGH THE AREA. CIGS AND VIS SHOULD REMAIN MVFR/IFR
OVERNIGHT TONIGHT. -MCCOY

&&

.MARINE...COMBINED SEAS TO STAY AROUND 10 TO 12 FT...POSSIBLY
BUILDING TO NEAR 15 FT AROUND 1 PM TO 8 PM AS WINDS OUT OF THE SOUTH
REACH GALE FORCE WITH GUSTS OF 40 TO 45 KTS LIKELY. SEAS WILL START
TO SUBSIDE TO 10 TO 12 FT LATE THIS EVENING. A 20-22 FT SWELL TRAIN
WITH IMPRESSIVELY LONG PERIODS MOVES IN EARLY FRIDAY MORNING. MODELS
TEND TO UNDER-DO THE WAVE HEIGHTS IN THIS TYPE OF SCENARIO...SO WAVE
HEIGHTS MAY WELL BE UNDERDONE. AS OF NOW...EXPECT SEAS ON FRIDAY
MORNING NEARLY DOUBLE IN HEIGHT BY FRIDAY AFTERNOON. BECAUSE OF
THIS...A HIGH SURF ADVISORY WILL LIKELY BE ISSUED WITH OUR AFTERNOON
PACKAGE TODAY. THE GOOD NEWS IS THAT THE LARGEST SWELL WILL LIKELY
ARRIVE DURING LOW TIDE AND DURING THE NIGHT TIME HOURS WHEN MOST
MARINERS AND BEACH GOERS ARE NOT IN HARMS WAY.

ON SATURDAY MORNING ANOTHER FRONT MOVES THROUGH THE WATERS.
CONFIDENCE IS INCREASING FOR ANOTHER ROUND OF SOUTHERLY GALES...WITH
POSSIBLY SOME LOW END STORM FORCE GUSTS. THIS ON TOP OF THE 20 FT
SWELL SATURDAY COULD EASILY BRING SEAS UP INTO THE MIDDLE 20S LATE
SATURDAY MORNING INTO THE AFTERNOON. BEHIND THIS FRONT...SMALL
CRAFT WINDS OUT OF THE WEST-SOUTHWEST WILL CONTINUE THROUGH THE
DAY ON SUNDAY.

HIGH PRESSURE BUILDS IN EARLY NEXT WEEK BRINGING RELATIVELY BENIGN
WEATHER THROUGH MIDWEEK. -MCCOY

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...NONE.

WA...NONE.

PZ...SMALL CRAFT ADVISORY FOR WINDS UNTIL 1 PM PST THIS AFTERNOON FOR
     COASTAL WATERS FROM CAPE SHOALWATER WA TO FLORENCE OR OUT
     60 NM.

     GALE WARNING FROM 1 PM THIS AFTERNOON TO MIDNIGHT PST TONIGHT
     FOR COASTAL WATERS FROM CAPE SHOALWATER WA TO FLORENCE OR OUT
     60 NM.

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

     SMALL CRAFT ADVISORY FOR ROUGH COLUMBIA RIVER BAR UNTIL 6 AM
     PST FRIDAY.

&&

$$

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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