Flooding and Heavy Rain in Oregon, December 17 to 19, 2015

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PORTLAND OR
957 AM PST THU DEC 17 2015

.SYNOPSIS... ANOTHER ATMOSPHERIC RIVER OF SUBTROPICAL MOISTURE WILL
COUPLE WITH AN OCCLUDING WARM FRONT TO BRING MORE HEAVY
PRECIPITATION THROUGH EARLY FRIDAY...POSSIBLY PUSHING SOME
RIVERS BACK ABOVE FLOOD STAGE. THIS SYSTEM WILL START OFF COOLER
WITH SNOW EXPECTED IN THE CASCADES AT LEAST INITIALLY. TEMPERATURES
ARE ALSO EXPECTED TO COOL ENOUGH IN THE UPPER HOOD RIVER VALLEY AND
CENTRAL COLUMBIA RIVER GORGE AND ADJACENT VALLEYS IN SKAMANIA COUNTY
TO BRING THE POTENTIAL FOR SIGNIFICANT WINTER WEATHER IN THESE AREAS
AS WELL. COOL AND UNSETTLED WEATHER IS EXPECTED TO FOLLOW INTO THE
WEEKEND WITH VALLEY RAIN AND MOUNTAIN SNOW SHOWERS CONTINUING.

&&

.SHORT TERM...UPDATE...PRIMARY FORECAST CHALLENGE REMAINS PRECIP TYPE
TODAY FOR THE CENTRAL COLUMBIA RIVER GORGE AND UPPER HOOD RIVER
VALLEY. AS OF 1630Z ALREADY HAVE HAD SEVERAL REPORTS OF 4+ INCHES OF
SNOW IN THE HOOD RIVER AREA...WITH HIGHER AMOUNTS TOWARD PARKDALE.
ODOT WEBCAMS SHOWED SNOW STARTING TO STICK ON I-84 AT CASCADE LOCKS.
WENT AHEAD AND UPGRADED THE ADVISORY FOR THE CENTRAL GORGE TO A
WARNING. NAM AND GFS MODEL SOUNDINGS FOR A POINT NEAR HOOD RIVER
SUGGEST A SNOW PROFILE THROUGH 18Z THEN A BRIEF TRANSITION TO
FREEZING RAIN FOLLOWED BY ALL RAIN IN THE AFTERNOON. MODELS TEND TO
MIX OUT THE COLD AIR TOO SOON AND HAVE HELD ON TO MAINLY SNOW THROUGH
EARLY AFTERNOON IN THE CENTRAL GORGE AND UPPER HOOD RIVER VALLEY.

THE OTHER CHANGE WAS TO UPGRADE THE HIGH WIND WATCH TO A WARNING.
HI-RES ARW...MM5 NAM VERSION AND 12K NAM ALL SHOW 50-60 KT 925 MB
WIND SPEEDS ALONG THE CENTRAL COAST 03Z FRI. STRONGEST WIND SPEEDS
THIS EVENING LOOK TO BE ALONG THE CENTRAL COAST...BUT WITH HIGH WIND
CRITERIA GUSTS SPREADING NORTH TO ABOUT TILLAMOOK LATER IN THE
EVENING. WILL ALSO NEED TO THINK ABOUT POTENTIAL FOR A WIND ADVISORY
IN THE SOUTH WILLAMETTE VALLEY FOR THIS EVENING. LATEST 3-HR RAINFALL
RATES ARE UP TO 0.75 TO 1.00 INCHES IN THE CENTRAL OREGON COAST
RANGE. PRECIPITABLE WATER RATES REACHING THE COAST THIS MORNING
EXPECTED TO BE AROUND 1.25 INCHES AS TROPICAL MOISTURE PLUME POKES
IN. IN CONJUNCTION WITH ISENTROPIC LIFT OVER THE WARM FRONT DRIVEN BY
AN INCREASING SW LOW LEVEL WIND FIELD...H8 WINDS EXPECTED AROUND 50
KT THIS MORNING...RAINFALL RATES EXPECTED PEAK THIS MORNING
ACROSS THE SOUTHERN HALF OF THE FORECAST AREA. AS LOW LEVEL WINDS
WEAKEN IN THE AFTERNOON...SO WILL RAINFALL RATES. COLD FRONT LOOKS TO
MAKE LANDFALL AROUND MIDNIGHT. MODEL PWATS DIMINISH QUITE A BIT ONCE
THE COLD FRONT MOVES ASHORE. WHILE THERE APPEARS TO BE A PERIOD OF
HEAVY RAIN ACCOMPANYING THE COLD FRONT TONIGHT...THE TOP END AMOUNTS
ARE LIKELY TO BE LIMITED BY THE LOWER MOISTURE CONTENT OF THE AIR
MASS AS WELL AS A SHORTER DURATION TO THE HEAVIEST RAINFALL PERIOD.

THE CASCADES WILL SEE HEAVY PRECIPITATION TODAY AND TONIGHT. THE
LIMITING POTENTIAL FOR HEAVY SNOW DEPENDS ON HOW FAST SNOW LEVELS
RISE. A COUPLE SNOTEL SITES AROUND 5000 FT IN THE LANE COUNTY
CASCADES HAVE WARMED TO NEAR 35 DEGREES AS OF 17Z. WILL TAKE LONGER
FOR SNOW LEVELS TO RISE ABOVE THE PASSES IN THE NORTH OREGON AND
SOUTH WASHINGTON CASCADES...ESPECIALLY CLOSER TO THE COLUMBIA GORGE
AND EAST FLANKS. MODELS MAKE THE COLD AIR AND LOWER SNOW LEVELS MORE
RESILIENT IN THE S WA CASCADES...ESPECIALLY OVER THE EASTERN SECTIONS
NEAR MT ADAMS. WILL NEED TO KEEP THE WINTER STORM WARNING GOING THERE
THROUGH TONIGHT.

PRECIPITATION BEGINS TO EASE UP FRI AS THE BAROCLINIC ZONE SLIPS OFF
TO THE SE. THE UPPER TROUGH REMAINS OFFSHORE FRI AND SAT...BUT DIGS
IN CLOSER TO THE COAST. THIS KEEPS AREA WITH A THREAT FOR SHOWERS
THROUGH SAT. WEISHAAR

.LONG TERM...SATURDAY NIGHT THROUGH WEDNESDAY...COOL AND WET WEATHER
IS EXPECTED TO CONTINUE ACROSS THE REGION THROUGH EARLY NEXT WEEK. A
BROAD UPPER TROUGH WILL EXIT TOWARDS THE ROCKIES SATURDAY NIGHT AND
SUNDAY...LEAVING BEHIND SOME COOL AIR WHICH WILL KEEP SNOW LEVELS
FAIRLY LOW IN THE CASCADES. MEANWHILE A PROGRESSIVE FLOW PATTERN
WILL RESUME ACROSS THE NORTH-CENTRAL AND NORTHEAST PACIFIC...KEEPING
OUR WEATHER ACTIVE AS A SERIES OF COLD FRONTS MOVE ACROSS THE PAC NW
FROM THE PACIFIC. ONE SUCH COLD FRONT IS EXPECTED LATE SATURDAY
NIGHT AND SUNDAY...AND ANOTHER MONDAY AND MONDAY NIGHT. THE SUNDAY
SYSTEM WILL LIKELY BE A FAST MOVER...KEEPING DOWN QPF TOTALS DESPITE
BEING A FAIRLY VIGOROUS FRONT. SNOW LEVELS WILL LIKELY BE AROUND
2500 FT WITH THIS FRONT SO IT WILL BE A GOOD SNOWMAKER FOR THE
CASCADES...ESPECIALLY WITH THE 30-40 KT WESTERLY PRODUCING HEALTHY
OROGRAPHICS BEHIND THE FRONT.

SNOW LEVELS WILL PROBABLY RISE A BIT HEAD OF THE NEXT...STRONGER
FRONT...PERHAPS UP TO NEAR PASS LEVEL IN THE CASCADES. 00Z
GFS/ECMWF/GEM ALL SUGGEST THIS SYSTEM WILL HAVE A BIT MORE MOISTURE
INVOLVED...WITH PW VALUES EXCEEDING AN INCH AND STRONG ONSHORE 850
MB FLOW RATCHETING UP THE VAPOR TRANSPORT ESPECIALLY INTO WESTERN
OREGON. WITH FLOW ALOFT BECOMING MORE ZONAL AGAIN IN THE PACIFIC...
THIS FRONTAL ZONE WILL LIKELY BE SLOWER MOVING AS WELL. WITH DEEP
MOISTURE...STRONG FORCING...AND SLOW MOVEMENT OF THE BAROCLINIC
ZONE...THERE IS THE POTENTIAL FOR SUBSTANTIAL RAIN TOTALS YET AGAIN
WITH THIS SYSTEM. LOOKS LIKE AREA RIVERS WILL NOT GET A CHANCE TO
SUBSIDE MUCH...SO LOCAL FLOODING REMAINS A THREAT...ALONG WITH THE
POSSIBILITY FOR LANDSLIDES IN THE REGION AS SOILS REMAIN VERY
SATURATED.  WEAGLE/27

&&

.AVIATION...MOIST SOUTHWESTERLY FLOW ALOFT WILL CONTINUE AS WARM
FRONT PUSHES ACROSS THE REGION THIS AM. COMBINATION OF VFR AND
MVFR CONDITIONS THIS MORNING WITH SOME BRIEF IFR WITH LOWER CIGS.
AFTER 00Z...CONDITIONS WILL DETERIORATE AS COLD FRONT APPROACHES
FROM THE WEST WITH INCREASING MVFR AND RAIN. WILL SEE SOUTH WINDS
ALONG OREGON COAST AFTER 02Z AND OVER HIGHER TERRAIN AFTER 04Z.
OTHER PROBLEM WILL BE AREAS OF SNOW AND LOWER VIS TO N AND E OF MT
HOOD TODAY AND THIS EVENING.

KPDX AND APPROACHES...MVFR CONDITIONS THROUGH THIS EVENING. MAY
SEE CIGS BRIEFLY IMPROVE TO LOW VFR BETWEEN 21Z AND 02Z...BUT NOT
ALL THAT CONFIDENT. SUSPECT WITH MOIST AIR MASS AND RAIN...CIGS
WILL LIKELY REMAIN MVFR...WITH ONLY BRIEF OCCASIONAL BREAKS IN
LOWER DECK. BOWEN/ROCKEY

&&

.MARINE...GUSTY WINDS HAVE SUBSIDED SOME THIS MORNING TO 15 TO 25 KT
AND THOSE WINDS ARE EXPECTED TO CONTINUE THIS AFTERNOON INTO THE
EARLY EVENING. BUT AS COLD FRONT FRONT APPROACHES THIS
EVENING...SOUTHERLY WINDS WILL INCREASE AGAIN. LIKELY WILL SEE
AREA OF LOW PRES MOVE ALONG THE FRONT THIS EVENING...WHICH WILL
INTENSIFY WINDS OVER THE OREGON WATERS. THIS LOW WILL PUSH ONSHORE
SOMEWHERE NORTH OF ASTORIA LATER TONIGHT. MODELS SUGGEST STORM
FORCE WINDS EXTENDING FROM S OREGON WATERS UP ALONG THE OREGON
COAST TO AS FAR NORTH AS ABOUT CASCADE HEAD.

AS A RESULT...WILL CONTINUE SMALL CRAFT ADVISORY THROUGH THIS
EVENING WHEN STORM WARNING SOUTH AND GALE WARNING NORTH GO INTO
EFFECT.

SEAS HAVE DECREASED SOME THIS MORNING TO AROUND 9 TO 12 FT.
HOWEVER THEY WILL BUILD AGAIN TONIGHT WITH THE WINDS AND A SLIGHT
INCREASE IN SWELL. LIKELY TO PEAK 12 TO 15 FT IN THE NORTH...AND
16 TO 19 FT FROM CASCADE HEAD SOUTHWARD LATER THIS EVENING AND
OVERNIGHT.

WINDS AND SEAS WILL DROP BACK LATE TONIGHT AND FRI...BUT EXPECT
ANOTHER ROUND OF GUSTY WINDS AND HIGHER SEAS THIS WEEKEND AS THE
PARADE OF FRONTS CONTINUES INTO THE PAC NW. BOWEN/ROCKEY

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...WINTER STORM WARNING UNTIL 4 PM PST THIS AFTERNOON FOR CENTRAL
     COLUMBIA RIVER GORGE-UPPER HOOD RIVER VALLEY.

     FLOOD WATCH THROUGH LATE FRIDAY NIGHT FOR CASCADE FOOTHILLS IN
     LANE COUNTY-CENTRAL COAST RANGE OF WESTERN OREGON-CENTRAL
     OREGON COAST-CENTRAL WILLAMETTE VALLEY-COAST RANGE OF
     NORTHWEST OREGON-GREATER PORTLAND METRO AREA-LOWER COLUMBIA-
     NORTH OREGON COAST-NORTHERN OREGON CASCADE FOOTHILLS-SOUTH
     WILLAMETTE VALLEY-WESTERN COLUMBIA RIVER GORGE.

     WINTER STORM WARNING UNTIL 6 PM PST THIS EVENING FOR NORTHERN
     OREGON CASCADES.

     HIGH WIND WARNING FROM 6 PM THIS EVENING TO 2 AM PST FRIDAY FOR
     CENTRAL OREGON COAST-NORTH OREGON COAST.

WA...WINTER STORM WARNING UNTIL 4 PM PST THIS AFTERNOON FOR CENTRAL
     COLUMBIA RIVER GORGE.

     FLOOD WATCH THROUGH LATE FRIDAY NIGHT FOR GREATER VANCOUVER
     AREA-I-5 CORRIDOR IN COWLITZ COUNTY-SOUTH WASHINGTON CASCADE
     FOOTHILLS-SOUTH WASHINGTON COAST-WESTERN COLUMBIA RIVER
     GORGE-WILLAPA HILLS.

     WINTER STORM WARNING UNTIL 4 AM PST FRIDAY FOR SOUTH WASHINGTON
     CASCADES.

PZ...SMALL CRAFT ADVISORY FOR WINDS UNTIL 6 PM PST THIS EVENING FOR
     COASTAL WATERS FROM CAPE SHOALWATER WA TO FLORENCE OR OUT 60 NM.

     GALE WARNING FROM 6 PM THIS EVENING TO MIDNIGHT PST TONIGHT FOR
     COASTAL WATERS FROM CAPE SHOALWATER WA TO CASCADE HEAD OR OUT
     60 NM.

     SMALL CRAFT ADVISORY FOR HAZARDOUS SEAS UNTIL 6 PM PST THIS
     EVENING FOR COASTAL WATERS FROM CAPE SHOALWATER WA TO FLORENCE
     OR OUT 60 NM.

     STORM WARNING FROM 6 PM THIS EVENING TO 1 AM PST FRIDAY FOR
     COASTAL WATERS FROM CASCADE HEAD OR 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.