Heavy Snow in Oregon, December 17, 2015

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PORTLAND OR
947 PM PST WED DEC 16 2015

.SYNOPSIS... ANOTHER ATMOSPHERIC RIVER OF SUBTROPICAL MOISTURE WILL
COUPLE WITH AN OCCLUDING WARM FRONT TO BRING MORE HEAVY PRECIPITATION
TONIGHT 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. TEMPS 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.

&&

.EVENING UPDATE...IR AND WATER VAPOR SATELLITE IMAGERY SHOW A SHIELD
OF INCREASING MOISTURE AND COOLER CLOUD TOPS PUSHING ONSHORE THIS
EVENING AHEAD OF A WARM FRONT APPROACHING JUST OFFSHORE. THE LATEST
RADAR IMAGERY ALSO SHOWS A SHIELD OF STRATIFORM RAIN SPREADING INLAND
AS WELL OVER THE PAST FEW HOURS. RAINFALL RATES WILL PICK UP
CONSIDERABLY AFTER MIDNIGHT AS ISENTROPIC LIFT INCREASES WITH THE
ARRIVAL OF THE WARM FRONT AND THE MID LEVEL WINDS TURN FROM SOUTHERLY
TO SOUTHWESTERLY...INCREASING PERPENDICULAR MOISTURE TRANSPORT INTO
THE COAST AND COAST RANGE. OVERALL...HAVE NOT CHANGED THE FCST
SIGNIFICANTLY THIS EVENING. QPF VALUES STILL LOOK REASONABLE AS THE
MODELS SEEM TO BE IN PRETTY GOOD GENERAL AGREEMENT ON THE PLACEMENT
AND TIMING OF THE HEAVIER PRECIPITATION. HOWEVER...CONSIDERABLE
UNCERTAINTY CONTINUES TO EXIST WITH RESPECT TO THE POTENTIAL FOR ICE
AND SNOW IN THE GORGE AND UPPER HOOD RIVER VALLEY AREAS LATE TONIGHT
AND THU. COOLER AND DRIER AIR IS TRYING TO WORK ITS WAY DOWN SOUTH
THROUGH THE COLUMBIA BASIN...BUT TEMPS AT KDLS AND IN THE HOOD RIVER
AREA STILL HOVERING IN THE 38 TO 40 DEG F RANGE WITH DEW POINTS NEAR
TO OR A COUPLE OF DEGREES BELOW 32. THE 00Z NAM AND UW WRF-GFS FCST
SOUNDINGS CONTINUE TO INDICATE THAT SURFACE TEMPS WILL COOL A FEW
MORE DEGREES LATER TONIGHT...AND WHEN COMBINED WITH WET BULB COOLING
AND DYNAMIC COOLING OF THE COLUMN WITH THE ARRIVAL OF THE HEAVIER
PRECIP LATE TONIGHT AND EARLY THU MORNING WILL BE ABLE TO PRODUCE AN
AIRMASS CONDUCIVE TO HEAVY SNOWFALL. GIVEN THESE LATEST
SOUNDINGS...WILL KEEP THE FCST TRENDED TOWARD THIS IDEA...BUT WOULD
NOT BE SURPRISED AT ALL IF WE CANT QUITE COOL OFF ENOUGH AND SEE MORE
RAIN THAN SNOW OR ICE IN THE GORGE WITH THIS EVENT. THE BETTER SNOW
AND ICE CHANCES APPEAR TO BE IN THE UPPER HOOD RIVER VALLEY...WHERE
EASTERLY UPSLOPE FLOW SHOULD PROVIDE ENOUGH ADDITIONAL COOLING TO
KEEP PRECIPITATION FALLING AS SNOW OR ICE THROUGH MOST OF THU. PYLE

.PREVIOUS SHORT TERM DISCUSSION ISSUED 331 PM WEDNESDAY...
.THIS AFTERNOON THROUGH THURSDAY NIGHT...THE FORECAST
WEST OF THE CASCADES REMAINS CONSISTENT WITH SHOWERS THIS AFTERNOON
THEN HEAVY RAIN MOVING INTO THE AREA STARTING AT THE COAST THIS
EVENING AND MOVING INLAND OVERNIGHT. THE FLOOD WATCH REMAINS IN
EFFECT FOR MOST OF THE LOCAL FORECAST AREA AND MULTIPLE RIVERS ARE
CURRENTLY FORECAST TO FLOOD. MADE ONLY MINOR CHANGES TO QPF
TODAY...MAINLY TO INCREASE IT SOME IN THE VALLEY INITIALLY DURING
SOUTHERLY FLOW. OTHERWISE NO MAJOR CHANGES TO AREAS WEST OF THE
CASCADES.

THE AREA FROM THE CASCADES EAST WILL BE AFFECTED BY THE SAME SYSTEM
WITH HEAVY PRECIPITATION BEGINNING LATE THIS EVENING AND TONIGHT.
WITH SNOW LEVELS CURRENTLY AROUND 2500 FT...PRECIP ABOVE THAT
ELEVATION WILL BEGIN AS SNOW. BECAUSE THERE WILL BE BURSTS OF INTENSE
PRECIP TOWARDS THE BEGINNING OF THE EVENT...CURRENT THOUGHT IS THAT
IT WILL NOT TAKE VERY MUCH TIME TO ACCUMULATE QUITE A BIT OF SNOW.
SNOW LEVELS WILL RISE QUICKLY IN THE LANE COUNTY CASCADES...SO WILL
STICK WITH WINTER WEATHER ADVISORY THERE AND HAVE UPGRADED TO A
WINTER STORM WARNING FOR THE NORTH OREGON AND SOUTH WASHINGTON
CASCADES WHERE SNOW LEVELS WILL BE SLOWER TO RISE.

DESPITE THE EVENT BEGINNING TONIGHT...CONFIDENCE REMAINS LOW IN THE
PRECIPITATION TYPE THROUGH THE EVENT FOR LOWER ELEVATIONS
PARTICULARLY IN THE COLUMBIA RIVER GORGE AND THE UPPER HOOD RIVER
VALLEY. MODELS CONTINUE TO INDICATE THAT COLD HIGH PRESSURE WILL MOVE
SOUTH INTO THE AREA EAST OF THE CASCADES THIS EVENING AND
TONIGHT...CREATING EASTERLY FLOW ON THE EAST SIDE AS WELL AS SOME
COLD AIR DAMMING. THE MODELS SHOW THE COLD AIR FUNNELING INTO THE
COLUMBIA RIVER GORGE SLIGHTLY BEFORE OR AROUND THE TIME THAT THE
PRECIPITATION BEGINS. AS SUCH...THEY SHOW PRECIP TYPE BEGINNING AS
SNOW FOR MUCH OF THE CENTRAL GORGE AND UPPER HOOD RIVER VALLEY THEN
TRANSITIONING TO RAIN SOMETIME ON THURSDAY.

THE ISSUES BEGIN WITH THE FACT THAT THE MODELS ARE RUNNING A COUPLE
OF DEGREES COOLER THAN OBSERVATIONS THIS AFTERNOON. PROBLEMS CONTINUE
WHEN LOOKING AT TEMPERATURES IN THE SUPPOSED LOCAL SOURCE REGION OF
THE COLD AIR IN NE WASHINGTON WHERE TEMPERATURES ARE AROUND OR ONLY A
FEW DEGREES BELOW FREEZING. SINCE SUNNY SKIES IN EASTERN OREGON AND
WASHINGTON HAVE ALLOWED SOME DAYTIME HEATING...THE COLD AIR WILL
INEVITABLY BE MODIFIED SOME BEFORE IT ARRIVES IN THE LOCAL AREA SO
WOULD FEEL BETTER ABOUT IT IF THE AIR WERE STARTING OUT MUCH COLDER.
COMBINED WITH THE FACT THAT TEMPERATURES IN THE GORGE AND HOOD RIVER
VALLEY ARE CURRENTLY IN THE 40S WITH DEWPOINTS IN THE MID TO UPPER
30S...THERE IS SOME DOUBT AS TO WHETHER TEMPERATURES WILL COOL ENOUGH
TO PRODUCE ANY FROZEN PRECIPITATION IN THE GORGE AND HOOD RIVER
VALLEY. FOR NOW HAVE GONE WITH A WINTER WEATHER ADVISORY FOR THE
WESTERN GORGE EAST OF MULTNOMAH FALLS AND THE CENTRAL GORGE AND A
WINTER STORM WARNING FOR THE UPPER HOOD RIVER VALLEY...BUT THE
EVENING SHIFT WILL BE WATCHING TEMPERATURES CLOSELY AND MAKING
CHANGES AS NECESSARY BASED ON OBSERVATIONS.

ANOTHER SOURCE OF UNCERTAINTY LIES IN THE TRANSITION FROM WINTRY
PRECIP TO PURE RAIN IN THE GORGE AND HOOD RIVER VALLEY ON THURSDAY
ASSUMING THE COLD AIR DOES INDEED ARRIVE TONIGHT. GIVEN THE FORECAST
WEATHER PATTERN...MODELS ARE INFAMOUS FOR SCOURING OUT THE COLD AIR
TOO QUICKLY WHICH MEANS THE TRANSITION COULD HAPPEN LATER THAN
EXPECTED OR TRANSITION P-TYPES OF SLEET OR FREEZING RAIN COULD LAST
LONGER THAN ANTICIPATED. FOR THIS REASON...ACCUMULATIONS ARE STILL
UP IN THE AIR.

ANOTHER SYSTEM WILL CONTINUE THE WET PATTERN THROUGH THURSDAY NIGHT
WITH INCREASING WINDS AS THE PRESSURE GRADIENT TIGHTENS AHEAD OF A
BAROCLINIC ZONE STRETCHING DOWN THE OREGON COAST WITH LOW PRESSURE
DEVELOPING OFF THE WASHINGTON COAST. MODELS ARE CURRENTLY SHOWING
BOUNDARY LAYER WINDS OF 50 TO 60 KT ALONG THE IMMEDIATE COAST
THURSDAY EVENING AND NIGHT WHICH MAY RESULT IN HIGH WINDS ALONG THE
COAST AND BREEZY CONDITIONS INLAND. FOR NOW HAVE INDICATED SOME LOW
LEVEL HIGH WINDS FOR THE BEACHES AND HEADLANDS IN THE FORECAST.
HOWEVER WILL HOLD OFF ON ISSUING ANY HIGH WIND PRODUCTS AT THIS TIME
SINCE THIS IS A RECENT DEVELOPMENT IN THE MODELS. BOWEN

.LONG TERM...FRIDAY THROUGH WEDNESDAY...VERY WET AND ACTIVE PATTERN
AHEAD FOR THE REGION THROUGH THE WEEKEND AND INTO EARLY NEXT WEEK. A
POTENT COLD FRONT STARTS OFF THIS WET STRETCH...PUSHING ONSHORE
FRIDAY AS THE MAIN UPPER LEVEL TROUGH DIGS SOUTHWARD. COLDER AIR IS
USHERED IN BEHIND THE FRONT...LOWERING SNOW LEVELS TO NEAR 2500 FT
LATE FRIDAY THROUGH NEXT MONDAY. THIS MEANS THE FOLLOWING FRONTAL
BANDS WILL BRING PLENTY OF SNOW TO THE CASCADES AND CASCADE
FOOTHILLS THROUGH THE ENTIRE WEEKEND...ESPECIALLY WITH ANOTHER COLD
FRONT ARRIVING SUNDAY. SNOW LEVELS RISE TO 4500 FT LATE MONDAY...IN
TIME FOR A STRONGER SYSTEM TO BRING NEARLY AN 1.5 INCHES OF RAIN IN
ABOUT 24 HRS TO THE AREA MONDAY INTO TUESDAY MORNING. 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. GOOD NEWS
THOUGH...NOT EXPECTING VERY STRONG WINDS WITH THIS SYSTEM ON
MONDAY/TUESDAY...ASIDE FROM BREEZY CONDITIONS ALONG THE COAST AND
ELEVATED SITES. /27

&&

.AVIATION...MOSTLY VFR CONDITIONS THIS EVENING. A WARM FRONT IS
BEGINNING TO BRING RAIN TO THE REGION...AND EXPECT FLIGHT
CONDITIONS TO FLUCTUATE BETWEEN VFR AND MVFR THROUGH 9Z. HEAVIER
RAIN WILL FURTHER DETERIORATE CONDITIONS TO A MIX OF MVFR AND IFR
AFT 09Z. SNOW LIKELY EAST OF HOOD RIVER TONIGHT AND
THURSDAY...WITH A MIX OF RAIN AND SNOW IN THE GORGE WEST OF HOOD
RIVER.

EAST WINDS THROUGH THE GORGE WILL GUST TO AROUND 30 KT AT KTTD
AND VCNTY. SOUTH WINDS WILL TO GUST AROUND 40 KT ALONG THE COAST
THU AFTERNOON AND EVENING. SOUTH WINDS GUSTING UP TO 30 KT IN THE
WILLAMETTE VALLEY THU EVENING.

KPDX AND APPROACHES...EAST WINDS WILL INCREASE TO AROUND 15 KT THU
MORNING...THEN 20 KT WITH GUSTS TO 30 KT POSSIBLE THU AFTERNOON.
PRIMARILY
VFR CONDITIONS THIS EVENING DETERIORATING TO PREDOMINANTLY MVFR
CONDITIONS WITH STEADY RAIN BETWEEN 09Z AND 15Z THU. BRIEF IFR
WITH HEAVY RAIN POSSIBLE AROUND 18Z THU. NEUMAN/TJ

&&

.MARINE...SOUTHEAST WINDS ARE INCREASING THIS EVENING AHEAD OF A
WARM FRONT...WITH GALES OBSERVED AT BUOY #89...AND ALMOST AT BUOY
#50. LOW END GALE FORCE WIND GUSTS WILL PERSIST THROUGH LATE
TONIGHT. WINDS SHOULD RELAX ACROSS THE WATERS THURSDAY
MORNING...ESPECIALLY NORTH OF CASCADE HEAD.

THE WINDS WILL RAPIDLY INCREASE THURSDAY EVENING AS A LOW
TRAILING THE COLD FRONT WILL PRODUCE A STRONG SURGE OF SOUTHERLY
WINDS. HAVE CONTINUED THE STORM WATCH AS THERE IS STILL A HIGH
PROBABILITY FOR STORM FORCE GUSTS WITH THIS SURGE. SEAS WILL BUILD
TO 15 TO 18 FT DUE TO THE INCREASED WINDS THURSDAY NIGHT...THEN
WILL SUBSIDE BACK TO 11 FT FRIDAY. THERE IS HIGHER FORECAST
CONFIDENCE FOR STORM FORCE GUSTS AND ABOVE 15 FT SEAS SOUTH OF
CASCADE HEAD AS ONLY ONE OF THE 3 MAIN MODELS FORECAST THAT STORM
FORCE GUSTS NORTH OF CASCADE HEAD. TJ

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...WINTER STORM WARNING UNTIL 10 PM PST THURSDAY FOR UPPER HOOD
     RIVER VALLEY.

     FLOOD WATCH FROM 4 AM PST THURSDAY 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 WEATHER ADVISORY UNTIL 7 PM PST THURSDAY FOR WESTERN
     COLUMBIA RIVER GORGE.

     WINTER WEATHER ADVISORY UNTIL 10 PM PST THURSDAY FOR CENTRAL
     COLUMBIA RIVER GORGE.

     WINTER STORM WARNING UNTIL 4 AM PST FRIDAY FOR NORTHERN OREGON
     CASCADES.

     WINTER WEATHER ADVISORY UNTIL 7 AM PST THURSDAY FOR CASCADES IN
     LANE COUNTY.

     HIGH WIND WATCH FROM THURSDAY EVENING THROUGH LATE THURSDAY
     NIGHT FOR CENTRAL OREGON COAST-NORTH OREGON COAST.

WA...FLOOD WATCH FROM 4 AM PST THURSDAY 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 WEATHER ADVISORY UNTIL 7 PM PST THURSDAY FOR WESTERN
     COLUMBIA RIVER GORGE.

     WINTER WEATHER ADVISORY UNTIL 10 PM PST THURSDAY FOR CENTRAL
     COLUMBIA RIVER GORGE.

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

PZ...STORM WATCH FROM THURSDAY AFTERNOON THROUGH FRIDAY MORNING FOR
     COASTAL WATERS FROM CAPE SHOALWATER WA TO FLORENCE OR OUT 60 NM.

     GALE WARNING UNTIL 10 AM PST THURSDAY FOR WATERS FROM CAPE
     SHOALWATER WA TO CASCADE HEAD OR FROM 10 TO 60 NM-WATERS
     FROM CASCADE HEAD TO FLORENCE OR FROM 10 TO 60 NM.

     SMALL CRAFT ADVISORY FOR ROUGH COLUMBIA RIVER BAR UNTIL NOON
    PST THURSDAY.

     GALE WARNING UNTIL 10 AM PST THURSDAY FOR COASTAL WATERS FROM
     CAPE SHOALWATER WA TO CASCADE HEAD OR OUT 10 NM-COASTAL WATERS
     FROM CASCADE HEAD TO FLORENCE OR OUT 10 NM.

 &&

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.