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000
FXUS66 KPQR 100721 CCA
AFDPQR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PORTLAND OR
857 PM PST MON DEC  9 2013

.SYNOPSIS...LOWER PRESSURE OFFSHORE CONTINUES TO DRIVE WEAK EASTERLY
FLOW THROUGH THE COLUMBIA GORGE AND INTO THE PORTLAND AREA. A WEAK
CLIPPER SYSTEM WILL PUSH DOWN FROM THE YUKON BRINGING A CHANCE OF
SNOW AND OR FREEZING DRIZZLE MAINLY FOR OUR NORTHERN ZONES INTO
TUESDAY. WEDNESDAY SHOULD REMAIN CHILLY AND DRY...THOUGH A LITTLE
WARMER THAN THE PAST COUPLE DAYS AS OUR AIR MASS SLOWLY MODERATES.
THE NEXT SYSTEM LOOKS STRONGER...AND MAY ENCOUNTER A STUBBORN LOW-
LEVEL COLD AIR MASS AS PRECIPITATION SPREADS ONSHORE. THIS MAY
RESULT IN MORE FROZEN OR FREEZING PRECIPITATION BEFORE A PACIFIC AIR
MASS FINALLY TAKES OVER LATE IN THE WEEK.

&&
.SHORT TERM...EVENING UPDATE FOR TONIGHT THROUGH THURSDAY...THE
THREAT FOR LIGHT FREEZING DRIZZLE CONTINUES WITH THE EVENING UPDATE
AS COASTAL SITES HAVE SHOWN FREEZING RAIN/UNKNOWN PRECIP THE PAST FEW
HOURS. CURRENT RADAR IMAGERY SHOWS SOME RETURNS OVER THE COAST AND
NORTHERN WILLAMETTE VALLEY...BUT SURFACE OBS ARE NOT PICKING UP ON
ANYTHING. MODELS ARE STILL SHOWING THE BEST TIME FRAME FOR FREEZING
DRIZZLE FOR INLAND LOCATIONS WILL BE EARLY TUESDAY MORNING...ROUGHLY
FROM 4-10AM PST...CREATING A RATHER SLICK MORNING COMMUTE. EXPECT THE
COAST TO SEE THINGS SLOWING DOWN AROUND 5-8AM PST.

GUSTY WESTERLY WINDS THROUGH THE GORGE...WHICH WERE INSTIGATED BY
SURFACE HIGH PRESSURE OVER THE WILLAMETTE VALLEY...KEPT TODAYS HIGH
TEMPERATURES SLIGHTLY ABOVE FREEZING. LATEST SURFACE OBS ARE STILL
SHOWING SOME GUSTS TO 20 MPH THIS EVENING...AND IF THESE WINDS
PERSIST...OVERNIGHT LOWS MAY NOT BE AS COLD EITHER. MODELS ARE
SHOWING WEST WINDS MAY CONTINUE THROUGH MIDDAY TUESDAY...SO HAVE
UPPED TONIGHTS LOW AND TUESDAYS HIGH TEMPERATURES FOR THE EAST GORGE
AND UPPER HOOD RIVER VALLEY. THIS WILL ALSO IMPACT THE TYPE OF PRECIP
THEY SEE OVERNIGHT INTO TUESDAY...WITH SOME LOCATIONS SEEING PATCHY
FREEZING DRIZZLE/LIGHT SNOW SHOWERS...WHILE SOME SPOTS MAY SEE MORE
OF A MIX IF THE WIND KEEPS THEM ABOVE FREEZING. ASIDE FROM THESE
CHANGES...THE REMAINDER OF THE FORECAST REMAINS VALID. /27

HAVE CONTINUED THE WINTER WEATHER ADVISORIES AS SCHEDULED BUT WILL
NEED TO LOOK AT EXTENDING THE NORTH COAST AND WILLAPA HILLS FURTHER
OUT IN TIME DEPENDING ON HOW FAST THE UPPER BOUNDARY LAYER AND LOWER
MID LEVEL AIR MASS COOLS. STILL LOOKS LIKE IT COULD BE A SLICK
COMMUTE WITH NOT NECESSARILY A WIDESPREAD FREEZING DRIZZLE EVENT.
CONSIDERED THIS TO BE A BIT MORE DANGEROUS AS PATCHY ICE COULD CATCH
OVERCONFIDENT DRIVERS BY SURPRISE. MAIN CONCERN WILL BE THE UPPER
WAVE DROPPING SOUTH FROM NORTHERN VANCOUVER ISLAND OVERNIGHT AND
THROUGH TUESDAY MORNING. IR IMAGERY DOES SHOW SOME CONVECTIVE
ENHANCEMENT NEAR THE NORTHERN TIP OF THE ISLAND. FOLLOWED THE
PREVIOUS SHIFTS LATE OVERNIGHT UPDATE TO KEEP THE CENTRAL VALLEY AREA
OUT OF THE ADVISORIES FOR THE TIME BEING. REALLY DONT EXPECT MUCH
BEYOND SOME VERY LIGHT SNOW ACCUMULATIONS, IF AT ALL, FOR AREAS SOUTH
OF SALEM. THE THREAT OF SNOW SHOWERS WILL DRIFT NORTH OVER THE COURSE
OF TUESDAY AS TRANSITIONAL UPPER RIDGING BUILDS IN.

DECIDED TO FOLLOW TODAYS TREND OF UNDERCUTTING TEMPERATURES THROUGH
WEDNESDAY DUE TO A LACK OF SIGNIFICANT ONSHORE FLOW. IN FACT,
CURRENT OBSERVATIONS SHOW A FAIRLY UNUSUAL SURFACE PRESSURE PATTERN
WITH HIGH PRESSURE OVER THE VALLEY RESULTING IN EASTERLY FLOW AT
ASTORIA AND LIGHT WESTERLY THROUGH THE WESTERN GORGE.

THE NEXT LOOMING THREAT CERTAINLY IS FOR WEDNESDAY NIGHT AND
THURSDAY WITH CARRY OVER TO THE LONG TERM PERIOD. AN UPPER LOW MOVES
EASTWARD ACROSS THE GULF OF ALASKA. THIS WILL INDUCE A FAIRLY STRONG
SOUTH TO SOUTHWESTERLY FLOW ALOFT ACROSS THE REGION. STRONG WARM AIR
ADVECTION WILL ACCOMPANY THE FLOW WITH A WARM NOSE ROUGHLY CENTERED
AROUND 900 MB. HAVE TAKEN A GUT FEELING APPROACH IN THINKING THE
COLD VALLEY AIR WILL BE SLOW TO SCOUR OUT. THIS LARGELY GOES AGAINST
ALL MODELS WITH SOME LINGERING WEST SIDE COLD POOL SUPPORT FROM THE
NAM12 BEING SLOWEST TO SCOUR. ARBITRARILY PICK A FREEZING RAIN
THREAT FOR INLAND LOCATIONS BELOW 750 TO 1000 FEET MSL. THIS
DECISION IS BASED ON MOSTLY A GUT FEELING BASED ON PAST POOR MODEL
PERFORMANCE IN SCOURING TOO FAST. ADDITIONALLY...THE SURFACE LOW
REMAINS WEST OF THE REGION WHICH WILL LIMIT THE FULL MIXING OF
SOUTHERLY WINDS TO THE SURFACE. WILL HOLD OFF ON ISSUING ANY
STATEMENTS WITH THE WEDNESDAY NIGHT AND THURSDAY EVENT UNTIL WE GET
PAST THE NEAR TERM THREAT AND HOPEFULLY WILL GET BETTER MODEL
SUPPORT...IF IT EVER COMES. AREAS ABOVE THE TRAPPED AIR WILL GET A
RAIN AND SNOW MIX THROUGH SNOW LEVEL LOOK TO DRAMATICALLY RISE AS
THE WARM AIR PUSHES IN. /JBONK


.LONG TERM...NO CHANGES. PREVIOUS DISCUSSION FOLLOWS...THURSDAY NIGHT
THROUGH MONDAY. THE EXTENDED PERIOD BEGINS WITH PRECIPITATION
ASSOCIATED WITH THE PACIFIC SYSTEM THAT ARRIVES EARLIER IN THE DAY
THURSDAY. THE MAIN UNCERTAINTY HERE LIES IN JUST HOW QUICKLY THE COLD
AIR WILL SCOUR OUT. AREAS SUCH AS THE COLUMBIA RIVER GORGE AND
WILLAMETTE VALLEY MAY SEE SOME FREEZING RAIN THROUGH THURSDAY NIGHT
WITH THIS SYSTEM AND HAVE INTRODUCED THIS TO THE GRIDS THROUGH
MIDNIGHT FRIDAY TO REFLECT THIS UNCERTAINTY. OTHERWISE...THIS WILL
CONTINUE AS RAIN AND CASCADE SNOW THROUGH FRIDAY MORNING.

AFTER THIS...LATE FRIDAY AND SATURDAY LOOK TO BE A BRIEF DRY PERIOD
AHEAD OF THE NEXT PACIFIC SYSTEM. MODEL GUIDANCE TODAY SLOWS THE
ARRIVAL OF THE SYSTEM ABOUT 12 HOURS. THE SYSTEM IS SIGNIFICANTLY
WEAKER IN THE 12Z GFS THAN ECMWF...BUT LOOKS LIKE ANOTHER ROUND OF
PRECIPITATION WILL ARRIVE SATURDAY NIGHT OR SUNDAY. THIS ONE WILL
LIKELY BE IN FORM OF RAIN THOUGH AS TEMPERATURES MODERATE TO AROUND
SEASONAL NORMALS FOR THE END OF THE EXTENDED PERIOD.  CULLEN

&&

.AVIATION...A WEAK SHORTWAVE MOVING INTO THE AREA FROM THE NW WILL
BRING LIGHT PCPN TONIGHT MAINLY TO THE NORTH COAST AND NORTH
INLAND AREAS. THE TYPE OF PCPN IS OF HIGH CONCERN. AT KAST AND
KHQM HAD AN HOUR OR TWO OF FZRA/FZDZ THIS AFTERNOON AND EARLY
EVENING AS THE RELATIVELY WARMER AIR ALOFT OVERRIDES THE SHALLOW
COLD AIR IN PLACE. IT APPEARS THE THREAT FOR FREEZING PCPN WILL
END BY 15Z TUE FOR THE COAST WHEN SOUTHERLY WINDS ARRIVE TO SCOUR
OUT THE COLD AIR AT THE SURFACE. INLAND AREAS NORTH OF ABOUT KSLE
WILL BE MORE OF A CHALLENGE...BUT EXPECT FREEZING DRIZZLE AND/OR SNOW
THROUGH MID MORNING TUE. FLIGHT CONDITIONS LIKELY IN THE VFR TO
MVFR RANGE WITH OCCASIONAL RESTRICTIONS TO VISIBILITY WITH PCPN.
SOUTH OF KSLE WILL BE VFR THROUGH TUESDAY...EXCEPT LOCAL IFR IN
PATCHY FOG POSSIBLE OVERNIGHT IN THE S WILLAMETTE VALLEY.

KPDX AND APPROACHES...VFR THROUGH 10Z THE INCREASING CHANCES FOR
LIGHT FREEZING DRIZZLE OR SNOW ALONG WITH MVFR CONDITIONS. MVFR
CONDITIONS LIKELY TO CONTINUE THROUGH TUE AFTERNOON AND
EVENING...WITH PCPN ENDING IN THE AFTERNOON.

&&

.MARINE...FEW UPDATES. BENIGN WEATHER THROUGH WED. HIGH PRES OVER
PAC NW MAINTAINS LIGHT OFFSHORE FLOW THROUGH WED. WINDS WILL TURN
MORE W TO SW TUE AND WED AS HIGH WEAKENS AND LOWER PRES APPROACHES
FROM THE W. STRONG COLD FRONT WILL ARRIVE ON THU...WITH S GALES
EXPECTED.

SEAS 3 TO 5 FT THROUGH LATE WED...THEN QUICKLY BUILDING TO 10 FT
EARLY THU AHEAD OF THE NEXT FRONT.

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...WINTER WEATHER ADVISORY UNTIL 1 PM PST TUESDAY FOR GREATER
     PORTLAND METRO AREA-LOWER COLUMBIA.

     WINTER WEATHER ADVISORY UNTIL 10 PM PST THIS EVENING FOR COAST
     RANGE OF NORTHWEST OREGON-NORTH OREGON COAST.

WA...WINTER WEATHER ADVISORY UNTIL 1 PM PST TUESDAY FOR GREATER
     VANCOUVER AREA-I-5 CORRIDOR IN COWLITZ COUNTY.

     WINTER WEATHER ADVISORY UNTIL 10 PM PST THIS EVENING FOR SOUTH
     WASHINGTON COAST-WILLAPA HILLS.

PZ...NONE.
&&


&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...WINTER WEATHER ADVISORY UNTIL 1 PM PST TUESDAY FOR LOWER
COLUMBIA.

     WINTER WEATHER ADVISORY UNTIL 10 PM PST THIS EVENING FOR COAST
      RANGE OF NORTHWEST OREGON-NORTH OREGON COAST.

     WINTER WEATHER ADVISORY UNTIL 1 PM PST TUESDAY FOR GREATER
      PORTLAND METRO AREA.

WA...WINTER WEATHER ADVISORY UNTIL 1 PM PST TUESDAY FOR I-5 CORRIDOR
      IN COWLITZ COUNTY.

     WINTER WEATHER ADVISORY UNTIL 10 PM PST THIS EVENING FOR SOUTH
      WASHINGTON COAST-WILLAPA HILLS.

     WINTER WEATHER ADVISORY UNTIL 1 PM PST TUESDAY FOR GREATER
      VANCOUVER AREA.

PZ...NONE.
&&

$$


MORE WEATHER INFORMATION ONLINE AT..
    HTTP://WEATHER.GOV/PORTLAND


INTERACT WITH US VIA SOCIAL MEDIA
    WWW.FACEBOOK.COM/US.NATIONALWEATHERSERVICE.PORTLAND.GOV
    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.