Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 281145
AFDPQR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PORTLAND OR
345 AM PST SUN DEC 28 2014

.SYNOPSIS...COOL AND SHOWERY WEATHER WILL CONTINUE TODAY THROUGH
MONDAY MORNING AS A SERIES OF DISTURBANCES PUSH THROUGH SOUTHWEST
WASHINGTON AND NORTHWEST OREGON FROM THE NORTH. BRISK ONSHORE FLOW
WILL CONTINUE TO ACCUMULATE MUCH-NEEDED SNOW IN THE CASCADES THROUGH
EARLY MONDAY. THE LAST OF THESE DISTURBANCES WILL MOVE THROUGH MONDAY
MORNING...AS A MODIFIED ARCTIC COLD FRONT PUSHES THROUGH THE FORECAST
AREA. A FEW WET SNOW FLAKES MAY MIX DOWN TO THE VALLEY FLOOR IN
HEAVIER SHOWERS WITH THE MODIFIED ARCTIC BOUNDARY MONDAY
MORNING...BUT ANY LIGHT ACCUMULATIONS SHOULD REMAIN ABOVE 500-1000
FEET. THE AIR MASS WILL THEN BECOME MUCH COLDER AND DRIER IN STRONG
NORTHEASTERLY FLOW BEHIND THE FRONT LATER MONDAY...WITH THE COLDEST
AIR MASS SO FAR THIS SEASON LIKELY TO AFFECT THE REGION THROUGH
MIDWEEK. WIND CHILLS MAY BECOME DANGEROUSLY COLD IN PORTIONS OF THE
CASCADES...ESPECIALLY MONDAY NIGHT AND TUESDAY MORNING. THE NEXT
CHANCE OF PRECIPITATION WILL COME LATE IN THE WEEK AND NEXT
WEEKEND...WITH A POSSIBILITY OF WINTER WEATHER AT LOW ELEVATIONS.

&&

.SHORT TERM...TODAY THROUGH TUESDAY...COOL ONSHORE FLOW CONTINUES TO
SPREAD SHOWERS ACROSS SOUTHWEST WASHINGTON AND NORTHWEST OREGON THIS
MORNING...WITH SNOW IN THE CASCADES. WITH A 100-120 KT NORTHWESTERLY
JET SITTING OVER THE FORECAST AREA THROUGH MONDAY MORNING...EXPECT
SHOWERY WEATHER TO CONTINUE FOR THE NEXT 24-36 HOURS. IT IS DIFFICULT
TO ASCERTAIN SNOW ACCUMULATIONS...BUT IT DOES APPEAR SOME SNOTEL
SITES ARE RECORDING ABOUT AN INCH EVERY OTHER HOUR...WHICH WOULD ADD
UP TO 6 INCHES IN A 12 HOUR PERIOD IF IT PERSISTS. SUSPECT
ACCUMULATION RATES WILL BE SLIGHTLY BELOW ADVISORY CRITERIA THE NEXT
36 HOURS...BUT WE WILL LEAVE THE WINTER WX ADVISORY UP FOR SNOW IN
THE CASCADES DUE TO THE BUSY HOLIDAY TRAVEL PERIOD.

WHILE THE COAST AND MOUNTAINS CONTINUE TO PICK UP PLENTY OF
PRECIPITATION...THERE WILL LIKELY BE SOME RAIN SHADOWING IN THE
VALLEYS. PARTICULARLY THE TUALATIN VALLEY AND WESTERN PORTIONS OF THE
WILLAMETTE VALLEY WILL LIKELY SEE LESS IN THE WAY OF SHOWER ACTIVITY
THAN CLARK COUNTY AND THE EAST METRO...MAINLY DUE TO DOWNSLOPING OFF
THE COAST RANGE IN W-NW FLOW. ATTEMPTED TO DEPICT THIS IN OUR GRIDDED
POP/QPF FORECASTS.

SNOW LEVELS WILL LOWER TONIGHT AND MONDAY MORNING AS COLDER AIR
BEGINS TO PUSH SOUTHWARD FROM WESTERN CANADA AND INTO THE PAC NW.
MODELS STILL SHOW FAIRLY DECENT AGREEMENT IN HAVING A DISTURBANCE NOW
DIGGING SOUTH ACROSS THE NORTHWEST TERRITORY OF CANADA BECOME
ABSORBED BY A BROADER UPPER TROUGH...WHICH WILL DIG S-SW INTO WA/OR
TONIGHT AND MONDAY. THIS WILL SET THE STAGE FOR A SIGNIFICANT SHOT OF
COLD AIR TO PUSH DOWN FROM NORTHERN CANADA INTO OUR FORECAST AREA. IT
APPEARS THIS SURGE OF COLD AIR WILL COME IN TWO WAVES... WITH ONE
COLD FRONT PUSHING SOUTH ACROSS WESTERN WA TONIGHT AND NORTHWEST
OREGON MONDAY MORNING. THIS FRONT WILL LIKELY BRING ONE LAST BURST OF
SHOWERS BEFORE THE SECOND SHOT OF EVEN COLDER AND MUCH DRIER AIR
PUSHES IN FROM THE N-NE LATER MONDAY.

THE PERIOD IN BETWEEN THE FIRST PUSH OF COLDER AIR EARLY MON MORNING
AND THE SECOND PUSH OF DRY AND VERY COLD CONTINENTAL AIR MON
AFTERNOON AND EVENING WILL NEED TO BE WATCHED CLOSELY. MODELS ARE
HINTING AT A WEAK LOW DEVELOPING ALONG THE FIRST FRONT MON
MORNING...WHICH COULD CAUSE PRECIPITATION TO FALL IN AREAS WHERE THE
850 MB TEMPS ARE IN THE -5 TO -7 DEG C RANGE. THIS COULD GENERATE A
BRIEF BURST LIGHT ACCUMULATING SNOW DOWN TO 500-1000 FEET...WITH WET
NON-ACCUMULATING SNOWFLAKES BRIEFLY MIXING WITH RAIN DOWN TO THE
VALLEY FLOOR IN HEAVIER SHOWERS. THAT SAID...NAM BUFR AND UW WRF-GFS
SOUNDINGS GENERALLY JUST INDICATE COLD RAIN SHOWERS FOR THE LOWLANDS.
EITHER WAY IMPACTS LOOK TO BE MINIMAL TO NONE IN THE LOWLANDS...WITH
ROADS LIKELY REMAINING WET BELOW 1000 FEET ELEVATION. SOME LIGHT
SLUSHY ACCUMULATIONS ARE POSSIBLE IN THE HILLS...LIKELY AN INCH OR
LESS FOR MOST.

THE AIR MASS BEHIND THE CONTINENTAL ARCTIC FRONT LATER MONDAY APPEARS
TO BE THE COLDEST WE HAVE SEEN THIS SEASON. DEPENDING ON ONES MODEL
OF CHOICE...850 MB TEMPS EAST OF THE CASCADES APPEAR TO GET DOWN AS
LOW AS -15 TO -20 DEG C MON NIGHT...WITH 850 MB TEMPS ON THE ORDER OF
-10 DEG C PUSHING SOUTHWARD FROM PUGET SOUND. THIS COLD AIR WILL
BLAST IN ON STRONG NORTHEASTERLY FLOW MON NIGHT/TUE MORNING....WITH
WIND CHILLS GETTING DOWN INTO THE SINGLE DIGITS OR TEENS FOR THE
LOWLANDS AND BITTER COLD WIND CHILLS OF AROUND 20 BELOW ZERO IN THE
CASCADES. WILL BE ISSUING A WIND CHILL WATCH FOR THE S WA/N OR
CASCADES WHERE OUR WIND CHILL GUIDANCE IS SUGGESTING -20 TO -25 DEG F
WIND CHILLS MON NIGHT/TUE MORNING. TUESDAY WILL SEE PLENTY OF
SUNSHINE...BUT WILL BE VERY COLD WITH BLUSTERY NE WINDS KEEPING TEMPS
NEAR FREEZING EVEN FOR THE LOWLANDS...WITH WIND CHILLS STRUGGLING TO
MAKE IT OUT OF THE TEENS IN THE PORTLAND METRO. EVEN THE COAST WILL
NOT BE IMMUNE TO THE CHILL...AS HIGHS TUE ARE EXPECTED TO STAY IN THE
30S WITH WIND CHILLS GENERALLY IN THE 20S. WITH THE NAM PUSHING A
1050 MB HIGH INTO THE COLUMBIA BASIN BY EARLY TUESDAY WE WILL NEED TO
KEEP AN EYE ON THE POTENTIAL NEED FOR A WIND ADVISORY NEAR THE GORGE
MON NIGHT/TUE. 00Z/06Z NAM CONTINUE TO SHOW KTTD-KDLS GRADIENTS
PUSHING -8 MB...THIS GRADIENT IS USUALLY UNDERDONE BY MODELS...AND
925 MB EAST WINDS PUSHING ABOVE 50 KT ACROSS THE EAST PORTLAND METRO
TUE MORNING. WITH DEEP COLD AIR POURING IN...STRONG E-NE WINDS MAY
NOT BE CONFINED TO THE GORGE.  /WEAGLE

.LONG TERM...TUESDAY NIGHT THROUGH SATURDAY...MODELS REMAIN IN VERY
GOOD AGREEMENT THAT COLD AND DRY WEATHER WILL CONTINUE THROUGH THE
MIDWEEK PERIOD...AS VERY STRONG HIGH PRESSURE SITS OVER THE AREA.
EAST WINDS SHOULD BECOME INCREASINGLY CONFINED TO THE GORGE AS THE
EASTSIDE COLD POOL BECOMES SHALLOWER DUE TO RIDGING ALOFT. BY THE END
OF THE WEEK INCREASING NW FLOW ALOFT WILL TAKE ITS TOLL ON THE
EASTSIDE HIGH AND ASSOCIATED COLD POOL...DUE TO A CHINOOK EFFECT OFF
THE EAST SLOPES OF THE CASCADES. THIS SAID...THE WILLAMETTE VALLEY
MAY HAVE ITS OWN COLD POOL ESTABLISHED BY THE END OF THE WEEK DUE TO
OVERNIGHT TEMPERATURES IN THE TEENS AND 20S AND STRONG INVERSIONS.
LINGERING COLD AIR COULD SET THE STAGE FOR A MIXED PRECIPITATION
EVENT FOR THE LOWLANDS LATE IN THE WEEK OR DURING THE WEEKEND AS A
COLD FRONT PUSHES DOWN THE COAST FROM BRITISH COLUMBIA. THE CHANCES
ARE SLIGHT THOUGH...AS MOST OF THE PRECIPITATION WITH FRONTS OF THIS
TRAJECTORY COME RIGHT ALONG THE FRONT WHERE ENOUGH MIXING USUALLY
OCCURS TO BREAK UP ANY INVERSIONS. THE 00Z/06Z GFS AND 00Z GEM HAVE
HINTED AT A FRONTAL ZONE STALLING NEAR OR JUST SOUTH OF THE PORTLAND
METRO AREA OVER THE WEEKEND FOR MORE PRECIPITATION...WHILE THE 00Z
ECMWF HAS STRONG RIDGING PUSHING THE FRONT BACK NORTH OF THE AREA
OVER THE WEEKEND. FOR NOW WE KEPT POPS AND TEMPS A LITTLE BELOW
NORMAL IN THE EXTENDED.  WEAGLE

&&

.AVIATION...WIDESPREAD VFR CONDITIONS WITH SHOWERS CONTINUING
EARLY THIS MORNING IN THE COOL POST FRONTAL AIR MASS. THIS WILL
GENERALLY CONTINUE THIS MORNING...WITH SOME LOCAL MVFR IN HEAVIER
SHOWERS. EXPECT THE ATMOSPHERE TO STABILIZE A BIT LATER THIS
MORNING WHICH MAY RESULT IN SOME AREAS WHERE CIGS LOWER INTO
HIGHER END MVFR. GENERALLY VFR CIGS REMAIN THROUGH THE DAY
SUN...WITH DETERIORATING CONDITIONS TO MVFR PROBABLE AFT 00Z MON
AS AN APPROACHING WEAK LOW PRESSURE SYSTEM MOVES THROUGH THE AREA
BRINGING ANOTHER ROUND OF SHOWERS.

KPDX AND APPROACHES...EXPECT VFR CIGS CONTINUING IN THE COOL
SHOWERY POST FRONTAL AIR MASS. BRIEF PERIODS OF MVFR CIGS POSSIBLE
THROUGH THIS MORNING...BUT EXPECT THESE TO BE MINIMAL IF ANY. A
BETTER CHANCE FOR MVFR CIGS AND ANOTHER ROUND OF SHOWERS AFT 00Z
MON AS WEAK LOW PRESSURE MOVES THROUGH THE AREA.  CULLEN

&&

.MARINE...THE COLD FRONT HAS CLEARED THE WATERS EARLY THIS MORNING
WITH GUSTY NW WINDS CONTINUING OVER THE WATERS...PARTICULARLY THE
NORTHERN ZONES. SEAS HAVE INCREASED AS WELL...CURRENTLY AROUND 13
FT AT BUOY 46029. WINDS AND SEAS WILL REMAIN HIGHEST OVER THE
NORTHERN COASTAL WATERS WITH GUSTS TO 30 KT CONTINUING THROUGH
THIS AFTERNOON...BEFORE EASING ONLY SLIGHTLY INTO THE 20-25 KT
RANGE THIS EVENING AND OVERNIGHT. WINDS OVER THE CENTRAL OREGON
WATERS WILL GUST TO 25 KT AT TIMES THIS MORNING...WITH THE
STRONGEST WINDS FROM NEWPORT NORTHWARD. THESE GUSTS WILL EASE IN
THE SOUTH THIS EVENING... EXPECT SEAS TO PEAK AROUND 11 FT OFF THE
CENTRAL OREGON COAST. SEAS WILL BEGIN TO SUBSIDE THIS AFTERNOON
AND EVENING...LIKELY DROPPING TO 8-9 FT BY MIDNIGHT TONIGHT.

A WEAK LOW PRES CENTER LOOKS TO DEVELOP AND DRIFT S ACROSS THE
INNER WATERS LATE TONIGHT OR EARLY MON WITH RATHER MINIMAL IMPACTS
TO MARINE INTEREST...THOUGH GUSTS OVER THE S WASHINGTON AND N
OREGON WATERS MAY CONTINUE TO FLIRT WITH 25 KT THROUGH THE DAY
MON. AS THE LOW PRES SYSTEM DROPS S OF THE REGION...EXPECT
AN OFFSHORE SURFACE PRES GRADIENT TO DEVELOP AND RAPIDLY INCREASE.
THIS WILL RESULT IN GUSTY EAST TO NORTHEAST WINDS TO 25-30 KT
ACROSS THE WATERS FROM MONDAY AFTERNOON OR EVENING THROUGH AT
LEAST THE MIDDLE PART OF THE WEAK. WITHOUT ANY LARGE INCOMING
SWELLS...SEAS WILL REMAIN RATHER SMALL THROUGH THIS WEEK...THOUGH
WIND WAVE DOMINATED AND MAY BE STEEP AT TIMES DURING THE PEAK OF
THE OFFSHORE WINDS...LIKELY MON NIGHT THROUGH TUE. CULLEN

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...WIND CHILL WATCH FROM MONDAY EVENING THROUGH TUESDAY MORNING
     FOR NORTHERN OREGON CASCADES.

     WINTER WEATHER ADVISORY UNTIL NOON PST MONDAY FOR CASCADES IN
     LANE COUNTY-NORTHERN OREGON CASCADES.

WA...WINTER WEATHER ADVISORY UNTIL 7 AM PST MONDAY FOR SOUTH
     WASHINGTON CASCADES.

     WIND CHILL WATCH FROM MONDAY EVENING THROUGH TUESDAY MORNING
     FOR SOUTH WASHINGTON CASCADES.

PZ...SMALL CRAFT ADVISORY FOR WINDS UNTIL 10 PM PST THIS EVENING FOR
     COASTAL WATERS FROM CAPE SHOALWATER WA TO CASCADE HEAD OR OUT
     60 NM.

     SMALL CRAFT ADVISORY FOR HAZARDOUS SEAS UNTIL 3 AM PST MONDAY
     FOR COASTAL WATERS FROM CAPE SHOALWATER WA TO FLORENCE OR OUT
     60 NM.

     SMALL CRAFT ADVISORY FOR WINDS UNTIL 2 PM PST THIS AFTERNOON
     FOR COASTAL WATERS FROM CASCADE HEAD OR TO FLORENCE OR OUT 60
     NM.

     SMALL CRAFT ADVISORY FOR ROUGH COLUMBIA RIVER BAR UNTIL
     MIDNIGHT PST TONIGHT.

&&

$$

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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