Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 271740
AFDPQR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PORTLAND OR
939 AM PST SAT DEC 27 2014

.SYNOPSIS...A COLD FRONT IS PUSHING DOWN THE BRITISH COLUMBIA COAST
THIS MORNING...AND WILL SPREAD RAIN AND MOUNTAIN SNOW SOUTHWARD
ACROSS THE FORECAST AREA TODAY AND TONIGHT. A TRAILING SECONDARY WAVE
WILL MAINTAIN SHOWERS SUNDAY INTO MONDAY. SNOW LEVELS WILL BE LOW
ENOUGH TO AFFECT THE CASCADE PASSES AND POSSIBLY SOME OF
THE HIGHER FOOTHILLS THIS WEEKEND. COLD AIR BEHIND THE FRONT WILL
DROP SNOW LEVELS FURTHER MONDAY MORNING...POSSIBLY MIXING SOME WET
SNOW DOWN TO THE VALLEY FLOOR AS SHOWERS TAPER OFF MONDAY. AT THIS
POINT VALLEY ACCUMULATIONS ARE NOT EXPECTED...BUT THE HILLS COULD GET
A DUSTING MONDAY MORNING. COLD AND DRY AIR WILL THEN DOMINATE MUCH OF
THE UPCOMING WEEK...WITH GUSTY NORTHEAST WINDS DEVELOPING LATE MONDAY
AND CONTINUING AT LEAST THROUGH MIDWEEK. PRECIPITATION CHANCES RETURN
FRIDAY AFTER A FEW DAYS OF DRY BUT COLD WEATHER.

&&

.UPDATE...RADAR THIS MORNING SHOWS PRECIP SPREADING ACROSS S
WASHINGTON AND INTO N OREGON AHEAD OF A COLD FRONT AND UPPER LEVEL
SYSTEM CURRENTLY LOCATED NEAR VANCOUVER ISLAND. BASED ON TEMP AND
PRECIP OBSERVATIONS OUT OF THE CASCADES THIS MORNING...SNOW LEVELS
APPEAR TO BE SLIGHTLY HIGHER THAN ANTICIPATED IN S WASHINGTON AND
SLIGHTLY LOWER FARTHER SOUTH INTO THE LANE COUNTY CASCADES...SO MADE
SOME ADJUSTMENTS TO SNOW LEVELS IN THE SHORT TERM VIA THE NAM WHICH
SEEMS TO HAVE A GOOD HANDLE ON THE CURRENT SITUATION. FORECAST SNOW
AMOUNTS CHANGED A LITTLE BIT WITH THIS UPDATE BASED ON TWEAKS TO
NEAR-TERM SNOW LEVEL AND QPF GRIDS...BUT ADVISORIES STILL LOOK GOOD
AND PREVIOUSLY STATED SNOW AMOUNT RANGES ARE STILL VALID. CASCADE
FOOTHILLS WILL SEE SOME SNOW WITH THIS SYSTEM...BUT IT STILL DOESN`T
LOOK LIKE SNOW LEVELS WILL BE LOW ENOUGH DURING THE HEAVIEST PRECIP
PERIODS TO MEET ADVISORY CRITERIA THERE. WILL BE SOMETHING TO KEEP AN
EYE ON THOUGH.

12Z SLE SOUNDING SHOWS WEAK SW 850 MB FLOW WHICH WILL ALLOW PRECIP TO
SPREAD INTO INTERIOR VALLEY AREAS THROUGH THIS AFTERNOON. HOWEVER 850
MB FLOW STRENGTHENS TO 30 TO 40 KT OUT OF THE WEST BY LATE THIS
AFTERNOON...WHICH SHOULD CREATE A STRONG RAIN SHADOW IN THE VALLEY.
HAVE DECREASED POPS AND QPF A BIT PARTICULARLY ON THE WEST SIDE OF
THE VALLEY TONIGHT FOR THIS REASON. AGAIN THE 12Z NAM APPEARS TO BE
IN TUNE WITH DOWNSLOPING/RAIN SHADOW SO TRENDED TOWARD IT. BOWEN

.PREVIOUS SHORT TERM...TODAY THROUGH MONDAY...INFRARED SATELLITE
IMAGERY SHOWS A ROBUST COLD FRONT PUSHING DOWN THE BRITISH COLUMBIA
COAST EARLY THIS MORNING...AS OF 245 AM IT WAS ABOUT HALF WAY ACROSS
VANCOUVER ISLAND. THIS FRONT WILL BE OUR MAIN WEATHER PLAYER THIS
WEEKEND AS IT HAS FAIRLY GOOD OVER WATER TRAJECTORY AND SHOULD
PRODUCE A DECENT AMOUNT OF PRECIPITATION AS IT PUSHES SOUTH ACROSS
THE DISTRICT. THE HEAVIEST PRECIPITATION WILL BE IN THE CASCADES...
WHERE 30-40 KT WESTERLY 850 MB WINDS ASSOCIATED WITH THE FRONT WILL
CAUSE OROGRAPHIC ENHANCEMENT AND A MUCH-NEEDED BURST OF HEAVY SNOW
FOR THE CASCADES. 00Z/06Z NAM AND UW WRF-GFS HAVE BEEN FAIRLY
CONSISTENT IN BRINGING 0.6-1.2 INCHES OF QPF TO THE SW WA/N OR
CASCADES...WITH SLIGHTLY LESS SOUTH OF SANTIAM PASS. THUS IT APPEARS
HIGH-END SNOW ADVISORY ACCUMULATIONS WILL OCCUR...WITH 8-14 INCHES IN
THE CASCADES N OF SANTIAM PASS AND 5-10 INCHES S OF SANTIAM PASS BY
SUNDAY.  SNOW LEVELS START OFF NEAR THE PASSES...AROUND 3500 TO 4500
FT AHEAD OF THE FRONT...BUT WILL FALL QUICKLY ALONG AND BEHIND THE
COLD FRONT WITH A COUPLE INCHES POSSIBLE FOR THE HIGHER FOOTHILLS
ABOVE 2000 FT BY SUNDAY.

THE FRONT WILL BRING A PERIOD OF MODERATE RAIN FOR THE LOWLANDS...
WITH ABOUT 0.25-0.50 INCH ON AVERAGE. HOWEVER WITH LOW-LEVEL FLOW
DISTINCTLY WESTERLY...THERE WILL BE CONSIDERABLE DOWNSLOPING AND RAIN
SHADOWING IN THE TUALATIN VALLEY AND WESTERN PORTIONS OF THE
WILLAMETTE VALLEY...POSSIBLY HOOD RIVER AS WELL. QPF IN THE PORTLAND
METRO COULD RANGE FROM ABOUT 0.10-0.20 INCH IN HILLSBORO AND
BEAVERTON WHILE TROUTDALE AND BATTLE GROUND GET UPWARDS OF ONE HALF
INCH.

WE WILL NEED TO KEEP AN EYE ON A SECONDARY VORT MAX PRESENTLY MOVING
ACROSS THE NORTH SLOPE OF ALASKA. 00Z ECMWF...AS WELL AS THE 00Z/06Z
GFS AND NAM...ALL SHOW THIS VORT MAX AND ASSOCIATED TROPOPAUSE FOLD
DIGGING SOUTH AROUND THE WESTERN PERIPHERY OF THE LONGWAVE TROUGH
SUN/EARLY MON. THIS DISTURBANCE WOULD PUSH SOUTH ACROSS BRITISH
COLUMBIA LATE SUN/SUN NIGHT...THEN INTO WESTERN WA/NW OREGON MON
MORNING. DEPENDING ON THE TRACK OF THIS VORT MAX...IT COULD DEVELOP
WEAK LOW PRESSURE JUST OFF THE PAC NW COAST. AT THE SAME TIME
MODIFIED ARCTIC HIGH PRESSURE WILL BE BUILDING SOUTHWARD INTO WA.
SNOW LEVELS WILL BE LOWERING MONDAY MORNING...WHILE THIS LOW MAY KEEP
PRECIPITATION AROUND AT THE SAME TIME. FOR NOW IT APPEARS SCATTERED
RAIN/WET SNOW SHOWERS COULD FALL DOWN TO THE LOWEST ELEVATIONS EARLY
MONDAY...BUT ANY LIGHT ACCUMULATIONS SHOULD BE LIMITED TO THE HILLS
ABOVE 500-1000 FT.

THERE ARE A COUPLE OF OTHER SCENARIOS WHICH NEED TO BE WATCHED FOR
MONDAY MORNING. ONE BEING THE MODIFIED ARCTIC HIGH MOVING SOUTH SO
FAST THAT THE AIR MASS DRIES OUT QUICKLY AND PRECIPITATION ENDS
BEFORE SNOW LEVELS CAN FALL TO THE VALLEY FLOOR. ANOTHER SCENARIO
WOULD BE COLD AIR ARRIVING MON MORNING...WHILE A RELATIVELY STRONGER
LOW DEVELOPS OFFSHORE AND THROWS MOISTURE BACK OVER THE COLD AIR...OR
A DEFORMATION BAND FORMING SOMEWHERE OVER THE FORECAST AREA. THIS
COULD RESULT IN ACCUMULATING SNOW FOR SOME OF OUR LOWLANDS. PAST
EXPERIENCE IN SIMILAR SITUATIONS SEEMS TO FAVOR THE DRIER
SOLUTIONS...BUT WE STILL DECIDED TO INCREASE POPS A BIT MONDAY AS IT
IS HARD TO IGNORE MODEL CONSENSUS SHOWING AT LEAST SOME QPF OVER THE
LOWLANDS MON MORNING. THE MAJORITY OF 03Z SREF ENSEMBLE MEMBERS SHOW
TRACE AMOUNTS OF SNOW ACCUMULATION AT KPDX MON MORNING.

MUCH COLDER AIR THEN SPILLS INTO THE REGION LATER MONDAY INTO MONDAY
NIGHT...WITH RAPID DRYING AND CLEARING. TEMPS MON NIGHT SHOULD EASILY
REACH THE 20S FOR MOST OF THE LOWLANDS...WITH COLDER TEMPS IN THE
HIGHER TERRAIN. CHILLY NE FLOW WILL ADD A BIT OF A BITE TO THE AIR
WITH WIND CHILLS LIKELY IN THE TEENS BY TUE MORNING.  WEAGLE

.LONG TERM...TUESDAY THROUGH FRIDAY...COLD AND DRY AIR WILL DOMINATE
SOUTHWEST WASHINGTON AND NORTHWEST OREGON THROUGH THE MIDWEEK PERIOD.
PRESSURE GRADIENTS...AT LEAST INITIALLY...ARE MORE NORTHERLY THAN
EASTERLY...SO BLUSTERY CONDITIONS WILL NOT BE CONFINED TO THE
COLUMBIA GORGE TUESDAY. WITH 850 MB TEMPS DOWN AS LOW AS -10 DEG C AT
KPDX ON SOME MODELS...TEMPS WILL STRUGGLE TO GET MUCH ABOVE FREEZING
ANYWHERE INLAND TUESDAY. SOME MODERATION OCCURS WED...BUT BY THEN A
ROBUST LOW-LEVEL COLD POOL WILL HAVE DEVELOPED OVER THE MID-COLUMBIA
BASIN. HIGH PRESSURE OFFSHORE WILL LIMIT EAST WINDS SOMEWHAT...BUT
ENOUGH EASTERLY FLOW SHOULD PERSIST THROUGH THE GORGE TO KEEP THE
PORTLAND METRO IN THE MID 30S WED WHILE THE COLD AIR MASS ACROSS REST
OF THE DISTRICT STARTS TO MODERATE. UPPER LEVEL RIDGING WILL BE
DEVELOPING DURING THE MIDWEEK PERIOD...WHICH WILL PROBABLY ONLY
STRENGTHEN VALLEY INVERSIONS. EXPECT PERSISTENT FOG IN THE
CENTRAL/SOUTHERN WILLAMETTE VALLEY TO KEEP SALEM AND EUGENE IN THE
30S THROUGH THURSDAY. THE STRONG UPPER RIDGING WILL LIKELY KEEP THE
FORECAST AREA DRY THROUGH THURSDAY...BUT THE 00Z GEM AND ECMWF SHOW
THE UPPER RIDGE DECAYING ENOUGH FOR A FRONTAL SYSTEM TO SPREAD SOME
PRECIP INLAND FRIDAY. DECIDED TO ADD A SLIGHT CHANCE OF FREEZING RAIN
TO THE INLAND VALLEYS FRI MORNING AS MOS GUIDANCE SHOWS TEMPS GETTING
DOWN INTO THE 20S THU NIGHT...AND THEY MAY HAVE A HARD TIME WARMING
ABOVE FREEZING BY THE TIME ANY RAIN STARTS TO FALL FRIDAY MORNING.
SOME MODELS SUCH AS THE 00Z ECMWF SHOW THE POTENTIAL FOR ANOTHER
SURGE OF COLD AIR BEHIND FRIDAYS FRONT...WHILE OTHERS SUCH AS THE
00Z/06Z GFS SHOW A MODERATING TREND WHILE THE COLDEST AIR DUMPS INTO
THE NORTHERN GREAT PLAINS.  WEAGLE

&&

.AVIATION...LIGHT RAIN WITH A FRONTAL STORM SYSTEM THAT IS
CURRENTLY DROPPING SOUTHWARD ACROSS THE AREA THIS MORNING WILL
BRING WIDESPREAD MVFR CONDITIONS TODAY. THERE IS A GOOD CHANCE
IFR CONDITIONS WILL DEVELOP AT COASTAL TAF SITES FOR A FEW HOURS AND
A COUPLE INLAND TAF SITES FOR AN HOUR OR TWO...WITH KHIO BEING OF
HIGHEST CONCERN FOR THESE LOWER FLIGHT CONDITIONS. LOW LEVEL COLD
ADVECTION SHOULD ALLOW MANY LOCATIONS TO TURN VFR WITH ONLY AN
OCCASIONAL MVFR CONDITION EXPECTED TO OCCUR THIS EVENING AND
OVERNIGHT. HOWEVER...AN EXTENDED CLEAR PERIOD COULD ALLOW PATCHY
LIFR FOG TO FORM...BUT CONFIDENCE IS LOW THAT THIS WILL OCCUR
BASED ON THE PATTERN.

KPDX AND APPROACHES...MVFR CONDITIONS MAY BRIEFLY LOWER AT TIMES
TO HIGH END IFR CONDITIONS TODAY. HOWEVER...AN INCREASINGLY
UNSTABLE LOWER ATMOSPHERE SHOULD ALLOW CONDITIONS TO TURN
PREDOMINANTLY VFR AFTER 00Z SUNDAY...LASTING THROUGH 18Z SUNDAY.
AN OCCASIONAL MVFR CIG WILL BE POSSIBLE FROM TIME TO TIME DURING
THIS PERIOD...BUT EXPECT THIS TO BE THE EXCEPTION AND NOT THE
NORM. /NEUMAN

&&

.MARINE...A FRONT WILL DROP SOUTHWARD ACROSS THE WATERS BRINGING
INCREASING NORTHWEST WINDS AND HIGHER SEAS TO THE WATERS. WINDS
AND SEAS SHOULD BE HIGHEST ACROSS THE NORTHERN WATERS...BUT WINDS
LOOK TO REMAIN IN SOLID SMALL CRAFT ADVISORY CRITERIA WITH GUSTS OF
25 TO 30 KT. SEAS MAY APPROACH 15 FT ACROSS THE FAR NORTHERN
WATERS...BUT SHOULD GENERALLY TOP OUT IN THE 11 TO 13 FT RANGE
TOWARDS THE CENTRAL OREGON COAST. EXPECT SEAS TO START SUBSIDING
SUNDAY AFTERNOON AND SUNDAY NIGHT...LIKELY DROPPING BELOW 10 FT
MONDAY MORNING.

A WEAK LOW PRESSURE MAY DEVELOP AND DRIFT SOUTHWARD ACROSS THE
INNER WATERS LATE SUNDAY NIGHT OR MONDAY MORNING...BUT IMPACTS TO
MARINE INTERESTS WILL BE FAIRLY MINIMAL AT FIRST. HOWEVER...AS THE
LOW PRESSURE DROPS SOUTH OF THE AREA...OFFSHORE PRESSURE GRADIENTS
WILL STRENGTHEN RAPIDLY. THIS WILL ALLOW GUSTY EAST TO NORTHEAST
WINDS TO DEVELOP ACROSS THE WATERS FOR THE SECOND HALF OF
MONDAY...LASTING THROUGH AT LEAST THE MIDDLE PART OF NEXT WEEK.
WITH NO MAJOR WESTERLY SWELLS...EXPECT SEAS TO RELAX CONSIDERABLY
DURING THIS TIME...BUT THEY COULD BECOME WIND DOMINATED IN PLACES
DURING THE PEAK EAST WIND TIMES. AT THIS POINT...THE STRONGEST EAST
WINDS WILL MOST LIKELY TO OCCUR MONDAY NIGHT INTO TUESDAY. /NEUMAN

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...WINTER WEATHER ADVISORY UNTIL 10 AM PST SUNDAY FOR CASCADES IN
     LANE COUNTY-NORTHERN OREGON CASCADES.

WA...WINTER WEATHER ADVISORY UNTIL 4 AM PST SUNDAY FOR SOUTH
     WASHINGTON CASCADES.

PZ...SMALL CRAFT ADVISORY FOR WINDS FROM NOON TODAY TO 10 PM PST
     SUNDAY FOR COASTAL WATERS FROM CAPE SHOALWATER WA TO CASCADE
     HEAD OR OUT 60 NM.

     SMALL CRAFT ADVISORY FOR HAZARDOUS SEAS UNTIL 9 AM PST MONDAY
     FOR COASTAL WATERS FROM CAPE SHOALWATER WA TO FLORENCE OR OUT 60
     NM.

     SMALL CRAFT ADVISORY FOR WINDS FROM 4 PM THIS AFTERNOON TO 6 PM
     PST SUNDAY FOR COASTAL WATERS FROM CASCADE HEAD OR TO FLORENCE
     OR OUT 60 NM.

     SMALL CRAFT ADVISORY FOR ROUGH COLUMBIA RIVER BAR FROM 4 PM THIS
     AFTERNOON TO 1 PM PST SUNDAY.

&&

$$

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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