339  
FXUS66 KPQR 220435  
AFDPQR  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE PORTLAND OR  
835 PM PST SUN JAN 21 2018  
   
SYNOPSIS
 
SHOWERS WITH SNOW IN THE MOUNTAINS IN ONSHORE FLOW WILL  
CONTINUE TONIGHT INTO MONDAY MORNING THEN DECREASE LATER MONDAY AND  
MONDAY NIGHT. THE NEXT FAIRLY STRONG SYSTEM WILL MOVE IN MAINLY ON  
TUESDAY FOR MORE RAIN, COASTAL WIND, AND CASCADE SNOW. THE ASSOCIATED  
FRONT WILL MOVE ONSHORE TUESDAY, STALL AS IT PIVOTS AND BECOMES  
NORTH-SOUTH ORIENTED TUESDAY NIGHT INTO WEDNESDAY, THEN MOVE EAST  
LATE WEDNESDAY AS THE NEXT FRONTAL BAND PUSHES ONSHORE LATE WEDNESDAY  
AND WEDNESDAY EVENING FOR MORE RAIN AND MOUNTAIN SNOW. WE COULD SEE  
SIGNIFICANT SNOWS IN THE CASCADES TUESDAY AND WEDNESDAY. THE TRAILING  
UPPER LOW WILL SPREAD MORE SHOWERS AND AN EVEN COOLER AIR MASS INTO  
THE FORECAST AREA WITH SNOW POSSIBLY DOWN INTO THE CASCADE FOOTHILLS  
AND THE COASTAL MOUNTAINS. SHOWERS LINGER INTO FRIDAY, THEN THE NEXT  
WARMER SYSTEM IS EXPECTED TO BRING RAIN TO THE AREA NEXT WEEKEND.  

 
   
SHORT TERM...TONIGHT THROUGH WEDNESDAY
 
THE FRONT FROM EARLIER  
TODAY HAS FINALLY MOVED EAST OF THE FORECAST AREA THIS EVENING, WITH  
ONSHORE FLOW AND SHOWERS TO CONTINUE TO SPREAD ONSHORE IN ITS WAKE  
THE REMAINDER OF TONIGHT INTO MONDAY. THE MODELS SUGGEST DECENT QPF  
AND SNOWS IN THE CASCADES IN THE SOUTH THROUGH A CHUNK OF TONIGHT AND  
IN THE NORTH INTO MONDAY MORNING, AND THUS HAVE EXTENDED THE SNOW  
ADVISORIES OUT ACCORDINGLY. THE MODELS SAY THAT THE SHOWERS SHOULD  
DECREASE BY LATER MONDAY AND ESPECIALLY MONDAY EVENING, AND THAT  
STILL SEEMS REASONABLE.  
 
WE HAD A FEW LIGHTNING STRIKES MOVE INTO PACIFIC COUNTY WASHINGTON  
EARLIER IN THE AFTERNOON, AND THE MODELS SUGGEST SOME INSTABILITY  
WILL CONTINUE FOR WHILE LONGER TONIGHT NEAR OUR NORTHERN COASTAL  
AREAS, SO WILL CONTINUE TO CARRY A FEW THUNDERSTORMS IN THE FORECAST  
ALONG THE SOUTH WASHINGTON AND FAR NORTH OREGON COASTS OVERNIGHT.  
 
THE NEXT SIGNIFICANT SYSTEM APPROACHES MONDAY NIGHT AND SPREADS AN  
OCCLUDED FRONT ONSHORE TUESDAY FOR MORE RAIN AND CASCADE SNOW. THIS  
FRONT WILL BRING WINDY CONDITIONS TO THE COAST AGAIN. THE FRONT  
STALLS OVER OUR AREA AS IT PIVOTS AND BECOMES MORE NORTH-SOUTH  
ORIENTED DUE TO THE UPPER FLOW ORIENTATION AND ENERGY DIGGING SOUTH  
TOWARD CALIFORNIA. THAT PUTS US IN A PROLONGED PERIOD OF RAIN AND  
MOUNTAIN SNOW FROM TUESDAY THROUGH A GOOD PART OF WEDNESDAY. AT THIS  
POINT, FLOODING IS NOT EXPECTED, BUT THIS WILL NEED TO BE MONITORED.  
IN ADDITION, SIGNIFICANT SNOWS COULD FALL IN THE CASCADES DURING THIS  
PERIOD, PERHAPS ONE OF OUR BETTER SNOWFALLS OF THE SEASON SO FAR.  
 
A SECONDARY FRONT WILL THEN APPROACH THE COAST WEDNESDAY AFTERNOON  
AND MOVE ONSHORE LATER WEDNESDAY AND WEDNESDAY EVENING FOR MORE RAIN  
AND MOUNTAIN SNOW. THIS FRONT WILL BE COLDER, AND COULD BE OF A  
CONVECTIVE NATURE, MEANING PRECIPITATION RATES COULD BE BRIEFLY  
HIGHER, WITH GUSTY CONVECTIVELY DRIVEN WINDS ACCOMPANYING IT.  
 
PRECIPITATION TOTALS FOR TUESDAY INTO WEDNESDAY EVENING COULD REACH  
AROUND 4 INCHES ALONG THE COAST, WITH 2-4 INCHES IN THE CASCADES,  
HIGHEST NORTH, AND 0.6-1.2 INCHES IN THE VALLEYS. SNOW IN THE  
CASCADES COULD BE 1-2 FT OR MORE.  
 
ALL IN ALL, IT LOOKS LIKE A PRETTY UNSETTLED WEEK COMING UP. TOLLESON  
   
LONG TERM...WEDNESDAY NIGHT THROUGH SUNDAY
 
ACTIVE WEATHER PATTERN  
TO CONTINUE THROUGH FRIDAY AS A SLOW-MOVING TROUGH KEEPS SWINGING  
MOISTURE INTO THE PACIFIC NORTHWEST.  
 
SNOW LEVELS DROP WEDNESDAY NIGHT INTO THURSDAY TO AROUND 1500 FT,  
THEN POSSIBLY LOWER BY FRIDAY MORNING.WITH 850-MB TEMPS LOWERING  
TO AROUND -6C ON BOTH THE GFS AND ECMWF, STRAIGHT MODEL GUIDANCE IS  
TRYING TO BRING SNOW LEVELS DOWN TO 500-800 FT, BUT THE PATTERN  
DOESN'T SEEM CONDUCIVE FOR LOW-ELEVATION SNOW (SOUTHERLY WINDS AND  
ONSHORE PRESSURE GRADIENT). LOOKING AT THE 1000-500-MB THICKNESSES,  
UNDER ONSHORE FLOW, WE TYPICALLY NEED THICKNESSES BELOW 522 DAM FOR  
SNOW LEVELS BELOW 1000 FT (LOCAL STUDY). BOTH THE GFS AND ECMWF HAVE  
THICKNESS AROUND 522-524 DAM. THE ECMWF IS THE LOWER OF THE TWO,  
WITH LOWEST THICKNESS AROUND 521.6 DAM FRIDAY MORNING. BECAUSE OF  
THE PATTERN AND THE UNCERTAINTY WITH IT BEING 6 DAYS OUT, FOR NOW I  
HAVE LIMITED SNOW LEVELS TO 1000 FT. "IF" THIS DOES PANS OUT WE MAY  
SEE SOME SNOW MIX DOWN TO AROUND 800 FT; HOWEVER, WITH PRECIPITATION  
FAIRLY LIGHT ON FRIDAY AND GROUND TEMPERATURES ABOVE FREEZING, WE  
WON'T SEE A LOT OF SNOW AND IT SHOULDN'T ACCUMULATE BELOW 1000 FT.  
 
SNOW LEVELS START TO COME BACK UP FRIDAY EVENING AS A MILDER SYSTEM  
STARTS TO MOVE IN WITH ANOTHER SURGE OF MOISTURE. SATURDAY MORNING  
WILL BE PRETTY WET, BUT RIDGING STARTS TO BUILD SATURDAY AFTERNOON,  
PUSHING THE MOISTURE PLUME NORTH BY LATE SATURDAY NIGHT OR EARLY  
SUNDAY. THERE IS SOME UNCERTAINTY WITH HOW FAR NORTH THIS RIDGE WILL  
BUILD. THIS WILL DETERMINE WHETHER WE DRY OUT ON SUNDAY. BOTH THE  
GFS AND ECMWF SEND THE PLUME OF MOISTURE NORTH INTO NORTHERN  
WASHINGTON AND BRITISH COLUMBIA, WHILE THE CANADIAN CMC SAGS THE  
MOISTURE PLUME FARTHER SOUTH OVER OUR AREA. HAVE LEANED TOWARD DRIER  
POPS, WHILE KEEPING A CHANCE DUE TO UNCERTAINTY IN HOW FAR NORTH THE  
RIDGE WILL BUILD OVER THE WEST COAST. -MCCOY  
 

 
   
AVIATION
 
VFR CONDITIONS PREVAIL WITH ISOLATED SHOWERS. MAINLY  
STATUS QUO FOR THE NEXT 24 HOURS, BUT COULD SEE SOME BRIEF PERIODS OF  
MVFR WITH SHOWERS.  
 
PDX AND APPROACHES...VFR THROUGH THE PERIOD. OCCASIONAL PASSING  
SHOWERS THROUGH THE NEXT 24 HOURS. BOWEN/BENTLEY  

 
   
MARINE
 
NO UPDATES NEEDED THIS EVENING. THE PREVIOUS DISCUSSION  
FOLLOWS. BOWEN  
 
WHILE EVERYTHING SETTLED DOWN TODAY, THERE WAS NOT MUCH TIME FOR REST  
AS EYES SHIFT TOWARDS THE NEXT FRONT TO BRING GALES TO THE AREA.  
GIVEN THE MODEL CONSISTENCY FOR THE PAST SEVERAL DAYS AND GUIDANCE  
SUGGESTING SOLID GALE FORCE WINDS TUESDAY, WENT AHEAD WITH A GALE  
WATCH. SEAS DO NOT LOOK QUITE AS HIGH ON TUESDAY AS THIS PAST SYSTEM  
WITH SEAS LIKELY PEAKING AT OR BELOW 20 FEET.  
 
THE NEXT STORM SYSTEM OF CONCERN ARRIVES WEDNESDAY NIGHT INTO  
THURSDAY. THERE IS NOW A BIT BETTER MODEL AGREEMENT NOW WITH AN  
OCCLUDING SURFACE LOW TRACKING TOWARDS VANCOUVER ISLAND. GALE FORCE  
WINDS ARE LIKELY AGAIN, BUT SEAS WILL LIKELY BE A BIT HIGHER AND MAY  
EXCEED 20 FEET GIVEN THE TRACK AND STRENGTH OF THIS LOW.  
 
THERE IS SURPRISINGLY GOOD MODEL AGREEMENT EVEN INTO NEXT WEEKEND  
WITH ENOUGH CONFIDENCE TO EXPECT ANOTHER ROUND OF GALE FORCE WINDS  
AND HIGH SEAS NEXT WEEKEND. /BENTLEY  

 
   
PQR WATCHES/WARNINGS/ADVISORIES
 
 
OR...WINTER WEATHER ADVISORY UNTIL 10 AM PST MONDAY FOR NORTHERN  
OREGON CASCADES.  
 
WINTER WEATHER ADVISORY UNTIL 4 AM PST MONDAY FOR CASCADES IN  
LANE COUNTY.  
 
WA...WINTER WEATHER ADVISORY UNTIL 10 AM PST MONDAY FOR SOUTH  
WASHINGTON CASCADES.  
 
PZ...GALE WATCH FROM TUESDAY MORNING THROUGH TUESDAY EVENING FOR  
COASTAL WATERS FROM CAPE SHOALWATER WA TO FLORENCE OR OUT  
10 NM.  
 
SMALL CRAFT ADVISORY FOR WINDS UNTIL 4 AM PST MONDAY FOR  
COASTAL WATERS FROM CAPE SHOALWATER WA TO FLORENCE OR OUT  
60 NM.  
 
SMALL CRAFT ADVISORY FOR HAZARDOUS SEAS UNTIL 10 PM PST MONDAY  
FOR COASTAL WATERS FROM CAPE SHOALWATER WA TO FLORENCE OR  
OUT 60 NM.  
 
GALE WATCH FROM LATE MONDAY NIGHT THROUGH TUESDAY EVENING FOR  
WATERS FROM CAPE SHOALWATER WA TO FLORENCE OR FROM 10 TO  
60 NM.  
 
SMALL CRAFT ADVISORY FOR ROUGH COLUMBIA RIVER BAR UNTIL 10 PM  
PST MONDAY.  
 

 
 

 
 
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. THIS AREA IS  
COMMONLY REFERRED TO AS THE CWA OR FORECAST AREA.  
 
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