690  
FXUS66 KSEW 221108  
AFDSEW  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE SEATTLE WA  
300 AM PST MON JAN 22 2018  
   
SYNOPSIS
 
SHOWERS WILL TAPER TODAY AND TONIGHT. A FRONT WILL CROSS  
WESTERN WASHINGTON TUESDAY NIGHT AND WEDNESDAY, BRINGING MORE RAIN  
TO THE LOWLANDS AND POSSIBLY HEAVY SNOW TO THE MOUNTAINS. A COOL AND  
SHOWERY UPPER TROUGH WILL REACH THE AREA THURSDAY. THE AIR MASS  
THURSDAY NIGHT AND FRIDAY MORNING COULD BE COLD ENOUGH FOR A LITTLE  
LOWLAND SNOW. WARMER AND WETTER CONDITIONS ARE LIKELY NEXT WEEKEND.  
 

 
   
SHORT TERM
 
AN UPPER TROUGH OVER THE AREA WILL BRING WIDESPREAD  
SHOWERS THIS MORNING, ALONG WITH ISOLATED THUNDERSTORMS ON THE COAST  
AND SNOW IN THE MOUNTAINS. SHOWERS WILL TAPER THIS AFTERNOON AND  
SNOW WILL MOSTLY END IN THE MOUNTAINS. THE AFTERNOON AND EVENING  
SHOULD BRING A BRIEF BREAK IN THE RAIN AND SHOWER ACTIVITY, AND THE  
SUN MIGHT EVEN PEEK OUT BRIEFLY. HIGHS WILL BE IN THE 40S.  
 
THE NEXT SYSTEM WILL REACH THE COAST TUESDAY MORNING AND SPREAD  
INLAND DURING THE DAY. ANOTHER 24 HOURS OR SO OF RAIN IN THE  
LOWLANDS AND SNOW IN THE MOUNTAINS IS LIKELY. THE SNOW LEVEL WILL BE  
NEAR THE PASS LEVEL AND CURRENT FORECASTS CALL FOR HEAVY SNOW,  
ESPECIALLY DURING THE PERIOD 18Z TUESDAY TO 06Z WEDNESDAY. HIGHS  
BOTH TUESDAY AND WEDNESDAY WILL BE IN THE 40S. BURKE  
   
LONG TERM
 
CONDITIONS WILL TURN MORE SHOWERY THURSDAY AS AN UPPER  
LEVEL TROUGH MOVES INTO THE REGION. THIS TROUGH WILL BE COLDER THAN  
RECENT ONES, WITH HEIGHTS IN THE 530S. THE SNOW LEVEL THURSDAY NIGHT  
WILL FALL TO BELOW 1000 FEET IN SOME AREAS, AND OVERNIGHT LOW  
TEMPERATURES WILL BE IN THE LOW TO MID 30S. THE UPSHOT IS THERE  
COULD BE A LITTLE LOWLAND SNOW BY FRIDAY MORNING. SHOWERS WILL  
TAPER CONSIDERABLY LATER FRIDAY AND WARM ADVECTION SHOULD MELT ANY  
SNOW ON THE GROUND.  
 
THE LATEST RUN OF THE GFS HAS PERIODS OF HEAVY RAIN OVER THE  
WEEKEND, THOUGH NOT AS MUCH AS IN SOME PREVIOUS RUNS. THE EURO IS  
SIMILAR. HIGH POPS WITH HIGH TEMPERATURES IN THE 50S SEEM LIKELY  
SATURDAY AND SUNDAY. BURKE  
 

 
   
HYDROLOGY
 
THE SKOKOMISH RIVER IN MASON COUNTY CRESTED LAST  
EVENING AND WILL FALL BELOW FLOOD STAGE THIS MORNING. MORE RAIN  
TUESDAY NIGHT COULD BRING THE SKOKOMISH BACK ABOVE FLOOD STAGE FOR  
ANOTHER ROUND OF MINOR FLOODING.  
 
RECENT RAINFALL HAS BROUGHT THE USGS LANDSLIDE INDICES CLOSER TO  
THRESHOLDS FOR SHALLOW LANDSLIDES, BUT THEY HAVE NOT YET EXCEEDED  
THOSE THRESHOLDS.  
 
FLOODING IS UNLIKELY ON RIVERS OTHER THAN THE SKOKOMISH THROUGH  
FRIDAY. SOME MODELS SHOW HEAVY RAIN IN THE OLYMPICS AND CASCADES  
NEXT WEEKEND FROM A STRONG WARM FRONT. THIS RAIN, IF IT OCCURS,  
COULD BE HEAVY ENOUGH FOR MORE WIDESPREAD FLOODING. BURKE  
 

 
   
AVIATION
 
A TROUGH OF LOW PRESSURE WILL PRODUCE SHOWERS OVER THE  
AREA EARLY THIS MORNING AS IT MOVES EASTWARD. SHOWERS WILL  
INCREASINGLY BECOME TIED TO THE TERRAIN BETWEEN SYSTEMS LATER THIS  
MORNING THROUGH THIS EVENING. A STRONG WARM FRONT WILL APPROACH  
THE AREA FROM THE WEST LATE TONIGHT AND REACH THE WASHINGTON  
COASTLINE LATE TUESDAY MORNING. FLOW ALOFT WILL BE WESTERLY AS A  
FLAT UPPER LEVEL RIDGE MOVES ACROSS THE AREA. THE AIR MASS WILL BE  
MOIST THROUGH TUESDAY. THE AIR MASS WILL BE SOMEWHAT UNSTABLE  
THIS MORNING AND WILL STABILIZE THIS AFTERNOON AND TONIGHT.  
 
NUMEROUS SHOWERS ARE MOVING NORTHEASTWARD ACROSS THE REGION THIS  
MORNING AS A TROUGH OF LOW PRESSURE AT THE SURFACE AND ALOFT MOVES  
ACROSS WESTERN WASHINGTON. CONDITIONS ARE GENERALLY ON THE LOW END  
OF VFR, EXCEPT MVFR IN HEAVIER SHOWERS. EXPECT CONDITIONS TO  
GRADUALLY IMPROVE BETWEEN 18Z AND 20Z AS THE TROUGH MOVES OFF TO  
THE EAST. CEILINGS IN THE INTERIOR AND ALONG THE IMMEDIATE COAST  
WILL BE HELD UP IN THE VFR CATEGORY TONIGHT AS LOW LEVEL EASTERLY  
PRESSURE GRADIENTS INCREASE IN ADVANCE OF THE INCOMING FRONTAL  
SYSTEM. ALBRECHT  
 
KSEA...SHOWERS WILL GIVE LOW END VFR TO MVFR CEILINGS THROUGH THIS  
MORNING. EXPECT IMPROVEMENT TO VFR THIS AFTERNOON THROUGH TONIGHT.  
SOUTH WINDS 7-10 KNOTS WILL GRADUALLY TURN SOUTHEASTERLY 5-8 KNOTS  
THIS EVENING. ALBRECHT  
 

 
   
MARINE
 
MODERATE SOUTHERLY ONSHORE FLOW EARLY THIS MORNING WILL  
GRADUALLY EASE THROUGH THIS EVENING AS A TROUGH OF LOW PRESSURE  
MOVING ACROSS THE WATERS SHIFTS EAST OF THE AREA. SMALL CRAFT  
ADVISORIES REMAIN IN EFFECT FOR WINDS OR SEAS ALONG THE COAST AND  
AT THE WEST ENTRANCE TO THE STRAIT THROUGH THIS EVENING, WHILE  
RESIDUAL SMALL CRAFT ADVISORY CONDITIONS CONTINUE OVER PORTIONS OF  
THE PUGET SOUND EARLY THIS MORNING.  
 
A STRONG FRONTAL SYSTEM WILL MOVE INTO THE COASTAL WATERS TUESDAY  
MORNING THEN WILL PUSH INLAND TUESDAY AFTERNOON. EXPECT GALES OVER  
THE COASTAL WATERS AND AT THE WEST ENTRANCE TO THE STRAIT AHEAD OF  
THIS FRONT LATE TONIGHT INTO TUESDAY. GALES ARE POSSIBLE AT THE  
EAST ENTRANCE TO THE STRAIT LATE TUESDAY MORNING INTO TUESDAY  
AFTERNOON. GALE WARNINGS HAVE BEEN ISSUED FOR THE COAST AND WEST  
ENTRANCE, AND A GALE WATCH IS IN EFFECT FOR THE EASTERN PORTIONS  
OF THE STRAIT ON TUESDAY.  
 
LATEST FORECAST MODELS SHOW A COUPLE OF SMALL, BUT VIGOROUS LOWS  
ROTATING AROUND AN UPPER LEVEL LOW IN THE GULF OF ALASKA AND INTO  
VANCOUVER ISLAND ON WEDNESDAY AND WEDNESDAY NIGHT. GALES ARE  
POSSIBLE WITH THESE FEATURES, AND SEAS IN EXCESS OF 20 FEET  
DEVELOPING ON THE SOUTH SIDE OF THESE LOWS WILL LIKELY AFFECT THE  
WATERS ON THURSDAY.  
 
YET ANOTHER IN A SERIES OF STRONG SYSTEMS WILL AFFECT THE WATERS  
LATER FRIDAY OR FRIDAY NIGHT INTO NEXT WEEKEND. ALBRECHT  
 

 
   
SEW WATCHES/WARNINGS/ADVISORIES
 
 
WA...WINTER WEATHER ADVISORY UNTIL 9 AM PST THIS MORNING FOR CASCADES  
OF PIERCE AND LEWIS COUNTIES-CASCADES OF SNOHOMISH AND KING  
COUNTIES.  
 
WINTER STORM WARNING UNTIL 9 AM PST THIS MORNING FOR CASCADES OF  
WHATCOM AND SKAGIT COUNTIES-OLYMPICS.  
 
PZ...SMALL CRAFT ADVISORY UNTIL MIDNIGHT PST TONIGHT FOR COASTAL  
WATERS FROM CAPE FLATTERY TO JAMES ISLAND 10 TO 60 NM-  
COASTAL WATERS FROM CAPE FLATTERY TO JAMES ISLAND OUT 10 NM-  
COASTAL WATERS FROM JAMES ISLAND TO POINT GRENVILLE 10 TO  
60 NM-COASTAL WATERS FROM JAMES ISLAND TO POINT GRENVILLE  
OUT 10 NM-COASTAL WATERS FROM POINT GRENVILLE TO CAPE  
SHOALWATER 10 TO 60 NM-COASTAL WATERS FROM POINT GRENVILLE  
TO CAPE SHOALWATER OUT 10 NM-WEST ENTRANCE U.S. WATERS  
STRAIT OF JUAN DE FUCA.  
 
GALE WARNING FROM MIDNIGHT TONIGHT TO 4 PM PST TUESDAY FOR  
COASTAL WATERS FROM CAPE FLATTERY TO JAMES ISLAND 10 TO 60  
NM-COASTAL WATERS FROM CAPE FLATTERY TO JAMES ISLAND OUT 10  
NM-COASTAL WATERS FROM JAMES ISLAND TO POINT GRENVILLE 10  
TO 60 NM-COASTAL WATERS FROM JAMES ISLAND TO POINT  
GRENVILLE OUT 10 NM-COASTAL WATERS FROM POINT GRENVILLE TO  
CAPE SHOALWATER 10 TO 60 NM-COASTAL WATERS FROM POINT  
GRENVILLE TO CAPE SHOALWATER OUT 10 NM-WEST ENTRANCE U.S.  
WATERS STRAIT OF JUAN DE FUCA.  
 
SMALL CRAFT ADVISORY FOR ROUGH BAR UNTIL 4 AM PST TUESDAY FOR  
GRAYS HARBOR BAR.  
 
GALE WATCH FROM TUESDAY MORNING THROUGH TUESDAY AFTERNOON FOR  
EAST ENTRANCE U.S. WATERS STRAIT OF JUAN DE FUCA.  
 
SMALL CRAFT ADVISORY UNTIL 9 AM PST THIS MORNING FOR PUGET SOUND  
AND HOOD CANAL.  
 

 
 

 
 
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