572  
FXUS66 KSEW 240437  
AFDSEW  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE SEATTLE WA  
937 PM PDT MON APR 23 2018  
   
SYNOPSIS
 
A RIDGE ALOFT WILL MOVE INTO EASTERN WASHINGTON  
TONIGHT AND PERSIST INTO THURSDAY. SURFACE HIGH PRESSURE OVER  
SOUTHEASTERN BRITISH COLUMBIA COMBINED WITH A DEVELOPING THERMAL  
TROUGH OF LOW PRESSURE ALONG THE COAST WILL GIVE LOW-LEVEL  
OFFSHORE FLOW. THIS PATTERN WILL BRING HIGHS IN THE 70S ACROSS  
MUCH OF THE AREA TUESDAY AND WEDNESDAY, WITH LOWER 80S POSSIBLE  
IN SOME LOCATIONS ON THURSDAY. AN UPPER LEVEL LOW WILL APPROACH  
FROM THE SOUTHWEST LATE IN THE WEEK BRINGING A RETURN TO  
SEASONABLY COOL TEMPERATURES AND A CHANCE OF SHOWERS FRIDAY  
THROUGH NEXT WEEKEND.  
 

 
   
SHORT TERM
 
AN UNCOMMONLY LONG STRETCH OF WARM AND DRY WEATHER  
IS EXPECTED THROUGH THURSDAY, AT LEAST BY APRIL STANDARDS.  
 
AN UPPER RIDGE AXIS OVER CENTRAL WASHINGTON THIS EVENING WILL  
MOVE A LITTLE FURTHER EAST TO EASTERN WASHINGTON ON TUE AND  
FLATTEN SLIGHTLY. IT WILL RE-BUILD OVER WESTERN WASHINGTON ON  
WED, AND THEN MOVE EAST TO THE NORTHERN ROCKIES ON THU. MEANWHILE,  
A THERMAL PRESSURE TROUGH WILL CONTINUE ALONG THE COAST UNTIL THU  
MORNING. HIGH UPPER HEIGHTS (500 MB HEIGHTS IN EXCESS OF 570 DAM)  
AND LOW-LEVEL OFFSHORE FLOW ARE THE PERFECT SETUP FOR WARMING OUR  
ATMOSPHERE. HIGHS WILL BE WELL INTO THE 70S FOR THE NEXT 3 DAYS,  
EXCEPT NEAR COOLER SHORELINES OF NORTHWEST WASHINGTON. COULD EKE  
OUT SOME 80-DEGREE REACHING ON THURSDAY FROM SEATTLE SOUTHWARD.  
 
LOOKS BREEZY OR POSSIBLY WINDY IN THE CASCADE FOOTHILLS ON TUE  
MORNING. EASTERLY GRADIENTS WILL PEAK ON TUESDAY MORNING, WITH THE  
SEA-EAT GRADIENT FORECAST TO BE AROUND -6 TO -7 MB. MORNING WIND  
GUSTS AROUND 35-40 MPH ARE POSSIBLE NEAR FOOTHILL COMMUNITIES SUCH  
AS NORTH BEND AND ENUMCLAW.  
 
ONCE THE RIDGE AXIS MOVES WELL EAST ON THURSDAY, SOUTHERLY FLOW  
ALOFT WILL DEVELOP. A SOUTHERLY MARINE SURGE WILL SPREAD UP THE  
OREGON COAST AND TRYING WORKING INTO BEACH COMMUNITIES ALONG THE  
WASHINGTON COAST BY MID-DAY THURSDAY, WITH MARINE CLOUDS AND  
COOLING AFTERNOON TEMPERATURES THERE. THE THERMAL PRESSURE TROUGH  
WILL MOVE INLAND THURSDAY AFTERNOON AND REACH THE CASCADES ON  
THURSDAY NIGHT. IT WILL BE FOLLOWED BY A SIGNIFICANT MARINE PUSH.  
WOULD EXPECT THE LOWLANDS TO WAKE UP FRIDAY MORNING TO SOLID, LOW  
MARINE STRATUS. THE WARM, SUNNY PARTY WILL BE OVER AT THAT TIME.  
HANER  
   
LONG TERM
 
FROM PREVIOUS DISCUSSION: THERE CONTINUES TO BE  
RATHER LARGE DIFFERENCES BETWEEN THE GFS SOLUTION AND THE  
ECMWF/CANADIAN SOLUTIONS TODAY. ALL MODELS AGREE THAT A MARINE  
PUSH WILL BEGIN THURSDAY EVENING, BUT THE ECMWF/CANADIAN SOLUTIONS  
HOLD MOST OF THE PRECIPITATION JUST TO THE SOUTH OF THE FORECAST  
AREA AS A WEAKER SHORTWAVE TROUGH ROTATES AROUND A LOW THAT WILL  
MOVE INLAND OVER THE SOUTHERN HALF OF OREGON. THE GFS IS MUCH MORE  
ROBUST WITH THE STRENGTH OF THE SHORTWAVE TROUGH AND IS FARTHER  
NORTH WITH ITS TRACK. THE GFS DEVELOPS A RATHER HEAVY RAIN BAND  
WITH A DEFORMATION ZONE OVER WESTERN WASHINGTON FRIDAY THROUGH  
SATURDAY NIGHT. THE GFS TOTAL PRECIPITATION AMOUNTS ARE WELL OVER  
AN INCH AT SEATAC AND SOME SOLUTIONS ARE WETTER STILL. THE ECMWF  
IS MUCH, MUCH MORE TAME. AT THIS TIME, A MODEL BLEND WAS USED TO  
POPULATE THE FORECASTS AND WERE WEIGHTED MORE HEAVILY TOWARD THE  
ECMWF. THE MODELS ARE ALL SHOWING GOOD CONTINUITY WITH THEIR  
PREVIOUS SOLUTIONS; MAYBE ONE OF THEM WILL 'GIVE IN' ON THE NEXT  
COUPLE OF FORECAST CYCLES.  
 
ONE SURE BET FOR THE UPCOMING WEEKEND WILL BE MUCH COOLER  
TEMPERATURES. TEMPERATURES WILL FALL SIGNIFICANTLY ON FRIDAY. A  
MODEL BLEND WOULD SUGGEST UPPER 50S FOR HIGHS ON FRIDAY WHILE THE  
ECMWF WOULD SUGGEST HIGHS MORE INTO THE MID 60S. A WEIGHTED BLEND  
TOWARD THE ECMWF WAS USED TODAY. AFTER FRIDAY, TEMPERATURES FALL  
TO A FEW DEGREES BELOW FOR THE WEEKEND AS DEEPER CLOUD COVER AND  
PRECIPITATION CHANCES INCREASE AND THE AIR MASS ALOFT CONTINUES TO  
COOL. ALBRECHT  
 

 
   
AVIATION
 
A STRONG UPPER RIDGE AXIS WILL MOVE FROM CENTRAL TO  
EASTERN WASHINGTON ON TUESDAY. LOW-LEVEL OFFSHORE FLOW IS EXPECTED  
GIVEN A THERMAL PRESSURE TROUGH NEAR THE COAST. EAST WINDS WILL BE  
STRONG ON TUE MORNING NEAR THE CASCADES OF SNOHOMISH, KING, AND  
LEWIS COUNTIES. THE AIR MASS WILL BE DRY AND STABLE. CLEAR SKIES  
FOR THE NEXT 24+ HOURS.  
 
KSEA...CLEAR SKIES FOR NEXT 30+ HOURS. ONLY FORECAST CHALLENGE IS  
THE POSSIBILITY OF THE NORTH SURFACE WIND BEING INTERRUPTED FOR A  
TIME LATE TUESDAY MORNING BY EAST-SOUTHEAST WIND. THIS WOULD  
EMANATE FROM EASTERLY GAP FLOW COMING THROUGH THE CASCADES. ANY  
CHANGE TO EAST-SOUTHEAST WIND WOULD BE TEMPORARY, LASTING  
ANYWHERE FROM 1-4 HOURS. THIS IS SHOWN IN THE TAF WITH A TEMPO  
EAST-SOUTHEAST WIND FROM 16Z-20Z. AFTER THAT TIME, WINDS WILL BE  
SOLIDLY OUT OF THE NORTH FOR TUE AFTERNOON. HANER  
 

 
   
MARINE
 
HIGH PRESSURE INLAND AND THERMALLY INDUCED LOW PRESSURE  
ALONG THE OREGON AND WASHINGTON COAST WILL PERSIST UNTIL ABOUT  
THURSDAY MORNING. THIS PATTERN LEADS TO NORTHERLY OFFSHORE FLOW.  
SMALL CRAFT ADVISORY STRENGTH EASTERLIES ARE FORECAST ON TUESDAY  
MORNING AT THE WEST ENTRANCE OF THE STRAIT OF JUAN DE FUCA AROUND  
CAPE FLATTERY. THE THERMALLY INDUCED LOW PRESSURE WILL MOVE  
INLAND ON THURSDAY WITH ONSHORE FLOW DEVELOPING THURSDAY NIGHT AND  
FRIDAY. HANER  
 

 
   
SEW WATCHES/WARNINGS/ADVISORIES
 
 
WA...NONE.  
PZ...SMALL CRAFT ADVISORY UNTIL 1 PM PDT TUESDAY FOR COASTAL WATERS  
FROM CAPE FLATTERY TO JAMES ISLAND OUT 10 NM-WEST ENTRANCE  
U.S. WATERS STRAIT OF JUAN DE FUCA.  
 

 
 

 
 
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