310  
FXUS66 KSEW 261031  
GPHAFD  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE SEATTLE WA  
320 AM PDT WED APR 26 2017  
   
SYNOPSIS
 
MOIST UNSETTLED FLOW FROM THE PACIFIC INTO WESTERN  
WASHINGTON WILL MAINTAIN SHOWERY WEATHER WITH SOME SUNBREAKS AT  
TIMES THROUGH THURSDAY. A DEVELOPING PUGET SOUND CONVERGENCE ZONE  
WILL ENHANCE SHOWERS LATER TODAY IN THE CENTRAL SOUND AREA. HIGHER  
PRESSURE IS EXPECTED TO BUILD FRIDAY FOR A RESPITE FROM THE WET  
WEATHER THAT MAY HANG ON INTO SATURDAY. ANOTHER WEATHER SYSTEM IS  
FORECAST TO ARRIVE BY SUNDAY FOR MORE RAIN BEFORE HIGHER PRESSURE  
REBUILDS EARLY NEXT WEEK FOR DRIER WEATHER HEADING INTO MAY.  
 

 
   
SHORT TERM
 
THE OLD SAYING - APRIL SHOWERS BRING MAY FLOWERS -  
WE CERTAINLY HAVE HAD PLENTY OF APRIL SHOWERS WITH MORE IN STORE THE  
NEXT FEW DAYS. A FLAT UPPER LEVEL RIDGE RESIDES SOUTHWEST OF THE  
PACIFIC NW THIS MORNING WITH A 140 KNOT PLUS JET EXTENDING FROM NEAR  
THE DATELINE INTO THE REGION. (PIC0) THERE ARE A SEQUENCE OF  
SHORTWAVES EMBEDDED IN THE JET THAT WILL MAINTAIN MOIST UNSETTLED  
WEATHER INTO THURSDAY.  
 
TUESDAY'S WEAKENING FRONT MOVED ONSHORE OVERNIGHT WITH SPOTTY RAIN.  
THE NEXT SHORTWAVE APPROACHING 130W APPEARING WELL ON WATER VAPOR  
SATELLITE IMAGERY AT THIS EARLY HOUR (PIC1) WILL MOVE ONSHORE AND  
ENHANCE SHOWERS LATER TODAY, PARTICULARLY IN SOUTHERN SECTIONS OF  
THE AREA. AT THE SURFACE, A RIDGE OF HIGHER PRESSURE OFF THE COAST  
WILL INCREASE ONSHORE FLOW AS WELL. (PIC2) THIS PATTERN WILL HELP  
CREATE A PUGET SOUND CONVERGENCE ZONE LATER TODAY (PIC2) THAT HAS  
THE POTENTIAL TO SAG SOUTH FROM NORTHERN KITSAP AND SNOHOMISH  
COUNTIES SOUTH WELL INTO KING COUNTY THIS EVENING (PIC3) THANKS TO  
THE STRONG FLOW TO DEVELOP IN THE STRAIT OF JUAN DE FUCA.  
 
ANOTHER EMBEDDED SHORTWAVE IS FORECAST TO SWING ONSHORE THURSDAY TO  
KEEP THE SHOWERS GOING, (PIC4) BUT MAINLY IN THE MOUNTAINS AND  
HIGHER TERRAIN. HIGH TEMPERATURES LOOK TO REMAIN A BIT BELOW LATE  
APRIL AVERAGES.  
 
GUIDANCE STRONGLY SUGGESTS THAT FRIDAY MAY OFFER A BREAK IN THE  
PRECIPITATION WITH SOME SUNSHINE AS A RIDGE OF HIGHER PRESSURE  
BUILDS OFF THE COAST. (PIC5) HAVE THOSE SUNGLASSES HANDY. BUEHNER  
   
LONG TERM
 
THE NEXT QUESTION IS WILL THE DRIER WEATHER HANG ON  
INTO SATURDAY. THE CANADIAN AND ECMWF CONCUR WITH THE RIDGE HOLDING  
FOR ANOTHER DRY DAY WHILE THE GFS IS QUICKER IN SPREADING  
PRECIPITATION ONSHORE DURING THE DAY. (PIC6) AT THIS POINT, HAVE  
INDICATED A GROWING THREAT OF RAIN FOR THE OLYMPIC PENINSULA AND  
COAST DURING THE DAY SATURDAY AND INCREASING CLOUDS ELSEWHERE. THEN  
THIS WEATHER SYSTEM SHOULD PROVIDE MORE RAIN SATURDAY NIGHT  
(PIC7) WITH DECREASING SHOWERS ON SUNDAY.  
 
AS THE MONTH OF MAY BEGINS, GUIDANCE AGAIN IS NOT CONSISTENT. THE  
ECMWF REBUILDS THE UPPER RIDGE WHILE THE CANADIAN AND GFS SWING YET  
ANOTHER SHORTWAVE ONSHORE FOR ADDITIONAL SHOWERS. (PIC8) BUT BEYOND  
MONDAY, RESTLESS NATIVES EAGER FOR SOME SUNSHINE MAY GET SOME AS IT  
APPEARS HIGHER PRESSURE ALOFT WILL BUILD OVER THE REGION. (PIC9)  
BUEHNER  
 

 
   
AVIATION
 
MODERATE WESTERLY FLOW ALOFT (PIC10) BECOMING LIGHT  
NORTHWESTERLY FLOW ALOFT TONIGHT AS A TROUGH SHIFT EASTWARD. STRONG  
ONSHORE FLOW TODAY WITH A MOIST AND WEAKLY UNSTABLE AIR MASS AIR  
MASS OVER WESTERN WASHINGTON THROUGH THIS EVENING. SHOWERS WILL  
DECREASE LATER TONIGHT. THE PUGET SOUND CONVERGENCE ZONE COULD  
BECOME ACTIVE THIS AFTERNOON AND EVENING IN THE PRONE CORRIDOR FROM  
SEA-PAE (PIC11). A WIND SHIFT TO THE NORTH-NORTHWEST WILL OCCUR OVER  
NORTH PUGET SOUND THIS AFTERNOON AND COULD CAUSE SOUTHWEST WINDS AT  
KBFI/KSEA TO TURN WESTERLY OR SWITCH TO NORTH-NORTHWEST FOR A FEW  
HOURS LATE IN AFTERNOON AND EARLY EVENING (PIC12). OPPOSING FLOW W/  
WIND SPEEDS TO 10 KT AT KSEA/KBLI POSSIBLE ROUGHLY 00Z-05Z EARLY  
THIS EVENING. MOSTLY MVFR CIGS AND ISOLATED IFR CIGS THIS MORNING  
BECOMING PREDOMINATELY VFR THIS AFTERNOON EXCEPT LOWER WITH SHOWERS  
OR NEAR THE CONVERGENCE ZONE.  
 
KSEA...THE AIR MASS IS SATURATED NEAR THE SURFACE WITH PERIODS OF  
IFR POSSIBLE UNTIL AROUND DAYBREAK. OTHERWISE PREDOMINATELY MVFR  
CIGS THIS MORNING IMPROVING TO MOSTLY VFR THIS AFTERNOON. BREEZY  
SOUTHWEST WINDS 10-16 KT W /GUSTS 25-28 KT. THE PUGET SOUND  
CONVERGENCE ZONE WILL DEVELOP DURING THE AFTERNOON AND COULD REACH  
THE TERMINAL BY AROUND 00Z LATE THIS AFTERNOON. WINDS COULD SHIFT  
MORE WESTERLY AND THEN NORTH-NORTHWESTERLY AT TIMES THROUGH 06Z  
BEFORE SWITCHING BACK TO A SOUTHERLY DIRECTION AND DECREASING. A  
PERIOD OF OPPOSING FLOW AT KSEA/KBFI COULD OCCUR LATE THIS AFTERNOON  
OR EVENING W/ WIND SPEED UP TO 10-12 KT OUT OF THE SOUTH AND LIGHTER  
FROM THE NORTH DIRECTION. DTM  
 

 
   
MARINE
 
ONSHORE FLOW WILL INCREASE TODAY BEHIND A FRONT AS AN  
UPPER LEVEL SYSTEM CROSSES THE AREA. WINDS SHOULD GRADUALLY EAST  
LATER TONIGHT. SMALL CRAFT WINDS EXPECTED MOST WATERS FOR POST  
FRONTAL ONSHORE WEST-SOUTHWESTERLY WINDS. A STRONGER WESTERLY PUSH  
IS ANTICIPATED IN THE CENTRAL/EAST STRAIT (PIC13), THUS A GALE  
WARNING IS UP FROM MIDDAY THROUGH MOST OF TONIGHT. SMALL CRAFT WINDS  
MAY CONTINUE OVER OTHER WATERS NEAR THE EAST ENTRANCE TO THE STRAIT.  
DTM  
 

 
   
SEW WATCHES/WARNINGS/ADVISORIES
 
 
WA...NONE.  
PZ...SMALL CRAFT ADVISORY ALL WATERS.  
GALE WARNING CENTRAL AND EASTERN STRAIT LATE THIS MORNING  
THROUGH LATE TONIGHT.  
 

 
 

 
 
WWW.WEATHER.GOV/SEATTLE/  
 
YOU CAN SEE AN ILLUSTRATED VERSION OF THIS DISCUSSION AT  
WWW.WRH.NOAA.GOV/SEW/GAFD/LATEST_WEBAFD.HTML  
 
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