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AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
352 PM PDT THU SEP 3 2015

.SYNOPSIS...AN UPPER LEVEL LOW WILL MOVE SLOWLY SOUTH ACROSS WESTERN 
WASHINGTON TONIGHT...THEN OVER OREGON ON FRIDAY. SCATTERED SHOWERS 
WILL CONTINUE IN THE COOL UNSTABLE AIR MASS ALONG WITH ISOLATED 
EVENING THUNDERSTORMS. AS THE UPPER LOW MOVES SOUTH OF THE AREA ON 
FRIDAY THE AIR MASS WILL GRADUALLY DRY AND WARM...LEADING TO A SUNNY 
SATURDAY. A WEAK FRONTAL SYSTEM WILL BRING A CHANCE OF RAIN LATE 
SATURDAY NIGHT AND SUNDAY...THEN A WEAK UPPER LEVEL RIDGE WILL 
PRODUCE A PARTLY SUNNY LABOR DAY. WEAK SYSTEMS MAY BRING A CHANCE OF 
SHOWERS MONDAY NIGHT THROUGH TUESDAY NIGHT. AN UPPER LEVEL RIDGE 
MAY BUILD OVER THE REGION WEDNESDAY.

&&

.SHORT TERM...SATELLITE IMAGERY SHOWED THE COLD UPPER LEVEL LOW WAS 
STILL OVER SW B.C. AS OF 3 PM THIS AFTERNOON. THE AIR MASS OVER W WA 
REMAINS MOIST AND UNSTABLE WITH WIDESPREAD SHOWERS. THERE HAD BEEN 
VERY LITTLE LIGHTNING UP THROUGH MID AFTERNOON...BUT WITH CELL ECHO 
TOPS STILL SHOOTING ABOVE 16KFT IN SPOTS...SOME ISOLATED 
THUNDERSTORMS ARE FINALLY DEVELOPING. THERE DO NOT APPEAR TO BE ANY 
MORE SHORT WAVE TROUGHS ABOUT TO TRAVERSE THE AREA TONIGHT...SO 
LACKING THAT SOURCE OF FORCING THE ONLY THING LEFT TO GENERATE 
SHOWERS IS AIR MASS INSTABILITY. THE CHANCE OF SHOWERS WILL CONTINUE 
TONIGHT...BUT SHOULD BEGIN DIMINISHING GRADUALLY LATE TONIGHT AND ON 
FRIDAY AS THE LOW DRIFTS S...AND DRIER N FLOW DEVELOPS BEHIND THE 
LOW. HOWEVER IF THE LOW TAKES LONGER THAN EXPECTED TO DEPART THE 
CHANCE OF SHOWERS MAY NEED TO BE EXTENDED INTO FRIDAY MORNING. 
GRADUAL CLEARING BEHIND THE LOW STILL LOOKS GOOD FOR FRIDAY 
AFTERNOON. 

THE COLD AIR MASS FROM THE LOW WILL BE RIGHT OVERHEAD TONIGHT SO LOW 
TEMPERATURES SHOULD DROP INTO THE LOWER 40S IN THE COLDEST SPOTS. 
FRIDAY NIGHT SHOULD BE SLIGHTLY COLDER WITH LESS CLOUD COVER OVER 
THE AREA. IT IS POSSIBLE THAT SOME OF THE COLDEST SPOTS LIKE OVER 
THE SW INTERIOR COULD SNEAK INTO THE UPPER 30S.

A WEAK UPPER LEVEL RIDGE WILL MOVE OVER THE AREA SATURDAY FOR A 
SUNNY AND WARMER DAY. MAX TEMPS WILL WARM A BIT...BUT ONLY TO 
SLIGHTLY BELOW NORMAL.

AFTER THE UPPER LEVEL LOW DEPARTS OREGON TO THE E ON SATURDAY...NW 
FLOW ALOFT WILL DEVELOP OVER THE REGION. THE LATEST MODEL SOLUTIONS 
HAVE STUCK WITH THE IDEA OF BRINGING AN UPPER LEVEL SHORTWAVE TROUGH 
AND ASSOCIATED WEAK COLD FRONT DOWN OVER W WA ON SUNDAY. THIS 
APPEARS TO BE THE SYSTEM THAT WAS OVER THE WESTERN ALEUTIANS NEAR 
170W AT 3 PM. THE 12Z GFS TIMING BRINGS SOME LIGHT RAIN OVER THE 
AREA LATE SATURDAY NIGHT THEN A LITTLE RAIN ON SUNDAY WITH THE WEAK 
FRONT AND TRAILING UPPER LEVEL TROUGH. KAM

.LONG TERM...THE GFS ECMWF AND CANADIAN ARE STILL HAVING A TOUGH 
TIME WITH EARLY NEXT WEEK. CONSISTENCY HAS BEEN ELUSIVE SO 
CONFIDENCE IS STILL LOW. THE 18Z GFS NOW SHOWS A HEALTHY UPPER LEVEL 
RIDGE BUILDING OVER W WA ON LABOR DAY FOR WHAT COULD BE A MOSTLY 
SUNNY DAY. THE 12Z ECMWF HAS A MUCH WEAKER RIDGE AND BRINGS SOME 
PRECIP ALMOST TO THE COAST. IN GENERAL THE MODELS STILL SHOW SOME 
WEAK TROUGHS EMBEDDED IN THE NW FLOW...WHICH AT THIS POINT MAY OR 
MAY NOT BRING PRECIP TO W WA ON ANY OF THE DAYS. EARLIER RUNS HAD A 
BROAD UPPER LEVEL RIDGE BUILDING OVER THE AREA AROUND MID-WEEK...BUT 
NOW THAT HAS EVOLVED TO WNW FLOW ALOFT WITH SOMEWHAT HIGHER 500 MB 
HEIGHTS. SO FOR NOW THE LOW CONFIDENCE FORECAST IS UNCHANGED... 
CHANCE OF SHOWERS MONDAY NIGHT THROUGH TUESDAY NIGHT AND THEN DRY 
AND WARMER WEDNESDAY AND THURSDAY. KAM 

&&

.AVIATION...AN UPPER LEVEL LOW SLOWLY DIGGING SOUTH INTO OREGON 
CONTINUES TO SUPPORT ENOUGH INSTABILITY FOR SCATTERED SHOWERS AND 
ISOLATED THUNDERSTORMS OVER THE AREA THIS AFTERNOON AND EVENING. VFR 
CONDITIONS OVER THE AREA WITH CEILINGS RANGING FROM 5000 TO 6500 
FEET...MAINLY BROKEN OVER THE OLYMPIC PENINSULA NORTH INTO BRITISH 
COLUMBIA...AND PARTLY SUNNY SKIES OVER THE CENTRAL SOUND SOUTHWARD 
INTO THE SOUTH INTERIOR. EXPECT SHOWERS AND THE VFR DECK TO CONTINUE 
TO SPREAD SOUTH AND EAST THROUGH THE SOUTH SOUND AND SOUTH INTERIOR 
EARLY THIS EVENING. A LINE OF SHOWERS EXTENDING FROM KSHN THROUGH 
KAWO INTO THE NORTH CASCADES IS LIKELY THE BEST BET FOR 
THUNDERSTORMS IN THE SHORT TERM...HAVING ALREADY PRODUCED A COUPLE 
OF CELLS WITH LIGHTNING OVER HOOD CANAL AND JUST NORTH EAST OF KAWO. 
CEILINGS WILL LOWER WITH SHOWERS...MVFR AND POSSIBLY LOWER. EXPECT 
SHOWERS AND THUNDERSTORMS TO TAPER OFF LATE THIS EVENING WITH 
CEILINGS REMAINING BKN BUT GENERALLY VFR THROUGH FRIDAY MORNING.  
LIGHT NORTHERLY FLOW ALOFT. THE MOIST AND SOMEWHAT UNSTABLE AIR MASS 
THIS EVENING WILL BECOME STABLE LATE IN THE EVENING. 
 
KSEA...VFR CEILINGS THIS AFTERNOON WILL PERSIST OVERNIGHT...A BAND 
MOVING INTO THE KITSAP PENINSULA AT 2230Z WILL MOVE INTO THE METRO 
AREA AFFECTING BOTH KBFI AND KSEA...MVFR CEILINGS ARE POSSIBLE AND 
THIS LINE HAS BEEN PRODUCING ISOLATED THUNDERSTORMS/LIGHTING AND 
GUSTY WINDS. OTHERWISE...CEILINGS WILL REMAIN BROKEN BUT VFR THROUGH 
THE NIGHT AS SHOWERS TAPER OFF. LIGHT SOUTHERLY WINDS WILL BECOME 
NORTHERLY LATE IN THE EVENING.
 
&&

.MARINE...A WEAK SURFACE LOW WILL WORK SOUTH ALONG THE OUTER COASTAL 
WATERS THIS EVENING...BRINGING LIGHT OFFSHORE FLOW LATE TONIGHT INTO 
FRIDAY MORNING. THE UPPER LEVEL TROUGH MOVING OVER WESTERN 
WASHINGTON WILL CONTINUE TO SUPPORT SCATTERED SHOWERS AND ISOLATED  
THUNDERSTORMS OVER AREA WATERS THIS EVENING...WITH GUSTY VARIABLE 
WINDS POSSIBLE. ONSHORE GRADIENTS WILL RETURN FRIDAY AND STRENGTHEN 
DURING THE AFTERNOON AND EVENING. HIGH PRESSURE WILL BUILD OVER THE 
WATERS FRIDAY INTO THE WEEKEND. 

&&

.SEW WATCHES/WARNINGS/ADVISORIES... 
WA...NONE.              
PZ...NONE.

&&

$$

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