Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
000
FXUS66 KSEW 030325
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
830 PM PDT WED SEP 2 2015

.SYNOPSIS...AN UPPER LEVEL LOW OVER SOUTHWEST BRITISH COLUMBIA IS
EXPECTED TO SAG SOUTH OVER WESTERN WASHINGTON THROUGH THURSDAY. THE
ASSOCIATED COOL UNSTABLE AIR MASS SHOULD MAINTAIN SOME SHOWERS ALONG
WITH A THREAT OF THUNDERSTORMS MAINLY DURING THE AFTERNOON AND
EVENING HOURS. THE UPPER LOW IS FORECAST TO CONTINUE SOUTH INTO
OREGON FRIDAY OFFERING A GRADUAL DRYING AND WARMING TREND FOR
WESTERN WASHINGTON FRIDAY AFTERNOON AND SATURDAY. ANOTHER WEAK
WEATHER SYSTEM MOVING INTO WESTERN CANADA MAY BRING SOME SHOWERS ON
SUNDAY WITH ANOTHER THREAT BY TUESDAY.

&&

.SHORT TERM...DOES IT FEEL LIKE FALL? AN UPPER LOW NEAR 52N/127W
OVER SW BRITISH COLUMBIA HAS QUITE A COOL UNSTABLE AIR MASS WITH IT.
THE SOUNDINGS FROM QUILLAYUTE AND PORT HARDY LATE TODAY HAD FREEZING
LEVELS OF 6100 AND 5200 FEET RESPECTIVELY. SATELLITE IMAGERY AND
DOPPLER WEATHER RADAR SHOW A LOT OF SHOWERS ROTATING AROUND THE LOW
AND CIRCULATING INTO WESTERN WASHINGTON.

WITH THE SUN DOWN...THE ISOLATED THUNDERSTORMS THIS AFTERNOON HAVE
DIED DOWN. YET AS OF 8 PM...THERE WAS QUITE A BIT OF LIGHTNING
ASSOCIATED WITH A SHOWER CLUSTER OFF NORTHERN VANCOUVER ISLAND
HEADING SOUTHEAST. HAVE SEEN THIS STORY BEFORE. WILL INCLUDE A
THREAT OF THUNDERSTORMS FOR THE COASTAL WATERS AND ONSHORE TO THE
BEACHES OVERNIGHT INTO THURSDAY MORNING FOR THE POTENTIAL OF
NOCTURNAL CONVECTION.

THE 00Z PROGS CONTINUE THE TREND OF SAGGING THE UPPER LOW SOUTH OVER
WESTERN WASHINGTON THURSDAY AND INTO OREGON FRIDAY THANKS TO THE
UPSTREAM BUILDING UPPER RIDGE NEAR 155W THIS EVENING. SO AFTER
ANOTHER COOL SHOWERY DAY WITH A CONTINUED THREAT OF CONVECTION
THURSDAY...SHOULD HAVE A CLEARING TREND ON FRIDAY INTO SATURDAY.
REST OF THE FORECASTS ARE ON TRACK. BUEHNER

.LONG TERM...FROM PREVIOUS DISCUSSION...NW FLOW ALOFT DEVELOPS OVER
THE REGION ON SUNDAY AND CONTINUES THROUGH TUESDAY. THE GFS AND
ECMWF HAVE BEEN PRETTY INCONSISTENT WITH THE TIMING AND STRENGTH OF
THE UPPER LEVEL SHORTWAVES EMBEDDED IN THE NW FLOW. THE MODELS AGREE
THAT A WEAK SHORTWAVE TROUGH WILL CROSS W WA ON SUNDAY...WHICH IS
JUST ENOUGH TO ADD A CHANCE OF SHOWERS TO THE FORECAST.
HOWEVER...CONFIDENCE IS STILL ONLY MODERATE BECAUSE THE MODEL
AGREEMENT WAS JUST REACHED OVER THE PAST 2-3 RUNS. FOR MONDAY AND
TUESDAY CONFIDENCE IS LOW. THE MODELS HAVE NOT REACHED ANY SORT OF
CONSENSUS AND EACH HAS BEEN INCONSISTENT FROM RUN-TO-RUN. I WILL
LEAVE LABOR DAY DRY...BUT KEEP THE CHANCE OF SHOWERS MONDAY NIGHT
THROUGH TUESDAY NIGHT. IT LOOKS LIKE AN UPPER LEVEL RIDGE MAY BE
BUILDING AROUND MIDWEEK SO I WENT DRY FOR WEDNESDAY. KAM

&&

.AVIATION...AN UPPER LEVEL TROUGH CONTINUES TO WORK SOUTH ALONG THE
BRITISH COLUMBIA COAST THIS EVENING. THERE HAS BEEN ENOUGH
INSTABILITY AND MOISTURE THAT IN ADDITION TO SCATTERED
SHOWERS...ISOLATED THUNDERSTORMS HAVE OCCURRED MAINLY NEAR AND OVER
THE MOUNTAINS AND ARE NOW ENDING.  THERE IS STILL A CHANCE OF
THUNDERSTORM OVERNIGHT NEAR THE COAST. VFR CONDITIONS WITH BROKEN TO
SCATTERED SKIES OVER MUCH OF THE AREA THIS EVENING. EXPECT VFR
CONDITIONS TO PREVAIL THIS EVENING WITH LOWER CEILINGS AND GUSTY
WINDS OCCURRING WITH SHOWERS. CEILINGS WILL LOWER OVERALL INTO
THURSDAY MORNING TO MVFR LEVELS.

KSEA...VFR CEILINGS...EXCEPT SOME MVFR WITH ANY SHOWERS. ABUNDANT
LOW LEVEL MOISTURE WILL HELP CEILINGS LOWER TO MVFR LEVELS EARLY
THURSDAY MORNING. WINDS WILL BE BRIEFLY NORTHERLY BEFORE BECOMING
VARIABLE AND THEN BACK TO SOUTHERLY OVERNIGHT.

&&

.MARINE...LOW LEVEL ONSHORE FLOW CONTINUING TODAY WITH NORTHWESTERLY
WESTERLY FLOW OVER THE COASTAL WATERS. SHOWERS OVER INLAND WATERS
WILL KEEP WINDS GUSTY BUT BELOW SMALL CRAFT ADVISORY CRITERIA
TONIGHT. THERE IS STILL A CHANCE OF THUNDERSTORMS OVERNIGHT IN THE
COASTAL WATERS. LOW LEVEL ONSHORE FLOW WILL WEAKEN SOMEWHAT LATE
TONIGHT INTO THURSDAY...AND AGAIN THURSDAY NIGHT INTO FRIDAY AS A
WEAK SURFACE LOW MOVES THROUGH THE OFFSHORE WATERS. HIGH PRESSURE
WILL BUILD OVER THE WATERS FRIDAY INTO THE WEEKEND.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY GRAYS HARBOR BAR.

&&

$$

WWW.WEATHER.GOV/SEATTLE/

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



USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.