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 041007
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
315 AM PDT TUE AUG 4 2015

.SYNOPSIS...AN UPPER TROUGH WILL BE OVER THE AREA THROUGH MIDWEEK.
WEAK HIGH PRESSURE ALOFT WILL MOVE OVERHEAD LATE IN THE
WEEK...FOLLOWED BY ANOTHER UPPER TROUGH NEXT WEEKEND.

&&

.SHORT TERM...AN UPPER LEVEL TROUGH WITH A LOW CENTER OVER B.C. WILL
CONTINUE TO DIG SWD TODAY LOWERING HEIGHTS OVER WRN WA. THE HEIGHT
FALLS AND CONTINUING ONSHORE FLOW WILL RESULT IN EVEN COOLER HIGH
TEMPERATURES TODAY...ESPECIALLY ACROSS THE INTERIOR WITH MAINLY 70S
FORECAST. SATELLITE FOG/STRATUS IMAGERY SHOWS AREAS OF HIGHER BASED
7-8K FT CLOUDS DEVELOPING OVER THE N PUGET SOUND REGION NEAR THE ERN
STRAIT WITH SOME LOWER STRATUS FORMING OVER THE SW INTERIOR AND FAR
S SOUND. THE LOWER STRATUS SURGE NORMALLY EXPECTED WITH AN UPPER
TROUGH INDUCED ONSHORE PUSH IS NOT VERY WIDESPREAD AT 2 AM...THOUGH
THE N COAST AND SW INTERIOR/S SOUND ARE SEEING SOME PATCHY LOW
CLOUDS FORMING IN PLACE. WILL PLAY WITH THE CLOUD WORDING A BIT TO
INDICATE AREAS OF MORNING CLOUDS. PARTLY SUNNY TO MOSTLY SUNNY SKIES
WILL DEVELOP DURING THE AFTERNOON BUT THIS COULD BE A SLOW PROCESS
GIVEN THE INCREASING ONSHORE GRADIENTS.

MODELS SHOW MORE SWLY ONSHORE GRADIENTS LATER TONIGHT/WEDNESDAY
MORNING WHICH WOULD BE BETTER FOR BRINING MORE SOLID LOW CLOUDS INTO
THE INTERIOR. WILL STICK WITH THE MOSTLY CLOUDY START TO THE
DAY...WITH PARTLY SUNNY SKIES AGAIN IN THE AFTERNOON. THE TROUGH
AXIS WILL BE OVER THE AREA WITH 500 MB HEIGHTS DOWN NEAR 5700M SO
WEDNESDAY MAY END UP THE COOLEST DAY OF THE WEEK. THERE COULD ALSO
BE SOME PATCHY MORNING DRIZZLE ON THE COAST. MODELS SHOW A SECONDARY
TROUGH AXIS DIGGING SWD ACROSS WRN WA DURING THE AFTERNOON SO WILL
MAINTAIN A THREAT OF SHOWERS...THOUGH QPF AMOUNTS SHOULD REMAIN
LIGHT. SHOWERS MAY LINGER INTO THE EVENING AS THE TROUGH PULLS E AND
THE FLOW BECOMES MORE NWLY. THERE COULD EVEN BE A BRIEF CONVERGENCE
ZONE OVER CENTRAL/N PUGET SOUND.

500 MB HEIGHTS REBOUND ON THURSDAY AS THE LOW PULLS E AND A RIDGE
OFFSHORE MOVES TOWARD THE AREA. LOW LEVEL NWLY ONSHORE FLOW WILL
PREVAIL BUT WEAKEN DURING THE DAY. THE TREND SHOULD BE TOWARD
SLIGHTLY WARMER TEMPERATURES...REACHING THE MID 70S TO LOWER 80S
AROUND PUGET SOUND AND THE SW INTERIOR.

.LONG TERM...VIRTUALLY ALL THE MODELS SHOW THE TRANSITORY RIDGE
SHIFTING EWD THURSDAY NIGHT AND FRIDAY. AN UPPER LOW WILL APPROACH
WITH LOWER HEIGHTS BY LATER IN THE DAY. THE MAIN ONSHORE INFLUENCE
WILL NOT BE FELT UNTIL THE WEEKEND SO ANOTHER PARTLY TO MOSTLY SUNNY
DAY APPEARS LIKELY. HIGHS SHOULD REACH SLIGHTLY ABOVE AVERAGE...INTO
THE LOW 80S AROUND PUGET SOUND AND THE SW INTERIOR...COOLER NEAR THE
WATER AND TOWARD THE N INTERIOR/COAST/STRAIT.

LONG RANGE MODELS DISAGREE ON THE TIMING...STRENGTH...AND TRACK OF
THE UPPER LOW FOR THE WEEKEND. THE GENERAL IDEA IS THAT UPPER
HEIGHTS WILL TREND LOWER AND ONSHORE FLOW WILL INCREASE ONCE AGAIN.
THIS SHOULD INTRODUCE MORE CLOUDS AND A THREAT OF SHOWERS TO WRN WA.
MADE LITTLE CHANGE TO THE EXTENDED WHICH HAS A CHANCE OF SHOWERS
INDICATED AT TIMES. THIS CAN BE FINE TUNED ONCE THE
GFS/ECMWF/CANADIAN COME INTO CLOSER AGREEMENT. TEMPERATURES WILL
PROBABLY COOL BACK TO NEAR AVERAGE OR EVEN SLIGHTLY BELOW. MERCER

&&

.AVIATION...A LARGE UPPER LEVEL LOW WILL REMAIN CENTERED OVER
CENTRAL BRITISH COLUMBIA THROUGH THIS AFTERNOON WITH AN UPPER LEVEL
TROUGH EXTENDING S OVER W WA. FLOW ALOFT WILL BE WESTERLY.

STRONG LOW LEVEL ONSHORE FLOW DEVELOPED ACROSS W WA LAST NIGHT
BRINGING COOLER MARINE AIR INLAND. THE ONSHORE FLOW WAS STRONG
ENOUGH FOR THE LOWER AIR MASS OR MARINE LAYER TO BE FAIRLY DEEP AND
WELL MIXED. IN ADDITION THERE WAS VERY LITTLE ACTUAL STRATUS PRESENT
ON THE COAST LAST EVENING. THIS SHOULD PRODUCE AREAS OF MAINLY MVFR
CIGS THIS MORNING...WHICH SHOULD BURN OFF BY 19Z.

ANOTHER ROUND OF ONSHORE FLOW TONIGHT WILL BRING MORE MARINE AIR
INLAND OVERNIGHT. THEN THE B.C. UPPER LEVEL LOW CENTER WILL MOVE
DOWN OVER S B.C. ON WEDNESDAY. THERE MAY BE MORE WIDESPREAD MVFR
CONDITIONS AND A FEW SHOWERS AS WELL.

KSEA...BKN022 STRATUS WAS STARTING TO FORM OVER KTCM AND KGRF AT 09Z
SO STRATUS SHOULD FORM AT KSEA BY 12Z. CIGS WILL BE MVFR BKN-OVC015-
025 AND SHOULD BURN OFF AROUND 18Z. SURFACE WINDS WILL BE S-SW 3-8
KT TODAY. KAM

&&

.MARINE...A LARGE PERSISTENT SURFACE RIDGE CENTERED WELL OFFSHORE...
COMBINED WITH LOWER PRESSURE OVER E WA WILL PRODUCE MODERATE TO
STRONG ONSHORE FLOW THROUGH THIS WEEK. THE WESTERLY ONSHORE FLOW
WILL BE STRONGEST DURING THE LATE AFTERNOON AND EVENING HOURS IN THE
CENTRAL AND E STRAIT OF JUAN DE FUCA.

LAST NIGHTS GALES IN THE STRAIT EASED QUICKLY AND NOW WINDS HAVE
EASED BELOW SCA LEVELS. THE GFS IS INDICATING A SIMILAR PRESSURE
GRADIENT THROUGH THE STRAIT TONIGHT AND THE CANADIAN LAM ALSO SHOWS
GALE FORCE WINDS...SO A GALE WILL BE IN EFFECT TONIGHT. THE N PART
OF ADMIRALTY INLET HAS A GOOD CHANCE AT GETTING SCA LEVEL WINDS
AGAIN TONIGHT...SO A SCA WILL BE IN EFFECT.

THE LEFTOVER BORDERLINE SCA FOR NW WINDS OVER THE OUTER COASTAL
WATERS WILL BE ALLOWED TO EXPIRE LATE THIS MORNING. MESOSCALE MODELS
SHOW THE WINDS EASING BELOW 20 KT...AND WITH NO OBSERVED WIND DATA
FROM THE AREA I WILL GO ALONG WITH THE MODEL SOLUTIONS.

THE GFS HAS THE ONSHORE FLOW A LITTLE WEAKER WEDNESDAY NIGHT THROUGH
FRIDAY NIGHT SO WINDS IN THE STRAIT WILL PROBABLY REMAIN IN THE SCA
RANGE. HOWEVER THERE IS ALWAYS A CHANCE OF A BRIEF GALE DURING THESE
EVENTS. KAM

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...GALE WARNING CENTRAL STRAIT AND EAST ENTRANCE TONIGHT.
     SMALL CRAFT ADVISORY ADMIRALTY INLET TONIGHT.

     SMALL CRAFT ADVISORY COASTAL WATERS CAPE FLATTERY TO CAPE
     SHOALWATER 10 NM TO 60 NM THROUGH 11 AM PDT.

&&

$$

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.