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 232201
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 PM PDT TUE SEP 23 2014

.SYNOPSIS...A COLD FRONT WILL SPREAD RAIN INLAND THIS EVENING WITH
THE FRONT CROSSING THE WESTERN WASHINGTON INTERIOR WEDNESDAY
MORNING. MOIST SOUTHWEST FLOW ALOFT WILL MAINTAIN MOSTLY CLOUDY
SKIES AND A CHANCE OF SHOWERS WEDNESDAY NIGHT AND THURSDAY. A LARGE
UPPER LEVEL TROUGH WILL MOVE OVER THE REGION ON SATURDAY FOR A
CHANCE OF SHOWERS. MOIST WESTERLY FLOW ALOFT WILL KEEP THE PATTERN
UNSETTLED EARLY NEXT WEEK.

&&

.SHORT TERM...THE DOMINANT WEATHER FEATURE THIS WEEK WILL BE THE
LARGE VERTICALLY STACKED UPPER LEVEL/SURFACE LOW CURRENTLY CENTERED
NEAR 46N/139W AT 21Z/2 PM.

THE MAIN RAIN EVENT WILL BE TONIGHT THROUGH WEDNESDAY MORNING AS A
COLD FRONT MOVES ACROSS THE AREA. MODELS ARE IN FAIRLY GOOD
AGREEMENT WITH THE TIMING. RAIN WILL BECOME MORE STEADY ON THE COAST
EARLY THIS EVENING THEN SPREAD OVER THE INTERIOR LATER THIS EVENING.
FROPA ON THE COAST WILL OCCUR AFTER MIDNIGHT AND OVER PUGET SOUND
WEDNESDAY MORNING. RAINFALL ACCUMULATION THROUGH 11 AM WEDNESDAY
SHOULD BE HEALTHY WITH 1-2 INCHES ON THE COAST AND ROUGHLY 0.50-1.00
INCHES OVER THE INTERIOR. WIND SHOULD NOT BE A BIG DEAL THIS TIME AS
PRESSURE GRADIENTS ARE TOO EASTERLY.

THE MAIN OFFSHORE LOW WILL HAVE DRIFTED NE TO AROUND 49N/135W BY
18Z/11 AM WEDNESDAY AND IT WILL REMAIN AROUND THAT AREA THROUGH
THURSDAY. THIS WILL KEEP W WA UNDER GENERALLY MOIST SW FLOW ALOFT.
WEAK EMBEDDED SHORTWAVE TROUGHS WILL PROBABLY GENERATE PERIODS OF
SHOWERS WEDNESDAY AFTERNOON THROUGH THURSDAY. CHANCE POPS WILL WORK
OUT BEST FOR THIS PERIOD. MAX TEMPS WILL REMAIN CLOSE TO NORMAL WITH
HIGHS IN THE 60S TO NEAR 70.

MODELS ALL WEAKEN THE LARGE UPPER LEVEL TROUGH ON FRIDAY AND SHIFT
IT E TOWARDS THE PACNW COAST. WHILE THE MODELS AGREE ON THE
EVOLUTION OF THE TROUGH THERE IS MORE DISCORD ABOUT PRECIP COVERAGE
AND AMOUNT. THE GFS LEANS TOWARDS DRY OVER THE LOWLANDS WHILE THE
ECMWF AND NAM SHOW A BIT MORE RAIN OVER THE AREA. I WILL KEEP THE
CHANCE OF SHOWERS FOR NOW. KAM

.LONG TERM...AS THE LARGE UPPER LEVEL TROUGH ALONG THE PACIFIC COAST
ON FRIDAY MOVES INLAND...THE GFS/ECMWF SHOW A CUT-OFF UPPER LEVEL
LOW FORMING IN THE SOUTHERN PART OF THE TROUGH ON SATURDAY. THE
ECMWF PLACES THE LOW CENTER OVER OREGON WITH SOME LIGHT RAIN PRETTY
CLOSE TO W WA. THE GFS KEEPS THE LOW CENTER DOWN OVER N CA WITH DRY
CONDITIONS OVER W WA. CUT-OFF UPPER LEVEL LOWS ARE CAPRICIOUS AND
THE MODELS HAVE NOT BEEN ALL THAT CONSISTENT...SO EVEN THOUGH
SATURDAY IS LOOKING LIKE A DRY DAY I WILL HANG ON TO A 20 PERCENT
CHANCE OF SHOWERS FOR NOW.

THE ECMWF IS FASTER THAN THE GFS IN MOVING THE UPPER LEVEL LOW
EASTWARD ON SUNDAY...ALLOWING MOIST WESTERLY FLOW ALOFT UPSTREAM TO
SPREAD INLAND OVER W WA. THE ECMWF HAS PRECIP FROM AN UPPER LEVEL
TROUGH EMBEDDED THE FLOW REACHING W WA SUNDAY AFTERNOON WHILE THE
GFS BRINGS A WEAK COLD FRONT INLAND SUNDAY NIGHT. UNSETTLED WEATHER
CONTINUES MONDAY AND TUESDAY IN BOTH MODELS. KAM

&&

.AVIATION...A LARGE UPPER TROUGH AND DEEP SURFACE LOW WILL MEANDER
ALONG 130 W THROUGH THU. THE ASSOCIATED SURFACE FRONT WILL REACH THE
COAST THIS EVENING THE VERY SLOWLY MOVE ACROSS INTERIOR WRN WA
OVERNIGHT INTO WED MORNING. UPPER LEVEL LOW PRESSURE WILL MOVE
CLOSER ON WED AS THE FRONT MOVES E OF THE CASCADES. STRONG SW FLOW
ALOFT THROUGH WED. MOIST AT ALL LEVELS AND WEAKLY UNSTABLE
ALOFT...BUT LIGHTNING IS NOT EXPECTED. SHOWERS WILL TRANSITION TO
STEADY RAIN THIS EVENING...HEAVY AT TIMES. CIGS ARE HIGHLY VARIABLE
THIS EVENING BUT MVFR CIGS WILL BECOME WIDESPREAD AS RAIN ARRIVES
WITH THE FRONT OVERNIGHT. CIGS IMPROVING TO VFR AND SCATTERING BY
18Z WED.

KSEA...RAIN AND LOWERING CIGS ARE SLOW TO DEVELOP BUT THE STRONGEST
PORTION OF THE FRONTAL SYSTEM WILL ARRIVE THIS EVENING AND
OVERNIGHT. VFR CIGS WITH SHOWERS WILL TRANSITION TO SOLID RAIN WITH
MAINLY MVFR CIGS AFTER 03Z THIS EVENING. RAIN TAPERING TO SHOWERS
WITH IMPROVING CIGS BY 18Z WED.

&&

.MARINE...STRONG SE PRESSURE GRADIENTS WILL BE CAUSED BY A DEEP 973
MB LOW NEAR 135 W. GALE FORCE WINDS ARE EXPECTED IN THE COASTAL
WATERS THROUGH TONIGHT AND THE WARNING REMAINS IN EFFECT. SE FLOW
ALSO PICKS UP AHEAD OF THE FRONT WITH SMALL CRAFT WINDS DEVELOPING
FROM THE E ENTRANCE TO THE STRAIT OF JUAN DE FUCA TO THE NRN INLAND
WATERS. THE W ENTRANCE WILL ALSO HAVE SMALL CRAFT WINDS AT TIMES.
WINDS WILL EASE ACROSS THE WATERS WED MORNING AS THE LOW BEGINS TO
FILL AND THE FRONT MOVES E. LIGHTER FLOW IS EXPECTED THE END OF THE
WEEK INTO THE WEEKEND.

THE LARGE 973 MB SURFACE LOW STALLED NEAR 135 W WILL SEND A LARGE
SWELL TRAIN ONTO THE WA COAST. EARLY SEASON CRITERIA FOR A HIGH
SURFACE ADVISORY IS 15 FT...BUT SWELL WILL PROBABLY BE JUST SHORT OF
THAT WHEN REACHING THE COAST. DECIDED AGAINST A HIGH SURF BUT SEA
STATE WILL NEED TO BE MONITORED. REGARDLESS...IT WILL BE HAZARDOUS
ALONG THE BEACH AS WAVES WELL OVER 10 FT WILL DEVELOP.  MERCER

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...GALE WARNING COASTAL WATERS TONIGHT.
     SMALL CRAFT ADVISORY ENTRANCES TO THE STRAIT AND NORTHERN
     INLAND WATERS THIS AFTERNOON THROUGH TONIGHT.
     SMALL CRAFT ADVISORY FOR ROUGH BAR CONDITIONS WEDNESDAY.

&&

$$

WWW.WEATHER.GOV/SEATTLE

YOU CAN SEE AN ILLUSTRATED VERSION OF THIS FORECAST 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.