Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 291614
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
900 AM PDT MON SEP 29 2014

.SYNOPSIS...A FRONT WILL BRING RAIN TO THE AREA TODAY. THE PUGET
SOUND CONVERGENCE WILL BECOME ACTIVE THIS EVENING BEHIND THE FRONT.
SHOWERS WILL TAPER TUESDAY AND WEDNESDAY AS AN UPPER TROUGH OVER THE
AREA WEAKENS. AN UPPER RIDGE WILL BRING DRY WEATHER LATE IN THE WEEK.

&&

.SHORT TERM...LATEST RADAR LOOP INDICATES RAIN WITH THE APPROACHING
FRONT IS STARTING TO PUSH INLAND FROM THE COAST. DRIZZLE IS ALSO
FALLING OVER PARTS OF THE INTERIOR AS INCREASING LIFT AHEAD OF THE
FRONT ACTS ON THE LOW LEVEL MOISTURE IN PLACE. THIS WILL BE A FAST
MOVING SYSTEM DRIVEN BY A 130 KT JET ON THE BACK SIDE OF THE UPPER
TROUGH. ALTHOUGH THE FRONT APPEARS WEAK AND DIFFUSE ON IR
IMAGERY...MODELS SHOW AN 80+ KT JET AT THE BASE OF THE TROUGH WITH
WRN WA UNDER THE FAVORED LEFT EXIT REGION FOR EXTRA LIFT. WITH THAT
IN MIND...THE 00Z WRF-GFS/12Z NAM QPF AMOUNTS OF ONE TENTH TO A
QUARTER INCH OF RAIN ACROSS WRN WA BY THE END OF THE DAY IS
REASONABLE. MODELS ALSO AGREE ON TIMING WITH STEADY RAIN REACHING
THE REST OF THE INTERIOR BY MIDDAY.

POST FRONTAL FLOW BECOMES FAVORABLE FOR A PUGET SOUND CONVERGENCE
ZONE. BOTH THE WRF/NAM ADVERTISE THIS DEVELOPING NEAR THE
KING/SNOHOMISH COUNTY LINE EARLY THIS EVENING...THEN DRIFTING
FURTHER SOUTH INTO THE REST OF THE SEATTLE AREA DURING THE EVENING.
THIS MAKES SENSE WITH THE 850 MB FLOW TURNING MORE NWLY. MESO MODELS
INDICATE THE CONVERGENCE ZONE DISSIPATES LATE TONIGHT AS IT REACHES
THE PIERCE/KING COUNTY LINE NEAR TACOMA. LOCALLY HIGHER RAIN AMOUNTS
SHOULD BE EXPECTED WITHIN THE FAVORED CONVERGENCE CORRIDOR.
ELSEWHERE...THERE WILL BE SHOWERS AROUND BUT ADDITIONAL AMOUNTS WILL
BE LIGHT AND VARIABLE.

THE COLD POOL ALOFT SEEN ON IR IMAGERY OFF THE CENTRAL B.C. COAST
WILL CLIP THE AREA LATE TONIGHT AND TUESDAY UNDER NW FLOW ALOFT.
CYCLONIC FLOW AND COLDER AIR ALOFT WILL CONTINUE TO FAVOR
INSTABILITY AND SHOWERS ACROSS THE AREA. SHOWERS WILL BEGIN TO TAPER
OFF TUESDAY NIGHT SO THE LOWER CHANCE POPS IN THE FORECAST LOOK
FINE. EXPECT GENERALLY DRY WEATHER BY WEDNESDAY EXCEPT FOR A FEW
LINGERING SHOWERS IN THE MOUNTAINS. HIGH TEMPERATURES WILL GENERALLY
RUN IN THE LOW 60S THROUGH MID WEEK WHICH IS A FEW DEGREES BELOW
AVERAGE FOR THE PUGET SOUND REGION.  MERCER

.LONG TERM...NO CHANGES FROM PREVIOUS DISCUSSION...A BROAD UPPER
RIDGE WILL BE OVER THE AREA THURSDAY THROUGH SUNDAY. MOISTURE COMING
OVER THE TOP OF THE RIDGE WILL GIVE A THREAT OF RAIN AT TIMES BUT
MODELS ARE HAVING A TOUGH TIME WITH THE DETAILS AND ARE NOT SHOWING
MUCH RUN TO RUN CONSISTENCY. FOR NOW...CONFIDENCE IS REASONABLY HIGH
THAT THURSDAY WILL BE DRY. A LITTLE RAIN COULD AFFECT THE NORTHERN
ZONES FRIDAY. THE GFS AND ECMWF HAVE FLIP-FLOPPED WITH THE GFS NOW
WETTER AND THE ECMWF MOSTLY DRY. THERE IS PROBABLY A BETTER CHANCE
OF RAIN SATURDAY FOR MOST AREAS AS AN UPPER SHORT WAVE PUSHES THE
MOISTURE A BIT FURTHER SOUTH. SCHNEIDER

&&

.AVIATION...A COLD FRONT WILL MOVE FROM THE COAST THIS MORNING
THROUGH THE I-5 CORRIDOR THIS AFTERNOON. THIS WILL HELP REPLACE THE
LOW CLOUDS FOG AND DRIZZLE WITH HIGHER CIGS AND AREAS OF
RAIN...TURNING TO SCATTERED SHOWERS IN THE WAKE OF THE FRONT. A PSCZ
MAY SET UP TONIGHT OR TUE...AND MOST OF THE SHOWERS BEHIND THE FRONT
WILL BE IN THE PSCZ OR IN THE CASCADES...WITH SOME CLEARING POSSIBLE
FOR THE LOWLANDS TONIGHT AND TUE.

KSEA...THE LOW CLOUDS AND FOG WILL GRADUALLY LIFT AS THE FRONT MOVES
IN AND STIRS THINGS UP. CURRENT TAF SEEMS TO BE IN THE BALLPARK WITH
AN INCREASING SLY BREEZE. THERE IS A GOOD CHANCE OF PSCZ ACTIVITY
TONIGHT THROUGH TUE NIGHT BUT IT IS LIKELY TO REMAIN NORTH OF THE
CITY.

&&

.MARINE...A COLD FRONT OVER THE COASTAL WATERS THIS MORNING WILL
MOVE THROUGH THE INLAND WATERS THIS AFTERNOON. A WESTERLY 25-35KT
WIND IS EXPECTED TO DEVELOP AND PUSH THROUGH THE STRAIT LATER THIS
AFTERNOON AND EVENING IN THE WAKE OF THE FRONT. HIGH PRESSURE
OFFSHORE AND LOW PRESSURE INLAND WILL GIVE ONSHORE FLOW THROUGH THE
MIDDLE OF THE WEEK.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...GALE WARNING CENTRAL AND EASTERN JUAN DE FUCA STRAIT.
     SMALL CRAFT ADVISORY NORTHERN INLAND WATERS AND PUGET SOUND
     AND HOOD CANAL.

&&

$$

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