Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 231626
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
925 AM PDT TUE SEP 23 2014

.SYNOPSIS...AN UPPER LEVEL TROUGH WILL BRING SHOWERS TO WESTERN
WASHINGTON TODAY AS IT CROSSES THE AREA. A COLD FRONT WILL SPREAD
RAIN INLAND THIS EVENING WITH THE FRONT CROSSING THE INTERIOR BY
LATE 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 MOVE OVER THE REGION INTO THE
WEEKEND FOR A CHANCE OF SHOWERS.

&&

.SHORT TERM...THE DOMINANT WEATHER FEATURE THIS WEEK WILL BE THE
LARGE VERTICALLY STACKED UPPER LEVEL/SURFACE LOW CURRENTLY CENTERED
NEAR 45N/140W AT 15Z/8 AM. THE FIRST SHOT OF SHOWERS WILL BE TODAY
AS AN UPPER LEVEL SHORTWAVE TROUGH EMBEDDED IN THE FLOW AROUND THE
MAIN LOW MOVES NE ACROSS W WA. THIS SHORTWAVE IS A LITTLE HARD TO
PICK OUT FROM THE MESS OF CLOUD COVER OFFSHORE...BUT RADAR SHOWS AN
ASSOCIATED WELL DEVELOPED LINE OF SHOWERS ALONG THE WA AND N OREGON
COAST THAT WILL MOVE OVER THE W WA INTERIOR BEFORE NOON. A 110 KT
300 MB JET STREAK NOSING INLAND OVER W WA BY 18Z WILL STRENGTHEN THE
SHORTWAVE TROUGH AS IT MOVES ACROSS THE AREA.

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 SPREADING OVER THE INTERIOR LATER
THIS EVENING. FROPA ON THE COAST WILL OCCUR AFTER MIDNIGHT AND OVER
PUGET SOUND MID TO LATE WEDNESDAY MORNING. COMBINED QPF FOR BOTH
TODAYS SHORTWAVE TROUGH AND COLD FRONT 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
PROBABLY WORK OUT BEST FOR THIS PERIOD. MAX TEMPS WILL REMAIN CLOSE
TO NORMAL WITH HIGHS IN THE MID 60S TO NEAR 70. KAM

.LONG TERM...PREVIOUS DISCUSSION FROM THE 3 AM AFD...AN UPPER TROUGH
OVER THE PACIFIC NORTHWEST WILL WEAKEN ON FRIDAY FOR A CONTINUED
CHANCE OF SHOWERS. FOR THE WEEKEND...THERE WILL BE AN UPPER LOW WELL
TO THE SOUTH OF THE AREA OVER THE SOUTHWEST U.S. AND A SECOND UPPER
LOW OVER THE GULF OF ALASKA. THIS PUTS WESTERN WASHINGTON IN BETWEEN
MOST OF THE ACTIVE WEATHER WITH WEAK HIGH PRESSURE ALOFT FOR
GENERALLY DRY WEATHER. THE FLOW ALOFT WILL LIKELY BECOME MORE
CONSOLIDATED ON MONDAY WITH AN UPPER TROUGH CARVING OUT OVER THE
AREA FOR A BETTER CHANCE OR RAIN. SCHNEIDER

&&

.AVIATION...A BROAD UPPER LOW WILL REMAIN OFFSHORE NEAR 135 W
THROUGH WEDNESDAY. THE ASSOCIATED SURFACE FRONT WILL REACH THE COAST
THIS EVENING...THEN PUSH THROUGH PUGET SOUND AFTER MIDNIGHT. THE AIR
MASS WILL BECOME VERY MOIST TODAY WITH SHOWERS TRANSITIONING TO
STEADY MODERATE RAINFALL THIS AFTERNOON AND THROUGH TONIGHT. WEAKLY
UNSTABLE CONDITIONS...THEREFORE ISOLATED LIGHTNING STRIKES ARE
POSSIBLE BUT UNCERTAINTY IS TOO HIGH INTO INCLUDE IN TAFS.

KSEA...RADAR INDICATES WIDESPREAD SHOWERS/RAIN FILLING IN ACROSS THE
AREA AND MOVING TOWARD THE KSEA/KBFI TERMINALS...ARRIVING WITHIN THE
HOUR. MAINLY VFR CEILINGS OF 3K TO 5K FEET AS SHOWERS INCREASE...
THEN LOWERING TO MVFR IN THE AFTERNOON AND EVENING WITH MORE STEADY
RAIN DEVELOPING. THE TIMING OF RAIN AND LOWER CIGS MAY OCCUR A
LITTLE FASTER THAN EXPECTED AND WILL MAKE SOME ADJUSTMENTS WITH THE
18Z TAFS. S WIND TO 10 KT.

&&

.MARINE...A DEEP 973 MB LOW WILL REMAIN STALLED NEAR 135 W OFFSHORE
THEN MIGRATE NWD TONIGHT AND WEDNESDAY. THE LOW WILL TIGHTEN
PRESSURE GRADIENTS OVER THE COASTAL WATERS WITH SMALL CRAFT WINDS
THROUGH THE DAY.

THE MAIN FRONT WILL REACH THE COAST THIS EVENING AND PUSH THROUGH
THE INLAND WATERS LATE TONIGHT. GALE FORCE WINDS ARE EXPECTED OVER
THE COASTAL WATERS WITH SMALL CRAFT WINDS AROUND THE E ENTRANCE AND
N INTERIOR.

WINDS WILL EASE OVER THE INTERIOR ON WED BUT SMALL CRAFT WINDS 15 TO
30 KNOTS WILL PERSIST OVER THE COASTAL WATERS. LIGHTER FLOW IS
EXPECTED BY FRI AND THE WEEKEND.

&&

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

&&

$$

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