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000
FXUS66 KSEW 210423
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
925 PM PDT MON MAY 20 2013

.SYNOPSIS...A COLD FRONT WILL MOVE THROUGH WESTERN WASHINGTON
TONIGHT AND TUESDAY MORNING. A COLD UPPER LEVEL LOW PRESSURE SYSTEM
WILL LINGER OVER THE PACIFIC NORTHWEST THROUGH THE WEEK. THE UPPER
LOW WILL GIVE A COOL SHOWERY WEATHER PATTERN INTO THE WEEKEND.

&&

.SHORT TERM...RAIN WILL CONTINUE TO SPREAD INLAND OVER WESTERN WA
TONIGHT. THIS FRONT WILL BE OVER WESTERN WA THROUGH TUE
MORNING...THEN THE PRECIP TURNS MORE SHOWERY TUESDAY AFTERNOON.
THERE COULD BE SOME AFTERNOON SUN BREAKS AFTER THE FRONT PASSES.
THE UPPER LEVEL LOW WILL WOBBLE AROUND OVER THE PACNW ALL WEEK. THIS
IS A SHOWERY PATTERN WITH HIGH POPS EACH PERIOD. THE AIR WILL BECOME
UNSTABLE...AT FIRST THE COLD AIR ALOFT IS DIRECTED INTO OREGON...FOR
INSTANCE THE GFS SHOWS THE TEMP OVER OREGON AT 700MB TUE NIGHT TO BE
-10 TO -14C OVER A GOOD PORTION OF OREGON. BY LATER WED AFTERNOON
THAT PUDDLE OF COLD AIR ALOFT HAS SPREAD OUT AND A GOOD PART OF BOTH
ORE AND WA ARE AROUND -10C. AT 500MB IT COOLS OFF TO AROUND -30C
OVER WESTERN WA WED AFTERNOON...RATHER COLD AIR ALOFT FOR THIS TIME
OF YEAR.

.LONG TERM...PREVIOUS DISCUSSION...ONLY MINOR DIFFERENCES BETWEEN
THE EURO AND GFS IN THAT BOTH KEEP AN UPPER LOW IN THE AREA. THE
EURO KEEPS IT MORE OR LESS OVER WESTERN WASHINGTON THROUGH MONDAY
WHILE THE GFS SPINS THE ORIGINAL LOW AWAY AND DEVELOPS A NEW ONE
OVER THE AREA. IN BOTH MODELS HEIGHTS SLOWLY RISE INTO THE 560S.
HAVE A BROAD BRUSH FORECAST FOR THE ENTIRE PERIOD FRIDAY THROUGH
MONDAY WITH CHANCE POPS AND HIGHS IN THE UPPER 50S TO MID 60S. BURKE

&&

.AVIATION...A DEEP UPPER TROUGH WILL SETTLE OVER THE PAC NW LATER
TONIGHT THROUGH MID WEEK. THE ASSOCIATED 1011 MB SURFACE LOW WILL
MOVING INTO OREGON LATE TONIGHT WITH THE NWD EXTENDING FRONT PUSHING
THROUGH THE WA WATERS. BY EARLY TUE MORNING...THE FRONT WILL PUSH
THROUGH THE INTERIOR WATERS. THIS WILL CAUSE STRONGER ONSHORE FLOW
ACROSS WRN WA. LIGHT SW FLOW ALOFT WILL BECOME LIGHT TO MODERATE WLY
FLOW. IT WILL BECOME MOIST AT ALL LEVELS ACROSS WRN WA LATER TONIGHT
AND TUE. RAIN DEVELOPS TONIGHT...THEN SHOWERS TUE. UNSTABLE
CONDITIONS TUE...ESPECIALLY AFTERNOON WITH ISOLATED THUNDERSTORMS
POSSIBLE. PROBABILITY TOO LOW TO PUT IN FORECAST.

PRECIPITATION ALREADY REACHED THE COAST THIS EVENING AND IS
SPREADING INLAND FASTER THAN EXPECTED. ADJUSTED TAFS FOR QUICKER
ARRIVAL OF RAINFALL AFTER MIDNIGHT TONIGHT WITH CONDITIONS
DETERIORATING ACROSS THE FORECAST AREA TO MVFR AND IFR. MARGINAL
IMPROVEMENT IS POSSIBLE LATE TUESDAY.

KSEA...N/NW WINDS QUICKLY SURGED SWD REACHING KSEA/KBFI AND THE
TACOMA AREA EARLY THIS EVENING. N WINDS WILL SWITCH AROUND EARLY TUE
MORNING AS RAIN INCREASES. IT APPEARS THE INITIAL ROUND OF RAINFALL
WILL REACH THE AREA FASTER THAN EXPECTED...BY AROUND 1-2 AM
TUE...AND CONDITIONS WILL LIKELY DETERIORATE TO MVFR OCCASIONALLY
IFR. DTM

&&

.MARINE...ONSHORE FLOW WILL INCREASE TONIGHT AND CONTINUE TUE AS LOW
PRESSURE MOVES INLAND S OF WRN WA. SMALL CRAFT WINDS WILL CONTINUE
THROUGH THE CENTRAL/E STRAIT OF JUAN DE FUCA AND WILL AFFECT THE SRN
PORTION OF THE NRN INLAND WATERS AND N PORTION OF ADMIRALTY INLET.
BY TUESDAY...MARGINAL SMALL CRAFT WINDS WILL BE LIMITED TO THE
CENTRAL/E STRAIT. NWLY ONSHORE FLOW WILL INCREASE OVER MOST THE
COASTAL WATERS TO WARRANT A SMALL CRAFT TUESDAY.

ANOTHER LOW WILL PROBABLY MOVE SOUTHEAST ACROSS THE WASHINGTON
OFFSHORE WATERS WEDNESDAY THROUGH THURSDAY. DTM

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY CENTRAL AND EAST STRAIT OF JUAN DE FUCA
     THROUGH TUESDAY AFTERNOON...
     SMALL CRAFT ADVISORY FOR ADMIRALTY INLET AND THE NORTHERN
     INLAND WATERS THROUGH 5 AM PDT TUESDAY.
     SMALL CRAFT ADVISORY COAST...EXCEPT THE NRN INNER
     PORTION...FROM 5 AM PDT TUESDAY THROUGH THE AFTERNOON.

&&

$$

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