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000
FXUS66 KSEW 182249
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
348 PM PDT SAT MAY 18 2013

.SYNOPSIS...AN UPPER TROUGH WILL PASS ACROSS WESTERN WASHINGTON THIS
EVENING...ACCOMPANIED BY A FEW SHOWERS. AN UPPER RIDGE WILL START TO
BUILD IN ON SUNDAY...THEN CROSS THE AREA ON MONDAY...LEADING TO
WARMER AND DRIER WEATHER. A LARGE UPPER LOW WILL DROP INTO THE AREA
ON TUESDAY...CAUSING A STRONG COLD FRONT TO PUSH INLAND DURING THE
DAY. THE UPPER LOW WILL SIT IN PLACE THROUGH THE END OF THE
WEEK...MAINTAINING SHOWERY WEATHER AND BELOW NORMAL TEMPERATURES.

&&

.SHORT TERM...A WEAK UPPER TROUGH AXIS IS MOVING ONSHORE...WITH
SCATTERED SHOWERS AND CLOUDS PRECEDING THE TROUGH AXIS. ONCE THE
TROUGH AXIS MOVES EAST OF THE CASCADES LATE TONIGHT...UPPER HEIGHTS
WILL RISE QUICKLY AND SHOWER ACTIVITY WILL DIMINISH GREATLY. IN
FACT...VISIBLE SATELLITE SHOWS RAPIDLY CLEARING SKIES OFFSHORE
BEHIND THE TROUGH AXIS. ON SUNDAY OVER THE INTERIOR
LOWLANDS...LOOKING FOR A LARGELY DRY SUNDAY. A SUBSIDENCE INVERSION
WILL DEVELOP AROUND 8000 FT MSL...SO ANY SHOWER ACTIVITY WILL BE
LIGHT AND SPOTTY AT MOST. IN FACT...WOULD EXPECT SHOWERS ON SUNDAY
AFTN TO BECOME CONFINED TO THE CASCADES...FOOTHILLS AND THE
PERIMETER OF THE OLYMPICS. A NORTH WIND WILL KICK IN THROUGH THE
PUGET SOUND LOWLANDS IN THE AFTN...A FAIR WEATHER DIRECTION FOR THE
LOWLANDS.

THE UPPER HEIGHT RISES WILL SET THE STAGE FOR AN UPPER RIDGE AXIS TO
PASS ACROSS WRN WA ON MON...MAKING MONDAY THE WARMEST AND DRIEST OF
THE NEXT 7 DAYS FOR WRN WA. AFTER A COOL START OWING TO MOSTLY CLEAR
SKIES AT SUNRISE...INTERIOR LOWLANDS WILL RISE INTO 70S ON MON AFTN.

ON TUE...A BIG CHANGE TOWARD COOL AND SHOWERY WEATHER WILL TAKE
PLACE. A HIGH AMPLITUDE UPPER TROUGH WILL SETUP NEAR 130W...BRINGING
A STRONG MARINE PUSH AND COLD FRONT THROUGH WRN WA DURING THE DAY.
AFTER A MILD START TO THE DAY...AFTN TEMPS WILL FALL IN MOST
PLACES...AND SHOWERS WILL QUICKLY BECOME WIDESPREAD. AN UNSEASONABLY
COOL AIR MASS WILL OVERSPREAD WRN WA. BY TUE NIGHT...850 MB TEMPS OF
-1C TO +1C WILL BE COMMON...AND SNOW LEVELS COULD FALL AS LOW AS
3000 FT.        HANER

.LONG TERM...ON WED...THE UPPER TROUGH WILL MORPH INTO A CUTOFF
LOW...DUE TO THE DEVELOPMENT OF AN UPPER RIDGE OVER THE YUKON AND
NRN B.C.. THIS WILL EFFECTIVELY BRING A BLOCKING PATTERN AND CUT THE
UPPER LOW OFF FROM ANY STEERING FLOW WHICH COULD PUSH IT OUT.
SO...WE WILL BE STUCK WITH THE UPPER LOW FROM WED THROUGH THE REST
OF THE NEW WEEK. SAFE TO SAY THAT A SHOWERY WEATHER REGIME WILL BE
IN PLACE WITH BELOW NORMAL TEMPERATURES. THIS PATTERN COULD ALSO
BRING A FEW INCHES OF SNOW TO THE HIGHER PASSES...SUCH AS RAINY PASS
AND CHINOOK PASS...WHICH HAVE ALREADY RE-OPENED FOR THE SEASON.
MODELS SHOW THE UPPER LOW BECOMING MORE RAG-TAG NEXT FRI AND SAT AS
IT SLOWLY FILLS AND THE LOW-LEVEL AIR MASS EVER SO SLOWLY MODIFIES.
TEMPS WILL SLOWLY RISE BACK TO WITHIN A FEW DEGREES OF NORMAL BY DAY
7...AND SHOWER COVERAGE MAY DECREASE. ONE CAVEAT FOR THU THROUGH SAT
IS THAT IF THE UPPER LOW CENTER MEANDERS FURTHER SOUTH INTO NW
ORE...IT WILL OPEN UP THE DOOR TO MORE WIDESPREAD DEFORMATION-STYLE
RAINFALL SPREADING IN FROM THE EAST. HAVE SEEN THIS IN PAST
MAYS...AND IT IS ONE WAY OF BRINGING A LIGHT ALL-DAY RAIN TO WRN WA
THIS TIME OF YEAR. HOWEVER...DETAILS OF INDIVIDUAL WAVES OR ROUNDS
OF SHOWERS WITHIN THE UPPER LOW ARE IMPOSSIBLE TO PIN DOWN 5-7 DAYS
IN THE FUTURE.   HANER

&&

.AVIATION...SCATTERED SHOWERS ARE OVER THE AREA AS A WEAK FRONT
BREAKS UP OVER THE REGION. THE AIR MASS IS MOIST AND SOMEWHAT
UNSTABLE WITH FAIRLY LIGHT SW FLOW ALOFT BECOMING NW TONIGHT. THE
MOUNTAINS AND FOOTHILLS ARE MOSTLY OBSCURED. MORNING CLOUDS WILL
GIVE WA TO AFTERNOON SUNSHINE SUNDAY.

KSEA...SCATTERED SHOWERS WILL CONTINUE THROUGH THE EVENING AS A
FRONT BREAKS UP OVER THE AREA. THE SHOWER ACTIVITY WILL END
OVERNIGHT AS THE WEAK UPPER TROUGH IS REPLACED BY A RIDGE THROUGH
SUNDAY. LIGHT PRESSURE GRADIENTS...A LIGHT AND VARIABLE WIND.

&&

.MARINE...A COLD FRONT IS BREAKING UP OVER THE COASTAL WATERS THIS
AFTERNOON. NW FLOW WILL EVENTUALLY INCREASE BEHIND THE FRONT...BUT
IT DOES NOT LOOK LIKELY THAT THE CENTRAL AND EASTERN STRAIT OF JUAN
DE FUCA WILL SEE MORE THAN ABOUT 20KT WLY WINDS LATER THIS EVENING.
HIGH PRES OVER THE NE PAC WILL MAINTAIN NW FLOW SUN AND MON. ANOTHER
FRONT WILL ARRIVE FOR TUE AND WED.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...NONE.

&&

$$

YOU CAN SEE AN ILLUSTRATED VERSION OF THE FORECAST DISCUSSION AT
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