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000
FXUS66 KSEW 082351
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
350 PM PST SUN DEC 8 2013

.SYNOPSIS...WEAK ONSHORE FLOW WILL START TO DEVELOP LATE MONDAY...
PUSHING A LITTLE PACIFIC MOISTURE INTO OUR COLD AIR MASS...FOR SOME
LIGHT SNOW...FREEZING DRIZZLE AND AN EVENTUAL TURNOVER TO LIGHT RAIN
BY TUESDAY AFTERNOON. TEMPERATURES WILL RISE TO NEAR NORMAL
WEDNESDAY. A PACIFIC FRONT WILL BRING MORE TYPICAL RAIN AND WINDS
THURSDAY...FOLLOWED BY ANOTHER PACIFIC FRONT SATURDAY.

&&

.SHORT TERM...THE FORECAST CHALLENGE TODAY IS DETERMINING THE PHASE
AND TYPE OF PRECIP ON MON AND TUE AS WRN WA TRANSITIONS OUT OF THE
COLD AND DRY CONTINENTAL POLAR AIR MASS THAT IS IN PLACE RIGHT NOW.
BEFORE DIVING INTO SPECIFICS OF THE FORECAST...HERE IS A QUICK
WEATHER LESSON. IT IS POSSIBLE TO GET LIQUID FALLING FROM THE
SKY...EVEN WHEN THE ENTIRE ATMOSPHERIC SOUNDING IS BELOW FREEZING.
THE KEY IS THAT PRECIP HAS TO BE GENERATED IN A LAYER THAT IS -5C OR
WARMER...IN WHICH CASE SUPERCOOLED WATER DROPLETS ARE FORMED...
RATHER THAN SNOWFLAKES. WHEN THESE CONTACT A SURFACE THAT IS BELOW
FREEZING...IT WILL FREEZE UPON CONTACT AND BE OBSERVED AS FREEZING
DRIZZLE OR FREEZING RAIN. THIS BECOMES A CONCERN IN A SETUP LIKE MON
AND TUE...WHERE PRECIP IS BEING GENERATED AT FAIRLY LOW AND
RELATIVELY WARM LAYERS OF THE ATMOSPHERE...EVEN IF THOSE LAYERS ARE
A LITTLE BELOW FREEZING.

OKAY...ON MONDAY THE UPPER RIDGE OUT AROUND 140W WILL START TO
FLATTEN...BACKING THE FLOW ALOFT FROM DUE NORTH TO NW. THIS WILL
INITIATE WEAK ONSHORE FLOW ABOVE THE LOW-LEVEL COLD AIR. MOISTURE
WILL RAPIDLY INCREASE FROM 850 MB DOWN TO THE SFC ON MON...AND WEAK
WARM ADVECTION LIFT WILL LEAD TO LIGHT PRECIP OUT OF THIS LOW-LEVEL
MOIST LAYER. LIGHT PRECIP WILL FIRST DEVELOP ON THE COAST DURING THE
DAY ON MON...THEN SPREAD INLAND ON MON EVNG. THE IMMEDIATE COAST OUT
NEAR OCEAN SHORES AND WESTPORT WILL LIKELY POP ABOVE FREEZING PRETTY
QUICK ON MON...MAKING MERE DRIZZLE OR LIGHT RAIN PREDOMINANT THERE.
BY GOING INLAND AS FAR AS ABERDEEN...BELOW FREEZING TEMPERATURES
WILL LIKELY SUPPORT A MIX OF LIGHT SNOW AND FREEZING DRIZZLE UNTIL
THE SURFACE AIR TEMPERATURE RISES ABOVE FREEZING ON MON EVNG OR
EARLY TUE MRNG. FURTHER INLAND ALONG THE I-5 CORRIDOR...THE STRETCH
FROM OLYMPIA SOUTH THROUGH LEWIS COUNTY IS LOOKING FAVORABLE FOR
FREEZING DRIZZLE ON MON NGT INTO TUE AFTN. PRECIP FROM TACOMA TO
EVERETT IS BEING GENERATED IN A LAYER THAT WILL BE COLD ENOUGH FOR A
MIX OF FREEZING DRIZZLE AND LIGHT SNOW. NORTH OF EVERETT...LIGHT
SNOW WILL BE THE DOMINANT PRECIP TYPE THROUGH TUE MRNG...AND
FREEZING DRIZZLE WILL BE MORE PATCHY. FOR THE I-5 CORRIDOR FROM
OLYMPIA NORTH...AIR TEMPS WILL RISE ABOVE FREEZING AROUND MID-DAY ON
TUE...ENDING THE THREAT OF FZDL. SOUTH OF OLYMPIA...SUB-FREEZING AIR
MAY HANG ON A BIT LONGER...PERHAPS TIL LATE AFTN OR EVNG.

WANT TO EMPHASIZE THAT PRECIP ON MON AND TUE IS EXPECTED TO BE
LIGHT...WITH LIQUID EQUIVALENTS OF ONE-TENTH OF AN INCH OR LESS.
SNOWFALL AMOUNTS SHOULD BE AN INCH OR LESS. AS FOR THE FREEZING
PRECIP...IT WILL ALSO BE LIGHT...BUT A LITTLE FREEZING PRECIP CAN GO
A LONG WAY WHEN IT FREEZES ON UNTREATED ROADWAYS...SIDEWALKS AND
OTHER SURFACES. HAVE INTRODUCED FREEZING DRIZZLE TO THE FORECAST FOR
MOST OF WESTERN WASHINGTON WITH THIS FORECAST ISSUANCE. IT WILL
MAINLY BE AN ASPECT DURING THE GRADUAL TRANSITION FROM LIGHT SNOW TO
LIGHT RAIN. OF NOTE IS THAT SOME OF THE LIGHTEST QPF IS OVER THE
KITSAP PENINSULA...WHERE A LITTLE DOWNWIND SHADOWING FROM THE
OLYMPICS WILL KEEP PRECIP EVEN LIGHTER YET.

ONSHORE FLOW WILL WEAKEN ON TUE NGT AND WED MRNG...SO PRECIP WILL
TAPER OFF. A WARM FRONT WILL APPROACH THE B.C. COAST ON WED. AN
INCREASE IN 850 MB WINDS TO 20 KT WILL HELP SCOUR OUT ANY REMAINING
HOLDOUTS OF COLD AIR. TEMPERATURES WED WILL RETURN CLOSE TO NORMAL
FOR THIS TIME OF YEAR.     HANER

.LONG TERM...EXTENDED MODELS SHOW A WARM FRONT SPREADING ACROSS WRN
WA ON THU...WITH A RETURN TO MORE ORDINARILY RAINY AND BREEZY
WEATHER. A LULL IN THE WEATHER WILL TAKE PLACE FRI. A PACIFIC COLD
FRONT WILL BRING ANOTHER ROUND OF RAIN...WIND AND MOUNTAIN SNOW ON
SAT...FOLLOWED BY POST-FRONTAL SHOWERS NEXT SUNDAY.     HANER

&&

.HYDROLOGY...RIVER FLOODING IS NOT EXPECTED FOR THE NEXT 7 DAYS.

&&

.AVIATION...STRONG NLY FLOW ALOFT WILL CONTINUE OVER THE REGION THRU
TONIGHT. VERY WEAK LOW LEVEL ONSHORE FLOW. ANTICIPATE VFR CONDITIONS
TODAY...WITH FLURRIES POSSIBLE MAINLY ON THE COAST. AREAS OF MVFR
CIGS /1-2K FT/ WILL DEVELOP TONIGHT...MAINLY AFTER 0600 UTC.

KSEA...VFR CONDITIONS WILL PREVAIL OVER THE TERMINAL. WINDS WILL BE
LIGHT.

&&

.MARINE...
A 1028 MB LOW OVER BRITISH COLUMBIA WILL RESULT IN LIGHT SOUTHERLY
FLOW OVER THE AREA THROUGH TONIGHT. EXPECT A WEAK SURFACE TROUGH TO
MOVE ACROSS THE COASTAL WATERS ON MON FOR CONTINUED WEAK ONSHORE
FLOW. A WEAK FRONTAL SYSTEM WILL MOVE ACROSS THE AREA ON TUE.

&&

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

&&

$$

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