Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 221151
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
350 AM PST MON DEC 22 2014

.SYNOPSIS...AN UPPER LEVEL RIDGE WILL MOVE EAST OVER WESTERN
WASHINGTON TODAY FOR A PARTLY SUNNY AND DRY DAY. A FRONTAL SYSTEM
WILL SPREAD LIGHT RAIN PARTWAY INLAND LATE TONIGHT AND ACROSS THE
AREA TUESDAY AND TUESDAY NIGHT. AN UPPER LEVEL TROUGH WILL KEEP THE
RAIN GOING ON WEDNESDAY. DRIER NORTHWEST FLOW ALOFT WILL KEEP THE
AREA COOL AND MAINLY DRY LATE THIS WEEK.

&&

.SHORT TERM...A MODERATE AMPLITUDE UPPER LEVEL RIDGE CONTINUES TO
BUILD JUST OFFSHORE THIS MORNING. THE SURFACE HIGH ASSOCIATED WITH
THE UPPER LEVEL RIDGE WILL MOVE OVER W WA THIS MORNING...FOLLOWED BY
THE RIDGE AXIS EARLY THIS EVENING. DRIER NW FLOW ALOFT CONTINUES
OVER W WA WHICH HAS ALLOWED SKIES OVER THE LOWLANDS TO CLEAR MORE
THAN EXPECTED. RADAR STILL SHOWS SOME RESIDUAL ISOLATED SHOWERS OVER
THE CASCADES...BUT THESE SHOULD END LATER THIS MORNING. I WILL STICK
WITH THE PARTLY SUNNY FORECAST FOR TODAY AS BOTH THE MODELS AND
SATELLITE IMAGERY HAVE SOME MID/HIGH LEVEL CLOUDS MOVING OVER THE
AREA THIS AFTERNOON.

THE UPPER LEVEL RIDGE WILL WEAKEN AND FLATTEN AS IT MOVES INLAND
THIS EVENING...WHICH WILL EVENTUALLY ALLOW THE FRONTAL SYSTEM
CURRENTLY OFFSHORE ALONG 150W TO SPREAD RAIN INLAND LATE TONIGHT
THROUGH TUESDAY NIGHT. THE MODELS REMAIN CONSISTENT IN DRAPING THE
COLD FRONT ACROSS THE REGION ON TUESDAY...THEN MOVING IT SLOWLY
SOUTH  ACROSS W WA TUESDAY NIGHT. MODERATELY STRONG SW FLOW ALOFT AT
850 MB WILL PROBABLY BRING A SHORT PERIOD OF MODERATE TO HEAVY RAIN
TO THE OLYMPICS AND N CASCADES ON TUESDAY.

SNOW LEVELS WILL RISE TO 6000-7000 FEET TUESDAY...BUT WILL LOWER TO
AROUND 3000 FEET TUESDAY NIGHT AS THE COLD FRONT MOVES SLOWLY S.
DEPENDING UPON HOW LONG THE HEAVIER PRECIP LASTS OVER THE MOUNTAINS
TUESDAY AFTERNOON AND EVENING...WHEN SNOW LEVELS START TO
LOWER...THERE IS SOME POTENTIAL FOR A SHORT PERIOD OF HEAVY MOUNTAIN
SNOW. THE CURRENT FORECAST HAS THE HEAVIER PRECIP FALLING WHILE SNOW
LEVELS REMAIN HIGH...BUT THIS WILL BE MONITORED FOR CHANGES.

ON WEDNESDAY A MODERATE AMPLITUDE UPPER LEVEL SHORTWAVE TROUGH WILL
MOVE SE ACROSS THE PACIFIC NORTHWEST. THIS FEATURE WILL INVIGORATE
THE OLD COLD FRONT A LITTLE AND SHOULD KEEP LIGHT RAIN GOING THROUGH
THE DAY. MODEL SOLUTIONS HAVE BEEN EVOLVING A LITTLE AND NOW TAKE
THE STRONGEST PART OF THE TROUGH TO THE SOUTH ACROSS OREGON. THE
WEAKER NORTHERN PART OF THE TROUGH APPROACHES W WA WEDNESDAY...WITH
THE AXIS CROSSING THE AREA WEDNESDAY EVENING. SNOW LEVELS WILL BE
BETWEEN 2000-3000 FEET ON WEDNESDAY...BUT WITH THE WEAKER PART OF
THE TROUGH AFFECTING THE AREA MOUNTAIN SNOW ACCUMULATIONS WILL BE
LOWER.

COOLER AIR MOVING S BEHIND THE COLD FRONT WILL PUSH MAX TEMPS
WEDNESDAY INTO THE MID 40S. KAM

.LONG TERM...BOTH THE GFS AND ECMWF AGREE THAT DRIER NW FLOW ALOFT
WILL SET UP OVER W WA THURSDAY AND FRIDAY. I HAVE STUCK WITH THE
CURRENT DRY FORECAST...BUT THE MODELS HAVE BEEN TRENDING SOMEWHAT
WETTER...PROBABLY AS SMALL WEAK SHORTWAVES MOVE SE ACROSS THE AREA.
IF THE MODEL TREND HOLDS THEN WE WILL PROBABLY NEED TO PUT A CHANCE
OF RAIN BACK INTO THE FORECAST.

THE 06Z GFS HAS MIGRATED TO THE 00Z ECMWF SOLUTION WITH RESPECT TO A
STRONGER UPPER LEVEL TROUGH THAT MAY AFFECT THE AREA FRIDAY NIGHT
AND SATURDAY. THE GFS SHOWING A STRONGER SHORTWAVE THAN THE
ECMWF...AND THE TIMING OF THE TROUGH IS STILL IN QUESTION. KAM

&&

.HYDROLOGY...FLOODING IS UNLIKELY OVER THE NEXT 7 DAYS.

&&

.AVIATION...NORTHWEST FLOW ALOFT WITH AN UPPER RIDGE BUILDING OVER
THE NORTHEAST PACIFIC TODAY AND TONIGHT. A WARM FRONT WILL MOVE INTO
THE AREA TONIGHT. AT THE SURFACE...WEAK HIGH PRESSURE WILL MOVE OVER
WESTERN WASHINGTON TODAY WITH INCREASING SOUTHERLY FLOW TONIGHT. AIR
MASS SOMEWHAT MOIST AND STABLE. AREAS OF LOW CLOUDS THIS MORNING
WITH SOME IMPROVEMENT LIKELY THIS AFTERNOON AS THE LOW LEVELS DRY.

KSEA...EXPECT LOW CLOUDS TO FILL IN THIS MORNING...THEN BREAK UP
THIS AFTERNOON. LIGHT WIND. SCHNEIDER

&&

.MARINE...WEAK HIGH PRESSURE WILL MOVE OVER WESTERN WASHINGTON TODAY
WITH EASING WINDS. WEST SWELL 10-13 FEET WILL CONTINUE OVER THE
COASTAL WATERS.

A FRONTAL SYSTEM WILL MOVE THROUGH THE WATERS TONIGHT AND
TUESDAY...BRINGING SMALL CRAFT ADVISORY WINDS TO MOST AREAS. A
TRAILING TROUGH OR SURFACE LOW WILL AFFECT THE WATERS WEDNESDAY
ALTHOUGH MODELS ARE NOT IN GOOD AGREEMENT WITH THE DETAILS ABOUT
THIS FEATURE.

SOME OF THE HIGHER ASTRONOMICAL HIGH TIDES OF THE YEAR WILL BE
OCCURRING THIS WEEK ALONG WITH SLIGHT TIDAL ANOMALIES DUE TO PASSING
WEATHER SYSTEMS. IT LOOKS LIKE MINOR TIDAL OVERFLOW IS POSSIBLE FOR
SOME LOCATIONS AROUND PUGET SOUND EARLY TUESDAY MORNING WITH A
BETTER CHANCE EARLY WEDNESDAY MORNING. SCHNEIDER

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY CENTRAL AND EAST ENTRANCE STRAIT OF JUAN
     DE FUCA.
    .SMALL CRAFT ADVISORY FOR HAZARDOUS SEAS COASTAL WATERS AND
     WEST ENTRANCE STRAIT OF JUAN DE FUCA.
    .SMALL CRAFT ADVISORY FOR ROUGH BAR GRAYS HARBOR BAR.

&&

$$

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