Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 230200 CCA
AFDSEW

AREA FORECAST DISCUSSION...CORRECTED
NATIONAL WEATHER SERVICE SEATTLE WA
600 PM PST MON DEC 22 2014

CORRECTED THE WATCHES/WARNINGS/ADVISORIES SECTION.

.SYNOPSIS...RAIN CHANCES WILL INCREASE ALONG THE COAST LATE TONIGHT
AS A PACIFIC FRONTAL SYSTEM APPROACHES THE AREA. PERIODS OF RAIN CAN
BE EXPECTED TUESDAY AND TUESDAY NIGHT AS THE FRONT MOVES SLOWLY
SOUTHEAST ACROSS THE AREA. AN UPPER LEVEL TROUGH WILL KEEP THE RAIN
GOING ON WEDNESDAY MORNING ALONG WITH COOLER CONDITIONS. DRIER
NORTHWEST FLOW ALOFT WILL GIVE COOLER WEATHER CHRISTMAS EVE THROUGH
CHRISTMAS DAY WITH ONLY A CHANCE OF SHOWERS. WEATHER SYSTEMS WILL
MOVE SOUTHEAST ACROSS THE AREA THURSDAY NIGHT THROUGH SATURDAY
GIVING SOME RAIN AT TIMES.

&&

.SHORT TERM...WEATHER PATTERN CHANGES ARE OCCURRING OVER THE PACIFIC
AND THE CHANGES ARE CAUSING DIFFICULTIES IN THE VARIOUS FORECAST
MODELS. THE MODELS CONTINUE TO HAVE A FAIRLY GOOD SENSE FOR TIMING
OF VARIOUS SHORT WAVE FEATURES AS THEY MOVE THROUGH WESTERN
WASHINGTON THROUGH THE WEEKEND. BUT THE MODELS CONTINUE TO BE HAVING
SIGNIFICANT PROBLEMS WITH THE DETAILS CONCERNING THE AMPLITUDE AND
EXACT POSITION OF A DEVELOPING BLOCKING RIDGE OFFSHORE. THE PROBLEMS
ARE BOTH MODEL-TO-MODEL AND RUN-TO-RUN. THERE APPEARS TO BE TWO
CULPRITS...A DEVELOPING SUBTROPICAL LOW NORTH OF HAWAII...AND A
DEEPENING LOW OVER THE FAR WESTERN BERING SEA. THESE TWO FEATURES
HAVE THE POTENTIAL TO DEVELOP A LARGE BLOCKING RIDGE ALONG ABOUT
145W...WHICH WOULD ALLOW MODIFIED ARCTIC AIR TO EVENTUALLY DROP
SOUTHWARD FROM THE YUKON EARLY NEXT WEEK...BUT IF THEY DO NOT
DEVELOP MUCH...COULD RESULT IN JUST HIGH PRESSURE AND COOL
CONDITIONS OVER THE AREA IN THE LONG TERM. THE DEVELOPMENT WILL ALSO
IMPACT THE STRENGTH AND TRACK OF SEVERAL SHORT WAVE
FEATURES...ESPECIALLY WEDNESDAY AND BEYOND...AS THEY DROP SE FROM
THE GULF OF ALASKA ONTO THE WEST COAST. CONTINUE TO EXPECT THE
FORECAST OVER THE NEXT COUPLE OF DAYS TO BE CHALLENGING...LOW
SKILL...AND DEPENDENT ON DETAILS THAT CANNOT BE RESOLVED WELL AT
THIS POINT.

A WARM FRONT SITS FROM OFF THE S COAST OF OREGON TO A 1003 MB LOW
NEAR 48N 143W. RAIN WITH THIS FRONT WILL MOVE ONTO THE COAST AROUND
OR SHORTLY AFTER 12Z THEN SHIFT INTO THE AREA DURING THE MORNING.
THE COLD FRONT WILL MOVE ACROSS THE AREA TUESDAY NIGHT. EXPECT ABOUT
AN INCH OF RAIN IN THE INTERIOR VALLEYS AND OVER AN INCH TO NEARLY
TWO INCHES ALONG THE COAST AND IN THE MOUNTAINS OVER A 24 HOUR
PERIOD ENDING 12Z WED. FREEZING LEVELS WILL RISE TO ABOUT 7000 FEET
IN THE WARM SECTOR TUESDAY.

THE 12Z AND 18Z MODEL SOLUTIONS BOTH COOL THE AIR MASS SIGNIFICANTLY
BEHIND THE FRONT LATE TUESDAY NIGHT INTO WEDNESDAY MORNING AS
PRECIPITATION CONTINUES TO FALL BEHIND THE FRONT. SNOW LEVELS FALL
TO AROUND 1500 FEET FOR A PERIOD WEDNESDAY MORNING. AT THIS TIME
SNOWFALL IS EXPECTED TO BE CONFINED TO THE MOUNTAIN ZONES WED
MORNING...BUT WE WILL NEED TO WATCH FOR THE POSSIBILITY THAT THE
LOWER LEVELS OF THE AIR MASS COOLS SLIGHTLY MORE AND SNOW LEVELS
OVER THE EASTERN PUGET SOUND LOWLANDS AND POSSIBLY THE SOUTH
INTERIOR DROP TO BELOW 1000 FT. AT THIS TIME FORECASTS WILL CALL FOR
A RAIN EVENT WITH TEMPERATURES JUST BELOW 40 DEGREES IN THAT AREA.

THE NAM12 CONTINUES TO SHOW A FEW SHOWERS WITH LOW SNOW
LEVELS...ABOUT 1000 FT AND POSSIBLY A BIT LOWER...CHRISTMAS MORNING.
LOW AND MID LEVEL FLOW WOULD INDICATE THAT THERE IS A CHANCE OF A
CONVERGENCE ZONE. WILL WAIT TO SEE THE WHITES OF THE EYES BEFORE
INCLUDING THIS IN THE FORECAST AT THIS TIME...WILL JUST INDICATE A
CHANCE OF SHOWERS WITH TEMPERATURES IN THE MID 30S.

A SHORT WAVE TROUGH IS EXPECTED TO DROP SE THROUGH THE AREA
CHRISTMAS EVENING. THIS FEATURE IS EXPECTED TO BE WEAK WITH MOST
PRECIPITATION FALLING ON THE COAST AND IN THE SOUTH INTERIOR. SNOW
LEVELS SHOULD BE NEAR 2000 FEET AS THIS FEATURE PASSES. ALBRECHT

.LONG TERM...THE 12Z AND 18Z GFS ALONG WITH THE ECMWF BRINGS A
STRONGER SHORT WAVE TROUGH THE AREA FRIDAY NIGHT OR SATURDAY
MORNING. ALL MODELS SHOW TEMPERATURES WITH THIS SYSTEM JUST ABOVE
SNOW CONCERNS IN THE LOWLANDS. STILL...THE FRIDAY NIGHT/SAT SYSTEM
MAY GIVE THE MOUNTAINS SOME MUCH NEEDED SNOWFALL. BEHIND FRIDAY
NIGHTS TROUGH...THE GFS BUILDS A RIDGE ALONG 140-145W THAT PROMISES
COOL AND DRY WEATHER FOR LATER IN THE WEEKEND THROUGH NEXT WEEK. THE
OPERATIONAL GFS APPEARS TO BRING COOL AIR IN QUICKLY FROM THE NORTH
WHILE THE PARALLEL RUN IS SLOWER AND STRONGER WITH A POTENTIAL
MODIFIED ARCTIC FRONT. THE CANADIAN SOLUTION WOULD GIVE A LITTLE
SNOW AND A STRONGER ARCTIC AIR MASS TO THE AREA. AT THIS POINT A
MODEL AVERAGE WAS USED THAT IS NOT AS COLD AS SOME MODEL SOLUTIONS.
IT WILL ALSO BE TURNING DRIER EARLY NEXT WEEK. ALBRECHT

&&

.HYDROLOGY...A PERIOD OF HEAVIER RAIN AND HIGH SNOW LEVELS ON
TUESDAY IS EXPECTED TO BE BRIEF ENOUGH TO NOT RESULT IN FLOODING
ACROSS WESTERN WASHINGTON AT THIS TIME.  FLOODING IS UNLIKELY LATER
WEDNESDAY THROUGH EARLY NEXT WEEK. ALBRECHT

&&

.AVIATION...HIGH PRESSURE ALOFT TONIGHT WILL FLATTEN ON TUESDAY. A
SURFACE WARM FRONT WILL MOVE INTO THE AREA LATE TONIGHT...FOLLOWED
BY A COLD FRONT ON TUESDAY AFTERNOON. AIR MASS SOMEWHAT MOIST AND
MOSTLY STABLE...BECOMING MOIST AT ALL LEVELS TUESDAY MORNING AS
WIDESPREAD RAIN DEVELOPS. MAINLY VFR CIGS TONIGHT WITH PREDOMINATELY
MVFR DEVELOPING TUESDAY.

KSEA...VFR THROUGH TONIGHT WITH MID/HIGH CLOUDS MORE DOMINATE. MVFR
CIGS SHOULD DEVELOP TUESDAY MORNING AS RAIN INCREASES ALONG A FRONT.

&&

.MARINE...WEST SWELL 10-13 FEET WILL CONTINUE OVER THE COASTAL
WATERS. WINDS GENERALLY LESS THAN 20 KT WITH HIGHER PRESSURE AT THE
SURFACE OVER THE WATERS THROUGH THIS EVENING.

A WARM FRONT WILL LIFT N ACROSS THE AREA LATER TONIGHT AND
TUESDAY...BRINGING SMALL CRAFT ADVISORY WINDS TO MOST AREAS EXCEPT
PUGET SOUND. ONSHORE FLOW STRENGTHENS BEHIND THE FRONT TUESDAY NIGHT
WITH STRONGER WESTERLIES IN THE STRAIT...NW WINDS IN ADMIRALTY AND
SWLY WINDS OVER PUGET SOUND POSSIBLY REACHING SMALL CRAFT.

SOME OF THE HIGHER ASTRONOMICAL HIGH TIDES OF THE YEAR WILL BE
OCCURRING THIS WEEK ALONG WITH SLIGHT TIDAL ANOMALIES. IT LOOKS LIKE
MINOR TIDAL OVERFLOW IS POSSIBLE FOR AREAS ALONG THE INLAND WATERS
AROUND 6 TO 7 AM TUESDAY MORNING...WITH A BETTER CHANCE OF TIDAL
OVERFLOW OCCURRING WITH EVEN HIGHER TIDES WEDNESDAY MORNING. TIDAL
OVERFLOW IS ALSO POSSIBLE ALONG THE BEACH WITH HIGH TIDES AROUND 100
PM TUESDAY AFTERNOON. A COASTAL FLOOD ADVISORY WAS ISSUED FOR BOTH
THE INLAND WATERS AND COAST...EXCLUDING AREAS ALONG THE STRAIT OF
JUAN DE FUCA.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...COASTAL FLOOD ADVISORY IN EFFECT FOR THE COAST AND
     SHORELINES BORDERING THE PUGET SOUND...ADMIRALTY INLET..AND
     NORTHERN INLAND WATERS FROM 10 AM TO 4 PM PST TUESDAY.

PZ...SMALL CRAFT ADVISORIES ARE IN EFFECT FOR THE COASTAL WATERS...
     ENTRANCES TO THE STRAIT OF JUAN DE FUCA...NORTHERN INLAND
     WATERS...AND ADMIRALTY INLET.

     SMALL CRAFT ADVISORY FOR ROUGH BAR IN EFFECT TIL 7 PM
     TUESDAY FOR THE GRAYS HARBOR BAR.

&&

$$

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