Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 160405
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
900 PM PST MON DEC 15 2014

.SYNOPSIS...A WEAK FRONTAL SYSTEM WILL BRING AREAS OF LIGHT RAIN TO
WESTERN WASHINGTON TONIGHT AND TUESDAY...AND THEN ANOTHER WEAK
SYSTEM WILL MOVE THROUGH THE AREA ON WEDNESDAY. A SOMEWHAT STRONGER
FRONTAL SYSTEM WILL ARRIVE THURSDAY AND THURSDAY NIGHT...WITH A
WETTER AND WARMER PATTERN DEVELOPING THIS WEEKEND.

&&

.SHORT TERM...SOUTHERLY FLOW ALOFT PREVAILS OVER THE PACIFIC
NORTHWEST THIS EVENING...BETWEEN THE UPPER RIDGE MOVING EAST OF THE
ROCKY MOUNTAINS AND THE UPPER TROUGH OFFSHORE. THE FIRST SHORTWAVE
AT THE LEADING EDGE OF THE TROUGH IS SPREADING AREAS OF LIGHT RAIN
INTO WESTERN WASHINGTON...MAINLY AT THE COAST AND OVER THE OLYMPIC
PENINSULA.

THIS INITIAL FEATURE WILL WEAKEN AS IT MOVES INLAND TONIGHT AND
TUESDAY...BUT IT WILL BRING LIGHT PRECIPITATION TO MUCH OF THE
FORECAST AREA LATER TONIGHT OR TUESDAY. ANOTHER WEAK SYSTEM WILL
MOVE THROUGH THE AREA ON WEDNESDAY...FOLLOWED BY A WEAK UPPER RIDGE
ON THURSDAY.   MCDONNAL

.LONG TERM...HERE IS THE LONG TERM SECTION FROM THE AFTERNOON
FORECAST DISCUSSION -- THE MEDIUM RANGE SOLUTIONS WERE IN GENERAL
AGREEMENT THAT A WET AND UNSETTLED PATTERN WILL PREVAIL ACROSS THE
PACIFIC NORTHWEST DURING THIS PERIOD. THE MOUNTAINS MIGHT RECEIVE
SIGNIFICANT SNOWFALL FROM THE SATURDAY SYSTEM.

BEYOND SATURDAY...IT APPEARED THAT A MILD AND QUITE MOIST SYSTEM
WILL IMPACT THE REGION ON SUNDAY. RELATIVELY HIGH SNOW LEVELS
COMBINED WITH STRONG UPSLOPE FLOW WILL RESULT IN THE POTENTIAL FOR
HEAVY RAIN ON THE COAST AND IN THE MOUNTAINS ON SUNDAY. UNLIKE THE
LAST SIGNIFICANT RAIN EVENT...THE FLOW WILL BE WESTERLY AND THIS
WOULD HAVE MORE OF AN IMPACT ON THE CASCADES.

THE MODELS BEGAN TO DIVERGE FOR MON...WITH THE ECMWF KEEPING
STRONG...MOIST AND MILD WESTERLY FLOW OVER THE PACIFIC NORTHWEST.
THIS WOULD MAINTAIN THE THREAT OF SIGNIFICANT RAINFALL FOR THE
CWA. THE GFS...ON THE OTHER HAND...WAS DRIER AND HAD LOWER 500 MB
HEIGHTS. THE FORECAST REFLECTED A COMPROMISE BETWEEN THESE TWO
SOLUTIONS.

&&

.HYDROLOGY...RIVER FLOODING IS NOT EXPECTED THROUGH AT LEAST
SATURDAY. A WETTER AND WARMER PATTERN WILL PROBABLY DEVELOP SUNDAY
AND MONDAY...AND SOME MODEL RUNS SHOW ENOUGH RAIN FOR A THREAT OF
FLOODING ON THE MOST FLOOD-PRONE RIVERS.   MCDONNAL

&&

.AVIATION...AN UPPER LEVEL TROUGH OVER THE OFFSHORE WATERS WILL
PRODUCE SOUTH TO SOUTHWEST FLOW ALOFT OVER WESTERN WASHINGTON
THROUGH TUESDAY. LOW LEVEL OFFSHORE FLOW CONTINUES WITH SURFACE HIGH
PRESSURE CENTERED OVER SOUTHERN BRITISH COLUMBIA AND A BROAD TROUGH
OFFSHORE. THE AIR MASS IS STABLE WITH MAINLY MID AND UPPER LEVEL
MOISTURE OVER WESTERN WASHINGTON. THE LOWER LEVELS ARE MAINLY DRY.
CEILINGS WILL GRADUAL LOWER WITH AREAS OF VERY LIGHT RAIN MOVING
INTO THE REGION. CEILINGS ARE EXPECTED TO REMAIN VFR...EXCEPT NEAR
THE COAST WHERE THEY COULD BECOME HIGH END MVFR AT TIMES IN LIGHT
RAIN OVERNIGHT AND TUESDAY MORNING.

KSEA...VFR. CEILING COULD LOWER TO BKN-OVC040 AT TIMES AFTER 09Z
WITH AREAS OF VERY LIGHT RAIN. EAST TO SOUTHEAST WIND 8-15 KNOTS
EASING TO 5-10 KNOTS OVERNIGHT.

&&

.MARINE...OFFSHORE FLOW TONIGHT WILL GRADUALLY EASE THROUGH TUESDAY
AS A TROUGH OF LOW PRESSURE OFFSHORE MOVES INTO THE WATERS AND
WEAKENS. SMALL CRAFT ADVISORY CONDITIONS WILL CONTINUE OVER THE
COASTAL WATERS AND THROUGH THE STRAIT OF JUAN DE FUCA TONIGHT...THEN
WILL BECOME LIMITED TO THE COASTAL WATERS TUESDAY. WEAKER PRESSURE
GRADIENTS WILL GIVE LIGHT FLOW ON WEDNESDAY.

A COLD FRONT WILL APPROACH THE WATERS LATER THURSDAY AND THURSDAY
NIGHT AND MOVE THROUGH THE REGION EARLY FRIDAY. GALE FORCE WINDS
ARE POSSIBLE WITH THIS FRONT...MAINLY OVER THE COASTAL WATERS.
A STRONGER SYSTEM ON SATURDAY COULD PRODUCES GALES OVER THE NORTHERN
INLAND WATERS AND STRAIT ENTRANCES IN ADDITION TO THE COASTAL
WATERS. 27

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY COAST...STRAIT OF JUAN DE FUCA.
     SMALL CRAFT ADVISORY FOR ROUGH GRAYS HARBOR BAR.

&&

$$

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




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