Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 302241 CCA
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 PM PDT TUE SEP 30 2014

.SYNOPSIS...A WEAK RIDGE WILL GRADUALLY BUILD TONIGHT WITH A
DECREASING CHANCE OF SHOWERS. THE NEXT SYSTEM SHOULD PASS TO THE
NORTH LEAVING WESTERN WASHINGTON DRY THROUGH THURSDAY. AN UPPER
RIDGE WILL HELP LIMIT RAIN TO THE NORTH COAST AND MOUNTAINS ON
FRIDAY. BY THE WEEKEND...A FRONT MAY BRING LIGHT RAIN TO SOME AREAS.
THE CHANCE OF LIGHT RAIN EXTENDS INTO EARLY NEXT WEEK AS A FRONT
LINGERS.

&&

.SHORT TERM...A SHORT WAVE RIDGE WILL TRAVERSE THE REGION LATER
TONIGHT AND WEDNESDAY MORNING. MOIST NORTHWESTERLY ONSHORE FLOW WILL
DECREASE TONIGHT IN RESPONSE. THE WEAKER FLOW WILL RESULT IN A
DIMINISHING THREAT OF SHOWERS EXCEPT FOR SOME LINGERING
PRECIPITATION IN THE CASCADES BY WEDNESDAY. NAM TIME-HEIGHTS SHOW
LOW CLOUDS ACROSS SOME AREAS DURING THE MORNING WHICH WILL LIMIT FOG
FORMATION. PARTLY SUNNY SKIES ARE LIKELY BY AFTERNOON WITH HIGHS
STILL STUCK IN THE LOW TO MID 60S.

DRY WEATHER SHOULD CARRY OVER INTO THURSDAY WITH THE UPPER RIDGE
BUILDING STRONGER OVER THE REGION. AREAS OF FOG ARE LIKELY TO
DEVELOP...ESPECIALLY ACROSS THE SW INTERIOR/S SOUND. HIGH CLOUDS
ASSOCIATED WITH A WARM FRONT LIFTING INTO B.C. SHOULD LIMIT FOG OVER
NORTHERN PORTIONS OF THE AREA.

MODELS HAVE FLIP FLOPPED SEVERAL TIMES ON THE STRENGTH OF A RIDGE
DURING THE FRIDAY PERIOD AND BEYOND. THE LATEST 12Z
GFS/ECMWF/CANADIAN AGREE FAIRLY WELL TODAY THROUGH THE FRIDAY
PERIOD. HIGH PRESSURE ALOFT SHOULD BEGIN TO FLATTEN ALLOWING
MOISTURE FROM A FRONTAL SYSTEM OVER B.C. TO DRIFT SWD TOWARD THE
BORDER. FRIDAY SHOULD GENERALLY REMAIN DRY BUT HIGH CLOUDS WILL
INFILTRATE THE AREA THROUGH THE DAY. THE GFS IS A BIT MORE
AGGRESSIVE WITH THE SYSTEM AND SHOWS SOME RAIN REACHING THE NORTH
COAST AND NORTH INTERIOR/CASCADES. THE ECMWF IS DRY AND THE MODEL OF
CHOICE BUT DID KEEP SOME LOW CHANCE POPS ALONG THE EXTREME N COAST/N
CASCADES. THE DAY COULD START OFF RATHER FOGGY AHEAD OF THE SYSTEM
BUT AFTERNOON MIXING WILL WARM UP THE INTERIOR WITH HIGHS NEAR 70.

.LONG TERM...THE FORECAST FOR THE WEEKEND AND EARLY NEXT WEEK
REMAINS IN FLUX. WHILE MODELS ARE IN BETTER AGREEMENT TODAY ON THE
LARGE SCALE FEATURES...THE LOCATION OF THE STALLED FRONTAL BAND IS
STILL UP IN THE AIR. NOTABLY...THE ECMWF HAS TRENDED TOWARD A DRY
SOLUTION KEEPING ALL THE RAIN CONFINED N OF THE BORDER UNTIL EARLY
NEXT WEEK. THE GFS DRAGS A DECAYING FRONT SWD INTO WRN WA ON
SATURDAY WHICH COULD PRODUCE LIGHT RAIN AT TIMES. MOST MODELS ARE
DRY ON SUNDAY BEFORE INDICATING SOME THREAT OF LIGHT RAIN RETURNING
EARLY NEXT WEEK. THE EXTENDED WAS ADJUSTED ACCORDINGLY WITH A HIGH
DEGREE OF UNCERTAINTY.  MERCER

&&

.AVIATION...THE UPPER TROUGH OVER THE PACNW WILL BE REPLACED BY A
WEAK UPPER RIDGE. ANOTHER UPPER TROUGH WILL MOVE THROUGH B.C. AND
ALBERTA WED NITE AND THU BUT HAVE LITTLE EFFECT ON WRN WA. THE FLOW
ALOFT WILL REMAIN WNW. THE LOW LEVEL FLOW WILL REMAIN WLY AND THE
AIR OVER WRN WA WILL REMAIN FAIRLY MOIST. SCATTERED SHOWERS ENDING
TONIGHT...MAINLY CASCADES.

KSEA...SCATTERED SHOWERS WILL END AND THERE MAY BE SOME CLEARING
PERIODS. THE LIGHT SW BREEZE SHOULD BECOME LIGHT AND VARIABLE THIS
EVENING.

&&

.MARINE...SURFACE HIGH PRESSURE OFFSHORE WITH LOWER PRESSURE INLAND
WILL RESULT IN WESTERLY FLOW THROUGH WED. THE HIGH WILL WORK EAST
OVER THE AREA WED NITE AND THU GIVING LIGHT GRADIENTS. A WARM FRONT
WILL BRUSH THE AREA LATER IN THE WEEK.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY CENTRAL AND EASTERN STRAIT OF JUAN DE FUCA.

&&

$$

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

















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