Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 252220
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
320 PM PDT THU SEP 25 2014

.SYNOPSIS...AN UPPER LEVEL LOW JUST OFFSHORE WILL MOVE SLOWLY EAST
TOWARD WESTERN WASHINGTON THROUGH FRIDAY...KEEPING SKIES MOSTLY
CLOUDY WITH SCATTERED SHOWERS. THERE IS A CHANCE OF THUNDERSTORMS
ACROSS THE AREA THROUGH EARLY THIS EVENING. WEAK HIGH PRESSURE WILL
BUILD OVER THE AREA THIS WEEKEND FOR PARTLY TO MOSTLY SUNNY SKIES. A
WEAK COLD FRONT WILL BRING SOME RAIN MONDAY. COOLER NORTHWEST FLOW
ALOFT WILL MAINTAIN MOSTLY CLOUDY SKIES AND A CHANCE OF SHOWERS
THROUGH MID NEXT WEEK.

&&

.SHORT TERM...A LARGE UPPER LEVEL LOW REMAINS JUST OFFSHORE AND WILL
CONTINUE WEAKENING WHILE MOVING SLOWLY TOWARD THE WA COAST THROUGH
FRIDAY...FINALLY MOVING INLAND FRIDAY NIGHT. A WEAK SHORTWAVE TROUGH
SWINGING AROUND THE SE SIDE OF THE MAIN LOW THIS AFTERNOON HAS BEEN
BRINGING THE REMNANTS OF THE OLD BACK-BENT OCCLUSION TOWARD THE
COAST. THE AIR MASS OVER THE COASTAL WATERS AND OVER W WA IS
UNSTABLE WITH LIFTED INDICES 0 TO -2 AND CAPE AROUND 500 J/KG. THERE
HAVE BEEN ENOUGH BREAKS IN THE CLOUD COVER THIS AFTERNOON FOR TEMPS
TO WARM INTO THE UPPER 60S TO NEAR 70...WARMER THAN THE CONVECTIVE
TEMPERATURE. NO LIGHTNING HAS SHOWN UP YET OVER LAND...BUT WITH
OCCASIONAL ECHO TOPS TO 22000 FEET...WELL ABOVE THE 17000 FOOT -20
ISOTHERM...THERE REMAINS A FAIR CHANCE FOR ISOLATED THUNDERSTORMS
THROUGH EARLY THIS EVENING.

SHOWERS WILL CONTINUE THIS EVENING THEN DIMINISH A LITTLE LATE
TONIGHT THROUGH FRIDAY. THE MUCH WEAKER UPPER LEVEL TROUGH WILL
STILL BE OVER W WA LATE FRIDAY NIGHT SO THE CHANCE OF SHOWERS WAS
EXTENDED PAST MIDNIGHT.

SATURDAY STILL LOOKS DRY. BUT THE LOW LEVEL AIR MASS WILL REMAIN
MOIST SATURDAY MORNING FOR A MOSTLY CLOUDY START TO THE DAY. WITH A
WEAK UPPER LEVEL RIDGE BUILDING OVER THE AREA...GRADUAL CLEARING CAN
BE EXPECTED SATURDAY AFTERNOON. MAX TEMPS SHOULD END UP NEAR
NORMAL...HIGHS IN THE 60S TO LOWER 70S.

THE NARROW POSITIVE TILT UPPER LEVEL RIDGE WILL CONTINUE BUILDING
OVER THE AREA SATURDAY NIGHT AND SUNDAY. WITH GRADUAL CLEARING
SATURDAY AFTERNOON THERE IS PROBABLY A BIT TOO MUCH CLOUD COVER IN
THE FORECAST FOR SATURDAY NIGHT. THE NAM TIME HEIGHT SECTION FOR
KSEA SHOWS A VERY SHALLOW LAYER OF SURFACE BASED MOISTURE SUNDAY
MORNING. WITH A WEAK SURFACE HIGH PRESENT FOR LIGHT WINDS AND
DIMINISHED CLOUD COVER...THIS IS A GOOD RECIPE FOR PATCHY MORNING
FOG OVER MUCH OF THE AREA. BURN OFF SHOULD OCCUR BY MIDDAY FOR A
MOSTLY SUNNY AND MILD SUNDAY AFTERNOON. MAX TEMPS WILL BE CLOSE TO
NORMAL ONCE AGAIN. KAM

.LONG TERM...GFS AND ECMWF ARE IN GOOD AGREEMENT WITH THE TIMING OF
THE NEXT WEAK COLD FRONT EXPECTED TO REACH THE AREA. BOTH BRING THE
LEADING EDGE OF PRECIP TO THE N COAST AROUND 12Z/5 AM MONDAY
MORNING...THEN MOVE IT INLAND DURING THE MORNING. THE FRONT LOOKS
RATHER SKINNY SO THE PERIOD OF CONTINUOUS RAIN SHOULD BE PRETTY
SHORT. HOWEVER ONSHORE FLOW BEHIND THE FRONT WILL CONTINUE PUMPING
LOW LEVEL MOISTURE INLAND...AND WILL ALSO SET UP A CONVERGENCE ZONE
PATTERN OVER PUGET SOUND.

COOLER AND MOIST NW FLOW ALOFT WILL CONTINUE OVER W WA MONDAY NIGHT
THROUGH THURSDAY. AT SOME POINT THERE WILL PROBABLY BE A WEAK UPPER
LEVEL SHORTWAVE RIDGE EMBEDDED IN THE FLOW CROSSING W WA...WHICH
SHOULD SUPPRESS SHOWERS FOR A SHORT PERIOD...HOWEVER THE GFS AND
ECMWF DO NOT AGREE ON THE TIMING. THE ECMWF IS ALSO QUITE A BIT
WETTER THAN THE GFS. THE EXISTING BROAD BRUSH CHANCE OF SHOWERS
FORECAST IS THE BEST FIT FOR NOW...UNTIL SOME BETTER MODEL AGREEMENT
ALLOWS FIRMER DETAILS TO EMERGE. KAM


&&

.AVIATION...UPPER LEVEL LOW PRESSURE OVER THE REGION WILL SPLIT AND
WEAKEN THROUGH FRIDAY. THE ASSOCIATED SURFACE TROUGH WILL DISSIPATE
NEAR THE COAST TONIGHT. MODERATE TO STRONG S/SW FLOW ALOFT. MOIST
AND SOMEWHAT UNSTABLE...ESPECIALLY THIS EVENING. SHOWERS
WILL PERSIST ACROSS THE AREA AHEAD OF THE SURFACE TROUGH WITH
ISOLATED THUNDERSTORMS POSSIBLE THIS EVENING. VCTS IS INDICATED FOR
SOME TERMINALS BUT NO PREVAILING INDICATED GIVEN THE HIGH DEGREE OF
UNCERTAINTY.

THE ATMOSPHERE SHOULD STABILIZE OVERNIGHT. THIS WILL ALLOW RESIDUAL
MOISTURE TO SETTLE INTO THE LOWER LEVELS. MOS GUIDANCE AND
TIME-HEIGHTS BOTH IMPLY THERE SHOULD BE CONSIDERABLE MVFR LOW CLOUDS
LATE TONIGHT THROUGH FRIDAY MORNING...AND PERHAPS EXTENDING
PARTIALLY INTO THE AFTERNOON. EVENTUALLY CLOUDS WILL BURN OFF FRIDAY
AFTERNOON.

KSEA...SOUTHERLY WIND 4-8 KT...EASING AFTER DARK. VFR THIS EVENING
WITH AREAS OF CB AND ISOLATED THUNDERSTORMS. MVFR CLOUDS REFORMING
AGAIN TONIGHT...MAINLY AFTER MIDNIGHT.  A 25 PERCENT CHANCE OF A
THUNDERSTORM UNTIL 05Z TODAY/THIS EVENING. DTM

&&

.MARINE...A SURFACE TROUGH STALLED OVER THE COASTAL WATERS WILL
SLOWLY DISSIPATE INTO FRI. STRONG SE GRADIENTS WILL STILL BE STRONG
ENOUGH TO PRODUCE SOLID SMALL CRAFT WINDS OVER THE COASTAL WATERS
THROUGH FRI MORNING AND AT THE E ENTRANCE TONIGHT.

WEAK HIGH PRESSURE WILL PREVAIL OVER THE FORECAST AREA FRIDAY
THROUGH SUNDAY. THE NEXT WEAK LOW PRESSURE SYSTEM WILL ARRIVE MONDAY
AND TUESDAY...WITH ONSHORE FLOW CONTINUING INTO WEDNESDAY.

WESTERLY SWELL OVER THE COASTAL WATERS IS NEAR 10 FEET AND WILL
SUBSIDE TO 8 OR 9 FT EARLY THIS EVENING. DTM

&&

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

&&

$$

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














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