Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 170957 CCA
AFDSEW

AREA FORECAST DISCUSSION...CORRECTED TYPO 3RD PARAGRAPH
NATIONAL WEATHER SERVICE SEATTLE WA
230 AM PDT WED SEP 17 2014

.SYNOPSIS...A LARGE UPPER LEVEL TROUGH OFFSHORE APPROACHING WESTERN
WASHINGTON WILL BRING A CHANCE OF SHOWERS TODAY. SHOWERS WILL BECOME
LIKELY TONIGHT AND THURSDAY MORNING AS THE TROUGH MOVES SLOWLY
ACROSS THE AREA. A SMALLER WEAKER UPPER LEVEL TROUGH WILL PROLONG
THE CHANCE OF SHOWERS THROUGH FRIDAY NIGHT. A STRONG UPPER LEVEL
RIDGE WILL BUILD OVER THE REGION THIS WEEKEND BRINGING SUNNY SKIES
AND WARM WEATHER. A WEAK FRONT MAY REACH THE AREA AROUND TUESDAY.

&&

.SHORT TERM...THE LARGE UPPER LEVEL TROUGH OFFSHORE ALONG 135W THIS
MORNING WILL BRING A SHOWERY PATTERN TO W WA TODAY THROUGH THURSDAY.
IR IMAGERY SHOWS A DISORGANIZED LOOKING LARGE BLOB OF MOISTURE
EMBEDDED IN THE S FLOW ALOFT ON THE E SIDE OF THE TROUGH...ALL
HEADED IN THE GENERAL DIRECTION OF THE PACNW. DISORGANIZED AREAS OF
MOISTURE ARE ALREADY BRINGING SPOTTY LIGHT SHOWERS TO W WA THIS
MORNING. A BIT MORE ORGANIZED BAND WAS W OF THE CENTRAL/S OREGON
COAST AT 09Z/2 AM AND SHOULD REACH W WA THIS AFTERNOON AND THIS
EVENING. MUCH OF THE MOISTURE MOVING N WILL INITIALLY END UP W OF
THE COAST...BUT WILL END UP MOVING ACROSS W WA LATE TONIGHT AND
THURSDAY MORNING AS THE WHOLE UPPER LEVEL TROUGH SHIFTS INLAND. THE
WETTEST PERIOD STILL LOOKS TO BE TONIGHT AND THURSDAY MORNING AS THE
TROUGH AXIS CROSSES THE AREA. LIKELY POPS LOOK GOOD FOR THE COAST
AND N INTERIOR WITH HIGH END CHANCE POPS AROUND THE PUGET SOUND
REGION.

THE UPPER LEVEL TROUGH WILL BE OVER E WA THURSDAY AFTERNOON...BUT A
SURFACE RIDGE BUILDING ALONG THE COAST MAY BRING ENOUGH ONSHORE FLOW
FOR A PUGET SOUND CONVERGENCE ZONE TO FORM. HOWEVER MESOSCALE MODELS
DO NOT OFFER A STRONG CASE FOR A CONVERGENCE ZONE AT THIS POINT.

W FLOW ALOFT OVER THE NE PACIFIC BEHIND THE TROUGH WILL DRIVE
ANOTHER UPPER LEVEL SHORTWAVE TROUGH ACROSS THE AREA THURSDAY NIGHT
AND FRIDAY MORNING FOR A CONTINUING CHANCE OF SHOWERS. THE 00Z
MODELS HAVE TRENDED A BIT STRONGER WITH THIS WAVE. THE CHANCE OF
SHOWERS HAS BEEN KEPT THROUGH FRIDAY NIGHT...ALTHOUGH THEY COULD
TAPER OFF BY FRIDAY EVENING. KAM

.LONG TERM...GFS AND ECMWF REMAIN IN GOOD AGREEMENT IN BUILDING A
STRONG UPPER LEVEL RIDGE OVER THE AREA THIS WEEKEND. 500 MB HEIGHTS
CLIMB BACK TO AROUND 5850 METERS SATURDAY. NE LOW LEVEL OFFSHORE
FLOW DEVELOPS ON SATURDAY AS A THERMALLY INDUCED SURFACE TROUGH
FORMS ALONG THE OREGON COAST. EASTERLY FLOW WILL DEVELOP SATURDAY
NIGHT AS THE SURFACE TROUGH EXTENDS NORTH ALONG THE WA COAST. MAX
TEMPS SHOULD EASILY CLIMB BACK INTO THE LOWER TO MID 80S ON SUNDAY.

THE UPPER LEVEL RIDGE AXIS WILL SHIFT E OVER ID ON SUNDAY. THIS WILL
ALLOW A SKINNY WEAKENING COLD FRONT TO APPROACH THE COAST ON MONDAY.
EARLIER MODELS HELD THE FRONT OFFSHORE UNTIL TUESDAY...BUT THE 00Z
RUNS HAVE IT JUST OFFSHORE ON MONDAY. THIS FRONT WILL NOT AMOUNT TO
MUCH...BUT THE SYSTEM BEHIND IT MAY BRING SOME RAIN BY TUESDAY OR
WEDNESDAY. KAM

&&

.AVIATION...AN UPPER TROUGH WILL APPROACH THE PACNW TODAY. THERE IS
SW FLOW ALOFT WITH CONSIDERABLE MID LEVEL MOISTURE AND SCATTERED
LIGHT SHOWERS. GRADIENTS HAVE BECOME LIGHT AND WITHOUT ONSHORE
FLOW IT IS NOT LIKELY THAT AREAS OF LOW CLOUDS WILL DEVELOP THIS
MORNING BUT THERE MIGHT BE PATCHY LOW CLOUDS AND FOG ON THE COAST
THIS MORNING.

KSEA...MID LEVEL CLOUDS. INCREASING AND THICKENING WITH A FEW LIGHT
SHOWERS. LIGHT AND VARIABLE WIND.

&&

.MARINE...LIGHT PRESSURE GRADIENTS TODAY AND THU AS A FAIRLY WEAK
WEATHER SYSTEM MOVES THROUGH THE REGION. OFFSHORE FLOW WILL DEVELOP
OVER THE WEEKEND.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...NONE.

&&

$$

WWW.WEATHER.GOV/SEATTLE

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