159
FXUS66 KSEW 082345
AFDSEW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
345 PM PST SUN NOV 8 2009
SYNOPSIS
A DEVELOPING LOW PRESSURE CENTER WILL MOVE TO CENTRAL
VANCOUVER ISLAND EARLY MONDAY MORNING. A TRAILING COLD FRONT WILL
SPREAD RAIN AND WIND TO THE COAST LATE TONIGHT AND INLAND ON MONDAY.
A LARGE UPPER LEVEL TROUGH WILL BRING SHOWERS TO THE AREA TUESDAY
THROUGH THURSDAY MORNING AS IT CROSSES THE REGION. A BRIEF PERIOD OF
DRY UPPER RIDGING IS POSSIBLE AROUND THURSDAY NIGHT...THEN ANOTHER
FRONTAL SYSTEM WILL REACH THE AREA FRIDAY NIGHT OR SATURDAY.
SHORT TERM
SATELLITE IMAGERY SHOWS A BAND OF CLOUDS ACROSS THE
ERN PAC...EXTENDING ROUGHLY ALONG 40N. THIS IS WHERE THE JET STREAM
AND A FRONT RESIDE. A SHORTWAVE TROUGH MOVING ALONG THE FRONT SHOULD
SPAWN LOW PRESSURE DEVELOPMENT THIS EVENING. THE DEEPENING LOW
CENTER SHOULD REACH THE COAST OF CENTRAL VANCOUVER ISLAND EARLY MON
MORNING IN THE 12Z-15Z TIME FRAME. AS THE LOW PASSES BY...WINDY
CONDITIONS WILL DEVELOP...MOST NOTABLY FOR THE COAST AND THE NORTH
INTERIOR. AT ITS PEAK AROUND 12Z MON...THE GFS SHOWS 850 MB WINDS
REACHING 70 KT ALONG THE NORTH COAST. THE NAM SHOWS 925 MB WINDS
REACHING 55-65 KT ALONG THE NORTH COAST AND NORTH INTERIOR AROUND
12-15Z. THE NAM-BASED MARINE MOS GUIDANCE BRINGS SUSTAINED WIND
SPEEDS OF 41-45 KT AT TATOOSH ISLAND DURING THIS TIME. HAVE OPTED TO
ISSUE A WIND ADVISORY FOR THE NORTH INTERIOR AND WESTERN STRAIT. FOR
THE NORTH INTERIOR...THE TRACK OF THE LOW FROM SSW-NNE WILL AT FIRST
MEAN AN EASTERLY GRADIENT WITH LIGHTER WIND...BUT THIS TRACK TENDS
TO CAUSE WIND TO PICK UP QUICKLY ONCE THE LOW IS LOCATED TO THE
NORTHWEST.
TRAILING THE DEEPENING LOW CENTER WILL BE A COLD FRONT...WHICH WILL
CROSS WRN WA DURING THE DAY ON MON. IT WILL BE ACCOMPANIED BY RAIN
AND FOLLOWED BY SHOWERS AND A DECREASE IN WIND. A SHORTWAVE RIDING
UP IN SW FLOW ALOFT WILL KEEP THE SHOWERS GOING ON MON NIGHT. SNOW
LEVELS SHOULD GENERALLY RUN NEAR 3500 FEET ON MON NIGHT AND TUE
FOLLOWING THE COLD FRONT.
THE UPPER PATTERN WILL AGAIN AMPLIFY ON TUE. THIS TIME...THE WRN
U.S. UPPER RIDGE WILL TRY NUDGING INTO WRN WA...SUPPORTING ON TUE
NIGHT A CHANCE OF DRY WEATHER SOUTH AND EAST OF SEATTLE. THE ERN PAC
TROUGH WILL THEN START TO MARCH EAST...PUSHING YET ANOTHER COLD
FRONT THROUGH WRN WA ON WED WITH ANOTHER ROUND OF RAIN. IN TYPICAL
FASHION...POST-FRONTAL SHOWERS WILL FOLLOW ON WED AND WED NIGHT.
HANER
LONG TERM
THE GFS AND ECMWF ARE AGAIN IN REASONABLE AGREEMENT
WITH THE EXTENDED FORECAST. THE MOST RECENT TREND HAS BEEN TO PINCH
OFF WEDNESDAYS UPPER TROUGH INTO A CLOSED LOW AND MOVE IT INTO THE
INTERMOUNTAIN WEST BY FRI MORNING. HERE IN THE PAC NW...THIS
SOLUTION WOULD SUPPORT WEAK UPPER RIDGING. HAVE ALREADY DRIED OUT
THE FORECAST ON THU NIGHT FROM SEATTLE TO THE SOUTH...BUT LATER
SHIFTS MAY WANT TO EXPAND THE DRY FORECAST IF THIS MODEL TREND
CONTINUES. DRY WEATHER COULD EVEN BEGIN DURING THE DAY THU AND
EXTEND INTO FRI.
THE UPPER RIDGE SHOULD EXIT ON FRI NIGHT...RE-OPENING THE DOOR TO
THE AFFECTS OF A BROAD UPPER TROUGH OVER THE ERN PAC WITH THE FIRST
IN A SERIES OF FRONT EXPECTED TO ARRIVE FRI NIGHT OR SAT. HANER
HYDROLOGY
NO SIGNIFICANT FLOODING IS EXPECTED IN WESTERN
WASHINGTON DURING THE NEXT WEEK. THIS INCLUDES THE GREEN RIVER.
WESTERN WASHINGTON WILL BE UNDER AN ACTIVE WEATHER PATTERN OF FRONTS
AND LOW PRESSURE TROUGHS THROUGH WED. SOME BRIEF PERIODS OF HEAVY
RAIN ARE POSSIBLE...BUT WEATHER SYSTEMS WILL BE MOVING QUICKLY
ENOUGH TO LIMIT THE DURATION AND AMOUNT OF RAINFALL. DRY WEATHER IS
LIKELY ON THU AND FRI.
THE WEATHER PATTERN SHOULD AGAIN BECOME ACTIVE NEXT WEEKEND. ONE
COMPUTER MODEL HAS STARTED SIGNALING A PATTERN THAT WOULD BRING
HYDROLOGICALLY SIGNIFICANT RAINFALL TO WESTERN BRITISH COLUMBIA NEXT
WEEKEND BUT KEEPS THE HEAVIER RAIN NORTH OF THE INTERNATIONAL
BORDER. IF THIS FORECAST WERE TO HOLD...FLOODING WOULD NOT BE A
CONCERN FOR WESTERN WASHINGTON...BUT THE EVOLUTION OF THIS PATTERN
WILL NEED TO BE WATCHED. HANER
AVIATION...BASIC PATTERN HAS CHANGED LITTLE
WEST-SOUTHWEST FLOW
ALOFT WITH A DEVELOPING LOW PRESSURE CENTER OFFSHORE. AIR MASS IS
MOIST AND SLIGHTLY UNSTABLE. SCATTERED SHOWERS WILL CONTINUE INTO
THIS EVENING BUT HIGH CLOUDS ARE MOVING IN AND THE SHOWERS SHOULD
END. CEILINGS AGAIN RANGE FROM NO CEILING TO MVFR IN OR NEAR
SHOWERS. AFTER 00Z HIGH AND MID LEVEL CLOUDS WILL INCREASE AS NEXT
SYSTEM BEGINS TO AFFECT THE AREA.
RAIN WILL DEVELOP ON THE COAST THIS EVENING AND SPREAD INLAND
OVERNIGHT. BY MORNING RAIN WILL BE WIDESPREAD WITH MVFR CEILINGS
EVERYWHERE. SOME VISIBILITY RESTRICTIONS ARE POSSIBLE FOR BRIEF
PERIODS IN HEAVIER RAIN. WIND IS PERHAPS THE BIGGER PROBLEM WITH
SOUTHERLY WINDS 20G30KT POSSIBLE OVER NORTHERN AREAS LATE TONIGHT
AND MONDAY MORNING.
KSEA...PARAGRAPH ABOVE APPLIES. SOUTH WIND 9-12 KT THIS EVENING WITH
HIGHER GUSTS IN SHOWERS. WIND WILL TURN SOUTHEAST AND INCREASE TO
10-15 KT AFTER 03Z. EARLY MONDAY AFTERNOON WIND SHOULD TURN MORE
SOUTHWESTERLY 10-20 KT. CHB
MARINE
NO REASON TO DOUBT MODELS WHICH SPIN UP A DEEP LOW
SOUTHWEST OF WASHINGTON AND BRING IT NORTHEAST THEN NORTH ALONG THE
COAST LATE TONIGHT. THIS WILL BRING GALES TO MOST WATERS AND HAVE
CONVERTED THE REMAINING GALE WATCHES TO WARNINGS. WIND IS
SURPRISINGLY STRONG THIS MORNING SO HAVE SIMPLIFIED MATTERS BY
HAVING THE GALE WARNED ZONES HAVE SMALL CRAFT ADVISORIES UNTIL THE
GALE WARNING GOES INTO EFFECT TONIGHT. MODELS HAVE BACKED OFF A BIT
ON THE PEAK STRENGTH OF THE SYSTEM...NO 50 KT SURFACE WINDS AROUND
TATOOSH. LATEST MM5-GFS MODEL RUNS PEAK ABOUT 45 KT...AND WINDS
DIMINISH RAPIDLY ALONG THE COAST AFTER 12Z.
ANOTHER IMPULSE MOVES THROUGH THE FLOW MONDAY NIGHT...WITH ANOTHER
FRONT WEDNESDAY. PATTERN IS QUITE PROGRESSIVE AND TIMING DETAILS
LIKELY TO CHANGE. AFTER WEDNESDAY SPLIT FLOW APPEARS TO DEVELOP
WHICH KEEPS WASHINGTON FAIRLY DRY. CHB
SEW WATCHES/WARNINGS/ADVISORIES
WA...WIND ADVISORY FOR THE NORTH INTERIOR AND WESTERN STRAIT.
PZ...GALE WARNING COAST...ENTRANCES...ADMIRALTY INLET...CAMANO
ISLAND TO POINT ROBERTS TONIGHT AND MONDAY.
SMALL CRAFT ADVISORY SAME ZONES AS ABOVE TODAY...AND CENTRAL STRAIT
AND PUGET SOUND AND HOOD CANAL TONIGHT.
WWW.WEATHER.GOV/SEATTLE
FOR AN ILLUSTRATED VERSION OF THE FORECAST DISCUSSION...PLEASE SEE
WWW.WEATHER.GOV/SEATTLE/GAFD/LATEST_WEBAFD.HTML.
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