Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 211558
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
900 AM PM PDT TUE OCT 21 2014

.SYNOPSIS...A STRONG FRONTAL SYSTEM WILL BRING RAIN AND WIND TO THE
COAST LATE THIS AFTERNOON AND TONIGHT AND TO THE INTERIOR OF WESTERN
WASHINGTON TONIGHT INTO WEDNESDAY MORNING. VERY HEAVY RAINFALL IS
POSSIBLE ALONG THE SOUTHWESTERN OLYMPIC MOUNTAINS. IT WILL REMAIN
WET ON WEDNESDAY...ESPECIALLY IN THE CASCADES...AS SOUTHWEST FLOW
ALOFT PERSISTS. UNSETTLED WEATHER WILL CONTINUE FOR THE REST OF THE
WEEK AS A SERIES OF WEATHER SYSTEMS MOVE ONSHORE.

&&

.SHORT TERM...IT APPEARS THAT THE UPCOMING WEEK WILL BE QUITE ACTIVE
WEATHERWISE AS A SERIES OF SYSTEMS CHARGE INTO THE PACIFIC
NORTHWEST. THE JET ACROSS THE NORTH PACIFIC IS BECOMING RATHER
STRONG AS COLD AIR DUMPS OUT OF THE BERING SEA INTO THE GULF OF
ALASKA AND SUBTROPICAL AND TROPICAL MOISTURE AND ENERGY FROM SYSTEMS
IN THE CENTRAL NORTH PACIFIC GETS ENTRAINED INTO THE FLOW.

FIRST FOR TODAY...A PASSING UPPER TROUGH IS CENTERED OVER NW
WASHINGTON. AN OCCLUSION THAT IS ASSOCIATED WITH A VERTICALLY
STACKED LOW NEAR 54N 141W EXTENDS SSW FROM NORTHERN VANCOUVER ISLAND
INTO THE WASHINGTON AND OREGON OFFSHORE WATERS. THIS OCCLUSION HAS
QUITE A BIT OF TSTM ACTIVITY ASSOCIATED WITH IT AS IT MOVES SLOWLY
EAST TOWARD THE COASTAL WATERS. WILL ADD SCATTERED TSTMS TO THE
OUTER COASTAL WATERS TODAY AS THE OCCLUSION NICKS THAT AREA. TSTM
ACTIVITY SHOULD DECREASE AS THE FRONT MOVES EAST TOWARD THE COAST
LATER THIS AFTERNOON AND THE MOST UNSTABLE AIR MOVES INTO VANCOUVER
ISLAND.

FOR TONIGHT...THE OCCLUSION STALLS OVER THE WESTERN OLYMPIC
PENINSULA LATE THIS AFTERNOON AND EVENING. THEN A DEVELOPING WAVE
NOW SEEN NEAR 43N 141W CHARGES EASTWARD INTO THE COASTAL WATERS THIS
EVENING AND DEVELOPS TO AROUND A 992 MB LOW AS IT HITS SOUTH CENTRAL
VANCOUVER ISLAND LATE TONIGHT. THIS LOW IS DEVELOPING IN THE LEFT
EXIT REGION OF A 150 KT 300 MB JET THAT IS CHARGING EASTWARD ACROSS
THE EAST CENTRAL NORTH PACIFIC AND HAS BEEN CHARGED WITH SUBTROPICAL
MOISTURE FROM AN UPPER TROUGH NOW SEEN NEAR 35N 165E. TWO ISSUES ARE
LIKELY WITH THIS LOW...WIND AND HEAVY RAINFALL. WIND IS LIKELY TO
REACH THE HIGHER END OF THE ADVISORY LEVEL ALONG THE COAST TONIGHT
AND THROUGH THE NORTH INTERIOR FROM ABOUT ADMIRALTY INLET NORTHWARD
LATE TONIGHT INTO WEDNESDAY MORNING. THE ADVISORY WAS ISSUED LAST
NIGHT AND WILL BE MAINTAINED. QPF FROM THE 12Z NAM12 AND THE 00Z
WRFGFS MESOSCALE MODELS...ESPECIALLY IN THE RAIN FOREST AREAS OF THE
SW OLYMPIC PENINSULA...IS QUITE IMPRESSIVE WITH 8 INCHES IN 12 HOURS
FROM THE NAM12 AND 9-12 INCHES FROM THE WRFGFS IN A 12-15 HR PERIOD.
WHILE THIS MAY BE OVERDONE IN THE MODELS...THIS IS HIGHER THAN WHAT
IS TYPICALLY SEEN IN EVEN HEAVIER EVENTS ALONG THE SW OLYMPICS. THE
FLOOD WATCH WILL BE EXTENDED FROM MASON COUNTY TO INCLUDE THE ENTIRE
OLYMPIC PENINSULA TONIGHT INTO WEDNESDAY AFTERNOON.

THE FRONT IS PROGRESSIVE AND WILL PUSH EASTWARD WED REACHING FROM
THE N CASCADES TO THE NW OREGON COAST. THE NAM12 HINTS THAT RATHER
HEAVY RAINFALL AMOUNTS ARE POSSIBLE OVER THE NORTH CASCADES WED
MORNING.

BREEZY AND SHOWERY WEATHER CAN BE EXPECTED WED AFTERNOON THROUGH THU
IN STRONG SW FLOW ALOFT.

FARTHER OUT...ALL OF THE LARGE SCALE MODELS...AS WELL AS THE LATEST
12Z NAM12...ARE SHOWING GOOD SURFACE DEVELOPMENT OVER THE EASTERN
PACIFIC THU NIGHT INTO FRI AS ENERGY DUMPING SOUTHWARD FROM THE
BERING SEA INTO THE NORTH PACIFIC PHASES WITH THE REMAINS OF
HURRICANE ANA THAT HAS RECENTLY IMPACTED WESTERN HAWAII. THIS LOW
DEVELOPS OFFSHORE THEN POSSIBLY DECAYS SOMEWHAT AS IT MOVES INLAND
OVER THE FIRST PART OF NEXT WEEKEND. THIS SYSTEM WILL NEED TO BE
CAREFULLY MONITORED. ALBRECHT

.LONG TERM...FROM THE PREVIOUS LONG TERM DISCUSSION...ANOTHER
VIGOROUS PACIFIC FRONTAL SYSTEM MAY IMPACT THE REGION ON SAT FOR
ANOTHER ROUND OF WET AND WINDY WEATHER. MODELS ARE SHOWING HIGH WIND
AS A SURFACE LOW TRACKS THROUGH WESTERN WA...BUT THE EXACT
STRENGTH...TRACK AND TIMING OF THE SYSTEM IS STILL UNCLEAR. IT IS
WORTH NOTING BUT NOT BITING ON ANY ONE SOLUTION JUST YET AS A LOT
CAN CHANGE. THE FORECAST LOOKS COOL AND SHOWERY THROUGH THE REST OF
THE WEEKEND WITH LONG WAVE TROUGHING ALONG THE WEST COAST. A RIDGE
MAY BRING DRIER AND CALMER WEATHER ON MON. 33

&&

.HYDROLOGY...THE FLOOD WATCH FOR THE SKOKOMISH RIVER IN MASON COUNTY
IS BEING EXPANDED TO INCLUDE THE ENTIRE OLYMPIC PENINSULA FOR
TONIGHT INTO WEDNESDAY. MESOSCALE MODELS ARE QUITE AGGRESSIVE IN
SHOWING RAINFALL RATES OF 0.70 TO 1.00 INCH PER HOUR FOR SEVERAL
HOURS TONIGHT AS A FRONT STALLS THEN A LOW PRESSURE SYSTEM MOVES BY
TO THE WEST OF THE AREA INTO VANCOUVER ISLAND. THE SKOKOMISH RIVER
IS LIKELY TO FLOOD. WHILE RIVERS SUCH AS THE QUINAULT AND CLEARWATER
CAN HANDLE A LOT OF WATER...THEY DONT TYPICALLY SEE 9-12 INCHES FALL
IN A 12 TO 15 HOUR PERIOD. IT THAT OCCURS...FLOODING IS LIKELY ON
PORTIONS OF THOSE RIVERS AND AN AREAL FLOOD WARNING WOULD BE ISSUED.
THE BOGACHIEL AND OTHER RIVERS FLOWING OFF THE OLYMPICS WILL ALSO
NEED TO BE WATCHED.

MESOSCALE MODELS ALSO SHOW A PERIOD OF RATHER HEAVY RAINFALL IN THE
NORTH CASCADES LATE TONIGHT INTO WEDNESDAY MORNING. WHILE MODELS DO
NOT SHOW FLOODING IN THIS AREA AT THIS TIME...IF THE INCOMING WRFGFS
4KM AND ECMWF SHOW VERY HEAVY RAINFALL IN THIS AREA...A FLOOD WATCH
MAY ALSO NEED TO BE ISSUED FOR SOME RIVERS IN WHATCOM AND SKAGIT
COUNTY LATER THIS AFTERNOON FOR THE WEDNESDAY AND WEDNESDAY NIGHT
TIME PERIOD.

ANOTHER PERIOD OF HEAVY RAINFALL IS EXPECTED FRIDAY INTO THE
WEEKEND...BUT AT THIS TIME FLOODING IS NOT EXPECTED ON WESTERN
WASHINGTON RIVERS. ALBRECHT

&&

.AVIATION...WESTERLY FLOW ALOFT BECOMING SOUTHWEST TONIGHT. A
VIGOROUS FRONTAL SYSTEM WILL MOVE INTO THE AREA LATER TODAY AND
TONIGHT...BRINGING RAIN AND WIND. AIR MASS MOIST AND STABLE.

CEILINGS MOSTLY 4000-7000 FEET TODAY WITH LOWER CEILING IN SCATTERED
SHOWERS. CEILINGS EXPECTED TO GRADUALLY LOWER TONIGHT AS STEADY RAIN
DEVELOPS.

KSEA...GENERALLY MID LEVEL CLOUDS TODAY BUT LOWER CEILINGS AT TIMES
IN SHOWERS. CEILINGS SHOULD GRADUALLY LOWER TONIGHT AS RAIN
DEVELOPS. SOUTH WIND 10 TO 15 KNOTS...INCREASING TO 10 TO 20 KNOTS
WEDNESDAY MORNING. SCHNEIDER

&&

.MARINE...A VIGOROUS FRONTAL SYSTEM WILL BRING GALE FORCE WINDS TO
MOST WATERS LATE THIS AFTERNOON AND TONIGHT WITH SMALL CRAFT
ADVISORY WINDS ALL OTHER WATERS.

THERE WILL BE A RELATIVE LULL IN THE STORMY WEATHER IN THE MIDDLE OF
THE WEEK...THEN ANOTHER STRONG WEATHER SYSTEM WILL AFFECT THE WATERS
FRIDAY AND SATURDAY. A DEEP SURFACE LOW IS EXPECTED TO TRACK THROUGH
THE WATERS DURING THIS TIME BUT THERE IS STILL A LOT OF UNCERTAINTY
WITH HOW STRONG THE LOW WILL BE AND WHERE IT WILL GO. SCHNEIDER

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...FLOOD WATCH MASON...CLALLAM...JEFFERSON...AND GRAYS HARBOR
      COUNTIES TONIGHT INTO WEDNESDAY AFTERNOON.
     WIND ADVISORY COAST...ADMIRALTY INLET...SAN JUANS...WESTERN
       WHATCOM...WESTERN SKAGIT.

PZ...GALE WARNING COASTAL WATERS...ENTRANCES TO THE STRAIT OF
      JUAN DE FUCA...NORTHERN INLAND WATERS...AND ADMIRALTY INLET
      LATE THIS AFTERNOON THROUGH TONIGHT.
     SMALL CRAFT ADVISORY COASTAL WATERS...ENTRANCES TO THE
      STRAIT  OF JUAN DE FUCA...NORTHERN INLAND WATERS...AND
      ADMIRALTY INLET TODAY.
     SMALL CRAFT ADVISORY CENTRAL STRAIT OF JUAN DE FUCA AND PUGET
      SOUND AND HOOD CANAL.
     SMALL CRAFT ADVISORY GRAY HARBOR BAR FOR ROUGH BAR.

&&

$$

WWW.WEATHER.GOV/SEATTLE

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