Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 212238
AFDSEW

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

.SYNOPSIS...A STRONG FRONTAL SYSTEM WILL BRING WIND AND RAIN TO THE
COAST TONIGHT AND TO THE INTERIOR OF WESTERN WASHINGTON LATE TONIGHT
INTO WEDNESDAY MORNING. VERY HEAVY RAINFALL IS POSSIBLE ALONG THE
SOUTHWESTERN OLYMPIC MOUNTAINS TONIGHT INTO EARLY WEDNESDAY AND TO
THE NORTH CASCADES LATE TONIGHT THROUGH WEDNESDAY. SHOWERS WILL
CONTINUE WEDNESDAY NIGHT THROUGH THURSDAY AS SOUTHWEST FLOW ALOFT
PERSISTS. A LOW PRESSURE SYSTEM WILL GIVE RAIN AND WINDY WEATHER TO
THE AREA FRIDAY NIGHT THROUGH THE WEEKEND.

&&

.SHORT TERM...THE UPCOMING WEEK WILL BE QUITE ACTIVE AS A SERIES OF
SYSTEMS MOVE THROUGH 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.

AN OCCLUDED FRONT SITS FROM CENTRAL VANCOUVER ISLAND SOUTHWESTWARD
INTO THE PACIFIC. THE AIR MASS THIS AFTERNOON OVER THE COASTAL
WATERS CONTINUES TO BE QUITE UNSTABLE AND THERE IS A GOOD DEAL OF
LIGHTNING BEING OBSERVED 40 TO 70 MILES OFF THE NORTH COAST. SOME
THUNDERSTORM ACTIVITY MAY CLIP THE NORTH COAST THIS EVENING...BUT
THE AIR MASS IS LIKELY TO BECOME A BIT MORE STABLE LATER TONIGHT AS
OVERRUNNING DEVELOPS AHEAD OF THE INCOMING DEVELOPING LOW.
A DEVELOPING WAVE NOW SEEN NEAR 46N 142W WILL DEVELOP TO A 992 MB
LOW AS IT MOVES INTO 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 37N 165E.

TWO ISSUES ARE LIKELY WITH THIS LOW TONIGHT...WIND AND HEAVY
RAINFALL. WIND IS LIKELY TO REACH 20 TO 35 MPH WITH GUSTS TO 50 MPH
ALONG THE COAST TONIGHT AND 20 TO 30 MPH WITH GUSTS TO 45 MPH
THROUGH THE NORTH INTERIOR FROM ABOUT ADMIRALTY INLET NORTHWARD LATE
TONIGHT INTO WEDNESDAY MORNING. A WIND ADVISORY WAS ISSUED LAST
NIGHT AND WILL BE MAINTAINED. QPF FROM THE 18Z NAM12 SHOWS 3 TO 6
INCHES OF RAIN OVER THE SOUTHWEST FACING OLYMPICS THROUGH TONIGHT
AND ANOTHER INCH OR TWO EARLY WEDNESDAY MORNING. THE 12Z WRFGFS
SHOWS UP TO 10 INCHES OF RAIN TONIGHT INTO WEDNESDAY MORNING IN THE
RAIN FOREST AREAS OF THE SW OLYMPIC PENINSULA...QUITE IMPRESSIVE.
THIS HEAVY RAIN APPEARS TO BE DUE TO A COMBINATION OF CONVECTION AND
STRONG UPSLOPE FLOW WITH SSW WINDS 55 KT AT 850 MB. RAINFALL IS
EXPECTED TO BE HEAVY VERY LATE TONIGHT INTO WEDNESDAY EVENING OVER
THE NORTH CASCADES...AND THE LATEST WRFGFS SHOWS SEVERAL INCHES OF
RAIN ON THE WEST SLOPES OF THE MOUNT BAKER AREA IN WHATCOM COUNTY
AND THE MOUNTAINS NW OF DARRINGTON.

A FLOOD WATCH IS IN EFFECT THROUGH WEDNESDAY FOR THE OLYMPIC
PENINSULA. THE FLOOD WATCH HAS BEEN EXPANDED AND EXTENDED TO INCLUDE
RIVERS FLOWING OFF THE NORTH CASCADES FROM SNOHOMISH COUNTY
NORTHWARD MIDDAY WEDNESDAY THROUGH WEDNESDAY NIGHT.

THE FRONT IS PROGRESSIVE AND WILL PUSH EASTWARD REACHING FROM THE N
CASCADES TO THE NW OREGON COAST WED AFTERNOON. THEN RAIN BECOMES
SHOWERY FROM THE NW WED EVENING AS THE FRONT CONTINUES TO SLOWLY
PUSH SOUTHEASTWARD.

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

LARGE SCALE MODELS CONTINUE TO SHOW DEVELOPMENT OF A RATHER DEEP LOW
OFF THE NORTHERN CALIFORNIA COAST THURSDAY NIGHT INTO FRIDAY AS
ENERGY DUMPING SOUTHWARD FROM THE BERING SEA INTO THE NORTH PACIFIC
PHASES SOMEWHAT WITH THE REMAINS OF HURRICANE ANA THAT HAS RECENTLY
IMPACTED WESTERN HAWAII. THE MODELS VARY SIGNIFICANTLY IN THE
EVENTUAL STRUCTURE OF THIS DEVELOPING SYSTEM AND WHERE IT MOVES
INLAND ON SATURDAY. IT IS A GOOD BET THAT SATURDAY WILL BE WET AND
WINDY OVER WESTERN WASHINGTON. THIS SYSTEM WILL NEED TO BE CAREFULLY
MONITORED. ALBRECHT

.LONG TERM...THE OFFSHORE LOW MOVES INLAND SOMEWHERE OVER THE
PACIFIC NW ON SATURDAY BRINGING WET AND WINDY WEATHER TO THE AREA.
POST FRONTAL SHOWERS DECREASE ON SUNDAY...THEN ANOTHER WARM FRONT
AND LOW APPEAR TO BE ON TRACK TO MOVE ACROSS THE AREA MONDAY AND
TUESDAY. MODEL DETAILS DIFFER...BUT IT APPEARS THAT NEXT WEEK WILL
BE WET AND BREEZY WITH MOUNTAIN SNOW LEVELS DECREASING TO AT LEAST
HIGHER PASS LEVEL MIDWEEK AND TEMPERATURES IN THE LOWLANDS COOLING
OFF WHERE HIGHS STRUGGLE TO GET OUT OF THE LOWER TO MID 50S. ALBRECHT

&&

.HYDROLOGY...FLOOD WATCHES ARE NOW IN EFFECT FOR THE OLYMPIC
PENINSULA TONIGHT AND WEDNESDAY...AND FOR SNOHOMISH COUNTY NORTHWARD
IN THE INTERIOR FROM MIDDAY WEDNESDAY INTO WEDNESDAY NIGHT.

THE SKOKOMISH RIVER IS LIKELY TO SEE MODERATE FLOODING. IF THE
HEAVIER PRECIPITATION SEEN IN THE WRFGFS 4KM VERIFIES...THE
SKOKOMISH RIVER AT POTLATCH COULD SEE MAJOR FLOODING DURING THE DAY
WEDNESDAY. OTHER RIVERS FLOWING OFF THE OLYMPICS SUCH AS THE
QUINAULT...CLEARWATER...HOH...AND BOGACHIEL COULD SEE MINOR
FLOODING LATE TONIGHT INTO WEDNESDAY.

BASED ON LATEST RAINFALL FORECASTS...PORTIONS OF THE STILLAGUAMISH
RIVER IN SNOHOMISH COUNTY AND THE NOOKSACK RIVER IN WHATCOM COUNTY
COULD SEE MINOR FLOODING LATER WEDNESDAY OR WEDNESDAY NIGHT. RATHER
INTENSE RAINFALL ON WEDNESDAY MAY ALSO CAUSE SMALL STREAMS AND
UNCONTROLLED RIVERS IN THE NORTH CASCADES TO RISE RAPIDLY AND
POSSIBLY FLOOD.  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 WITH SOME PEEKS OF SUN EAST OF PUGET
SOUND THE REMAINDER OF THIS AFTERNOON. LOWER CEILINGS EXPECTED IN
SCATTERED SHOWERS AS THEY SPREAD EASTWARD FROM OLYMPIC PENINSULA.
CEILINGS EXPECTED TO GRADUALLY LOWER TONIGHT AS STEADY RAIN DEVELOPS.

KSEA...GENERALLY MID LEVEL CLOUDS FOR THE REMAINDER OF THE AFTERNOON
AND INTO EARLY EVENING. CEILINGS SHOULD GRADUALLY LOWER TONIGHT AS
RAIN DEVELOPS. SOUTH WIND 10 TO 15 KNOTS...INCREASING TO 10 TO 20
KNOTS WEDNESDAY MORNING. SCHNEIDER/SMR

&&

.MARINE...A VIGOROUS FRONTAL SYSTEM WILL BRING GALE FORCE WINDS TO
MOST WATERS OVER THE NEXT 24 HOURS WITH SMALL CRAFT ADVISORY WINDS
EXPECTED IN THE CENTRAL STRAIT AND PUGET SOUND.

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/SMR


&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...FLOOD WATCH MASON...CLALLAM...JEFFERSON...AND GRAYS HARBOR
      COUNTIES TONIGHT INTO WEDNESDAY AFTERNOON.
     FLOOD WATCH SNOHOMISH...SKAGIT...AND WHATCOM COUNTIES MIDDAY
      WEDNESDAY THROUGH WEDNESDAY NIGHT.
     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 THROUGH TONIGHT.
     SMALL CRAFT ADVISORY CENTRAL STRAIT OF JUAN DE FUCA AND
      PUGET SOUND AND HOOD CANAL.
     SMALL CRAFT ADVISORY GRAY HARBOR BAR FOR ROUGH BAR.

&&

$$

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