Winter Storm Warning 6, NWS Seattle, WA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 PM PDT SAT OCT 23 2010

.SYNOPSIS...A STRONG STORM WILL AFFECT THE AREA TONIGHT THROUGH
MONDAY. HEAVY RAIN WILL FALL IN THE OLYMPICS...GUSTY WINDS WILL
AFFECT THE AREA...AND GIANT WAVES WILL REACH THE COAST. TEMPERATURES
WILL FALL AND BY SUNDAY NIGHT SNOW WILL BE FALLING IN THE CASCADES.
CONDITIONS SHOULD DRY OUT TUESDAY AND WEDNESDAY. VIGOROUS WEATHER
COULD RETURN BY THE END OF NEXT WEEK.

&&

.SHORT TERM...NUMEROUS RAIN BANDS HAVE MOVED THROUGH WESTERN
WASHINGTON TODAY. RAINFALL AMOUNTS HAVE NOT BEEN IMPRESSIVE IN THE
LOWLANDS BUT THE OLYMPICS HAVE RECEIVED MODERATE AMOUNTS. THIS IS
MERELY A PRECURSOR TO THE STRONG STORM DUE TONIGHT.

A SURFACE LOW FAR OFFSHORE WILL DEEPEN TO AROUND 960 MB AS IT MOVES
NORTHEAST TOWARD THE QUEEN CHARLOTTE ISLANDS. THE RESULTING STRONG
PRESSURE GRADIENTS WILL BRING STRONG WINDS TO THE COAST TONIGHT.
CURRENTLY MODELS KEEP THE WIND STRENGTH BELOW HIGH WIND OVER LAND
AREAS AND THIS IDEA ACCEPTED. THERE WILL BE HEAVY RAIN ON THE COAST
AND IN THE MOUNTAINS TONIGHT. HYDROLOGICAL CONCERNS ON THE SKOKOMISH
RIVER ARE DISCUSSED IN THE HYDROLOGY SECTION BELOW.

A COLD FRONT WILL SWEEP THROUGH WESTERN WASHINGTON SUNDAY MORNING
BRINGING A TEMPORARY END TO THE HEAVY RAIN IN THE OLYMPICS. FLOW
ALOFT WILL TURN WESTERLY AND THE SNOW LEVEL WILL DROP TO AROUND 4000
FEET. THEN...AS THE LOW CENTER MOVES SOUTHEAST DOWN VANCOUVER ISLAND
AND FILLS...THE MODELS KEEP MODERATE PRECIPITATION IN THE MOUNTAINS
SUNDAY NIGHT THROUGH MONDAY NIGHT. THIS WILL MOSTLY BE IN THE FORM
OF SNOW AS THE SNOW LEVEL JUMPS AROUND BETWEEN 3000 AND 4000 FEET.
HOWEVER IN THE SOUTHERN OLYMPICS...WITH GENERALLY LOWER TERRAIN...
ANOTHER ROUND OF MODERATE TO HEAVY RAIN WILL AGAIN POSE A THREAT OF
MINOR FLOODING ON THE SKOKOMISH RIVER.

MONDAY WILL ALSO BE THE DAY THE NORTH-SOUTH PRESSURE GRADIENT IS
STRONGEST THROUGH THE PUGET SOUND AREA. SEATTLE WILL PROBABLY HAVE
ITS WINDIEST DAY ON MONDAY. MODELS WITH THE STRONGEST KPDX-KBLI
GRADIENT HAVE NO MORE THAN +10 MB...WHICH SHOULD KEEP THE WIND
SAFELY BELOW THE HIGH WIND CATEGORY. WINDS WILL DIMINISH MONDAY
NIGHT.

BY TUESDAY THE SHOWER THREAT WILL DIMINISH AND BE MAINLY LIMITED TO
THE MOUNTAINS. HOWEVER...THE CHANGE FROM INDIAN SUMMER LAST WEEK TO
CONDITIONS MORE LIKE LATE FALL WILL BE COMPLETE AS HIGH TEMPERATURES
WILL BE IN THE LOW TO MID 50S FOR THE REST OF THE WEEK. BURKE

.LONG TERM...IN COORDINATION WITH ADJACENT OFFICES HAVE GIVEN THE
PACIFIC NORTHWEST ONE DRY DAY WITH SOME SUN...WEDNESDAY. A TRANSIENT
RIDGE BUILDS AND THE AREA WILL BE BETWEEN SYSTEMS. THE FORECAST
BECOMES MURKY AFTER THAT. THE EURO AND GFS DIVERGE SIGNIFICANTLY
WITH THE EURO TAKING SUBSEQUENT SYSTEMS ON A MORE SOUTHERLY TRACK
AND THE GFS SENDING SYSTEM AFTER SYSTEM INTO WASHINGTON. HAVE BROAD
BRUSHED THE FORECAST. HAVE LITTLE CONFIDENCE IN THE DETAILS ABOUT
THE END OF NEXT WEEK AND THE WEEKEND BUT IT WILL PROBABLY BE WET AT
TIMES. BURKE

&&

.HYDROLOGY...THE SOUTHERN OLYMPICS REMAIN THE FOCUS OF FORECAST
HEAVY RAIN. THE LATEST MODELS BRING BETWEEN 2.5 AND 3.0 INCHES OF
RAIN TO THE SKOKOMISH BASIN BETWEEN 18Z TODAY AND 18Z SUNDAY...WITH
THE HEAVIEST RAIN LATE TONIGHT. THIS SEEMS LIKE ENOUGH TO BRING THE
RIVER ABOVE THE ADVISORY STAGE OF 15.5 FT BUT PERHAPS NOT TO THE
FLOOD STAGE OF 16.5 FT.

THERE IS A DISTINCT LULL IN THE RAINFALL SUNDAY AFTERNOON. THEN AS
THE SURFACE LOW TRACKS SOUTHEAST SUNDAY NIGHT AND MONDAY...AND
STRONG SOUTHERLY WINDS DEVELOP...THERE IS LIKELY TO BE SIGNIFICANT
OROGRAPHIC RAINFALL ON THE SOUTHERN SLOPES OF THE OLYMPICS. THIS
PROBABLY WILL NOT BE ENOUGH TO DRIVE THE RIVER TO FLOOD STAGE IF IT
HAS NOT ALREADY GOTTEN THERE...BUT IT WILL CERTAINLY KEEP IT RUNNING
HIGH. OTHER RIVERS IN THE SAME AREA WILL ALSO RISE AND THEN RUN AT
HIGH LEVELS...BUT ANY OTHER FLOODING IS UNLIKELY.

WILL ISSUE A FLOOD WATCH THIS AFTERNOON FOR THE SKOKOMISH RIVER.
THIS WILL COVER THE POSSIBILITY OF MINOR FLOODING FROM THE RAIN
TONIGHT AND FROM THE ADDITIONAL RAIN SUNDAY NIGHT AND MONDAY.

THE PATTERN IS TOO COOL AND PROGRESSIVE TO BE A REAL FLOOD THREAT
FOR ANY WESTERN WASHINGTON RIVER EXCEPT THE SKOKOMISH. IN PARTICULAR
THE CASCADES WILL RECEIVE CONSIDERABLY LESS PRECIPITATION...AND MUCH
OF THAT WILL BE IN THE FORM OF SNOW BEGINNING SUNDAY NIGHT.

THE OVERALL WEATHER PATTERN INDICATES THE GREEN RIVER WILL NOT FLOOD
IN THE NEXT TEN DAYS. BURKE

&&

.AVIATION...A STRONGER FRONTAL SYSTEM WILL CROSS WRN WA TONIGHT WITH
FLOW ALOFT BECOMING STRONG SOUTHWESTERLY. LOW-LEVEL WIND SHEAR IS
POSSIBLE WHERE SURFACE WINDS REMAIN LIGHT. LOCALLY HEAVY RAIN
EXPECTED TONIGHT...LEADING TO REDUCED VSBYS IN RAIN OVERNIGHT. AN
EASTERLY COMPONENT TO THE LOW-LEVEL FLOW WILL BE AN INHIBITING
FACTOR TO IFR CIGS ALONG THE I-5 CORRIDOR THROUGH TONIGHT...BUT IT
WILL NOT BE MUCH HELP WITH CIGS FOR THE COAST AND OLYMPICS. EVEN
SO...REDUCED VSBYS IN HEAVY RAIN ALONG WITH STRONG WINDS ALOFT WILL
POSE CHALLENGES ENOUGH.

KSEA...LIGHT RAIN WILL GRADUALLY INCREASE OVER THE TERMINAL THROUGH
06Z...BUT AN EASTERLY COMPONENT TO THE LOW-LEVEL FLOW WILL HOLD OFF
ANYTHING LESS THAN VFR CIGS THROUGH 06Z. AFT 06Z...RAIN WILL BECOME
MODERATE TO OCCASIONALLY HEAVY RAIN WILL SPREAD IN AS THE STRONG
FRONTAL SYSTEM ROLLS IN. WINDS AT 5000 FT MSL WILL GRADUALLY INCRS
FROM 30 KT AT 00Z TO 50 KT AT 12Z SUNDAY.     HANER

&&

.MARINE...A VERY ACTIVE PERIOD IS IN STORE THROUGH TUE AS A SERIES
OF STRONG SYSTEMS THAT HAVE DEVELOPED OVER THE CENTRAL AND EASTERN
PACIFIC MOVE ACROSS THE AREA.

THE BIG STORY WILL BE A RAPIDLY DEEPENING LOW CENTER WHICH WILL
REACH PEAK INTENSITY ON SUNDAY MORNING AS A 960 MB LOW NEAR 50N
132W. THIS WILL LIFT A STRONG FRONTAL SYSTEM NE THROUGH THE WATERS
TONIGHT WITH THE BIGGEST WIND IMPACTS ALONG THE COAST...THOUGH
SOUTHEAST-PRONE PARTS OF THE NORTH INTERIOR WILL ALSO GET AN
INCREASE IN WIND. ALONG THE COASTAL WATERS...S-SE WINDS TONIGHT WILL
INCREASE TO AT LEAST HIGH-END GALES...BUT WOULD NOT BE SURPRISED IF
WINDS BRIEFLY TOUCH STORM FORCE (48+ KT).

THE WRN WA WATERS WILL LIKELY GET A MERELY BREEZY LULL IN THE WIND
ON SUNDAY...THEN A STRONG COLD FRONT WILL SPREAD ACROSS THE AREA
LATE SUN NIGHT OR EARLY MON MORNING. PUGET SOUND WILL LIKELY GET ITS
STRONGEST WIND FOLLOWING THIS FEATURE ON MON. HIGH-END SMALL CRAFT
IS CURRENTLY FORECAST...BUT WOULD NOT BE SURPRISED FOR A FEW
OBSERVATION SITES TO BRIEFLY REACH GALE FOR THE FIRST TIME THIS
SEASON ON MON.

LAST BIG MARINE STORY IS THE GIANT SWELL THAT WILL DEVELOP WITH THE
POWERFUL LOW CENTER. SWELL IN EXCESS OF 30 FEET WILL REACH THE
COASTAL WATERS ON MONDAY. SLIGHTLY LESSER SWELL LAST WINTER BROUGHT
FLOODING IN DOWNTOWN WESTPORT. PLUS...ASTRONOMICAL HIGH TIDES
ALREADY REACH THEIR HIGHEST POINT FOR THIS LUNAR HALF-CYCLE ON
SUNDAY AND MONDAY.   HANER

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...COASTAL FLOOD WARNING FOR THE CENTRAL WASHINGTON COAST.
     HIGH SURF WARNING FOR THE NORTH WASHINGTON COAST.
     COASTAL FLOOD WATCH FOR THE NORTH WASHINGTON COAST.
     WINTER STORM WATCH FOR THE MOUNTAINS.
PZ...GALE WARNING COAST...STRAIT...ADMIRALTY INLET...AND
     NORTHERN INLAND WATERS.
     SMALL CRAFT ADVISORY FOR PUGET SOUND AND HOOD CANAL.
     SMALL CRAFT ADVISORY FOR ROUGH BAR CONDITIONS GRAYS HARBOR BAR.

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