Updated: Tue, 06-Jan-2009 10:12am PST
HEAVY RAIN TO DRAMATICALLY INCREASE RISK FOR RAINFALL INDUCED
LANDSLIDES...
PREVIOUS RAINFALL AMOUNTS HAVE BEEN AROUND TWO TO THREE INCHES IN
THE SEATTLE AREA OVER THE PAST 18 DAYS. THIS HAS RESULTED IN
MODERATE LEVELS OF SOIL MOISTURE AROUND WESTERN WASHINGTON. HEAVY
RAINFALL OF NEARLY TWO INCHES IN 24 HOURS AND OVER THREE INCHES IN
48 HOURS IS EXPECTED THIS AFTERNOON THROUGH WEDNESDAY NIGHT. THIS
AMOUNT OF RAIN WILL PUT EXTRA PRESSURE ON SOIL INSTABILITY LEADING
TO AN INCREASED RISK OF LANDSLIDES IN SLIDE PRONE AREAS. A
DIMINISHING RISK OF LANDSLIDES WILL CONTINUE FOR SEVERAL DAYS
AFTER THE RAIN ENDS.
THE SLOW MELT OF THE SNOW THAT HAS FALLEN OVER THE LAST COUPLE OF WEEKS
HAS HAD AN ADDITIONAL SOIL WETTING EFFECT. IN AREAS OUTSIDE OF
SEATTLE...STORM TOTAL RAINFALL COULD BE 3 TO 8 INCHES IN THE
LOWLANDS AND 10 TO 20 INCHES IN THE MOUNTAINS. FOR THESE REASONS
THE LANDSLIDE RISK IS HIGHER THAN THE USGS CHARTS WOULD SUGGEST.
THE AMOUNT OF RAINFALL FORECAST IN 24 HOURS...ON TOP OF
THE CUMULATIVE RAINFALL OVER THE LAST THREE WEEKS...WILL PUSH
WESTERN WASHINGTON SOILS TO THE POINT WHERE THEY ARE ABOVE THE
USGS LANDSLIDE INDEX. WHEN CUMULATIVE RAINFALL EXCEEDS THE USGS
LANDSLIDE CUMULATIVE THRESHOLD INDEX...RAINFALL INDUCED LANDSLIDES
ARE POSSIBLE DURING INTENSE RAINFALL OR WHEN RAINFALL AMOUNTS
EXCEED TWO INCHES IN 24 HOURS.
WHILE THE USGS RAINFALL THRESHOLDS WERE DESIGNED FOR THE SEATTLE
AREA OF THE PUGET SOUND...MOST OF THE LOWLANDS OF WESTERN
WASHINGTON ARE SIMILARLY SUSCEPTIBLE TO LANDSLIDES CAUSED BY WET
SOILS. AREAS MOST SUSCEPTIBLE TO LANDSLIDES UNDER THESE CONDITIONS
ARE STEEP COASTAL BLUFFS AND OTHER STEEP HILLSIDES.
FOR MORE INFORMATION ABOUT CURRENT CONDITIONS...VISIT
WWW.WEATHER.GOV/SEATTLE...SELECT HYDROLOGY...AND THEN SCROLL DOWN
FOR THE LINK TO THE USGS LANDSLIDE INFORMATION PAGE.
$$
WEATHER.GOV/SEATTLE
JBB
Issuing Weather Forecast Office Homepage
Updated: Tue, 06-Jan-2009 10:12am PST
BIG STORM BRINGING SOME WIND TO THE SEATTLE AREA...
WAZ505-507>509-511-070530-
/O.NEW.KSEW.WI.Y.0002.090106T2121Z-090107T1800Z/
EAST PUGET SOUND LOWLANDS-EVERETT AND VICINITY-
SEATTLE/BREMERTON AREA-TACOMA AREA-HOOD CANAL AREA-
121 PM PST TUE JAN 6 2009
...WIND ADVISORY IN EFFECT UNTIL 10 AM PST WEDNESDAY...
THE NATIONAL WEATHER SERVICE IN SEATTLE HAS ISSUED A WIND
ADVISORY...WHICH IS IN EFFECT UNTIL 10 AM PST WEDNESDAY.
A STRONG STORM WHICH IS BRINGING HEAVY RAIN TO THE MOUNTAINS IS
ALSO CAUSING STRONG WINDS IN THE SEATTLE AREA. THESE WINDS WILL
LAST OFF AND ON THROUGH THE NIGHT AND DIMINISH WEDNESDAY MORNING.
PRECAUTIONARY/PREPAREDNESS ACTIONS...
A WIND ADVISORY IS ISSUED WHEN SUSTAINED WINDS OF 30 TO 39 MPH OR
GUSTS OF 45 TO 57 MPH ARE LIKELY. WINDS THIS STRONG CAN SNAP
SMALL TREE BRANCHES...TOPPLE SMALL OR SHALLOW-ROOTED TREES...AND
CAUSE LOCAL POWER OUTAGES.
$$
Issuing Weather Forecast Office Homepage
