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PUBLIC INFORMATION STATEMENT
NATIONAL WEATHER SERVICE SEATTLE WA
400 PM PDT FRI MAY 16 2025
...NOTIFICATION OF MAJOR CHANGES TO FLOOD STAGES DUE TO CHANGES IN
STREAM GAUGE REFERENCE LEVELS FOR THE SOUTH FORK STILLAGUAMISH
RIVER...
THE NATIONAL WEATHER SERVICE WILL BE ADJUSTING THE FLOOD WARNING
LEVELS FOR THE RIVER FORECAST POINT, THE SOUTH FORK STILLAGUAMISH
RIVER AT JORDAN ROAD BRIDGE GAUGE (WRAW1), EFFECTIVE JUNE 4 , 2025.
THE STREAM GAUGE OWNER WASHINGTON STATE DEPARTMENT OF ECOLOGY WILL
CONVERT THE BASE REFERENCE POINT (DATUM) FOR THIS GAUGE. THIS MEANS
THE RIVER STAGE READING WILL BE DIFFERENT THAN WHAT YOU ARE USED TO
SEEING AND THAT THE NATIONAL WEATHER SERVICE (NWS) FLOOD LEVELS NEED
TO BE RENUMBERED TO MATCH THE CHANGES.
THE STAGE HEIGHT OF THE WATER LEVEL REPORTED BY A GAUGE IS MEASURED
FROM A BASE REFERENCE POINT WHICH CAN BE AN ARBITRARY NUMBER. FOR
EXAMPLE, IF A GAUGE HAS A BASE REFERENCE LEVEL OF 6.0 FEET AND THE
WATER LEVEL AT THE GAUGE READS 10.0 FEET, IT MEANS THE HEIGHT OF THE
WATER LEVEL IS 4.0 FEET ABOVE THE BASE. SINCE THE BASE LEVEL IS
USUALLY GIVEN FOR SOME LEVEL BELOW THE BOTTOM OF THE STREAMBED, THE
ACTUAL STREAM DEPTH WOULD TYPICALLY BE SOMETHING LESS THAN 4 FEET.
IN THE NEW SYSTEM, CALLED NORTH AMERICAN VERTICAL DATUM 88 (NAVD88),
THE BASE REFERENCE LEVEL IS NEARLY MEAN SEA LEVEL AND THE MEASURED
WATER LEVEL IS THE ELEVATION OF THE WATER SURFACE ABOVE MEAN SEA
LEVEL. THAT MEANS THAT, FOR THE SAME EXAMPLE GAUGE, INSTEAD OF
READING 10.0 FEET, MAY READ 170.0 FT.. A WATER LEVEL READING OF
170.0 FEET OF THIS EXAMPLE GAUGE MEANS THAT THE WATER LEVEL IS 170.0
FEET ABOVE SEA LEVEL WHILE IT IS STILL THE SAME 4 FEET DEEP.
FOR THE NWS RIVER FLOOD WARNING SYSTEM, THE DEFINED FLOOD LEVELS FOR
MINOR, MODERATE AND MAJOR FLOOD CATEGORIES HAVE TO BE CONVERTED TO
NAVD88 AS WELL IN ORDER TO MATCH. THE CONVERSION FACTOR FOR THIS
GAUGE IS 196.54 FEET. BELOW PLEASE FIND THE LISTING OF THIS FLOOD
WARNING POINT'S OLD FLOOD LEVELS, AND THE NEWLY CONVERTED FLOOD
LEVELS. LEVELS WERE ROUNDED TO NEAREST ONE-HALF FOOT AS NEEDED
HERE ARE THE NEW AND OLD FLOOD STAGES:
S.F. STILLAGUAMISH RIVER AT R STREET
FLOOD CATEGORY NEW STAGE OLD STAGE
FLOOD STAGE/MINOR FLOOD 211.0 FT. 14.5 FT.
MODERATE FLOOD 212.5 FT. 16.0 FT.
MAJOR FLOOD 213.5 FT. 17.0 FT.
IF YOU WOULD LIKE MORE INFORMATION ABOUT THE NATIONAL WEATHER
SERVICE HYDROLOGIC SERVICES IN THE SEATTLE AREA, PLEASE CONTACT
BRENT BOWER, SENIOR SERVICE HYDROLOGIST, AT 206-526-6095.
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