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NOUS46 KSEW 051430
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PUBLIC INFORMATION STATEMENT
NATIONAL WEATHER SERVICE SEATTLE WA
730 AM PDT THU JUN 5 2025
...NOTIFICATION OF MAJOR CHANGES TO FLOOD STAGES DUE TO CHANGES IN
STREAM GAUGE REFERENCE LEVELS FOR THE SOUTH FORK STILLAGUAMISH
RIVER...
THIS ISSUANCE CORRECTS THE STATION ID AND LOCATION DESCRIPTION.
THE NATIONAL WEATHER SERVICE HAS ADJUSTED THE FLOOD WARNING LEVELS
FOR THE RIVER FORECAST POINT, THE SOUTH FORK STILLAGUAMISH RIVER AT
JORDAN ROAD BRIDGE GAUGE (SSFW1), EFFECTIVE JUNE 4 , 2025.
THE STREAM GAUGE OWNER WASHINGTON STATE DEPARTMENT OF ECOLOGY
CONVERTED THE BASE REFERENCE POINT (DATUM) FOR THIS GAUGE. THIS
MEANS THE RIVER STAGE READING IS DIFFERENT THAN WHAT YOU ARE USED TO
SEEING AND THAT THE NATIONAL WEATHER SERVICE (NWS) FLOOD LEVELS
NEEDED 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 WERE 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 JORDAN RD BRIDGE
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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