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FGUS76 KSEW 122132
ESFSEW
WATER SUPPLY/SPRING FLOOD OUTLOOK
NATIONAL WEATHER SERVICE SEATTLE WA
227 PM PDT FRI APRIL 12, 2024
...WESTERN WASHINGTON WATER SUPPLY AND SPRING SNOWMELT FLOOD
POTENTIAL...
SUMMARY: THE LATEST FORECASTS OF WATER SUPPLY FOR THE SUMMER WERE
FOR BELOW TO MUCH BELOW NORMAL FOR WESTERN WASHINGTON RIVERS.
THE SNOWPACK IS MUCH BELOW NORMAL AS WELL. AS A RESULT, THERE IS
LITTLE TO NO CHANCE OF SNOWMELT FLOODING IN WESTERN WASHINGTON AS IS
TYPICAL. IN ADDITION, WATER SUPPLY FORECASTS COULD DROP EVEN FURTHER
THROUGH THE SPRING.
PRECIPITATION SUMMARY
FOR WESTERN WASHINGTON IN MARCH, PRECIPITATION WAS WELL BELOW
NORMAL. THE PERCENT OF NORMAL PRECIPITATION WAS 35 TO 45 PERCENT FOR
THE CASCADES, 45 TO 65 FOR THE LOWLANDS FROM THE PUGET SOUND
EASTWARD, AND 50 TO 85 PERCENT FOR THE OLYMPIC PENINSULA.
SNOWPACK CONDITIONS
-------------------
THE SNOWPACK WAS MUCH BELOW NORMAL FOR WESTERN WASHINGTON AS OF
APRIL 11. WEST OF THE CASCADES, THE BASIN AVERAGE WATER CONTENT OF
THE SNOWPACK RANGED FROM 60 TO 68 PERCENT OF THE MEDIAN.
SNOW DEPTHS FOR NORTHWEST AVALANCHE CENTER LOCATIONS IN WESTERN
WASHINGTON AS OF APRIL 1 RANGED FROM 52 TO 95 PERCENT OF NORMAL,
WITH ALL BUT WHITE PASS BELOW 80 PERCENT.
STREAMFLOWS SUMMARY
-------------------
STREAMFLOWS ON WESTERN WASHINGTON RIVERS FOR LAST MONTH WERE MOSTLY
NORMAL TO BELOW NORMAL. THE STREAMS IN THE CENTRAL CASCADES WERE
BELOW NORMAL.
RESERVOIR STORAGE SUMMARY
-------------------------
STORAGE FOR RESERVOIRS AS OF THE FIRST OF APRIL AS A PERCENTAGE OF
PERIOD OF RECORD MEDIAN:
ROSS RESERVOIR 130%
UPPER BAKER RESERVOIR 130%
HOWARD HANSON RESERVOIR 150%
MAYFIELD LAKE 97%
WEATHER OUTLOOK
---------------
THE OUTLOOK FOR APRIL AND BEYOND FOR WASHINGTON STATE, FOR THE NEXT
TWO WEEKS THE OUTLOOK IS FOR BELOW NORMAL PRECIPITATION. THE
MONTHLY OUTLOOK FOR APRIL CALLS FOR GREATER CHANCES OF BELOW NORMAL
PRECIPITATION. THE THREE MONTH OUTLOOK FOR MAY THROUGH JULY IS FOR
GREATER ODDS OF BELOW NORMAL PRECIPITATION FOR WASHINGTON STATE.
WATER SUPPLY OUTLOOK
--------------------
LONG RANGE HYDROLOGIC MODELS ARE FORECASTING BELOW TO MUCH BELOW
NORMAL RIVER FLOWS AND WATER SUPPLY FOR WESTERN WASHINGTON RIVERS
THROUGH SUMMER. THESE FORECASTS ARE LOWER THAN LAST MONTH'S
FORECASTS AND COULD CONTINUE TO DROP DURING THE SPRING. WATER
SUPPLY FORECASTS FOR WESTERN WASHINGTON RANGED FROM A LOW OF 62%
PERCENT FOR THE NORTH FORK SKOKOMSIH RIVER TO 88 PERCENT FOR THE
PUYALLUP RIVER
WATER SUPPLY FORECASTS THAT INCLUDE REGULATION ARE USED FOR
LOCATIONS WHERE FORECASTS ARE LISTED BELOW AS REGULATED, FOR ALL
OTHER LOCATIONS FORECASTS ARE FOR NATURAL VOLUMES. HERE ARE THE
STREAM FLOW VOLUME FORECASTS FOR SPECIFIC RIVERS AND SITES AS OF
APRIL 11.
NOTE THAT THE NORMAL CLIMATOLOGY IS NOW THE 1991-2020 PERIOD.
WATER SUPPLY FORECASTS
NATURAL FLOW UNLESS OTHERWISE SPECIFIED
(IN THOUSANDS OF ACRE FEET)
RIVER AND GAUGING SITE PERIOD FORECAST NORMAL PERCENT
(1991-2020)
NOOKSACK RIVER
AT NORTH CEDARVILLE APR-SEP 936 1171 80
SKAGIT RIVER
NEAR CONCRETE (REGULATED) APR-SEP 4423 5955 74
SAMISH RIVER
NEAR BURLINGTON APR-SEP 34 46 75
BAKER RIVER
UPPER BAKER RESERVOIR INFLOW APR-SEP 684 823 83
SULTAN RIVER
SPADA LAKE INFLOW APR-SEP 148 190 78
PILCHUCK RIVER
NEAR SNOHOMISH APR-SEP 71 101 71
TOLT RIVER
TOLT RESERVOIR APR-SEP 40 46 87
ISSAQUAH CREEK
NEAR ISSAQUAH APR-SEP 17 26 65
SNOQUALMIE RIVER
NEAR CARNATION APR-SEP 875 1040 84
CEDAR RIVER
CHESTER MORSE LAKE INFLOW APR-SEP 118 136 87
GREEN RIVER
HOWARD HANSON DAM INFLOW APR-SEP 181 262 69
PUYALLUP RIVER
AT PUYALLUP APR-SEP 958 1092 88
NISQUALLY RIVER
ALDER RESERVOIR INFLOW APR-SEP 316 395 80
DESCHUTES RIVER
NEAR RAINIER APR-SEP 29 41 70
COWLITZ RIVER
MAYFIELD RESERVOIR (REGULATED) APR-SEP 1601 1864 86
CHEHALIS RIVER
NEAR GRAND MOUND APR-SEP 274 406 68
NEWAUKUM RIVER
NEAR CHEHALIS APR-SEP 58 85 68
CALAWAH RIVER
NEAR FORKS APR-SEP 122 154 79
ELWHA RIVER
MCDONALD BRIDGE APR-SEP 378 465 81
DUNGENESS RIVER
NEAR SEQUIM APR-SEP 106 145 73
WYNOOCHEE RIVER
WYNOOCHEE DAM INFLOW APR-SEP 82 99 83
NF SKOKOMISH RIVER
CUSHMAN DAM INFLOW APR-SEP 95 183 62
SPRING AND SUMMER SNOWMELT AND FLOODING CLIMATOLOGY OF SPRING
FLOODS:
CLIMATOLOGY OF SPRING FLOODS:
FLOODING IN WESTERN WASHINGTON IS UNLIKELY DURING THE PERIOD OF
MOUNTAIN SNOWPACK RUNOFF, WHICH PEAKS FROM APRIL THROUGH JUNE.
RIVERS WEST OF THE CASCADES CREST USUALLY REACH THEIR HIGHEST PEAK
FLOWS DURING THE WINTER SEASON FROM THE HEAVY RAIN FROM WINTER
STORMS. THE VAST MAJORITY OF RIVER FLOODING IN WESTERN WASHINGTON,
AND ALMOST ALL MAJOR FLOODING, OCCURS BETWEEN NOVEMBER AND MARCH.
HEAVY RAINFALL, RATHER THAN SNOWMELT, IS THE PRIMARY CAUSE OF THESE
EVENTS.
THE HISTORICAL RECORD DOES NOT SHOW MAJOR FLOODING IN WESTERN
WASHINGTON DURING THE PERIOD WHEN THE MOUNTAIN SNOWPACK RUNS OFF.
THE RUNOFF FROM SNOWMELT, EVEN DURING UNUSUALLY HOT WEATHER, IS
SMALL COMPARED TO THE RUNOFF DURING HEAVY WINTER RAINS. THIS IS TRUE
REGARDLESS OF THE SIZE OF THE MOUNTAIN SNOWPACK.
WHILE FLOOD-PRODUCING RAINFALL IS NOT COMMON AFTER MARCH, HEAVY, OR
EVEN MODERATE RAIN IN SPRING, WHILE RIVERS ARE SWOLLEN WITH SNOWMELT
RUNOFF, OCCASIONALLY WILL DRIVE THE MOST FLOOD PRONE RIVERS ABOVE
MINOR FLOOD STAGE. TYPICALLY THESE ARE RIVERS SUCH AS THE SKOKOMISH
AND SNOQUALMIE RIVERS. HEAVY RAIN IN SUMMER, WHEN ROSS LAKE IS FULL,
CAN ALSO CAUSE THE SKAGIT RIVER TO FLOOD. WHILE THESE FLOODS ARE
MINOR COMPARED TO THE WINTER EVENTS, THEY SOMETIMES CAUSE
SUBSTANTIAL DAMAGE TO FARM CROPS SINCE THE FLOOD PLAINS ARE OFTEN IN
USE DURING THE SPRING AND SUMMER.
FORECASTS: AS IN MOST YEARS, THE THREAT FOR SPRING SNOWMELT FLOODING
THIS YEAR IS EXTREMELY LOW, WITH NO SNOWMELT FLOODING EXPECTED BASED
ON RIVER MODELING, THE CURRENT SNOW PACK, COMBINED WITH THE EXPECTED
PRECIPITATION AND TEMPERATURES. THERE IS LESS THAN A 5% CHANCE OF
EXCEEDING FLOOD STAGE ON THE MAJOR RIVERS IN WESTERN WASHINGTON THIS
SPRING WITH THE EXCEPTION OF THE SAMISH, SNOQUALMIE, AND WHITE
RIVERS WHICH HAVE ONLY A 5% CHANCE.
HERE ARE THE PEAK FLOW FORECASTS FOR APRIL 11 THROUGH SEPTEMBER 31
FOR SOME WESTERN WASHINGTON RIVERS. STATISTICALLY,THERE IS A 70
PERCENT CHANCE THAT THE ACTUAL SPRING CREST WILL EXCEED THE LOWER
VALUE AND A 30 PERCENT CHANCE OF EXCEEDING THE HIGHER VALUE.
RIVER AND SITE FLOOD STAGE MOST LIKELY RANGE OF
THE SPRING CREST
SKAGIT RIVER
NEAR MT. VERNON 28.0 FT 18.0 FT TO 21.0FT
STILLAGUAMISH RIVER
AT ARLINGTON 14.0 FT 4.9 FT TO 6.2 FT
SNOQUALMIE RIVER
NEAR SNOQUALMIE 20000 CFS 7790 CFS TO 11700CFS
WHITE RIVER
MUD MTN DAM INFLOW 3180 CFS TO 4600 CFS
COWLITZ RIVER
NEAR RANDLE 18.0 FT 9.3 FT TO 11.0 FT
S.F. SKOKOMISH RIVER
NEAR UNION 770 CFS TO 1270 CFS
DUNGENESS RIVER
NEAR SEQUIM 7.0 FT 4.2 FT TO 4.5 FT
FORECASTS ARE SELECTED FROM THOSE PREPARED BY THE NWRFC.
FOR FURTHER DETAILS, GRAPHICS, AND STATISTICS REGARDING THE WATER
SUPPLY FORECASTS VISIT:
HTTPS://WWW.NWRFC.NOAA.GOV/WS
HTTPS://WWW.NWRFC.NOAA.GOV/NATURAL
FOR FURTHER DETAILS, GRAPHICS, AND STATISTICS REGARDING THE PEAK
FLOW FORECASTS VISIT:
HTTPS://WWW.NWRFC.NOAA.GOV/PEAK/
THE NEXT WATER SUPPLY FOR WESTERN WASHINGTON WILL BE ISSUED THE WEEK
OF MAY 6.
WEATHER.GOV/SEATTLE
JBB
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