563  
FGUS76 KSEW 232332  
ESFSEW  
 
WATER SUPPLY/SPRING FLOOD OUTLOOK  
NATIONAL WEATHER SERVICE SEATTLE WA  
431 PM PDT WED MAY 23 2018  
 
...WESTERN WASHINGTON WATER SUPPLY AND SPRING SNOW MELT  
FLOOD POTENTIAL...  
 
OVERVIEW: THE FORECASTS OF WATER SUPPLY FOR THE SUMMER WERE FOR  
ABOVE NORMAL TO MUCH ABOVE NORMAL FOR WESTERN WASHINGTON RIVERS.  
THIS IS DUE TO ABUNDANT RAIN THIS WINTER SEASON AND A HEALTHY  
SNOWPACK.  
 
FLOODING IN WESTERN WASHINGTON IS UNLIKELY DURING THE PERIOD OF  
MOUNTAIN SNOWPACK RUNOFF, WHICH PEAKS FROM APRIL THROUGH JUNE. THIS  
YEAR WILL BE SIMILAR DESPITE GREATER THAN NORMAL SNOW PACK. BASED ON  
THE CURRENT SNOWPACK AND EXPECTED PRECIPITATION AND TEMPERATURES,  
THE THREAT OF SPRING AND SUMMER SNOWMELT FLOODING IN WESTERN  
WASHINGTON IS LOW AS IS TYPICAL. HEAVY RAIN FROM A LATE SEASON  
RAINSTORM CAN OCCASIONALLY PRODUCE MINOR FLOODING.  
 
PRECIPITATION SUMMARY  
---------------------  
 
AFTER A BELOW NORMAL MARCH FOR MANY, APRIL BROUGHT MUCH ABOVE NORMAL  
PRECIPITATION TO ALL OF WASHINGTON. FOR SOME AREAS, APRIL 2018 WAS  
IN THEIR TOP TEN WETTEST APRILS. FOR THE WATER YEAR, ALL OF  
WASHINGTON CONTINUES TO BE NEAR TO ABOVE NORMAL.  
 
IN WESTERN WASHINGTON, THE MONTHLY PERCENTAGE OF NORMAL FOR  
PRECIPITATION RANGED FROM 161 PERCENT IN THE WESTERN FOOTHILLS OF  
THE CASCADES TO 209 PERCENT IN THE PUGET SOUND LOWLANDS. THE  
GREATEST AMOUNT OF PRECIPITATION AT THE CLIMATE STATIONS FOR THE  
MOUNTAINS, COAST AND INTERIOR LOWLANDS WAS 16.73 INCHES AT CEDAR  
LAKE, 12.47 AT ABERDEEN, AND 14.78 INCHES AT GRAYS RIVER  
RESPECTIVELY.  
 
THE TABLE BELOW GIVES PRECIPITATION FIGURES AS A PERCENT OF NORMAL  
FOR REGIONS OF WASHINGTON. THE CURRENT WATER YEAR BEGAN 1 OCTOBER  
2017 AND ENDS 30 SEPTEMBER 2018.  
 
APRIL WATER YEAR PAST 3 PAST 12  
2018 TO DATE MONTHS MONTHS  
WESTERN WASHINGTON  
COAST 175 109 98 105  
OLYMPICS 162 108 80 101  
NORTHWEST INTERIOR 168 140 142 121  
PUGET SOUND LOWLANDS 209 118 112 111  
SOUTHWEST INTERIOR 162 104 98 101  
WEST FOOTHILLS CASCADES 161 119 116 107  
CASCADES WEST 179 121 123 113  
 
SNOWPACK CONDITIONS  
-------------------  
 
THE SNOWPACK WAS NEAR TO WELL ABOVE NORMAL FOR ALL OF WESTERN  
WASHINGTON. AS OF MAY 22, THE WATER CONTENT OF THE MOUNTAIN SNOWPACK  
IN THE RIVER BASIN GROUPS RANGED FROM 89 TO 112 PERCENT OF NORMAL.  
 
SNOW DEPTH LEVELS FROM THE NORTHWEST AVALANCHE CENTER RANGED FROM 63  
TO 127 PERCENT OF NORMAL AS OF MAY 1. ALL WESTERN WASHINGTON SITES  
EXCEPT FOR CRYSTAL MOUNTAIN WERE AT OR ABOVE NORMAL.  
 
STREAMFLOWS SUMMARY  
-------------------  
 
STREAMFLOWS ON WESTERN WASHINGTON RIVERS FOR LAST MONTH RANGED  
FROM ABOVE NORMAL TO NEAR DAILY HIGH LEVELS.  
 
RESERVOIR STORAGE SUMMARY  
-------------------------  
 
STORAGE FOR ROSS RESERVOIR FOR MAY 1 WAS AT 13% OF AVERAGE. THIS LOW  
VALUE IS LIKELY A REFLECTION OF HOLDING SPACE OPEN FOR THE EXPECTED  
LARGE SPRING MELT INFLOWS.  
 
WEATHER OUTLOOK  
---------------  
 
THE OUTLOOK FOR MAY AND BEYOND FOR WASHINGTON STATE: THE MONTHLY  
OUTLOOK FOR MAY CALLS FOR BELOW NORMAL PRECIPITATION. THE THREE  
MONTH OUTLOOK FOR MAY THROUGH JULY IS ALSO CALLING FOR BELOW NORMAL  
PRECIPITATION FOR THE STATE.  
 
WATER SUPPLY OUTLOOK  
--------------------  
 
LONG RANGE HYDROLOGIC MODELS ARE FORECASTING NEAR NORMAL TO MUCH  
ABOVE NORMAL RIVER FLOWS AND WATER SUPPLY FOR MOST RIVERS THROUGH  
THIS SPRING AND SUMMER. WATER SUPPLY FORECASTS FOR WESTERN  
WASHINGTON RANGE FROM A LOW OF 95 PERCENT FOR THE TOLT AND CALAWAH  
RIVERS TO 146 PERCENT FOR THE PILCHUCK RIVER. ALMOST ALL RIVERS HAVE  
FORECASTS ABOVE 100% OF NORMAL.  
 
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  
MAY 22.  
 
WATER SUPPLY FORECASTS  
NATURAL FLOW UNLESS OTHERWISE SPECIFIED  
(IN THOUSANDS OF ACRE FEET)  
 
RIVER AND GAUGING SITE PERIOD FORECAST NORMAL PERCENT  
(1981-2010)  
NOOKSACK RIVER  
AT NORTH CEDARVILLE APR-SEP 1282 1159 111  
 
SKAGIT RIVER  
NEAR CONCRETE (REGULATED) APR-SEP 6547 5934 110  
 
SAMISH RIVER  
NEAR BURLINGTON APR-SEP 48 43 111  
 
BAKER RIVER  
UPPER BAKER RESERVOIR INFLOW APR-SEP 860 806 107  
 
SULTAN RIVER  
SPADA LAKE INFLOW APR-SEP 219 189 116  
 
PILCHUCK RIVER  
NEAR SNOHOMISH APR-SEP 140 96 146  
 
TOLT RIVER  
TOLT RESERVOIR INFLOW APR-SEP 44 46 95  
 
ISSAQUAH CREEK  
NEAR ISSAQUAH APR-SEP 28 25 113  
 
CEDAR RIVER  
CHESTER MORSE LAKE INFLOW APR-SEP 176 152 116  
 
GREEN RIVER  
HOWARD HANSON DAM INFLOW APR-SEP 306 260 118  
 
NISQUALLY RIVER  
ALDER RESERVOIR INFLOW APR-SEP 441 378 117  
 
COWLITZ RIVER  
MAYFIELD RESERVOIR (REGULATED) APR-SEP 1713 1835 97  
 
CHEHALIS RIVER  
NEAR GRAND MOUND APR-SEP 495 390 127  
 
CALAWAH RIVER  
NEAR FORKS APR-SEP 150 158 95  
 
ELWHA RIVER  
MCDONALD BRIDGE APR-SEP 517 472 109  
 
DUNGENESS RIVER  
NEAR SEQUIM APR-SEP 177 145 122  
 
WYNOOCHEE RIVER  
WYNOOCHEE DAM INFLOW APR-SEP 124 98 126  
 
NF SKOKOMISH RIVER  
CUSHMAN DAM INFLOW APR-SEP 246 191 129  
 
SNOW MELT  
----------  
 
AT THIS POINT IN MAY, THERE IS NORMAL TO GREATER THAN NORMAL SNOW  
PACK IN MOST AREAS AT MID TO UPPER ELEVATIONS, WITH HEALTHY AMOUNTS  
OF WATER BEING STORED THERE. THIS WILL RESULT IN HIGH SPRING RUNOFF  
IN THE RIVERS. WITH THE WARM AND DRY MAY WE HAVE HAD, A LOT OF SNOW  
HAS MELTED OFF. DESPITE THE RIVERS RUNNING HIGH, THE FLOWS WILL BE  
NOWHERE NEAR A FLOOD THREAT. HOWEVER, WITH RIVERS FULL THIS SPRING,  
THEY WILL BE MORE SUSCEPTIBLE TO FLOODING FROM SPRING RAINSTORMS.  
 
CLIMATOLOGY:  
RIVERS WEST OF THE CASCADES CREST USUALLY REACH THEIR HIGHEST PEAK  
FLOWS DURING THE WINTER SEASON. THE VAST MAJORITY OF RIVER FLOODING  
IN WESTERN WASHINGTON, AND ALMOST ALL MAJOR FLOODS, OCCUR BETWEEN  
OCTOBER AND MARCH. HEAVY RAINFALL, RATHER THAN SNOW MELT, 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 SNOW MELT, 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. RARELY, UNDER JUST  
THE RIGHT CONDITIONS OF GREATER THAN NORMAL SNOW PACK, GREATER  
COVERAGE TO LOWER ELEVATIONS, AND NEAR RECORD WARM TEMPERATURES,  
RIVER FLOWS MIGHT RISE TO NEAR MINOR FLOOD STAGE.  
 
WHILE FLOOD PRODUCING RAINFALL IS NOT COMMON AFTER MARCH, MODERATE  
TO HEAVY RAIN IN SPRING OR SUMMER, WHILE RIVERS ARE SWOLLEN WITH  
SNOW MELT RUNOFF, OCCASIONALLY DRIVE THE MOST FLOOD PRONE RIVERS  
ABOVE FLOOD STAGE. TYPICALLY THESE ARE RIVERS SUCH AS THE SKOKOMISH  
AND SNOQUALMIE. HEAVY RAIN IN THE SPRING OR SUMMER, WHEN ROSS LAKE  
IS FULL, CAN ALSO CAUSE THE SKAGIT RIVER TO FLOOD. WHILE THESE  
FLOODS ARE TYPICALLY MINOR COMPARED TO THE WINTER EVENTS, THEY  
SOMETIMES CAUSE SUBSTANTIAL DAMAGE TO FARM CROPS.  
 
SPRING AND SUMMER SNOW MELT PEAK FLOW FORECASTS  
------------------------------------------------  
 
HERE ARE THE LATEST SPRING AND SUMMER CREST FORECASTS FOR WESTERN  
WASHINGTON RIVERS AS OF MAY 11. STATISTICALLY THERE IS A 67 PERCENT  
CHANCE THAT THE ACTUAL SPRING CREST WILL FALL WITHIN THE MOST LIKELY  
RANGE.  
 
RIVER AND SITE FLOOD STAGE MOST LIKELY RANGE OF  
THE SPRING/SUMMER CREST  
SKAGIT RIVER  
NEAR MT. VERNON 28.0 FT 19.7 FT TO 20.8 FT  
 
STILLAGUAMISH RIVER  
AT ARLINGTON 14.0 FT 3.8 FT TO 5.2 FT  
 
SNOQUALMIE RIVER  
NEAR SNOQUALMIE 20000 CFS 4760 CFS TO 7400 CFS  
 
COWLITZ RIVER  
AT RANDLE 18.0 FT 12.0 FT TO 12.1 FT  
 
WHITE RIVER  
AT R STREET 5500 CFS 3450 CFS TO 3450 CFS  
 
SF SKOKOMISH RIVER  
NEAR UNION 320 CFS TO 570 CFS  
 
ELWHA RIVER  
AT MCDONALD BRIDGE 20.0 FT 11.8 FT TO 12.0 FT  
 
DUNGENESS RIVER  
NEAR SEQUIM 7.0 FT 4.6 FT TO 4.9 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 AND SPRING FLOOD OUTLOOK FOR WESTERN  
WASHINGTON WILL BE ISSUED AROUND THE WEEK OF JUNE 4.  
 

 
 
WEATHER.GOV/SEATTLE  
JBB  
 
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