698  
FGUS76 KSEW 202243  
ESFSEW  
 
WATER SUPPLY/SPRING FLOOD OUTLOOK  
NATIONAL WEATHER SERVICE SEATTLE WA  
344 PM PDT MON MAY 20 2019  
 
...WESTERN WASHINGTON WATER SUPPLY AND SPRING SNOW MELT FLOOD  
POTENTIAL...  
 
##################################################################  
 
NOTICE: THIS SEASONAL TEXT PRODUCT IS BEING CONSIDERED FOR  
ELIMINATION IN LIEU OF THE CONTENT BEING AVAILABLE ON THE INTERNET  
FROM THE SAME OFFICIAL SOURCES. FOR EXAMPLE,  
HTTPS://WWW.ERH.NOAA.GOV/SEW/HYDRO/DROUGHTSEW-A1-PHP.  
IF THERE ARE ANY COMMENTS OR QUESTIONS ABOUT THIS PRODUCT BEING  
ELIMINATED, PLEASE CONTACT BRENT BOWER AT THE SEATTLE NATIONAL  
WEATHER SERVICE OFFICE AT BRENT.BOWER@NOAA.GOV.  
 
##################################################################  
 
OVERVIEW: DESPITE WATER YEAR PRECIPITATION IN THE 75% TO 95% RANGE,  
LOW WATER SUPPLY FORECASTS ABOUND DUE TO BEING DRY AND WARN SINCE  
THE FIRST OF THE YEAR. ABUNDANT PRECIPITATION EARLY IN THE WATER  
YEAR HELPS LITTLE FOR THE FOLLOWING SUMMER. THE FORECASTS OF WATER  
SUPPLY FOR THE SUMMER WERE FOR BELOW NORMAL TO MUCH BELOW NORMAL FOR  
WESTERN WASHINGTON RIVERS. THIS IS MOSTLY DUE TO LOWER THAN NORMAL  
PRECIPITATION IN RECENT MONTHS AND A VERY LOW SNOW PACK. IF THE  
SPRING AND SUMMER WERE TO TURN OUT TO BE UNUSUALLY WET AND COOL,  
THAT WOULD ALTER THE OUTCOME.  
 
FLOODING IN WESTERN WASHINGTON IS UNLIKELY DURING THE PERIOD OF  
MOUNTAIN SNOWPACK RUNOFF, WHICH PEAKS FROM APRIL THROUGH JUNE. THIS  
YEAR, SPRING FLOODING WILL BE EVEN LESS LIKELY THAN USUAL. BASED ON  
THE CURRENT SNOWPACK AND EXPECTED PRECIPITATION AND TEMPERATURES,  
THE THREAT OF SPRING AND SUMMER SNOWMELT FLOODING IN WESTERN  
WASHINGTON IS EXTREMELY LOW. HOWEVER, HEAVY RAIN FROM A LATE SEASON  
RAINSTORM CAN OCCASIONALLY PRODUCE MINOR FLOODING.  
 
PRECIPITATION SUMMARY  
---------------------  
 
IN CONTRAST TO THE DRY MARCH, APRIL SAW NEAR TO ABOVE NORMAL  
PRECIPITATION FOR WASHINGTON STATE. FOR THE WATER YEAR, MOST OF  
WASHINGTON STATE WAS NEAR TO BELOW NORMAL, WITH A MAJORITY OF THE  
BELOW NORMAL REGIONS ON THE WEST SIDE. WITHOUT THE ABUNDANT  
PRECIPITATION EARLY IN THE WATER YEAR, THESE NUMBERS WOULD BE EVEN  
LOWER.  
 
FOR WESTERN WASHINGTON, THE MONTHLY PERCENTAGE OF NORMAL FOR  
PRECIPITATION RANGED FROM 90 PERCENT ON THE COAST TO 143 PERCENT IN  
THE NORTHWEST INTERIOR. THE GREATEST AMOUNT OF PRECIPITATION AT THE  
CLIMATE STATIONS FOR THE MOUNTAINS, COAST AND INTERIOR LOWLANDS WAS  
12.54 INCHES AT CEDAR LAKE ON THE WESTERN SLOPES OF THE CASCADES,  
8.27 AT QUILLAYUTE, AND 6.00 INCHES AT CLEARBROOK IN THE NORTHWEST  
INTERIOR.  
 
THE TABLE BELOW GIVES PRECIPITATION FIGURES AS A PERCENT OF NORMAL  
FOR REGIONS OF WASHINGTON. THE CURRENT WATER YEAR BEGAN 1 OCTOBER  
2018 AND ENDS 30 SEPTEMBER 2019.  
 
APR WATER YEAR PAST 3 PAST 12  
2019 TO DATE MONTHS MONTHS  
WESTERN WASHINGTON  
COAST 90 83 55 79  
OLYMPICS 95 82 53 80  
NORTHWEST INTERIOR 143 95 86 87  
PUGET SOUND LOWLANDS 92 82 71 78  
SOUTHWEST INTERIOR 101 76 76 71  
WEST FOOTHILLS CASCA 123 83 77 78  
CASCADES WEST 127 86 76 81  
 
SNOWPACK CONDITIONS  
-------------------  
 
THE SNOWPACK WAS BELOW TO MUCH BELOW NORMAL FOR ALL OF WESTERN  
WASHINGTON. AS OF MAY 20, THE WATER CONTENT OF THE MOUNTAIN SNOWPACK  
IN THE RIVER BASIN GROUPS RANGED FROM 22 TO 53 PERCENT OF MEDIAN.  
 
AS OF MAY 1, THE SNOW DEPTH LEVELS FOR WESTERN WASHINGTON FROM THE  
NORTHWEST AVALANCHE CENTER RANGED FROM 0 TO 75 PERCENT OF NORMAL  
WITH A MEDIAN VALUE OF 58 PERCENT.  
 
STREAMFLOWS SUMMARY  
-------------------  
 
STREAMFLOWS ON WESTERN WASHINGTON RIVERS PICKED UP IN APRIL TO  
MOSTLY NEAR NORMAL. EVEN WITH THE INCREASE IN PRECIPITATION IN  
APRIL, A NUMBER OF GAGES STILL SHOWED WELL BELOW NORMAL STREAM  
FLOWS, WHILE A SMALL FEW SHOWED ABOVE NORMAL STREAM FLOWS..  
 
RESERVOIR STORAGE SUMMARY  
-------------------------  
 
STORAGE FOR ROSS RESERVOIR FOR MAY 1 WAS AT 72% OF AVERAGE.  
 
WEATHER OUTLOOK  
---------------  
 
THE OUTLOOK FOR MAY AND BEYOND FOR WASHINGTON STATE: THE MONTHLY  
OUTLOOK FOR MAY IS FOR BELOW NORMAL PRECIPITATION FOR MUCH OF  
WASHINGTON, EXCEPT FOR EQUAL CHANCES OF ABOVE, BELOW OR NEAR NORMAL  
PRECIPITATION FOR THE EXTREME NORTHEAST CORNER OF STATE. THE THREE  
MONTH OUTLOOK FOR MAY THROUGH JULY CALLS FOR BELOW PRECIPITATION FOR  
WESTERN WASHINGTON.  
 
WATER SUPPLY OUTLOOK  
--------------------  
 
LONG RANGE HYDROLOGIC MODELS ARE FORECASTING BELOW NORMAL TO MUCH  
BELOW NORMAL RIVER FLOWS AND WATER SUPPLY FOR RIVERS THROUGH THIS  
SPRING AND SUMMER. WATER SUPPLY FORECASTS FOR WESTERN WASHINGTON  
RANGE FROM A LOW OF 58 PERCENT FOR THE NORTH FORK SKOKOMISH RIVER TO  
95 PERCENT FOR ISSAQUAH CREEK. MOST LOCATIONS HAD FORECASTS BETWEEN  
60% AND 80% OF NORMAL. LOSERS WAY OUTNUMBERED GAINERS AS THE PERCENT  
OF NORMAL DROPPED FOR MOST GAGES OVER LAST MONTH BY AS MUCH AS 15%  
TO 25%.  
 
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 20.  
 
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 797 1159 69  
 
SKAGIT RIVER  
NEAR CONCRETE (REGULATED) APR-SEP 4538 5934 76  
 
SAMISH RIVER  
NEAR BURLINGTON APR-SEP 29 43 68  
 
BAKER RIVER  
UPPER BAKER RESERVOIR INFLOW APR-SEP 647 806 80  
 
SULTAN RIVER  
SPADA LAKE INFLOW APR-SEP 129 189 68  
 
PILCHUCK RIVER  
NEAR SNOHOMISH APR-SEP 72 96 75  
 
TOLT RIVER  
TOLT RESERVOIR APR-SEP 34 46 73  
 
ISSAQUAH CREEK  
NEAR ISSAQUAH APR-SEP 24 25 95  
 
CEDAR RIVER  
CHESTER MORSE LAKE INFLOW APR-SEP 111 152 73  
 
GREEN RIVER  
HOWARD HANSON DAM INFLOW APR-SEP 204 260 79  
 
NISQUALLY RIVER  
ALDER RESERVOIR INFLOW APR-SEP 326 378 86  
 
COWLITZ RIVER  
MAYFIELD RESERVOIR (REGULATED) APR-SEP 1467 1835 80  
 
CHEHALIS RIVER  
NEAR GRAND MOUND APR-SEP 235 390 60  
 
CALAWAH RIVER  
NEAR FORKS APR-SEP 96 158 61  
 
ELWHA RIVER  
MCDONALD BRIDGE APR-SEP 300 472 64  
 
DUNGENESS RIVER  
NEAR SEQUIM APR-SEP 114 145 79  
 
WYNOOCHEE RIVER  
WYNOOCHEE DAM INFLOW APR-SEP 67 98 68  
 
NF SKOKOMISH RIVER  
CUSHMAN DAM INFLOW APR-SEP 110 191 58  
 
SNOW MELT  
----------  
 
AS OF THE FIRST OF MAY, THERE IS LESS THAN NORMAL SNOW PACK. THIS  
WILL RESULT IN A SPRING RUNOFF IN THE RIVERS THAT WILL BE BELOW  
NORMAL AND NO CHANCE OF FLOODING WITHOUT SUBSTANTIAL RAIN.  
 
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 20, FOR TODAY THROUGH SEPTEMBER 1. THE  
PEAK FLOW FORECASTS ARE ALL SUBSTANTIALLY BELOW FLOOD LEVELS.  
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 16.6 FT TO 18.3 FT  
 
STILLAGUAMISH RIVER  
AT ARLINGTON 14.0 FT 2.5 FT TO 4.0 FT  
 
SNOQUALMIE RIVER  
NEAR SNOQUALMIE 20000 CFS 4053 CFS TO 6348 CFS  
 
COWLITZ RIVER  
AT RANDLE 18.0 FT 8.3 FT TO 9.1 FT  
 
WHITE RIVER  
AT R STREET 5500 CFS 2696 CFS TO 3346 CFS  
 
SF SKOKOMISH RIVER  
NEAR UNION 238 CFS TO 571 CFS  
 
ELWHA RIVER  
AT MCDONALD BRIDGE 20.0 FT 10.9 FT TO 11.4 FT  
 
DUNGENESS RIVER  
NEAR SEQUIM 7.0 FT 4.4 FT TO 4.7 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 3.  
 

 
 
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