Flood Potential Outlook
Issued by NWS Seattle/Tacoma, WA

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WATER SUPPLY OUTLOOK
NATIONAL WEATHER SERVICE SEATTLE WA
127 PM PDT FRI MAY 8 2015

...WESTERN WASHINGTON WATER SUPPLY AND SPRING SNOW MELT
FLOOD POTENTIAL...

OVERVIEW: THE FORECASTS OF WATER SUPPLY FOR THE SUMMER WERE FOR
MUCH BELOW NORMAL TO RECORD LOWS FOR WESTERN WASHINGTON RIVERS. MOST
RIVERS HAVE FORECASTS THAT ARE AT OR WELL BELOW 75 PERCENT OF
NORMAL. THIS IS MOSTLY DUE TO VERY LOW TO NEAR RECORD LOW SNOW PACK
IN THE OLYMPIC AND CASCADE MOUNTAINS. IN ADDITION...APRIL AND THE
FIRST PART OF MAY HAVE BEEN DRIER THAN NORMAL. THE U.S. DROUGHT
MONITOR FOR MAY 5 DEPICTS MODERATE DROUGHT INTENSITY FOR THE OLYMPIC
MOUNTAINS AND FOR THE WASHINGTON CASCADES. THIS IS MOSTLY DUE TO THE
SNOW DROUGHT. THE GOVERNOR HAS DECLARED A DROUGHT FOR MULTIPLE
REGIONS IN WESTERN AND EASTERN WASHINGTON.

FLOODING IN WESTERN WASHINGTON IS UNLIKELY DURING THE PERIOD OF
MOUNTAIN SNOWPACK RUNOFF...WHICH PEAKS FROM APRIL THROUGH JUNE. THIS
YEAR WILL BE EVEN LESS LIKELY WITH RECORD LOW SNOW PACK. MOST AREAS
PEAKED IN SNOW PACK WATER EQUIVALENT IN JANUARY TO EARLY FEBRUARY
INSTEAD OF APRIL. BASED ON THE CURRENT SNOWPACK AND EXPECTED
PRECIPITATION AND TEMPERATURES... THE THREAT OF SPRING AND SUMMER
SNOWMELT FLOODING IN WESTERN WASHINGTON IS EXTREMELY LOW TO ZERO.
MANY AREAS HAVE ALREADY MELTED OFF.

PRECIPITATION SUMMARY
---------------------

APRIL SAW BELOW NORMAL PRECIPITATION FOR ALL OF WASHINGTON
STATE...WITH NEARLY ALL AREAS EAST OF THE CASCADES RECEIVING LESS
THAN 50 PERCENT OF NORMAL FOR THE MONTH. IN REGARDS TO THE WATER
YEAR...MOST OF THE REGIONS ARE SEEING NEAR TO ABOVE NORMAL VALUES
WITH THE EXCEPTION OF BELOW NORMAL VALUES IN THE SOUTHWEST INTERIOR
AND NORTHEAST REGIONS.

THE MONTHLY PERCENTAGE OF NORMAL FOR PRECIPITATION RANGED FROM 63
PERCENT IN THE NORTHWEST INTERIOR TO 71 PERCENT IN THE OLYMPICS AND
WESTERN FOOTHILLS OF THE CASCADES. THE GREATEST AMOUNT OF
PRECIPITATION AT THE CLIMATE STATIONS FOR THE MOUNTAINS...COAST...
AND INTERIOR LOWLANDS WAS 7.26 INCHES AT FORKS IN THE
OLYMPICS...6.19 INCHES AT QUILLAYUTE...AND 3.37 INCHES AT BATTLE
GROUND IN THE SOUTHWEST INTERIOR.

THE TABLE BELOW GIVES PRECIPITATION FIGURES AS A PERCENT OF NORMAL
FOR REGIONS OF WASHINGTON.  THE CURRENT WATER YEAR BEGAN 1 OCTOBER
2014 AND ENDS 30 SEPTEMBER 2015.

                        APRIL      WATER YEAR     PAST 3     PAST 12
                         2015       TO DATE       MONTHS      MONTHS
WESTERN WASHINGTON
  COAST                    66          102           92          101
  OLYMPICS                 71          100           90           97
  NORTHWEST INTERIOR       63          116          106          117
  PUGET SOUND LOWLANDS     69           98          100          102
  SOUTHWEST INTERIOR       66           89           84           92
  WEST FOOTHILLS CASCADES  71          100           93          102
  CASCADES WEST            64          101           91          102


SNOWPACK CONDITIONS
-------------------

THE SNOWPACK WAS WELL BELOW NORMAL FOR WESTERN WASHINGTON AS OF MAY
8 AND CONTINUED TO DETERIORATE. MANY AREAS ARE ALREADY SNOW FREE
AND/OR AT RECORD LOWS FOR THIS DATE. THE WATER CONTENT OF THE
MOUNTAIN SNOWPACK IN THE BASINS THAT FEED THE MAJOR RIVERS WEST OF
THE CASCADES AND OLYMPICS RANGED FROM 0 TO 39 PERCENT OF NORMAL.

DATA FROM THE NORTHWEST WEATHER AND AVALANCHE CENTER AS OF MAY 1
SHOWED THAT SNOW DEPTHS IN WESTERN WASHINGTON WERE 0 TO 39 PERCENT
OF NORMAL. THESE ARE RECORD LOW SNOW DEPTHS FOR ALL BUT THE PARADISE
SITE.


STREAMFLOWS SUMMARY
-------------------

STREAMFLOWS FOR OF WESTERN WASHINGTON RIVERS FOR LAST MONTH ROUGHLY
RANGED FROM ABOVE NORMAL IN THE NORTH CENTRAL PART OF THE STATE TO
BELOW NORMAL EVERYWHERE ELSE. HOWEVER...THE CURRENT FLOWS ON MANY
RIVERS AS OF MAY 8 WERE MUCH BELOW TO RECORD LOW DAILY FLOWS.


RESERVOIR STORAGE SUMMARY
-------------------------

STORAGE FOR ROSS RESERVOIR FOR MAY 1 WAS AT 94% OF AVERAGE.

WEATHER OUTLOOK
---------------

THE OUTLOOK FOR WESTERN WASHINGTON FOR APRIL AND BEYOND...OVER THE
NEXT 7 DAYS THE PRECIPITATION FORECAST IS FOR GENERALLY 0.1 TO 0.50
INCHES WITH UP TO 1.25 INCHES IN THE MOUNTAINS. FOR THE NEXT TWO
WEEKS THE OUTLOOK IS FOR NORMAL PRECIPITATION. THE MONTHLY OUTLOOK
FOR MAY IS CALLING FOR BELOW NORMAL PRECIPITATION AND ABOVE NORMAL
TEMPERATURES. THE THREE MONTH OUTLOOK FOR THE SUMMER FROM  JUNE
THROUGH AUGUST HAS GREATER ODDS OF DRIER THAN NORMAL AND WARMER THAN
NORMAL CONDITIONS AND THE SAME IS TRUE FOR SEPTEMBER THROUGH
NOVEMBER. IF THAT PANS OUT IT WOULD NOT BE HELPFUL IN ENDING THE
DROUGHT OF THIS SUMMER.


WATER SUPPLY OUTLOOK
--------------------

FORECASTS TRENDED SIGNIFICANTLY DOWNWARD FOR ALL RIVERS FROM A FEW
PERCENT UP TO 10 PERCENT LOWER FROM A MONTH AGO. LONG RANGE
HYDROLOGIC MODELS ARE FORECASTING MUCH BELOW NORMAL AND RECORD LOW
RIVER FLOWS AND WATER SUPPLY FOR MOST RIVERS THROUGH THIS SPRING AND
SUMMER. WATER SUPPLY FORECASTS FOR WESTERN WASHINGTON RANGE FROM A
LOW OF 27 PERCENT FOR THE NORTH FORK SKOKOMISH RIVER TO 67 PERCENT
FOR THE CHEHALIS RIVER AT GRAND MOUND. THE FORECASTS FOR MOST OF THE
RIVERS RANK AS THE LOWEST OR NEXT TO THE LOWEST VOLUMES FOR THE
PERIOD OF RECORD WHICH ARE AS LONG AS 64 YEARS.

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 7.

                           WATER SUPPLY FORECASTS
                  NATURAL FLOW UNLESS OTHERWISE SPECIFIED
                        (IN THOUSANDS OF ACRE FEET)

RIVER AND GAUGING SITE            PERIOD   FORECAST   NORMAL  PERCENT

NOOKSACK RIVER
   AT NORTH CEDARVILLE            APR-SEP      534      1195      45

SKAGIT RIVER
   NEAR CONCRETE (REGULATED)      APR-SEP      3738     5934      63

BAKER RIVER
   UPPER BAKER RESERVOIR INFLOW   APR-SEP       451      805      56

SULTAN RIVER
   SPADA LAKE INFLOW              APR-SEP        94      191      49

TOLT RIVER
   TOLT RESERVOIR INFLOW          APR-SEP        16       51      32

CEDAR RIVER
   CHESTER MORSE LAKE INFLOW      APR-SEP        51      154      33

GREEN RIVER
   HOWARD HANSON DAM INFLOW       APR-SEP       108      260      42

NISQUALLY RIVER
   ALDER RESERVOIR INFLOW         APR-JUL       229      376      61

COWLITZ RIVER
   MAYFIELD RESERVOIR (REGULATED) APR-SEP       950     1835      52

CHEHALIS RIVER
   NEAR GRAND MOUND               APR-SEP       261      398      67

ELWHA RIVER
   MCDONALD BRIDGE                APR-SEP       185      472      39

DUNGENESS RIVER
   NEAR SEQUIM                    APR-SEP        54      145      37

WYNOOCHEE RIVER
   WYNOOCHEE DAM INFLOW           APR-SEP        61       99      62

SKOKOMISH RIVER
   NF SKOKOMISH RIVER
   BELOW STAIRCASE RAPIDS         APR-SEP        36      135      27


SNOW MELT

WITH RECORD LOW SNOW PACK IN MOST AREAS...THERE IS LITTLE CONCERN
FOR SPRING FLOODING THIS YEAR.

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 RARE 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 APRIL 6. 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            15.5 FT TO 17.7 FT

STILLAGUAMISH RIVER
  AT ARLINGTON            14.0 FT             2.8 FT TO 3.9 FT

SNOQUALMIE RIVER
  NEAR SNOQUALMIE        20000 CFS          3295 CFS TO 5537 CFS

COWLITZ RIVER
  AT RANDLE               18.0 FT               6.4 TO 7.0 FT

SF SKOKOMISH RIVER
  NEAR UNION                                 455 CFS TO  777 CFS

FORECASTS ARE SELECTED FROM THOSE PREPARED BY THE NWRFC.
FOR FURTHER DETAILS...GRAPHICS...AND STATISTICS REGARDING THE WATER
SUPPLY FORECASTS VISIT:
HTTP://WWW.NWRFC.NOAA.GOV/WS (LOWER CASE)
HTTP://WWW.NWRFC.NOAA.GOV/ESPNATURAL (LOWER CASE)

FOR FURTHER DETAILS...GRAPHICS...AND STATISTICS REGARDING THE PEAK
FLOW FORECASTS VISIT:
HTTP://WWW.NWRFC.NOAA.GOV/PEAK (LOWER CASE)

THE NEXT WATER SUPPLY AND SPRING FLOOD OUTLOOK FOR WESTERN
WASHINGTON WILL BE ISSUED AROUND THE WEEK OF JUNE 10.

$$

WEATHER.GOV/SEATTLE
JBB






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