National Weather Service Text Product Display

Flood Potential Outlook
Issued by NWS Seattle/Tacoma, WA
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WATER SUPPLY OUTLOOK
NATIONAL WEATHER SERVICE SEATTLE WA
849 AM PDT FRI APR 19 2013

...WESTERN WASHINGTON WATER SUPPLY AND SPRING THROUGH SUMMER SNOW
MELT FLOOD POTENTIAL IN WESTERN WASHINGTON...

OVERVIEW: THE FORECASTS OF WATER SUPPLY FOR THE SUMMER IN WESTERN
WASHINGTON WERE MOSTLY FOR NEAR NORMAL VOLUMES FOR WASHINGTON RIVERS.

FLOODING IN WESTERN WASHINGTON IS UNLIKELY DURING THE PERIOD OF
MOUNTAIN SNOWPACK RUNOFF...WHICH PEAKS FROM APRIL THROUGH JUNE. THIS
YEAR WILL BE NO EXCEPTION EVEN WHERE THERE IS ABOVE NORMAL SNOWPACK
IN SOME AREAS. BASED ON THE CURRENT SNOWPACK AND EXPECTED
PRECIPITATION AND TEMPERATURES...THE THREAT OF SPRING AND SUMMER
SNOWMELT FLOODING IN WESTERN WASHINGTON IS NORMAL...WHICH IS VERY
LOW.

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

MARCH SAW ABOVE NORMAL PRECIPITATION SOME PARTS OF
WASHINGTON...MOSTLY ON THE COAST...OLYMPIC MOUNTAINS AND NORTHWEST
INTERIOR. AS FOR THE WATER YEAR...THE STATE CONTINUES TO BE NEAR OR
ABOVE NORMAL.

IN WESTERN WASHINGTON...THE MONTHLY PERCENTAGE OF NORMAL FOR
PRECIPITATION RANGED FROM 64 PERCENT FOR THE SOUTHWEST INTERIOR TO
118 PERCENT OVER THE COAST. THE GREATEST AMOUNT OF PRECIPITATION AT
THE CLIMATE STATIONS FOR THE MOUNTAINS...COAST...AND INTERIOR
LOWLANDS WAS 16.35 INCHES AT THE HOH RANGER STATION IN THE
OLYMPICS...19.02 INCHES AT CLEARWATER...AND 7.09 INCHES AT
CLEARBROOK IN THE NORTHWEST INTERIOR.

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

THE SNOWPACK WAS ABOVE NORMAL FOR WESTERN WASHINGTON AS OF APRIL 15.
THE WATER CONTENT OF THE  MOUNTAIN SNOWPACK IN THE BASINS THAT FEED
THE MAJOR RIVERS WEST OF THE CASCADES RANGED FROM 112 TO 137 PERCENT
OF NORMAL.

DATA FROM THE NORTHWEST WEATHER AND AVALANCHE CENTER AS OF APRIL 15
SHOWED THAT SNOW DEPTHS IN WESTERN WASHINGTON WERE 94 TO 114 PERCENT
OF NORMAL.


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

STREAMFLOWS FOR LAST MONTH WERE NEAR TO ABOVE NORMAL FOR MOST OF
WESTERN WASHINGTON RIVERS.


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

STORAGE FOR MAJOR RESERVOIRS AROUND THE WESTERN WASHINGTON WERE
BELOW NORMAL SINCE THE END OF MARCH. NOTE THAT DURING THE WINTER
MOST RESERVOIRS ARE KEPT RELATIVELY LOW UNTIL THE SPRING RUNOFF AND
STORAGE LEVELS ARE NOT INDICATIVE OF FUTURE WATER AVAILABILITY.

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

THE OUTLOOK FOR THE REST OF APRIL AND BEYOND...MONTHLY OUTLOOK FOR
APRIL AND THE THREE MONTH OUTLOOK FROM APRIL TO JUNE BOTH CALL FOR
NORMAL CONDITIONS FOR THE STATE.


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

LONG RANGE HYDROLOGIC MODELS ARE FORECASTING NEAR 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 90
PERCENT FOR THE SKOKOMISH RIVER TO 109 PERCENT OF NORMAL FOR THE
COWLITZ 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 17.

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

RIVER AND GAUGING SITE            PERIOD   FORECAST   NORMAL  PERCENT

SKAGIT RIVER
   NEAR CONCRETE (REGULATED)      APR-SEP      5929     6267      95

BAKER RIVER
   UPPER BAKER RESERVOIR INFLOW   APR-SEP       820      797     103

SULTAN RIVER
   SPADA LAKE INFLOW              APR-SEP       161      171      94

TOLT RIVER
   TOLT RESERVOIR INFLOW          APR-SEP        49       46     106

CEDAR RIVER
   CHESTER MORSE LAKE INFLOW      APR-SEP       128      132      97

NISQUALLY RIVER
   NEAR NATIONAL                  APR-SEP       232      251      92

COWLITZ RIVER
   MAYFIELD RESERVOIR (REGULATED) APR-SEP      1722     1585     109

ELWHA RIVER
   MCDONALD BRIDGE                APR-SEP       456      484      94

DUNGENESS RIVER
   NEAR SEQUIM                    APR-SEP       152      145     105

WYNOOCHEE RIVER
   WYNOOCHEE DAM INFLOW           APR-SEP        88       87     101

SKOKOMISH RIVER
   CUSHMAN DAM INFLOW             APR-SEP       195      217      90


SNOW MELT 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 16. 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            21.1 FT TO 23.5 FT

STILLAGUAMISH RIVER
  AT ARLINGTON            14.0 FT             6.2 FT TO 9.3 FT

SNOQUALMIE RIVER
  NEAR SNOQUALMIE        20000 CFS          9015 CFS TO 14000 CFS

PUYALLUP RIVER
  NEAR ORTING             4500 CFS          1689 CFS TO 2327 CFS

SF SKOKOMISH RIVER
  NEAR UNION                                1258 CFS TO 2554 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/OUTLOOK (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 MAY 6.

$$

WEATHER.GOV/SEATTLE
National Weather Service Weather Forecast Office
