National Weather Service Text Product Display

Flood Potential Outlook
Issued by NWS Seattle/Tacoma, WA
Versions: 1 2 3
000
FGUS76 KSEW 142245
ESFSEW

WATER SUPPLY OUTLOOK
NATIONAL WEATHER SERVICE SEATTLE WA
345 PM PDT TUE MAY 14 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 TO ABOVE 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
---------------------

AFTER A DRIER THAN NORMAL MARCH FOR MOST OF THE STATE...APRIL 2013
DID BRING APRIL SHOWERS THAT BROUGHT NEAR TO WETTER THAN NORMAL
CONDITIONS FOR ALL OF WASHINGTON. EVERY REGION SAW 100 PERCENT OF
NORMAL OR GREATER PRECIPITATION FOR THE MONTH EXCEPT THE NORTHEAST
REGION...WHICH SAW 94 PERCENT. THE STATE CONTINUES TO SEE NEAR OR
ABOVE NORMAL VALUES FOR THE WATER YEAR.

IN WESTERN WASHINGTON...THE MONTHLY PERCENTAGE OF NORMAL FOR
PRECIPITATION RANGED FROM 105 PERCENT FOR THE SOUTHWEST INTERIOR TO
174 PERCENT IN THE NORTHWEST INTERIOR. THE GREATEST AMOUNT OF
PRECIPITATION AT THE CLIMATE STATIONS FOR THE
MOUNTAINS...COAST...AND INTERIOR LOWLANDS WAS 16.46 INCHES AT CEDAR
LAKE IN THE WESTERN CASCADES...10.93 INCHES AT QUILLAYUTE...AND
11.53 INCHES AT PALMER IN THE WEST FOOTHILLS OF THE CASCADES.

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

THE SNOWPACK WAS ABOVE NORMAL FOR WESTERN WASHINGTON AS OF MAY 9.
THE WATER CONTENT OF THE  MOUNTAIN SNOWPACK IN THE BASINS THAT FEED
THE MAJOR RIVERS WEST OF THE CASCADES RANGED FROM 112 TO 152 PERCENT
OF NORMAL.

DATA FROM THE NORTHWEST WEATHER AND AVALANCHE CENTER AS OF MAY 1
SHOWED THAT SNOW DEPTHS IN WESTERN WASHINGTON WERE 71 TO 124 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 HAD NO
REPORT THIS MONTH. 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 MAY AND BEYOND...FOR THE NEXT TWO WEEKS THE OUTLOOK
IS FOR NEAR NORMAL PRECIPITATION FOR ALL OF WASHINGTON. THE MONTHLY
OUTLOOK FOR MAY IS CALLING FOR DRIER THAN NORMAL CONDITIONS FOR ALL
OF WASHINGTON...WITH A HIGHER PROBABILITY OF DRIER THAN NORMAL
CONDITIONS FOR THE WESTERN HALF OF THE STATE. THE THREE MONTH
OUTLOOK FROM MAY TO JULY CALLS FOR DRIER THAN NORMAL CONDITIONS FOR
THE ENTIRE STATE.


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

LONG RANGE HYDROLOGIC MODELS ARE FORECASTING NEAR NORMAL TO 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 84 PERCENT FOR THE NISQUALLY RIVER TO 118
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 MAY 9.

                           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      6806     5934     115

BAKER RIVER
   UPPER BAKER RESERVOIR INFLOW   MAY-SEP       788      710     111

SULTAN RIVER
   SPADA LAKE INFLOW              MAY-SEP       124      132      94

TOLT RIVER
   TOLT RESERVOIR INFLOW          MAY-SEP        38       40      96

CEDAR RIVER
   CHESTER MORSE LAKE INFLOW      MAY-SEP        98      102      96

NISQUALLY RIVER
   ALDER RESERVOIR INFLOW         MAY-JUL       186      221      84

COWLITZ RIVER
   MAYFIELD RESERVOIR (REGULATED) APR-SEP      2166     1835     118

ELWHA RIVER
   MCDONALD BRIDGE                MAY-SEP       486      428     114

DUNGENESS RIVER
   NEAR SEQUIM                    APR-SEP       159      145     110

WYNOOCHEE RIVER
   WYNOOCHEE DAM INFLOW           MAY-SEP        66       64     103

SKOKOMISH RIVER
   CUSHMAN DAM INFLOW             MAY-SEP       177      188      94


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 MAY 8. 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            21.8 FT TO 22.9 FT

STILLAGUAMISH RIVER
  AT ARLINGTON            14.0 FT             5.8 FT TO 7.4 FT

SNOQUALMIE RIVER
  NEAR SNOQUALMIE        20000 CFS          8436 CFS TO 10610 CFS

PUYALLUP RIVER
  NEAR ORTING             4500 CFS          1618 CFS TO 2239 CFS

SF SKOKOMISH RIVER
  NEAR UNION                                 657 CFS TO 1548 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 JUNE 10.

$$

WEATHER.GOV/SEATTLE
National Weather Service Weather Forecast Office
