Flood Potential Outlook
Issued by NWS Seattle/Tacoma, WA

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WATER SUPPLY OUTLOOK
NATIONAL WEATHER SERVICE SEATTLE WA
254 PM PDT THU MARCH 5 2015

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

OVERVIEW: THE FORECASTS OF WATER SUPPLY FOR THE SUMMER IN WESTERN
WASHINGTON WERE MOSTLY FOR MUCH BELOW NORMAL VOLUMES FOR WESTERN
WASHINGTON RIVERS. MANY RIVERS HAVE FORECASTS THAT ARE FOR 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.

FLOODING IN WESTERN WASHINGTON IS UNLIKELY DURING THE PERIOD OF
MOUNTAIN SNOWPACK RUNOFF...WHICH PEAKS FROM APRIL THROUGH JUNE. THIS
YEAR THERE IS A MUCH BELOW 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 EXTREMELY LOW.

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

FEBRUARY WAS ABOVE NORMAL TO MUCH ABOVE NORMAL PRECIPITATION ACROSS
WESTERN WASHINGTON EXCEPT FOR THE COASTAL REGION WHICH WAS BELOW
NORMAL. THE WATER YEAR TOTALS ARE ALL NEAR NORMAL TO ABOVE NORMAL.

IN WESTERN WASHINGTON...THE MONTHLY PERCENTAGE OF NORMAL FOR
PRECIPITATION RANGED FROM 80 PERCENT FOR THE COAST TO
135 PERCENT FOR THE PUGET SOUND LOWLANDS. MUCH OF THE HIGH
PRECIPITATION VALUES CAME FROM A VERY LARGE FLOOD EVENT THE FIRST
WEEK OF THE MONTH THAT SAW WIDESPREAD FLOODING WITH SOME MODERATE
TO MAJOR FLOODING.

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

THE SNOWPACK WAS WELL BELOW NORMAL FOR WESTERN WASHINGTON AS OF
MARCH 5. THE WATER CONTENT OF THE  MOUNTAIN SNOWPACK IN THE BASINS
THAT FEED THE MAJOR RIVERS WEST OF THE CASCADES AND OLYMPICS RANGED
FROM ONLY 9 TO 49 PERCENT OF NORMAL.

DATA FROM THE NORTHWEST WEATHER AND AVALANCHE CENTER AS OF MARCH 1
SHOWED THAT SNOW DEPTHS IN WESTERN WASHINGTON WERE 5 TO 44 PERCENT
OF NORMAL. THESE ARE RECORD LOW SNOW DEPTHS FOR MANY SITES.


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

STREAMFLOWS FOR LAST MONTH WERE NEAR TO ABOVE NORMAL FOR MOST OF
WESTERN WASHINGTON RIVERS. HOWEVER...THE CURRENT FLOWS AS OF MARCH 5
ON MANY RIVERS ARE MUCH BELOW TO RECORD DAILY LOW FLOWS.


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

STORAGE FOR ROSS RESERVOIR FOR MARCH 1 WAS AT 96% OF AVERAGE. NOTE
THAT DURING THE WINTER MOST RESERVOIRS ARE KEPT RELATIVELY LOW UNTIL
THE SPRING RUNOFF. RESERVOIR STORAGE LEVELS ARE NOT INDICATIVE OF
FUTURE WATER AVAILABILITY UNTIL THE SPRING REFILL HAS OCCURRED.

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

THE OUTLOOK FOR MARCH AND BEYOND...FOR THE NEXT TWO WEEKS THE
OUTLOOK IS FOR ABOVE NORMAL PRECIPITATION FOR WESTERN WASHINGTON.
THE MONTHLY OUTLOOK FOR MARCH IS CALLING FOR GREATER ODDS OF DRIER
THAN NORMAL AND WARMER THAN NORMAL CONDITIONS FOR WESTERN
WASHINGTON. THE THREE MONTH OUTLOOK FROM MARCH TO MAY CALLS FOR
GREATER ODDS OF DRIER THAN NORMAL AND WARMER THAN NORMAL CONDITIONS
FOR WESTERN WASHINGTON.


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 MARCH 5. 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            19.3 FT TO 22.0 FT

STILLAGUAMISH RIVER
  AT ARLINGTON            14.0 FT             6.4 FT TO 11.2 FT

SNOQUALMIE RIVER
  NEAR SNOQUALMIE        20000 CFS          6817 CFS TO  9676 CFS

COWLITZ RIVER
  AT RANDLE                18.0 FT               7.9 TO 10.0 FT

SF SKOKOMISH RIVER
  NEAR UNION                                1584 CFS TO 6095 CFS



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

LONG RANGE HYDROLOGIC MODELS ARE FORECASTING MUCH BELOW NORMAL RIVER
FLOWS AND WATER SUPPLY FOR ALL RIVERS THROUGH THIS SPRING AND
SUMMER. FORECASTS VOLUMES HAVE DROPPED LOWER THE LAST SEVERAL WEEKS.
WATER SUPPLY FORECASTS FOR WESTERN WASHINGTON RANGE FROM A LOW OF 45
PERCENT FOR THE DUNGENESS RIVER RIVER TO 88 PERCENT FOR THE SATSOP
RIVER. THE FORECAST VOLUMES FOR THE SKAGIT AND COWLITZ RIVERS RANK
AS NEXT TO THE LOWEST FOR THE 45 YEAR HISTORY. THE DUNGENESS RIVER
FORECAST VOLUME RANKS AS A RECORD LOW OUT OF ALL 45 HISTORICAL 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
MARCH 5.

                           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       628     1195      53

SKAGIT RIVER
   NEAR CONCRETE (REGULATED)      APR-SEP      4220     5934      71

BAKER RIVER
   UPPER BAKER RESERVOIR INFLOW   APR-SEP       537      805      67

SULTAN RIVER
   SPADA LAKE INFLOW              APR-SEP       129      191      67

TOLT RIVER
   TOLT RESERVOIR INFLOW          APR-SEP        32       51      62

CEDAR RIVER
   CHESTER MORSE LAKE INFLOW      APR-SEP        80      154      52

GREEN RIVER
   HOWARD HANSON DAM INFLOW       APR-SEP       162      262      62

NISQUALLY RIVER
   ALDER RESERVOIR INFLOW         APR-SEP       274      376      73

COWLITZ RIVER
   MAYFIELD RESERVOIR (REGULATED) APR-SEP      1084     1835      59

CHEHALIS RIVER
   NEAR GRAND MOUND               APR-SEP       314      397      79

ELWHA RIVER
   MCDONALD BRIDGE                APR-SEP       224      472      47

DUNGENESS RIVER
   NEAR SEQUIM                    APR-SEP        65      145      45

WYNOOCHEE RIVER
   WYNOOCHEE DAM INFLOW           APR-SEP        77       99      78

SKOKOMISH RIVER
   CUSHMAN DAM INFLOW             APR-SEP        93      191      49



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 APRIL 6.

$$

WEATHER.GOV/SEATTLE











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