Flood Potential Outlook
Issued by NWS Seattle/Tacoma, WA

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PROBABILISTIC HYDROLOGIC OUTLOOK
NATIONAL WEATHER SERVICE SEATTLE WA
127 PM PDT FRI MAY 13 2016

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

OVERVIEW: THE FORECASTS OF WATER SUPPLY FOR THE SUMMER VARIED
GREATLY FROM MUCH BELOW NORMAL TO NEAR NORMAL FOR WESTERN WASHINGTON
RIVERS. MOST RIVERS HAD FORECASTS THAT WERE NEAR OR ABOVE 75 PERCENT
OF NORMAL. DESPITE NEAR TO ABOVE NORMAL RAINFALL AND SNOW PACK INTO
MARCH...A VERY WARM AND DRY APRIL AND FIRST PART OF MAY HAS LEFT
SNOW PACK THAT IS MELTING FAST AND IS NOW BELOW NORMAL FOR THIS TIME
OF YEAR.

FLOODING IN WESTERN WASHINGTON IS UNLIKELY DURING THE PERIOD OF
MOUNTAIN SNOW PACK RUNOFF...WHICH PEAKS FROM APRIL THROUGH JUNE.
BASED ON THE CURRENT SNOW PACK AND EXPECTED PRECIPITATION AND
TEMPERATURES...THE THREAT OF SPRING AND SUMMER SNOWMELT FLOODING
IN WESTERN WASHINGTON IS BELOW ITS NORMAL LOW LEVEL.


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

APRIL WAS VERY DRY WITH NEARLY ALL OF WASHINGTON STATE RECEIVING
MUCH BELOW NORMAL PRECIPITATION. EVEN WITH THE VERY DRY APRIL...THE
WATER YEAR REMAINS IN THE ABOVE TO MUCH ABOVE NORMAL CATEGORY FOR
NEARLY ALL OF WASHINGTON STATE.

THE MONTHLY PERCENTAGE OF NORMAL FOR PRECIPITATION RANGED FROM 36
PERCENT ON THE COAST TO 67 PERCENT IN THE NORTHWEST INTERIOR. THE
GREATEST AMOUNT OF PRECIPITATION AT THE CLIMATE STATIONS FOR THE
MOUNTAINS...COAST...AND INTERIOR LOWLANDS WAS 4.65 INCHES AT STARTUP
IN THE CASCADE FOOTHILLS...2.73 AT ABERDEEN...AND 3.85 INCHES AT
MONROE IN THE PUGET SOUND LOWLANDS.

                        APRIL     WATER YEAR      PAST 3       PAST 12
                         2016       TO DATE       MONTHS        MONTHS
WESTERN WASHINGTON
  COAST                    36          118          117           110
  OLYMPICS                 40          115          111           108
  NORTHWEST INTERIOR       67          121          127           108
  PUGET SOUND LOWLANDS     56          133          125           122
  SOUTHWEST INTERIOR       63          122          109           110
  WEST FOOTHILLS CASCADES  48          124          113           111
  CASCADES WEST            38          125          116           114


SNOW PACK CONDITIONS
-------------------

THE SNOW PACK WAS BELOW NORMAL TO WELL BELOW NORMAL FOR WESTERN
WASHINGTON AS OF MAY 12. THE NEAR NORMAL SNOW PACK AS OF THE END OF
MARCH HAS MELTED MORE RAPIDLY THAN NORMAL AND THERE WERE FEWER
REPLENISHING SNOW STORMS THAN NORMAL DURING THE DRY APRIL. A FEW
AREAS ARE ALREADY SNOW FREE. THE WATER CONTENT OF THE MOUNTAIN
SNOW PACK IN THE BASINS THAT FEED THE MAJOR RIVERS WEST OF THE
CASCADES AND OLYMPICS RANGED FROM 35 PERCENT OF NORMAL FOR THE
CENTRAL PUGET SOUND BASIN TO 74 PERCENT OF NORMAL FOR OLYMPICS.
SINCE THE MELTING IS OCCURRING DURING THE MELT SEASON...RESERVOIRS
ARE ABLE TO CAPTURE MUCH OF THE RUNOFF.

DATA FROM THE NORTHWEST WEATHER AND AVALANCHE CENTER AS OF MAY 1
SHOWED THAT SNOW DEPTHS IN WESTERN WASHINGTON WERE 0 TO 86 PERCENT
OF NORMAL. ONLY THE 0 PERCENT AT WHITE PASS IS NEAR THE RECORD LOW
FOR THE DATE.


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

STREAMFLOWS FOR MANY OF WESTERN WASHINGTON RIVERS WERE RUNNING AT
NEAR NORMAL TO ABOVE NORMAL LEVELS AS OF MAY 12. BUT A Significant
NUMBER OF STREAMS WITH LOW ELEVATION WATERSHEDS ARE RUNNING AT BELOW
TO MUCH BELOW NORMAL LEVELS. THIS WAS BECAUSE OF THE VERY WARM AND
DRY APRIL. THE WARMTH DEPLETED THE LOW ELEVATION SNOW AND THE DRY
CONDITIONS ALLOWED THE STREAMS WITH NO SNOW PACK TO HAVE FALLING
LEVELS. THIS WAS LARGELY THE CENTRAL PUGET SOUND AND SOUTHWEST
REGIONS AND THE WASHINGTON COAST.

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

STORAGE FOR ROSS RESERVOIR FOR MAY 1 WAS AT 93 PERCENT OF AVERAGE.
THE COMBINED RESERVOIR STORAGE OF CHESTER MORSE LAKE...MASONRY
POOL...LAKE YOUNGS...AND SOUTH FORK TOLT IS ABOVE AVERAGE.


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

THE OUTLOOK FOR MAY AND BEYOND FOR WASHINGTON STATE...THE MONTHLY
OUTLOOK FOR MAY CALLS FOR EQUAL CHANCES OF ABOVE...BELOW OR NEAR
NORMAL PRECIPITATION AND GREATER CHANCES OF ABOVE NORMAL
TEMPERATURES. THE THREE MONTH OUTLOOK FOR MAY THROUGH JULY IS ALSO
CALLING FOR EQUAL CHANCES OF ABOVE...BELOW OR NEAR NORMAL
PRECIPITATION AND GREATER CHANCES OF ABOVE NORMAL TEMPERATURES.


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

FORECASTS TRENDED SIGNIFICANTLY DOWNWARD FOR ALL RIVERS FROM A FEW
PERCENT UP TO 30 PERCENT LOWER FROM A MONTH AGO. LONG RANGE
HYDROLOGIC MODELS ARE FORECASTING A WIDE RANGE OF AVAILABLE
WATER FROM MUCH BELOW NORMAL TO 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 54 PERCENT FOR
THE CHEHALIS RIVER NEAR GRAND MOUND TO 96 PERCENT FOR THE ELWHA
RIVER AT MCDONALD BRIDGE.

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 12.

                           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      863      1195      74

SKAGIT RIVER
   NEAR CONCRETE (REGULATED)      APR-SEP      5657     5934      95

BAKER RIVER
   UPPER BAKER RESERVOIR INFLOW   APR-SEP       710      806      88

SULTAN RIVER
   SPADA LAKE INFLOW              APR-SEP       126      189      67

TOLT RIVER
   TOLT RESERVOIR INFLOW          APR-SEP        34       46      73

CEDAR RIVER
   CHESTER MORSE LAKE INFLOW      APR-SEP       110      152      73

GREEN RIVER
   HOWARD HANSON DAM INFLOW       APR-SEP       187      260      72

NISQUALLY RIVER
   ALDER RESERVOIR INFLOW         APR-JUL       299      378      79

COWLITZ RIVER
   MAYFIELD RESERVOIR (REGULATED) APR-SEP      1582     1835      86

CHEHALIS RIVER
   NEAR GRAND MOUND               APR-SEP       211      390      54

ELWHA RIVER
   MCDONALD BRIDGE                APR-SEP       451      472      96

DUNGENESS RIVER
   NEAR SEQUIM                    APR-SEP       139      145      96

WYNOOCHEE RIVER
   WYNOOCHEE DAM INFLOW           APR-SEP        63       98      65

SKOKOMISH RIVER
   NF SKOKOMISH RIVER
   BELOW STAIRCASE RAPIDS         APR-SEP       116      135      86


SNOW MELT

WITH LOW SNOW PACK IN MOST AREAS...THERE IS LITTLE CONCERN FOR
SPRING SNOW MELT 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 SNOW PACK 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 SNOW PACK. 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 SPRING AND SUMMER CREST FORECASTS FOR WESTERN
WASHINGTON RIVERS AS OF MAY 12. 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             3.1 FT TO 4.5 FT

SNOQUALMIE RIVER
  NEAR SNOQUALMIE        20000 CFS          4590 CFS TO 6800 CFS

COWLITZ RIVER
  AT RANDLE               18.0 FT              10.0 TO 10.0 FT

SF SKOKOMISH RIVER
  NEAR UNION                                 500 CFS TO  830 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/NATURAL (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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