Flood Potential Outlook
Issued by NWS Seattle/Tacoma, WA

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2
000
FGUS76 KSEW 072009
ESFSEW

WATER SUPPLY OUTLOOK
NATIONAL WEATHER SERVICE SEATTLE WA
107 PM PDT TUE APRIL 7 2015

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

OVERVIEW: THE FORECASTS OF WATER SUPPLY FOR THE SUMMER WERE FOR
BELOW TO MUCH BELOW NORMAL 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. THE U.S. DROUGHT MONITOR FOR APRIL 2
DEPICTS MODERATE DROUGHT INTENSITY FOR THE OLYMPIC PENINSULA...
ACROSS THE CENTRAL AND SOUTHERN PORTIONS OF THE EAST SLOPES OF THE
CASCADES...AND MUCH OF THE SOUTHERN HALF OF EASTERN WASHINGTON. THIS
IS MOSTLY DUE TO THE SNOW DROUGHT.

THE GOVERNOR HAS DECLARED A DROUGHT FOR THREE REGIONS IN WASHINGTON:
THE EAST SLOPE OF THE CENTRAL CASCADES INCLUDING THE UPPER AND LOWER
YAKIMA...NACHES...WENATCHEE AND ENTIAT RIVER WATERSHEDS...THE WALLA
WALLA RIVER WATERSHED INCLUDING PORTIONS OF WALLA WALLA AND COLUMBIA
COUNTIES...AND THE OLYMPIC PENINSULA...PRIMARILY IN JEFFERSON AND
CLALLAM COUNTIES.  SPECIFIC WATERSHEDS INCLUDE THE QUILCENE-SNOW...
ELWHA-DUNGENESS...LYRE-HOKO...SOL DUC-HOH AND QUEETS-QUINAULT.

MOST AREA SKI RESORTS IN WESTERN WASHINGTON WERE CLOSED AT LEAST
TEMPORARILY BY MID MARCH DUE TO LACK OF SNOW. A FEW WITH HIGHER
ELEVATION TERRAIN HAVE REMAINED OPEN ON A LIMITED BASIS.

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. 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
---------------------

MARCH BROUGHT NEAR TO ABOVE NORMAL PRECIPITATION FOR NEARLY ALL OF
WASHINGTON STATE. THE EXCEPTIONS WERE THE SOUTHWEST INTERIOR AND
EAST SLOPES OF THE CASCADES...WHICH SAW BELOW NORMAL PRECIPITATION
FOR THE MONTH. REACHING THE HALFWAY POINT FOR THE WATER YEAR AND THE
MOST RAINY MONTHS...ALL OF THE REGIONS ARE SEEING NEAR TO ABOVE
NORMAL PRECIPITATION.

THE MONTHLY PERCENTAGE OF NORMAL FOR PRECIPITATION RANGED FROM 89
PERCENT IN THE SOUTHWEST INTERIOR TO 137 PERCENT IN THE NORTHWEST
INTERIOR. THE GREATEST AMOUNT OF PRECIPITATION AT THE CLIMATE
STATIONS FOR THE MOUNTAINS...COAST...AND INTERIOR LOWLANDS WAS 15.39
INCHES AT FORKS IN THE OLYMPICS...16.75 INCHES AT CLEARWATER...AND
9.89 INCHES AT GRAYS RIVER 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.

                        MARCH      WATER YEAR     PAST 3       PAST
12                       2015       TO DATE       MONTHS      MONTHS
WESTERN WASHINGTON
  COAST                   120          106           99           105
  OLYMPICS                 99          103           87           101
  NORTHWEST INTERIOR      137          121          122           121
  PUGET SOUND LOWLANDS     97          101           94           107
  SOUTHWEST INTERIOR       89           91           86            96
  WEST FOOTHILLS CASCADES 103          103           96           107
  CASCADES WEST           101          104           94           108


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

THE SNOWPACK WAS WELL BELOW NORMAL FOR WESTERN WASHINGTON AS OF
APRIL 6 AND CONTINUED TO DETERIORATE INSTEAD OF ACCUMULATE. THE
WATER CONTENT OF THE  MOUNTAIN SNOWPACK IN THE BASINS THAT FEED THE
MAJOR RIVERS WEST OF THE CASCADES AND OLYMPICS RANGED FROM ONLY 3 TO
42 PERCENT OF NORMAL. MANY SITES ARE EXPERIENCING RECORD LOW SNOW
PACK.

DATA FROM THE NORTHWEST WEATHER AND AVALANCHE CENTER AS OF APRIL 1
SHOWED THAT SNOW DEPTHS IN WESTERN WASHINGTON WERE 4 TO 41 PERCENT
OF NORMAL. THESE ARE RECORD LOW SNOW DEPTHS FOR MANY SITES.


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

STREAMFLOWS FOR OF WESTERN WASHINGTON RIVERS FOR LAST MONTH ROUGHLY
RANGED FROM ABOVE NORMAL IN THE NORTH TO BELOW NORMAL IN THE SOUTH
BUT MOST WERE WERE NEAR NORMAL. HOWEVER...THE CURRENT FLOWS ON MANY
RIVERS AS OF APRIL 6 WERE NORMAL TO MUCH BELOW WITH SOME RECORD LOW
DAILY FLOWS.


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

STORAGE FOR ROSS RESERVOIR FOR APRIL 1 WAS AT 98% OF AVERAGE. NOTE
THAT DURING THE WINTER MOST RESERVOIRS ARE KEPT RELATIVELY LOW UNTIL
THE SPRING RUNOFF. STORAGE LEVELS ARE NOT INDICATIVE OF FUTURE
WATER AVAILABILITY UNTIL THE SPRING REFILL HAS OCCURRED.

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

THE OUTLOOK FOR WESTERN WASHINGTON FOR APRIL AND BEYOND...OVER THE
NEXT 7 DAYS THE PRECIPITATION FORECAST IS FOR GENERALLY 0.25 TO 0.75
INCHES WITH UP TO 1.5 INCHES IN THE MOUNTAINS. FOR THE NEXT TWO
WEEKS THE OUTLOOK IS FOR BELOW NORMAL PRECIPITATION AND ABOVE NORMAL
TEMPERATURES. THE MONTHLY OUTLOOK FOR APRIL IS CALLING FOR EQUAL
CHANCES OF ABOVE...NORMAL...OR BELOW NORMAL PRECIPITATION AND ABOVE
NORMAL TEMPERATURES. THE THREE MONTH OUTLOOK FROM APRIL TO JUNE
HAS GREATER ODDS OF DRIER THAN NORMAL AND WARMER THAN NORMAL
CONDITIONS AND THE SAME IS TRUE FOR JULY THROUGH SEPTEMBER.


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

FORECASTS TRENDED 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 RIVER FLOWS AND WATER SUPPLY FOR MOST
RIVERS THROUGH THIS SPRING AND SUMMER. WATER SUPPLY FORECASTS FOR
WESTERN WASHINGTON RANGE FROM A LOW OF 40 PERCENT FOR THE ELWHA
RIVER TO 84 PERCENT FOR THE CHEHALIS RIVER. 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
APRIL 6.

                           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      657      1195      55

SKAGIT RIVER
   NEAR CONCRETE (REGULATED)      APR-SEP      4097     5934      69

BAKER RIVER
   UPPER BAKER RESERVOIR INFLOW   APR-SEP       529      805      66

SULTAN RIVER
   SPADA LAKE INFLOW              APR-SEP       115      191      60

TOLT RIVER
   TOLT RESERVOIR INFLOW          APR-SEP        26       51      52

CEDAR RIVER
   CHESTER MORSE LAKE INFLOW      APR-SEP        71      154      46

GREEN RIVER
   HOWARD HANSON DAM INFLOW       APR-SEP       141      260      54

NISQUALLY RIVER
   ALDER RESERVOIR INFLOW         APR-JUL       276      376      74

COWLITZ RIVER
   MAYFIELD RESERVOIR (REGULATED) APR-SEP      1075     1835      59

CHEHALIS RIVER
   NEAR GRAND MOUND               APR-SEP       327      398      84


ELWHA RIVER
   MCDONALD BRIDGE                APR-SEP       195      472      41

DUNGENESS RIVER
   NEAR SEQUIM                    APR-SEP        56      145      39

WYNOOCHEE RIVER
   WYNOOCHEE DAM INFLOW           APR-SEP        73       99      75

SKOKOMISH RIVER
   NF SKOKOMISH RIVER
   BELOW STAIRCASE RAPIDS         APR-SEP        54      135      40


SNOW MELT

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

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            17.2 FT TO 20.1 FT

STILLAGUAMISH RIVER
  AT ARLINGTON            14.0 FT             4.0 FT TO 7.0 FT

SNOQUALMIE RIVER
  NEAR SNOQUALMIE        20000 CFS          5491 CFS TO 8283 CFS

COWLITZ RIVER
  AT RANDLE               18.0 FT               7.3 TO 8.7 FT

SF SKOKOMISH RIVER
  NEAR UNION                                 835 CFS TO 1299 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





USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.