573
FGUS76 KSEW 151135
ESFSEW
WAC009-027-029-031-033-035-041-045-053-055-057-061-067-073-312300-
PROBABILISTIC HYDROLOGIC OUTLOOK
NATIONAL WEATHER SERVICE SEATTLE WA
435 AM PDT MON MAR 15 2010
..SPRING AND SUMMER SNOW MELT FLOOD POTENTIAL IN WASHINGTON
BASED ON THE CURRENT SNOWPACK AND EXPECTED PRECIPITATION AND
TEMPERATURES...THE THREAT OF FLOODING IS VERY LOW THROUGHOUT
WASHINGTON. THE BELOW NORMAL SPRING FLOOD RISK MEANS IT WILL BE MUCH
HARDER FOR SOME SMALLER STREAMS AND FLOOD PRONE RIVERS TO HAVE
FLOODING EVEN WITH A SUDDEN LARGE WARMUP OR THE OCCURRENCE OF
THUNDERSTORMS OVER THOSE WATERSHEDS THAT TYPICALLY HAPPENS SOMETIME
DURING THE SPRING MELT SEASON.
THE SNOWPACK WAS BELOW NORMAL FOR MOST OF WASHINGTON. AS OF MARCH
14...THE WATER CONTENT OF THE MOUNTAIN SNOWPACK IN THE BASINS THAT
FEED THE MAJOR RIVERS EAST OF THE CASCADES CREST RANGED FROM 52 TO
83 PERCENT OF NORMAL. WEST OF THE CASCADES...THE WATER CONTENT OF
THE SNOWPACK RANGED FROM 52 TO 97 PERCENT OF NORMAL.
SNOW DEPTH LEVELS FROM THE NORTHWEST AVALANCHE CENTER RANGED FROM 52
TO 109 PERCENT OF NORMAL AS OF MARCH 1.
EAST OF THE CASCADES CREST
FLOODING DURING THE SNOW MELT SEASON CAN OCCUR ANYWHERE WHEN HEAVY
RAIN FALLS IN A RIVER BASIN IF THE RAIN IS INTENSE ENOUGH.
MANY RIVERS EAST OF THE CASCADES CREST REACH THEIR ANNUAL PEAK IN
LATE SPRING OR EARLY SUMMER WHEN THE MOUNTAIN SNOWPACK MELTS AND
RUNS OFF. THE SNOWPACK USUALLY REACHES ITS ANNUAL MAXIMUM IN
APRIL...AND THE RIVERS TYPICALLY CREST BETWEEN MID MAY AND MID JULY.
AS A GENERAL RULE THE LARGER THE SNOWPACK IS AT THE END OF THE
SEASON...THE HIGHER THE CRESTS WILL BE AND VICE VERSA.
HERE ARE THE LATEST SPRING AND SUMMER CREST FORECASTS FOR EASTERN
WASHINGTON RIVERS AS OF MARCH 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 CREST
SNAKE RIVER
NEAR ANATONE 20.0 FT 9.3 FT TO 14.8 FT
PEND OREILLE RIVER
AT NEWPORT 100.0 KCFS 26.1 KCFS TO 46.1 KCFS
SPOKANE RIVER
AT SPOKANE 27.0 FT 22.0 FT TO 23.8 FT
SIMILKAMEEN RIVER
NEAR NIGHTHAWK 13.0 FT 8.0 FT TO 10.8 FT
OKANOGAN RIVER
NEAR TONASKET 15.0 FT 10.6 FT TO 13.1 FT
METHOW RIVER
NEAR PATEROS 10.0 FT 5.8 FT TO 8.3 FT
WENATCHEE RIVER
AT PESHASTIN 13.0 FT 7.7 FT TO 10.1 FT
COLUMBIA RIVER BELOW
PRIEST RAPIDS DAM 32.0 FT 13.8 FT TO 23.4 FT
YAKIMA RIVER
AT CLE ELUM 9.0 FT 5.5 FT TO 7.6 FT
NACHES RIVER
NEAR NACHES 17.0 FT 13.7 FT TO 15.6 FT
YAKIMA RIVER
AT PARKER 10.0 FT 4.7 FT TO 7.7 FT
YAKIMA RIVER
AT KIONA 13.0 FT 5.2 FT TO 7.7 FT
WALLA WALLA RIVER
NEAR TOUCHET 13.0 FT 4.2 FT TO 7.6 FT
WEST OF THE CASCADES CREST
FLOODING IN WESTERN WASHINGTON IS UNLIKELY DURING THE PERIOD OF
MOUNTAIN SNOWPACK RUNOFF...WHICH PEAKS FROM APRIL THROUGH JUNE.
THIS YEAR IT WILL BE VERY UNLIKELY WITH BELOW NORMAL SNOWPACK.
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
NOVEMBER 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.
HOWEVER...OCCASIONALLY WHEN A MUCH ABOVE NORMAL SNOWPACK IS COMBINED
WITH NEAR RECORD HEAT...SOME MINOR FLOODING CAN OCCUR ON THE
MOST FLOOD PRONE RIVERS.
WHILE FLOOD PRODUCING RAINFALL IS RARE AFTER MARCH...MODERATE TO
HEAVY RAIN IN SPRING...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 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.
THE NEXT SPRING FLOOD OUTLOOK FOR WASHINGTON WILL BE ISSUED AROUND
APRIL 9.
33
WEATHER.GOV/SEATTLE
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