HYDROLOGIC OUTLOOK
NATIONAL WEATHER SERVICE SEATTLE WA
210 PM PDT WED MAY 10 2006
...SPRING AND SUMMER SNOW MELT FLOOD POTENTIAL IN WASHINGTON...
EAST OF THE CASCADES CREST
THE MOUNTAIN SNOWPACK IS NEAR NORMAL TO ABOVE NORMAL. IN THE MAJOR
BASINS OF EASTERN WASHINGTON...THE WATER CONTENT OF THE SNOWPACK
RANGED FROM 106 TO 122 PERCENT OF NORMAL AS OF MAY 9. THE NORTH
WEST WEATHER AND AVALANCHE CENTER REPORTED SNOW DEPTHS OF 86 TO 173
PERCENT OF NORMAL IN THE OLYMPIC AND CASCADE MOUNTAINS AS OF MAY 1.
STREAM FLOWS WERE MOSTLY NEAR NORMAL. BASED ON THE CURRENT SNOWPACK
AND EXPECTED PRECIPITATION...THE THREAT OF FLOODING IS NEAR NORMAL
IN ALL AREAS WHICH MEANS THAT SOME SMALLER STREAMS AND FLOOD PRONE
RIVERS COULD HAVE MINOR FLOODING. RECENT DRY AND MILD CONDITIONS
HAVE ALLOWED THE SNOWPACK TO BE REDUCED SLOWLY AND THEREFORE HAS
LOWERED THE FLOOD THREAT SOMEWHAT. IF CONDITIONS SUCH AS AN
EXTENDED HOT PERIOD PUSH THE RIVER LEVELS TO THE HIGH END OF THE
FORECAST RANGE...THE RIVERS MOST AT RISK OF SPRING FLOODING ARE THE
NATCHES...UPPER YAKIMA...AND THE SNAKE RIVER AT ANATONE.
FLOODING DURING THE SNOW MELT SEASON CAN OCCUR ANYWHERE WHEN HEAVY
RAIN FALLS IN A RIVER BASIN IF THE RAIN IS INTENSE ENOUGH. WITH
ABOVE NORMAL STREAM FLOWS THIS SEASON IT WOULD NOT NEED TO BE VERY
HEAVY RAIN TO RESULT IN FLOODING.
MANY RIVERS EAST OF THE CASCADES 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.
HERE ARE THE LATEST SPRING AND SUMMER CREST FORECASTS FOR EASTERN
WASHINGTON RIVERS AS OF MAY 9. 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 17.0 FT TO 21.0 FT
PEND OREILLE RIVER
AT NEWPORT 100000 CFS 56100 CFS TO 76100 CFS
SPOKANE RIVER
AT SPOKANE 27.0 FT 24.2 FT TO 25.8 FT
SIMILKAMEEN RIVER
NEAR NIGHTHAWK 9.3 FT TO 12.8 FT
OKANOGAN RIVER
NEAR TONASKET 15.0 FT 12.5 FT TO 14.8 FT
METHOW RIVER
NEAR PATEROS 10.0 FT 7.1 FT TO 9.1 FT
WENATCHEE RIVER
AT PESHASTIN 13.0 FT 9.0 FT TO 11.2 FT
COLUMBIA RIVER BELOW
PRIEST RAPIDS DAM 32.0 FT 20.0 FT 25.5 FT
YAKIMA RIVER
AT CLE ELUM 9.0 FT 8.0 FT TO 9.3 FT
NACHES RIVER
NEAR NACHES 17.0 FT 15.8 FT TO 17.5 FT
YAKIMA RIVER
AT PARKER 10.0 FT 6.8 FT TO 9.0 FT
YAKIMA RIVER
AT KIONA 13.0 FT 8.7 FT TO 11.9 FT
WALLA WALLA RIVER
NEAR TOUCHET 13.0 FT 8.3 FT TO 10.8 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 WILL BE NO EXCEPTION WITH NORMAL TO SLIGHTLY ABOVE 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 A CASE OF 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.
WHILE FLOOD PRODUCING RAINFALL IS RARE AFTER MARCH...MODERATE TO
HEAVY RAIN IN SPRING...WHILE RIVERS ARE SWOLLEN WITH SNOW MELT
RUNOFF...OCCASIONALLY DRIVES 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
JUNE 9.
JBB
WEATHER.GOV/SEATTLE