FGUS76 KSEW DDHHMM ESFSEW WAC009-027-029-031-033-035-041-045-053-055-057-061-067-073-312300- HYDROLOGIC OUTLOOK NATIONAL WEATHER SERVICE SEATTLE WA 830 AM PDT MON MAR 16 2009 ...SPRING AND SUMMER SNOW MELT FLOOD POTENTIAL IN WASHINGTON... BASED ON THE CURRENT SNOWPACK AND EXPECTED PRECIPITATION AND TEMPERATURES...THE THREAT OF FLOODING IS LOWER THAN IT USUALLY IS FOR WASHINGTON. THE SNOWPACK WAS BELOW NORMAL FOR MOST OF WASHINGTON AS OF MARCH 11. THE WATER CONTENT OF THE MOUNTAIN SNOWPACK IN THE BASINS THAT FEED THE MAJOR RIVERS EAST OF THE CASCADES CREST RANGED FROM 62 TO 90 PERCENT OF NORMAL...EXCEPT FOR THE LOWER COLUMBIA BASIN WHICH IS 101 PERCENT OF NORMAL. MANY OF LOCATIONS FOR THAT GROUP ARE IN OREGON. WEST OF THE CASCADES...THE WATER CONTENT OF THE SNOWPACK RANGED FROM 63 TO 95 PERCENT OF NORMAL. DATA FROM THE NORTHWEST WEATHER AND AVALANCHE CENTER AS OF MARCH 1 SHOW THAT SNOW DEPTHS WERE 59 TO 91 PERCENT OF NORMAL EXCEPT FOR WHITE PASS WHICH WAS AT 114 PERCENT. 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. HERE ARE THE LATEST SPRING AND SUMMER CREST FORECASTS FOR EASTERN WASHINGTON RIVERS AS OF MARCH 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 11.1 FT TO 13.3 FT PEND OREILLE RIVER AT NEWPORT 100.0 KCFS 43.5 KCFS TO 63.5 KCFS SPOKANE RIVER AT SPOKANE 27.0 FT 23.6 FT TO 25.0 FT SIMILKAMEEN RIVER NEAR NIGHTHAWK 13.0 FT 7.8 FT TO 10.8 FT OKANOGAN RIVER NEAR TONASKET 15.0 FT 9.7 FT TO 12.8 FT METHOW RIVER NEAR PATEROS 10.0 FT 5.6 FT TO 8.0 FT WENATCHEE RIVER AT PESHASTIN 13.0 FT 8.1 FT TO 10.1 FT COLUMBIA RIVER BELOW PRIEST RAPIDS DAM 32.0 FT 16.8 FT TO 25.8 FT YAKIMA RIVER AT CLE ELUM 9.0 FT 6.0 FT TO 7.8 FT NACHES RIVER NEAR NACHES 17.0 FT 14.1 FT TO 16.0 FT YAKIMA RIVER AT PARKER 10.0 FT 5.4 FT TO 8.0 FT YAKIMA RIVER AT KIONA 13.0 FT 6.5 FT TO 10.3 FT WALLA WALLA RIVER NEAR TOUCHET 13.0 FT 6.4 FT TO 9.4 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. 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...A VERY LARGE SNOWPACK LATE INTO SPRING COMBINED WITH NEAR RECORD WARM TEMPERATURES CAN BRING THE MOST FLOOD PRONE RIVERS TO AROUND FLOOD STAGE AND CAUSE MINOR FLOODING ON RARE OCCASIONS. 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 8. $$ JBB WEATHER.GOV/SEATTLE