659  
FGUS76 KSEW 042149  
ESFSEW  
 
PROBABILISTIC HYDROLOGIC OUTLOOK  
NATIONAL WEATHER SERVICE SEATTLE WA  
150 PM PST THU MAR 4 2021  
 
...SPRING AND SUMMER SNOWMELT FLOOD POTENTIAL IN WASHINGTON IS NEAR  
NORMAL TO SLIGHTLY ABOVE NORMAL IN A FEW LOCATIONS...  
 
OVERVIEW:  
 
BASED ON THE CURRENT NORMAL TO MUCH ABOVE NORMAL SNOWPACK, AND  
EXPECTED PRECIPITATION AND TEMPERATURES, THE THREAT OF SPRING  
SBOWMELT FLOODING IS NEAR NORMAL IN MOST AREAS OF EASTERN  
WASHINGTON, WITH A FEW EXCEPTIONS. THERE IS A SLIGHTLY ABOVE NORMAL  
THREAT OF FLOODING FOR THE EAST SIDE OF THE NORTH AND CENTRAL  
WASHINGTON CASCADES INTO THE OKANOGAN VALLEY, AND THE SOUTHEAST  
CORNER OF THE STATE SURROUNDING THE BLUE MOUNTAINS. FOR WESTERN  
WASHINGTON, THE FLOOD THREAT IS MINIMAL AS IS NORMAL.  
 
WHILE THE SPRING FLOOD THREAT IS LOW, THE RIVERS AT MOST THREAT IN  
WASHINGTON ARE THE OKANOGAN, STEHEKIN, AND THE NATCHES RIVERS. MILL  
CREEK WOULD BE ANOTHER STREAM WITH AN ELEVATED THREAT. SIMILARLY AS  
IN THE TABLE BELOW, THE FORECAST FOR THE MOST LIKELY RANGE OF THE  
SPRING CREST FOR MILL CREEK IS 600 CFS TO 1100 CFS.  
 
THERE IS, AS IS TYPICAL, A SMALL SPRING FLOOD THREAT FOR SMALLER  
STREAMS IN EASTERN WASHINGTON MOST YEARS SINCE THERE COULD BE MINOR  
FLOODING WITH A SUDDEN LARGE WARMUP OR THE OCCURRENCE OF  
THUNDERSTORMS OVER THOSE WATERSHEDS. AT THIS TIME, THE THREAT FOR  
SMALL STREAM AND OVERLAND FLOODING IS ABOUT NORMAL DUE TO NOT HAVING  
VERY MUCH SNOW IN THE LOW TO MID ELEVATIONS.  
 
CURRENTLY, THE THREAT OF ICE JAM FLOODING ON THE EAST SIDE IS NEAR  
NORMAL TO SLIGHTLY BELOW NORMAL.  
 
SNOWPACK CONDITIONS:  
 
THE BASIN AVERAGED SNOWPACK RANGED NEAR FROM NORMAL TO MUCH ABOVE  
NORMAL FOR MOST OF WASHINGTON AS OF MARCH 4. THE WATER CONTENT OF  
THE MOUNTAIN SNOWPACK IN THE BASINS THAT FEED THE MAJOR RIVERS EAST  
OF THE CASCADE CREST RANGED FROM 97 TO 130 PERCENT OF THE MEDIAN,  
WITH MOST BASINS 115 PERCENT OR GREATER. WEST OF THE CASCADES, THE  
WATER CONTENT OF THE SNOWPACK RANGED FROM 128 TO 169 PERCENT OF THE  
MEDIAN.  
 
SNOWDEPTH AMOUNTS FROM THE NORTHWEST AVALANCHE CENTER SITES IN THE  
WASHINGTON CASCADES RANGED FROM 109 TO 167 PERCENT OF NORMAL AS OF  
MARCH 1.  
 
CLIMATOLOGY OF SPRING FLOODS:  
 
EAST OF THE CASCADE CREST  
 
MANY RIVERS EAST OF THE CASCADE CREST REACH THEIR ANNUAL PEAK FLOW  
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. WHEN SPRING SNOWMELT FLOODING DOES  
OCCUR, IT IS USUALLY THE COMBINATION OF A MUCH GREATER THAN NORMAL  
SNOWPACK THAT EXPERIENCES A RAPID WARM UP OF HOT TEMPERATURES.  
 
FLOODING DURING THE SNOWMELT SEASON CAN OCCUR ANYWHERE WHEN HEAVY  
RAIN FALLS IN A RIVER BASIN IF THE RAIN IS INTENSE ENOUGH. TYPICALLY  
THIS CAN OCCUR WITH THUNDERSTORMS. THIS IS ESPECIALLY TRUE IF THE  
RAIN FALLS DURING A TIME WHEN THE STREAMS ARE SWOLLEN WITH SNOWMELT  
RUNOFF.  
 
WEST OF THE CASCADE 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 FROM THE HEAVY RAIN FROM WINTER  
STORMS. THE VAST MAJORITY OF RIVER FLOODING IN WESTERN WASHINGTON,  
AND ALMOST ALL MAJOR FLOODS, OCCUR BETWEEN NOVEMBER AND MARCH. HEAVY  
RAINFALL, RATHER THAN SNOWMELT, 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 SNOWMELT, 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 NOT COMMON AFTER MARCH, HEAVY, OR  
EVEN MODERATE RAIN IN SPRING, WHILE RIVERS ARE SWOLLEN WITH SNOWMELT  
RUNOFF, OCCASIONALLY WILL DRIVE THE MOST FLOOD PRONE RIVERS ABOVE  
MINOR FLOOD STAGE. TYPICALLY THESE ARE RIVERS SUCH AS THE SKOKOMISH  
AND SNOQUALMIE RIVERS. HEAVY RAIN IN SUMMER, WHEN ROSS LAKE IS FULL,  
CAN ALSO CAUSE THE SKAGIT RIVER TO FLOOD. WHILE THESE FLOODS ARE  
MINOR COMPARED TO THE WINTER EVENTS, THEY SOMETIMES CAUSE  
SUBSTANTIAL DAMAGE TO FARM CROPS SINCE THE FLOOD PLAINS ARE OFTEN IN  
USE DURING THE SPRING AND SUMMER.  
 
FORECASTS:  
 
HERE ARE THE SPRING AND SUMMER CREST FORECASTS AS OF MARCH 4 FOR  
SOME WASHINGTON RIVERS. STATISTICALLY THERE IS A 67 PERCENT CHANCE  
THAT THE ACTUAL SPRING CREST WILL FALL WITHIN THE MOST LIKELY RANGE  
(70% TO 30% EXCEEDANCE).  
 
RIVER AND SITE FLOOD STAGE MOST LIKELY RANGE OF  
THE SPRING CREST  
SNAKE RIVER  
NEAR ANATONE 20.0 FT 12.1 FT TO 15.0 FT  
 
PEND OREILLE RIVER  
AT NEWPORT 95.0 KCFS 64.9 KCFS TO 78.5 KCFS  
 
SPOKANE RIVER  
AT SPOKANE 27.0 FT 20.7 FT TO 23.9 FT  
 
SIMILKAMEEN RIVER  
NEAR NIGHTHAWK 14.0 FT 10.6 FT TO 11.5 FT  
 
OKANOGAN RIVER  
NEAR TONASKET 15.0 FT 13.7 FT TO 15.2 FT  
 
METHOW RIVER  
NEAR PATEROS 10.0 FT 7.8 FT TO 8.5 FT  
 
STEHEKIN RIVER  
AT STEHEKIN 24.0 FT 23.3 FT TO 24.4 FT  
 
WENATCHEE RIVER  
AT PESHASTIN 13.0 FT 9.8 FT TO 11.1 FT  
 
COLUMBIA RIVER BELOW  
PRIEST RAPIDS DAM 32.0 FT 23.6 FT TO 26.1 FT  
 
YAKIMA RIVER  
AT HORLICK 36.0 FT 32.0 FT TO 32.6 FT  
 
NACHES RIVER  
NEAR NACHES 17.8 FT 17.0 FT TO 18.3 FT  
 
YAKIMA RIVER  
AT PARKER 10.0 FT 6.9 FT TO 7.7 FT  
 
YAKIMA RIVER  
AT KIONA 13.0 FT 8.9 FT TO 10.3 FT  
 
WALLA WALLA RIVER  
NEAR TOUCHET 13.0 FT 8.0 FT TO 9.6 FT  
 
SKAGIT RIVER  
NEAR MT. VERNON 28.0 FT 22.4 FT TO 25.2 FT  
 
STILLAGUAMISH RIVER  
AT ARLINGTON 14.0 FT 8.9 FT TO 11.2 FT  
 
SNOQUALMIE RIVER  
NEAR SNOQUALMIE 20000 CFS 10200 CFS TO 15800 CFS  
 
WHITE RIVER  
MUD MTN DAM INFLOW 3800 CFS TO 5200 CFS  
 
COWLITZ RIVER  
NEAR RANDLE 18.0 FT 11.7 FT TO 14.6 FT  
 
S.F. SKOKOMISH RIVER  
NEAR UNION 2200 CFS TO 7200 CFS  
 
DUNGENESS RIVER  
NEAR SEQUIM 7.0 FT 4.6 FT TO 4.9 FT  
 

 
 
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