####018006753#### FGUS76 KSEW 111840 ESFSEW PROBABILISTIC HYDROLOGIC OUTLOOK NATIONAL WEATHER SERVICE SEATTLE WA 1140 AM PDT Thu Mar 11 2024 ...SPRING AND SUMMER SNOWMELT FLOOD POTENTIAL IN WASHINGTON IS LOWER THAN NORMAL FOR WASHINGTON... The threat for spring melt flooding is lower than normal based on river modeling, the current low snow pack, combined with the expected precipitation and temperatures. There is generally less than a 10% chance of exceeding flood stage on the major rivers in Washington this spring. The risk for small stream and overland flooding at this time is below normal. There is a small spring flood risk for smaller streams in eastern Washington most years since there could be a sudden large warmup or the occurrence of thunderstorms over those watersheds that may result in minor flooding. There is no increased risk of ice jam flooding on the east side. For western Washington, the flood risk is minimal as is normal. However, is a low chance of river flooding during the spring but that is due to moderate to heavy rainstorms bringing rivers to flood, and not from snowmelt. Snow pack conditions: the snowpack ranged from well below normal to below normal for most of Washington, with a few isolated gages showing above normal as of March 11. The average water content of the mountain snowpack in the basins that feed the major rivers east of the Cascade crest ranged from 63 to 89 percent of normal with most basins between 68 and 87 percent. West of the Cascades, the water content of the snowpack ranged from 57 to 69 percent, with the Olympics being the lowest. Snow depth amounts from the Northwest Avalanche Center sites in the Washington Cascades ranged from 66 to 113 percent of normal, with most sites below 100 percent 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 rapid warm up of hot temperatures and a much greater than normal snow pack. But it can happen with lower snow packs as well due to the very large amounts of snow typically in the Washington mountains. Flooding during the snowmelt season can occur anywhere when heavy rain falls in a river basin if the rain is intense or lasts long enough. Typically this can occur with thunderstorms. This is especially true if the rain falls during a time when the streams are running higher due to 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 rare after March, moderate to heavy rain in spring, while rivers are swollen with snowmelt runoff, occasionally 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 spring/summer 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 latest spring and summer crest forecasts for some Washington rivers as of March 11. Statistically,there is a 70 percent chance that the actual spring crest will exceed the lower value and a 30 percent chance of exceeding the higher value. RIVER AND SITE FLOOD STAGE MOST LIKELY RANGE OF THE SPRING CREST SNAKE RIVER NEAR ANATONE 20.0 FT 12.3 FT TO 14.9 FT PEND OREILLE RIVER AT NEWPORT 95.0 KCFS 52.7 KCFS TO 63.1 KCFS SPOKANE RIVER AT SPOKANE 27.0 FT 23.7 FT TO 24.3 FT SIMILKAMEEN RIVER NEAR NIGHTHAWK 14.0 FT 8.2 FT TO 9.1 FT OKANOGAN RIVER NEAR TONASKET 15.0 FT 10.5 FT TO 12.0 FT METHOW RIVER NEAR PATEROS 10.0 FT 5.8 FT TO 6.7 FT WENATCHEE RIVER AT PESHASTIN 13.0 FT 8.3 FT TO 9.5 FT STEHEKIN RIVER AT STEHEKIN 24.O FT 22.0 FT TO 22.7 FT COLUMBIA RIVER BELOW PRIEST RAPIDS DAM 32.0 FT 19.4 FT TO 21.4 FT YAKIMA RIVER AT HORLICK 36.0 FT 31.4 FT TO 31.8 FT NACHES RIVER NEAR NACHES 17.8 FT 16.2 FT TO 16.8 FT YAKIMA RIVER AT PARKER 10.0 FT 5.9 FT TO 6.5 FT YAKIMA RIVER AT KIONA 13.0 FT 7.2 FT TO 8.0 FT WALLA WALLA RIVER NEAR TOUCHET 13.0 FT 7.4 FT TO 9.2 FT SKAGIT RIVER NEAR MT. VERNON 28.0 FT 19.0 FT TO 21.4 FT STILLAGUAMISH RIVER AT ARLINGTON 14.0 FT 6.0 FT TO 7.9 FT SNOQUALMIE RIVER NEAR SNOQUALMIE 20.0 KCFS 9.1 KCFS TO 13.8 KCFS COWLITZ RIVER NEAR RANDLE 18.0 FT 9.7 FT TO 12.4 FT S.F. SKOKOMISH RIVER NEAR UNION N/A KCFS TO 5.7 KCFS DUNGENESS RIVER NEAR SEQUIM 7.0 FT 4.3 FT TO 4.6 FT $$ Forecasts are selected from those prepared by the NWRFC. For further details, graphics, and statistics regarding the peak flow forecasts visit: https://www.nwrfc.noaa.gov/peak/ && WEATHER.GOV/SEATTLE jbb