FGUS76 KSEW 122132 ESFSEW Water Supply/Spring Flood Outlook National Weather Service Seattle WA 227 PM PDT Fri April 12, 2024 ...Western Washington Water Supply and Spring Snowmelt Flood Potential... Summary: The latest forecasts of water supply for the summer were for below to much below normal for western Washington rivers. The snowpack is much below normal as well. As a result, there is little to no chance of snowmelt flooding in western Washington as is typical. In addition, water supply forecasts could drop even further through the spring. PRECIPITATION SUMMARY For Western Washington in March, precipitation was well below normal. The percent of normal precipitation was 35 to 45 percent for the Cascades, 45 to 65 for the lowlands from the Puget Sound eastward, and 50 to 85 percent for the Olympic Peninsula. Snowpack Conditions ------------------- The snowpack was much below normal for western Washington as of April 11. West of the Cascades, the basin average water content of the snowpack ranged from 60 to 68 percent of the median. Snow depths for Northwest Avalanche Center locations in western Washington as of April 1 ranged from 52 to 95 percent of normal, with all but White Pass below 80 percent. Streamflows Summary ------------------- Streamflows on western Washington rivers for last month were mostly normal to below normal. The streams in the central Cascades were below normal. Reservoir Storage Summary ------------------------- Storage for Reservoirs as of the first of April as a percentage of period of record median: Ross Reservoir 130% Upper Baker Reservoir 130% Howard Hanson Reservoir 150% Mayfield Lake 97% Weather Outlook --------------- The outlook for April and beyond for Washington state, for the next two weeks the outlook is for below normal precipitation. The monthly outlook for April calls for greater chances of below normal precipitation. The three month outlook for May through July is for greater odds of below normal precipitation for Washington state. Water Supply Outlook -------------------- Long range hydrologic models are forecasting below to much below normal river flows and water supply for western Washington rivers through summer. These forecasts are lower than last month's forecasts and could continue to drop during the spring. Water supply forecasts for Western Washington ranged from a low of 62% percent for the North Fork Skokomsih River to 88 percent for the Puyallup River Water supply forecasts that include regulation are used for locations where forecasts are listed below as regulated, for all other locations forecasts are for natural volumes. Here are the stream flow volume forecasts for specific rivers and sites as of April 11. Note that the Normal climatology is now the 1991-2020 period. Water Supply Forecasts Natural Flow Unless Otherwise Specified (in thousands of acre feet) River and Gauging Site Period Forecast Normal Percent (1991-2020) Nooksack River at North Cedarville Apr-Sep 936 1171 80 Skagit River near Concrete (regulated) Apr-Sep 4423 5955 74 Samish River near Burlington Apr-Sep 34 46 75 Baker River Upper Baker Reservoir Inflow Apr-Sep 684 823 83 Sultan River Spada Lake Inflow Apr-Sep 148 190 78 Pilchuck River near Snohomish Apr-Sep 71 101 71 Tolt River Tolt Reservoir Apr-Sep 40 46 87 Issaquah Creek near Issaquah Apr-Sep 17 26 65 Snoqualmie River near Carnation Apr-Sep 875 1040 84 Cedar River Chester Morse Lake Inflow Apr-Sep 118 136 87 Green River Howard Hanson Dam Inflow Apr-Sep 181 262 69 Puyallup River at Puyallup Apr-Sep 958 1092 88 Nisqually River Alder Reservoir Inflow Apr-Sep 316 395 80 Deschutes River near Rainier Apr-Sep 29 41 70 Cowlitz River Mayfield Reservoir (regulated) Apr-Sep 1601 1864 86 Chehalis River near Grand Mound Apr-Sep 274 406 68 Newaukum River near Chehalis Apr-Sep 58 85 68 Calawah River near Forks Apr-Sep 122 154 79 Elwha River McDonald Bridge Apr-Sep 378 465 81 Dungeness River near Sequim Apr-Sep 106 145 73 Wynoochee River Wynoochee Dam Inflow Apr-Sep 82 99 83 NF Skokomish River Cushman Dam Inflow Apr-Sep 95 183 62 Spring and Summer snowmelt and Flooding Climatology of Spring Floods: Climatology of Spring Floods: 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 flooding, occurs 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: As in most years, the threat for spring snowmelt flooding this year is extremely low, with no snowmelt flooding expected based on river modeling, the current snow pack, combined with the expected precipitation and temperatures. There is less than a 5% chance of exceeding flood stage on the major rivers in western Washington this spring with the exception of the Samish, Snoqualmie, and White Rivers which have only a 5% chance. Here are the peak flow forecasts for April 11 through September 31 for some western Washington rivers. 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 SKAGIT RIVER NEAR MT. VERNON 28.0 FT 18.0 FT TO 21.0FT STILLAGUAMISH RIVER AT ARLINGTON 14.0 FT 4.9 FT TO 6.2 FT SNOQUALMIE RIVER NEAR SNOQUALMIE 20000 CFS 7790 CFS TO 11700CFS WHITE RIVER MUD MTN DAM INFLOW 3180 CFS TO 4600 CFS COWLITZ RIVER NEAR RANDLE 18.0 FT 9.3 FT TO 11.0 FT S.F. SKOKOMISH RIVER NEAR UNION 770 CFS TO 1270 CFS DUNGENESS RIVER NEAR SEQUIM 7.0 FT 4.2 FT TO 4.5 FT $$ Forecasts are selected from those prepared by the NWRFC. For further details, graphics, and statistics regarding the water supply forecasts visit: https://www.nwrfc.noaa.gov/ws https://www.nwrfc.noaa.gov/natural For further details, graphics, and statistics regarding the peak flow forecasts visit: https://www.nwrfc.noaa.gov/peak/ The next water supply for western Washington will be issued the week of May 6. $$ weather.gov/seattle jbb