Flood Potential Outlook
Issued by NWS Seattle/Tacoma, WA
Issued by NWS Seattle/Tacoma, WA
576 FGUS76 KSEW 101702 ESFSEW Water Supply/Spring Flood Outlook National Weather Service Seattle WA 1002 AM PDT Fri May 10, 2024 ...Western Washington Water Supply and Spring Snowmelt Flood Potential... Summary: The latest forecasts of water supply for the summer were lower than last month`s forecasts and were for below to much below normal for western Washington rivers. In addition, the water supply forecasts could drop even further through the spring. The snowpack was much below normal as well. As a result, there is little to no chance of snowmelt flooding in western Washington as is typical. PRECIPITATION SUMMARY For Western Washington in April, precipitation was well below normal. The percent of normal precipitation was generally 30 to 70 percent for the Cascades, 20 to 80 percent for the Olympic Peninsula, and 30 to 70 for the western Washington lowlands with a few areas near 100 percent north of King County. Snowpack Conditions ------------------- The snowpack was much below normal for western Washington as of May 10. West of the Cascades, the basin average water content of the snowpack ranged from 60 to 67 percent of the median. Snow depths for Northwest Avalanche Center locations in western Washington as of May 1 ranged from 33 to 98 percent of normal, with all but White Pass around 80 percent or lower. 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 May as a percentage of period of record median: Ross Reservoir 123% Upper Baker Reservoir 106 Howard Hanson Reservoir 125 Mayfield Lake 100 Weather Outlook --------------- The outlook for May and beyond for Washington state: for the next two weeks and the month of May, month of May is for equal chances of above or below normal precipitation and temperature. The three month outlook for June through August is for greater odds of below normal precipitation and above normal temperatures. 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 once again lower than last month`s forecasts and was generally a few percent to 10 percent. The forecasts could continue this trend and drop further during the spring. The forecast Water supply forecasts for Western Washington ranged from a low of 65% percent for the Issaquah Creek to 83 percent for the Snoqualmie 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 May 10. 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 904 1171 77 Skagit River near Concrete (regulated) Apr-Sep 4042 5955 68 Samish River near Burlington Apr-Sep 35 46 76 Baker River Upper Baker Reservoir Inflow Apr-Sep 613 823 74 Sultan River Spada Lake Inflow Apr-Sep 147 190 77 Pilchuck River near Snohomish Apr-Sep 73 101 73 Tolt River Tolt Reservoir Apr-Sep 38 46 82 Issaquah Creek near Issaquah Apr-Sep 17 26 65 Snoqualmie River near Carnation Apr-Sep 864 1040 83 Cedar River Chester Morse Lake Inflow Apr-Sep 111 136 81 Green River Howard Hanson Dam Inflow Apr-Sep 173 262 66 Puyallup River at Puyallup Apr-Sep 861 1092 79 Nisqually River Alder Reservoir Inflow Apr-Sep 277 395 70 Deschutes River near Rainier Apr-Sep 27 41 65 Cowlitz River Mayfield Reservoir (regulated) Apr-Sep 1477 1864 79 Chehalis River near Grand Mound Apr-Sep 292 406 72 Newaukum River near Chehalis Apr-Sep 62 85 73 Calawah River near Forks Apr-Sep 118 154 77 Elwha River McDonald Bridge Apr-Sep 355 465 76 Dungeness River near Sequim Apr-Sep 101 145 69 Wynoochee River Wynoochee Dam Inflow Apr-Sep 79 99 79 NF Skokomish River Cushman Dam Inflow Apr-Sep 135 183 74 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, and in combination 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. Here are the peak flow forecasts for May 10 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 16.0 FT TO 19.0FT STILLAGUAMISH RIVER AT ARLINGTON 14.0 FT 3.8 FT TO 5.7 FT SNOQUALMIE RIVER NEAR SNOQUALMIE 20000 CFS 5890 CFS TO 10500CFS WHITE RIVER R St. Bridge 2770 CFS TO 4160 CFS COWLITZ RIVER NEAR RANDLE 18.0 FT 9.1 FT TO 9.4 FT S.F. SKOKOMISH RIVER NEAR UNION 450 CFS TO 770 CFS DUNGENESS RIVER NEAR SEQUIM 7.0 FT 4.2 FT TO 4.4 FT $$ Forecasts are selected from those prepared by the NWRFC. For further details, graphics, and statistics regarding the water supply forecasts visit: https://www.nwrfc.noaa.gov/ws https://www.nwrfc.noaa.gov/natural For further details, graphics, and statistics regarding the peak flow forecasts visit: https://www.nwrfc.noaa.gov/peak/ The next water supply for western Washington will be issued the week of June 3. $$ weather.gov/seattle jbb