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<br />Basin Outlook Reports <br />and <br />Federal - State - Private <br />Cooperative Snow Surveys <br /> <br />For more water supply and resource management information, contact: <br />Michael A. Gillespie <br />Data Collection Office Supervisor <br />USDA, Natural Resources Conservation Service <br />655 Parfet St., Rm E200C <br />Lakewood, CO 80215-5517 <br />Phone (303) 236-2910 <br /> <br />How forecasts are made <br /> <br />Most of the nnnual streamflow in the western United States originates as snowfall that has accumulated in the mountains <br />during the winter and early spring. As the snowpack accumulates, hydrologists estimate the runoff that will occur when it <br />melts. Measurements of snow water equivalent at selected manual snow courses and automated SNOTEL sites, along with <br />precipitation, antecedent streamflow, and indices of the EI Niiio / Southern Oscillation are used in computerized statistical <br />and simulation models to prepare runoff forecasts. These forecasts are coordinated between hydrologists in the Natural <br />Resources Conservation Service and the National Weather Service. Unless otherwise specified, all forecasts are for flows <br />that would occur naturally without any upstream influences. <br /> <br />Forecasts ofany kind, of course, are not perfect. Strearnflow forecast uncertainty arises from three primary sources: (I) <br />uncertain knowledge offuture weather conditions, (2) uncertainty in the forecasting procedure, and (3) errors in the data, <br />The forecast, therefore, must be interpreted not as a single value but rather as a range of values with specific probabilities <br />of occurrence, The middle of the l1IIl8e is expressed by the 50% exceedance probability forecast, for which there is a 50% <br />chance that the actual flow will be above, and a 50% chance that the actual flow will be below, this value, To describe the <br />expected l1IIl8e around this 50% value, four other forecasts are provided, two smaller values (90% and 70% exceedance ' <br />probability) and two larger values (30%, and 10% exceedance probability). For example, there is a 90% chance that the <br />actual flow will be more than the 90% exceedance probability forecast. The others can be interpreted similarly. <br /> <br />The wider the spread among these values, the more uncertain the forecast, As the season progresses, forecasts become <br />more accurate, primarily because a greater portion of the future weather conditions become known; this is reflected by a <br />narrowing of the range around the 50% exceedance probability forecast, Users should take this uncertainty into <br />consideration when making operational decisions by selecting forecasts corresponding to the level of risk they are willing to <br />assume about the amount of water to be expected, If users anticipate receiving a lesser supply of water, or if they wish to <br />increase their chances of having an adequate supply of water for their operations, they may want to base their decisions on <br />the 90% or 70% exceedance probability forecasts, or something in between. On the other hand, if users are concerned <br />about receiving too much water (for example, threat of flooding), they may want to base their decisions on the 30% or 10% <br />exceedance probability forecasts, or something in between. Regardless of the forecast value users choose for operations, <br />they should be prepared to deal with either more or less water, (Users should remember that even if the 90% exceedance <br />probability forecast is used, there is still a 10% chance of receiving less than this amount.) By using the exceedance <br />probability information, users can easily detennine the chances of receiving more or less water. <br /> <br />The Un/led Sta(eo Department o( Agriculture (USDA) prohibits dioalminat;on In all Its programs "" It>e basls o( race, color, national o~gin, gender, religloo, <br />age, disability, political beliefs, sexual orie"lstIon snd marltsl or family status, (Not sll prohibited ba... apply 10 all programs.) Persons with disabilities v.t<> <br />require sllemotiYe means (or communication of program Information (Braille, largo print, sudlotape, sic,) should contact USDA'. TARGET Center at 202.)20- <br />2600 (voice snd TOO), <br /> <br />To file a complslnt of discrimination, write USDA, Director, Office of Civil Rights, Rocxn 326-W, Whitten Building, 14th & Independence Avenue, SW, <br />Washington, DC, 20250-9410, or call (202) 720-5964 (voice snd TOO), USDA Is on equal opportunity provider and omplo)'e<. <br />