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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 annual 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 El Nino / 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 of any kind, of course, are not perfect, Streamflow forecast uncertainty arises from three primary sources: (I)
<br />uncertain knowledge of future 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 range 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 range 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, Ifusers 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, ifusers 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 determine the chances of receiving more or less water.
<br />
<br />The United Stales Department of Agriculture (USDA) prohibits discrimination in all its programs on the basis of race, color, national origin, gender, religion,
<br />age. disability, political beliefs, sexual orientation and marital Of family status. (Not all prohibited bases apply to all programs.) Persons with disabilities who
<br />require a~ernatjve means for communication of program information (Braille, large print, audiotape, etc.) shOUld contact USDA's TARGET Center at 202-720-
<br />2600 (voice and TDD),
<br />
<br />To file a complaint of discrimination, write USDA, Director, Office of Civil Rights, Room 326-W, Whitten Building, 14th & Independence Av~nue, SW,
<br />Washington, DC, 20250-9410, or call (202) 720-5964 (voice and TOO). USDA is an equal opportunity provider and employer.
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