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<br />..
<br />
<br />M1891
<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 (720) 544.2852
<br />
<br />.
<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
<br />it melts. Measurements of snow water equivalent at selected manual snow courses and automated SNOTEL sites, along
<br />with precipitation, antecedent streamflow, and indices of the EI Nifio / Southern Oscillation are used in computerized
<br />statistical and simulation models to prepare runoff forecasts. These forecasts are coordinated between hydrologists in the
<br />Natural Resources Conservation Service and the National Weather Service. Unless otherwise specified, all forecasts are
<br />for flows 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: (1)
<br />uncertain knowledge of future weather conditions, (2) uncertainty in the forecasting procedure, and (3) errors ill the data.
<br />The forecast, therefore, must be interpreted not as a single value but rather as a range of values with specific prc;lbabilities
<br />of occurrence. The middle of the range is expressed by the 50% exceedance probability forecast, for which there is a
<br />50% chance that the actual flow will be above, and a 50% chance that the actual flow Will be below, this value. To
<br />describe the expected range around this 50% value, four other forecasts are provided, two smaller values (90% and 70%
<br />exceedance probability) and two larger values (30%, and 10% exceedance probability). For example, there is a 90%
<br />chance that the actual flow will be more than the 90% exceedance probability forecast. The others can be interpreted
<br />. 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
<br />to assume about the amount of water to be expected, If users anticipate receiving a lesser supply of water, or if they wish
<br />to increase their chances of having an adequate supply of water for their operations, they may want to base their decisions
<br />on 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
<br />10% exceedance probability forecasts, or something in between. Regardless of the forecast value users choose for
<br />operations, they should be prepared to deal with either more or less water. (Users shouldrernernber that even if the 90%
<br />exceedance probability forecast is used, there is still a 10% chance of receiving less than this amount.) By using the
<br />exceedance probability infoimation, users can easily determine the chances of receiving more or less water.
<br />
<br />
<br />The United States Department of Agriculture (USDA) prohibits discrimination In Its programs and activities on the basis of race, color, national origin,
<br />gender, religion, age, disability, political beliefs, sexual orientation and marital or family status. (Not all prohibited bases apply to all programs.)
<br />Persons with disabilities who require al!emative means for communication of program Information (Braille, large print, audio tape, etc.) shOUld contact
<br />USDA's TARGET Center at202-72()'2600 (voice or TOO),
<br />
<br />To file a complaint of discrimination, write USDA, Director, Office of Civil Rights, Room 326-W, Whltlen Building, 14th & Independence Avenue, SW,
<br />Washington, DC, 20250-9410, or call (202) 720-5964 (voice or TDD). USDA Is an equal opportunity provider and employer.
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