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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 Rosources Conservation Service
<br />655 Partel 51., Rm E200C
<br />Lakewood, CO 80215-5517
<br />Phone (720) 544-2852
<br />
<br />How forecasts are made
<br />
<br />Most orthe annual strcamOow in the western United Stales 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 prccipibtion, antecedent streamflow, and indices of the EI Nillo I Southern Oscillation arc used in computerized
<br />statistical and simulation models to pTt.'pare runoff forecasts. These forecasts are coordmated 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. Stream110w forecast uncertainty arises from three primary sources: (I)
<br />uncertain knowledge of future weather conditions, (2) uncertainty in the forecastmg 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
<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% exeeedance probability). For example, there is a 90%
<br />chance that the actual flow will be more than the 90% exeeedance probability forecas!. lne 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 1a1O\\o11; this is reflected by a
<br />narrowing of the range around the 50% excecdance probability forecas!. 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 />10 assume about the amount of water to be eXpe1;:ted. If users anticipate receiving a lesser supply of water, or if they wish
<br />to increase their chances of having an adequate supply ofwatt:r 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, ifusers arc 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 should remember 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 information, users can easily determine the chances of receiving more or less water.
<br />
<br />The United Slales Departmenl of Agriculture (USDA) prohibits discrimination In its programs and activities on the basis 01 race, c:oIot', national origin.
<br />gender. religion, age, disability, political beliefs. sexual orientation and marital or family slalus. (Not all prohibited bases apply 10 all programs.)
<br />Pltfsons with disabilities who require alternative means lor communication 0' program Information (Braille. large prinl audio Iape, etc.) should conlacl
<br />USDA'. TARGET Center 411202.720-2600 (voice or TOO).
<br />
<br />To file a complaint of dl5Ctlmination, 'Mile USDA, Director, Office 01 CMI RIghts. Room 326-W, Whitten Building, 14th & Independence Avenue. SW,
<br />Washington, DC, 20250-9410, or call (202) 720-5964 (voice or TOD). USDA is an eQual opportunity provider and employer.
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