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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 />Dala Collection Office Supervisor
<br />USDA, Natural Resources Conservation Service
<br />655 Partel St., Rm E200C
<br />Lakewood, CO 80215-5517
<br />Phone (720) 544-2852
<br />
<br />How forecasts are made
<br />
<br />Mosl of the annual streamflow in the western United Stales originates as snowfall thai has accumulated in the mountains
<br />during the winter and early spring. As the snO\vpack accumulates, hydrologists estimate the runoff that will occur when
<br />It melts. !\Icasurements of snow water equivalent at selected manual snow courses and automated SNOTEL sites, along
<br />with precipitation, antecedent streamflow, and indices oClhe El Nino I Southern Oscillation are used in computerized
<br />statistical and simulation models to prepare runoff forecasts. These forecasts arc coordinated betwecn hydrologists in the
<br />Natural Resources Conservation Service and the National Weather Service. Unless otherwise specified, all forecasts are
<br />for !lows that would occur naturally without any upstream influences.
<br />
<br />Forecasts of any kind, of course, are not perfect. Streaml10w 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% exccedance 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 mnge around this 50% value, four other forecasts are provided, two smaller values (90% and 70%
<br />excccdance 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 />slInllarly.
<br />
<br />'[be wider the spread among these values, the more uncertain the forecast, As the season prob'Tesses, 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% exceedancc probability forecast. Users should take this uneert<linty into
<br />eonsidcmtion when making operational decisions by selecting forecasts corresponding 10 Ihe level of risk they are willing
<br />to assume about the amount of water to be expected. If users anticipate receiving a lesser supply ofwaler, 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 nooding), they may want to base their decisions on the 30% or
<br />10% exceed:mce 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 />exceed:mce probability forecast is used. there is still a 10% chance of n:ceiving less than this amount) By using the
<br />e,'(cecdance probability information, users can easily determine the chances of receiving more or less water,
<br />
<br />The United SIaI$S Department 01 Agriculture (USDA) pl"ohibits dlscriminatloo in 115 programs and activities 00 lhe basis of race, color, national origin,
<br />pender, religion, age, disability, political beliefs. sexual orientallon and marital or family status. (Not all prohibited bases apply to all progmms.)
<br />Persoos wilh disabilities who require alternative moans lor communicalioo of program In!ormatiOn (Braille, large print. audio tape, etc.) should contact
<br />USDA's TARGET Cenler at202-72()"2600 (voice or 100)_
<br />
<br />To file a complaint of dlsaiminatioo, write USDA, Director, Qlfice of Civil Rights, Room 32$-W, Whitten Building, 14lh & Independence Avenue, SW,
<br />Washington, DC. 2025().9410, or call (202) 720.5964 (voice or TOO). USDA Is an equal opportunity provider and employer.
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