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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 informa/ion, 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 />Most of the :mnual strcamllow in the western United Slates originates as snowfall that has accumulated in the mount:l.Ins
<br />during the winler and early spring. As the snowpack accumulates, hydrologists estimate the runoffthut will occur when
<br />it melts. Measurements of snow water equivalent at selected ma.nual snow courses and automated SNOTEL siles, along
<br />with precipitation, antecedent streamflow, and indices of the El Nino I Southern Oscillation are used in computeri7..ed
<br />statistical and simulation models to prepare runoff forecasts. These forecasts arc coordinated between hydrologists in the
<br />Natural Resources Conservation Service and the National Weather Semce. 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: (I)
<br />unccrtain knowledge of future weather conditions, (2) uncertainty in the forecasting procedure, and (3) errors in the data.
<br />The forecast, thercfore, 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% excccdance probability forecast, for which there is a
<br />50% chance that the actual Oow will be above, and a 50% chance that the actual Oow will be below, this value, To
<br />describe the expected range around this 50% valuc, four other forecasts arc provided, Iwo smaller values (90% and 70%
<br />exceedanee 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 reOected 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 bct,....een. On the other hand, ifusers arc concerned
<br />about receiving too much water (for example, threat of Oooding), 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 Ufltled S!.iltos Oopartfrwfll at Agriculture (USDA) prohibits discrimination in its programs and activities on the basis of race, color, national origin,
<br />gEH'Ider, religion, age, disability, political beliefs, seltual orientation and marilal Of lamily status. (Not all prohibited bases apply to all progfams.)
<br />Pellloos with disabilities who require alternative means IOf communication of program information (Braille, larva pOnt, audio lape, etc.) should contact
<br />USDA's TARGET Center at 202-720-2600 (voice Of TOD).
<br />
<br />To file iI complaInt of discrimination, Wlite USDA, Direclof, Office 01 Civil Rights, Room 326-W, Whtt!en Building, 14th & Independence Avenue, SW,
<br />Washington. DC, 20250-9410, Of call (202) 720-5964 (voice Of TOO). USDA 1$ an equal opportunity provider and employer.
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