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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 SI., Rm E200C <br />Lakewood, CO 80215.5517 <br />Phone (720) 544-2852 <br /> <br />How forecasts are made <br /> <br />J\1ost of the annual streamflow in the western United States originates as snowfall that has accumulated in the mountains <br />dunng the winter and early spring. As the snowpack accumulates. hydrologists estimate the runoff that will occur whl"n <br />it melts. Measurements of snow water eqUivalent at selected manual snow courses and autonutcd SNOTEL sites, along <br />with precipitation. antecedent strcamllow, and indices of the El Nino I Southern Oscillation are used III compulenzed <br />statislical and simulation models 10 prepare runoff forecasts. These forecasts are coordinated between hydrologists in Ihe <br />Nalural Resources Conservation Service and the National Weather Scrvlce. Unless otherwise specified, all forecasts arc <br />for flows that would. occur naturally without any upstream influences. <br /> <br />. <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 III the dala. <br />The forecast, therefore, must be interpreted not as a single value but rather as a range of values with specllic 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, Iwo smaller values (90% and 70% <br />excccdance probability) and two larger values (30%, and 10% exceedance probabihty), For example, there IS a 90% <br />chance that the actual flow will be more than the 90% exceedance probability forecast. The othcrs can be intcrpreted <br />similarly, <br /> <br />The widcr the sprcad among these values, Ihe more uncertain the forecast. As the season progresses, forecasts become <br />more accurate, pnmarily because a greater portion of the future weather conditions become known; this is rellccted by a <br />narrowing of the range around the 50% cxceedance probability forecast. Users should take this uncertainty IIlto <br />consideration when making operational dccisions by selecting forecasts corresponding to the level of risk they arc wilhng <br />to assume about the amount ofwatcr to be expected. If users antiCipate rcccivmg a lesser supply ofwaler, or Iflhey WIsh <br />10 increase their chances of having :m adcquate supply of water for their opcrations, they may want to base Ihelf decisions <br />on the 90% or 70% exceedance probability forecasts, or something in between. On the other hand, If users are cOllccrned <br />about receiving too much water (for example, threat of nooding), they may want 10 base Ihelr decisions on the 30% or <br />10% excecdance probability forecasts, or something III between. Regardless of the forecast value users choose for <br />operations, they should be prepared to deal with ellher mOfe or less water. (Users should remember that even if the 90% <br />exceedance probability forecast is uscd, there is still a 10% chance ofrecciving less than this amount.) By uSlIlg the <br />exceedance probability infonnation, users can easily detennine the chances of receiving more or less water. <br /> <br />The Uniled SlaltiS [)&parln",nt 01 AgricvllUfe (USDA) prohibits discnmlnalloo In its programs and aelivibes on the basis of ra<:e, color, nntiOoat 0I1Qlr1. <br />oender. religion, age, disability. pohlical beliefs, sexual orientalion and marital or family status. (NOI all prohibited bases apply 10 an programs.) <br />Persons with disabilities who require allemallve means for communicalioo of program inlormalion (Braille,large pllnl, audio tape, ele.) shOlJld contaCI <br />USDA's TARGET Cenler al202.720-2600 (VOice or TOO). <br /> <br />To file a complaint 01 discrlminalion, wrile USDA. Olreclof, Office 01 Civil Rights, Room 32f5.W, Whillen Building, 14lh & Independence Avenue, SW. <br />Washington, DC, 20250-9410, or call (202) 72()"5964 (voice or TOO). USDA is an equal opportunily provider and employer. <br />