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To Recipients of Water Supply Outlook Reports This report presents a broad picture of water supply outlook conditions for the W'eslem United Simes, including selec- ted streamflow forecasts, summary of snow accumulation to date. and stor. age in hJrger reservoirs. Most of the useable water in the western slates originates as mOllntain snowfall. This snowfall accumulates during winter and spring. several months before the snow melts and appears as streamflow. Since the runoff from precipitation as snow is delayed. estimates of snowmelt runoff can be made well in advance of its occurrence. Fall precipitation influ- ences the soil moisture conditions prior to fooniltion of the snowpack and ex- plains, in p..m, the effectiveness of the snow pack in producing nmoff. The forecasts of naluml nmoff in this outlook are based principally on mea- surements of precipitation. snow water equivalent. and antecedent runoff. Forecasts become more accurate as more of the data affecting runoff are measured. All forecasts assume that climatic factors during the remainder of the snow uccumulation and Illelt season will interact with a resultant average affect on runoff. Early season forecasbi are therefore subject to a greater change than those made on later dates. Probability Forecasts and Adjustments Precipition and snowfall accumulation of known probability as detennined by analysis of past records are utilized in the preparation of probability runoff foreca.sts. The forecasts include an evaluation of the standard error of the prediction model. The forecasts are presented at three levels of probability as follows: Most Probable Forecast. Given the CUITent hydrometeorological conditions to date, this is the best estimate of what the actual runoff volume will be this season. Reasonable Maximum Forecast. Given current hydrometeorologicaJ conditions, the seasonal runoff that has a ten ( 10) percent chance of being exceeded. Real;lonahle Minimum Forecast. Given current hydrollleteorological conditions. the seasonal runoff that has a ninety (90) percent chance of being exceeded. Adjustments. Runoff forecasts at all points are for full natural or unimpaired runoff corrected for evaporation. upstream diversions, and adjusted for other hydrologic changes as they are developed. Reference should be made to the U.S. Geological Survey water supply papers for detailed infonnation concerning diversions and adjustments at the v:.uious forecast points. For More Information The Water Supply Outlook is published jointly by the NOAA National Weather Service and the USDA Soil Conserva- tion Service, following the principal snow survey dates from January I, through May I. Some basic data and streamflow forec(jsls prepared by cooperating agencies are presented in this bulletin. These agencies include the Bureau of Reclamation, Corps of Engineers. Forest Service. National Park Service. Geological Survey, British Columbia Ministry of the Environment, and the California Department of Water Resources. Reqoest copies of this publication from: National Weather Service National Oceanic and Atmospheric Administration Attention: Office of Hydrology Silver Spring. Maryland 20910 Soil Conservation Service, West National Technical Center 511 N.W. Broadway, Room 248 Portland, Oregon 97209. Similar reports are available from the following agencies: Snow Surveys Branch California Department of Water Resources P.O. Box 942836 Sacramento. California 94236 Ministry of the Environment Water Management Branch Parliament Buildings Victoria. British Columbia V8V I X5 Department of Indian and Northern Affairs Northern Operations Branch 200 Range Roud, Whilehorse Yukon Territory Y I A 3V I Alberta Environment Technical Services Division 9820 106th Street Edmonton. Alberta T5K 2J6. State and regional contacts for SCS and NWS are listed on the back cover. Programs and assistance of the United States Department of Agriculture are available without regard to race, creed, color, sex, age, or national origin.