To Recipients of Water Supply Outlook Reports

This report presents a broad picture of water supply outlook conditions for the Western United States, including selected streamflow forecasts, summary of snow accumulation to date, and storage in larger reservoirs.

Most of the usable water in the western states originates as mountain 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 influences the soil moisture conditions prior to formation of the snowpack and explains, in part, the effectiveness of the snowpack in producing runoff.

The forecasts of natural runoff in this outlook are based principally on measurements 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 accumulation and melt season will interact with a resultant average affect on runoff. Early season forecasts are therefore subject to a greater change than those made on later dates.

Probability Forecasts and Adjustments

Precipitation and snowfall accumulation of known probability as determined by analysis of past records are utilized in the preparation of probability runoff forecasts. 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 current hydrometeorological conditions to date, this is the best estimate of what the actual runoff volume will be this season.

Reasonable Maximum Forecast. Given current hydrometeorological conditions, the seasonal runoff that has a ten (10) percent chance of being exceeded.

Reasonable Minimum Forecast. Given current hydrometeorological 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 information concerning diversions and adjustments at the various forecast points.

For More Information

The Water Supply Outlook is published jointly by the NOAA National Weather Service and the USDA Natural Resources Conservation Service, following the principal snow survey dates from January 1, through May 1.

Some basic data and streamflow forecasts 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.

Information on this publication can be obtained from:

National Weather Service
National Oceanic and Atmospheric Administration
Attention: Office of Hydrology
Silver Spring, Maryland 20910

Natural Resources Conservation Service
West National Technical Center
101 Southwest Main, Suite 1700
Portland, Oregon 97204-3221

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 Division
Parliament Buildings
Victoria, British Columbia V8V 1X5

Department of Indian and Northern Affairs
Northern Operations Branch
200 Range Road, Whitehorse
Yukon Territory Y1A 3V1

Alberta Environment
Technical Services Division
9820 106th Street
Edmonton, Alberta T5K 2J6.

Last Modified: Tuesday, 14-Dec-1999 21:44:58 UTC