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New streamflow forecasts for expert users | CLIMAS

New streamflow forecasts for expert users

TitleNew streamflow forecasts for expert users
Publication TypeFeature Articles
AuthorsWerner, K
JournalSouthwest Climate Outlook
Start Page3
Date Published02/2008
Full Text

The National Weather Service (NWS) has combined a number of forecast tools into a powerful new collection of web-based, site-specific maps and graphs that provide expert forecast users with a much greater depth of information about streamflow than ever before.

Streamflow forecasts are critical for making the best decisions related to water management, recreation, hydropower, agriculture, and natural hazards, like floods. Streamflow is water from rainfall or snowmelt that flows over land, usually in rivers and streams. The NWS’s new suite of tools, available at, allows users to access NWS predictions and historical forecasts to help make these decisions and more effectively manage the limited water resources of the western United States.

Released this month, the new online service uses interactive maps, forecast analysis tools, and forecast verification tools to provide improved access to the official water supply forecasts, and forecast uncertainty—the probability that inflow will fall within a certain range, for example. It also allows users to access the long-term accuracy of forecasts. An overview of the new tools is presented here.

Water Supply Maps

Forecasts are presented in both space and time through the NWS western water supply map (Figure 1a–b). The map, color-coded by percent of average runoff levels, presents the most recent forecast or observed spring runoff from the current water year. By clicking on a state or entering or selecting keywords in one of the search fields, users may zoom into geographical areas to find individual forecast points and basin boundaries and can apply New streamflow forecasts for expert users continued on page 4 National Weather Service western water supply services Figure 1a. The NWS western water supply map before any options are selected.

Figure 1a. The NWS western water supply map zoomed in on Arizona and the Colorado River. optional lake, stream, state, and other layers. When users click on a forecast point, a mouse-over box displays the name of the location, the forecast in thousands of acre-feet (kaf ), the date and period of the forecast, and other basin data, including the historical mean, minimum, and maximum runoff from that basin. Users can navigate between the various other tools by clicking the “Change Application” button at the top of the screen and then the desired application. Guides explaining how to use and interpret the maps and graphs are available for each application under the “About Western Water” tab.

Forecast Evolution

The forecast evolution plot shows a graph displaying the time evolution of individual forecasts (Figure 2). This plot was developed in coordination with forecast users in the Pacific Northwest who needed to know how forecasts change over time with monthly and seasonally observed streamflow. This configurable plot allows users to graph forecasts; Ensemble Streamflow Prediction (ESP) forecasts, which are used to make probabilistic forecasts of up to ninety days for streamflow volume; and observed streamflow on a single plot for the current water year and past water years. A water year runs from October 1 through September 30. The graphs display the monthly statistical range of the forecasts for seasonal streamflow volume, the accumulation of monthly average volumes for the water year and the forecast period, and the accumulation of monthly observed volumes for the water year. Users can access this tool by clicking on the desired forecast location box on the map.

Ensemble Forecasts

Ensemble streamflow forecasts are presented through a highly interactive application that allows users to create plots showing forecast probabilities of streamflow volume month by month or for a season of their choice (Figure 3). This application also incorporates historical streamflow data, El Niño Southern Oscillation (ENSO) conditions, and forecast ensemble information.

Users can combine information to address questions about forecast uncertainty and the effects of the current soil moisture states and ENSO conditions on the current forecast. One of the useful aspects of this tool is the ability to overlay different years and ENSO conditions and see the results simultaneously, allowing for easy comparisons.


A simple, interactive forecast evaluation tool is included for users to assess the quality of past coordinated, NWS, Natural Resources Conservation Service, and Statistical Water Supply forecasts. Simple forecast verification metrics including error and skill scoreare included as well as an ability to visually explore past forecasts and observed streamflow. The verification application is important; research by the CLIMAS project found that stakeholders will not use a forecast that does not include an indication of its skill.

Data Check Out

Actual forecast and observed streamflow data is provided in text format to users through this application. Users may download data for their own purposes. In particular, the ensemble streamflow forecast data may be used for downstream applications involving decision support software.

The NWS is exploring the expansion of these services. Future development possibilities include a climate change scenario application that would leverage climate change scenarios from the International Panel on Climate Change or similar sources to produce water supply time series. In addition, development is underway for an adjustment procedure for the ensemble forecasts to remove forecast biases.