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For example, warm phases of ENSO in the Pacific Northwest of the United States are generally (but not always) characterized by anomalously warm and dry conditions. A nowcast of evolving warm conditions in the tropical Pacific implies a number of conditions to be expected regionally, such as less snowpack in the mountains and earlier peaking and overall decreased streamflow in the major river systems fed by mountain snowpack. Since large parts of the Pacific Northwest depend on streamflow for irrigation, hydroelectric power, river transport, and city reservoirs, actions can be taken in advance to mitigate the effects of reduced streamflow.

Specific ENSO Forecast Needs—Time and Space Resolution

Forecast needs depend on the sector that may use the forecast and on the particular use within the sector to which the forecast is applied. Many users desire precipitation forecasts, averaged over the weather time scales—this usually means monthly averaged precipitation predicted a season to a year in advance. Such forecasts are useful for agriculture, sanitation and sewer management, hydroelectric power generation, river transportation, flood control, forest fire control, and mosquito control. Some users desire monthly averaged temperature forecasts for a season to a year in advance. Such forecasts are useful for coastal fishery management, fuel distribution and storage planning, construction involving concrete pouring, and the tourism, recreation, and retail sales industries. Climate scientists believe there are fewer practical applications of forecasts of other physical quantities (we regard the winds that go with hurricanes as part of hurricane prediction rather than wind prediction). The match between forecast information and its users' needs is discussed further in Chapter 4.

Applications that require averaged precipitation or temperature can benefit from ENSO forecasts, but applications that require information on when forecast events will occur cannot benefit, because of limitations in forecasting capability. Agriculture in India, for example, depends on planting relatively soon before the onset of the summer monsoon rains. Planting too soon means the seeds will die in the ground, whereas waiting too long to plant means that the ground may be too soft or muddy for planting. ENSO climate models may forecast the intensity of the monsoon rainfall in advance, but they cannot (and probably will never be able to) forecast the specific date of onset of the monsoon rains, because such onsets depend strongly on the details of weather patterns that are essentially unpredictable more than a week or so in advance.

In general, forecasts of averaged precipitation and temperature are made on the same spatial scale as the atmospheric model that is directly