A decade of multiseasonal climate forecasts

Latest spotlight on IRI’s work.

October marks the tenth anniversary of IRI’s monthly net assessment forecasts, which are a series of freely available maps that show temperature and rainfall predictions for every region of the world.

The IRI Forecast group looks back on the evolution of this popular service, and shares a few of its favorite anecdotes from the past ten years. But first, a bit about what the IRI forecasts are all about.

On the third Thursday of each month, IRI releases a series of regional and global maps that show the probability of any particular area having above-normal, near-normal or below-normal temperatures and rainfall in a three-month period. The forecasts extend six months into the future.

In October, for example, users can view the forecasts for the November-December-January, December-January-February, January-February-March, and February-March-April periods. Of course, the further away from the current month one gets, the less confident the forecast.

Still, it’s important to go out six months because many of IRI’s forecast users need a long lead time to take action, says Tony Barnston, who runs the forecast operations.

“During La Niña, for example, there is a greater likelihood of above average rainfall in Botswana, which could pose an increased threat to epidemic malaria. Several months of lead time provide a needed margin of safety in planning mitigative action such as spraying or immunization,” he says. Moreover, there are some situations where forecasts can still have fairly good confidence at longer lead times. “For example, an El Niño-Southern Oscillation episode that has developed by August is very likely to still be occurring in December and January,” says Barnston.

The IRI produces its net assessments by using current and forecast sea-surface temperature data to drive model-based predictions of how the atmosphere will change during the next several months. But the atmosphere is a chaotic system, and small changes in current conditions can result in big differences in the seasonal patterns that models predict, so a single prediction doesn’t tell the whole story. Instead, the IRI runs multiple models numerous times to get its best estimate of the upcoming season’s mean climate.

Read the rest of this story here.

Visit the Net Assessments main page, to sign up to receive monthly email alerts when new forecasts are posted, access ten years of archives, a tutorial on forecasts and other resources.