IRI Seasonal Temperature and Precipitation Forecasts

The tercile probabilities are the forecast probabilities that the temperature or precipitation will be in the lower 33.3% of the climatological distribution (cold or dry), the middle 33.3% (near normal temperature or precipitation), or the upper 33.3% (warm or wet).

The probability of the dominant tercile is the probability of the tercile that has the highest forecast probability. The color indicates which tercile has this highest probability, and the intensity of the color indicates the probability (see the color bar). Gray indicates that the middle category (near normal) is the dominant forecast category, and the probability is always 40% (never higher). The white indicates the climatology forecast (33.3% for each tercile), in which case no tercile is dominant.

The lead time is defined as one month less than the difference between the month of the latest observed data, and the first month of the period being forecast. For example, a forecast made using observations through July, for the period of September through November, has a lead time of 1 month. Such a forecast would typically be made in early August. Lead time is intended to show the time between the END of the latest observed data and the BEGINNING of the period being forecast.