Seasonal Precipitation Outlook

The NOAA CPC seasonal precipitation outlooks forecast the likelihood (percent chance) of the 1-month or 3-month temperatures occurring in the above-average, near-average, and below-average categories, based on the historic (1981-2010) distribution of precipitation.  These three categories, or terciles, are bounded so that an equal fraction (33.3%) of the historic precipitation values is in each category.

The numeric values on the maps do not refer to actual precipitation values, but to the probability, in percent, that precipitation will be in the most likely of these three terciles, if that probability is different than 33.3%. The shading indicates areas in which the odds are tilted towards wetter-than-average (A, green shading) or drier-than-average (B, brown shading) condition. The numeric values indicate the probability that precipitation will be in the above-average or below-average tercile--with a corresponding decrease in the opposite category. The near-average category remains at 33.3% likelihood, unless the forecast probability is very high for the above- or below-average tercile, or a near-average forecast is more likely than the other two categories. 

Thus, areas with dark brown shading indicate a 40.0–50.0% chance of below-average, a 33.3% chance of near-average, and a 16.7–26.6% chance of above-average precipitation. Light brown shading displays a 33.3–39.9% chance of below-average, a 33.3% chance of near-average, and a 26.7–33.3% chance of above-average temperatures, and so on. Equal Chances (EC) represents equal odds or a 33.3% probability for each tercile, indicative of areas where forecast signals are weak or conflicting, or the forecast is believed to have poor skill. “N” indicates an increased chance of near-average conditions, but this is not forecast very often. 

The skill of the precipitation outlooks largely comes from the status of ENSO. 

Additional CPC forecast maps, at lead times longer than 1.5 months.