The University of Arizona

Precipitation Outlook | CLIMAS

 SW Climate Outlook

Precipitation Outlook

(August 2013–January 2014)
Data Source(s): NOAA-Climate Prediction Center (CPC)

The seasonal precipitation outlooks issued by the NOAA-Climate Prediction Center (CPC) in July call for slightly increased chances for above-average precipitation in western New Mexico and eastern Arizona during the August–October periods (Figure 11a). This region, particularly in southwest New Mexico and southeast Arizona, is where the monsoon is most vigorous. This region is also off to a good start to the monsoon, with many areas experiencing above-average rain (see Monsoon Summary). However, forecasting the monsoon is difficult because many phenomena can influence the amount of rain that falls, including the El Niño Southern Oscillation (ENSO) and tropical hurricane activity. In September, for example, monsoon precipitation can be boosted by incursion of moisture driven by tropical storms, and this year there is some indication that hurricane activity will be above average.

For the 3-month seasons following the August–October period, the CPC forecasts equal chances that precipitation will be above, below, or near average (Figures 11b–d). These forecasts reflect uncertainty in the fate of ENSO which exerts a large influence on Southwest precipitation during the winter. The current forecast calls for ENSO-neutral conditions (see El Niño Status and Forecast), which can deliver either above- or below-average rain to Arizona and New Mexico (whereas La Niña and El Niño events tend to deliver below-average and above-average precipitation, respectively).


These outlooks predict the likelihood (chance) of above-average, average, and below-average precipitation, but not the magnitude of such variation. The numbers on the maps do not refer to inches of precipitation.

The NOAA-CPC outlooks are a 3-category forecast. As a starting point, the 1981–2010 climate record is divided into 3 categories, each with a 33.3 percent chance of occurring (i.e., equal chances, EC). The forecast indicates the likelihood of one of the extremes—above-average (A) or below-average (B)—with a corresponding adjustment to the other extreme category; the “average” category is preserved at 33.3 likelihood, unless the forecast is very strong.

Thus, using the NOAA-CPC precipitation outlook, areas with light green shading display a 33.3–39.9 percent chance of above-average, a 33.3 percent chance of average, and a 26.7–33.3 percent chance of below-average precipitation. A shade darker green indicates a 40.0–50.0 percent chance of above-average, a 33.3 percent chance of average, and a 16.7–26.6 percent chance of below-average precipitation, and so on.

Equal Chances (EC) indicates areas where no forecast skill has been demonstrated or there is no clear climate signal; areas labeled EC suggest an equal likelihood of above-average, average, and below-average conditions, as a “default option” when forecast skill is poor.