The University of Arizona

Precipitation Outlook | CLIMAS

 SW Climate Outlook

Precipitation Outlook

Data between October 2013–March 2014. Data Source: NOAA-Climate Prediction Center (CPC)

The seasonal precipitation outlooks issued by the NOAA-Climate Prediction Center (CPC) in September call for equal chances that precipitation will be above, below, or near average for the Southwest during the October–December period (Figure 10a). This forecast in part reflects the expectation that the El Niño-Southern Oscillation (ENSO) will remain in a neutral state. ENSO-neutral conditions, however, have been accompanied by both above- and below-average rain and snow in Arizona and New Mexico, which makes forecasting based on ENSO alone difficult. The possibility that remnants of Pacific Ocean storms and hurricanes that could waft into the region add uncertainty in the October–December outlooks. It is difficult projecting the frequency of these events as well as their storm tracks. For seasons spanning November–March period, the CPC is calling for increased chances for below-average precipitation even though ENSO is expected to remain neutral during the winter (see ENSO Status; Figures 10b–d). These forecasts are based primarily on decadal drying trends.








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 40.0–49.9 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.