Home » November 2012 Southwest Climate Outlook » Precipitation Outlook
Forecasts
Published November 21, 2012

Precipitation Outlook

 (December 2012–May 2013)
Data Source(s): NOAA-Climate Prediction Center (CPC)

The seasonal precipitation outlooks issued by the NOAA-Climate Prediction Center (CPC) in November call for equal chances that precipitation during the December–February period will be above , below , or near average (Figure 10a). However, the CPC indicates that precipitation during the three-month seasons that follow have a greater chance of being drier than average (Figures 10b–d). Only a few months ago, the CPC considered an El Niño event likely, which would have increased chances for above-average precipitation. However, the El Niño recently sputtered and neutral conditions are now favored. During neutral ENSO events and when the Pacific Decadal Oscillation (PDO) is negative—which is the current state—the West historically receives  about 91 percent of average precipitation. Also, during these conditions, all climate divisions in Arizona experience, on average, less than 80 percent of average rain and snow. Although the physical influence of the PDO remains a topic of hot scientific debate, most of the CPC models simulate precipitation patterns similar to the signature of a negative PDO measured in historical data.

Precipitation Outlook
Notes:

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.

 

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