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Forecasts
Published September 20, 2011

Precipitation Outlook

 (October 2011–March 2012)
Data Source(s): NOAA-Climate Prediction Center (CPC)

The seasonal precipitation outlooks issued by the NOAA-Climate Prediction Center (CPC) in September call for increased chances that precipitation through the fall and winter will be similar to the driest 10 years of the 1981–2010 period (Figures 10a–d). In September the CPC issued a La Niña Advisory, which means that a La Niña event has developed and likely will persist for the next six months. Because La Niña events historically bring dry conditions to the Southwest, this tendency has heavily influenced the forecasts. The odds for drier-than-average conditions are highest for the January–March period (Figure 10d). There is uncertainty in the ultimate strength and longevity of the La Niña event. Forecasts issued by the International Research Institute for Climate and Society state a 52 percent chance that La Niña will continue through the November–January period, with a 48 percent chance that neutral conditions will emerge (see page 16).

Seasonal precipitation forecasts for the U.S.
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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