Home » April 2012 Southwest Climate Outlook » Precipitation Outlook
Forecasts
Published April 25, 2012

Precipitation Outlook

 (May–October 2012)
Data Source(s): NOAA–Climate Prediction Center (CPC)

The seasonal precipitation outlooks issued by the NOAA-Climate Prediction Center (CPC) in April call for equal chances that precipitation will be above, below, or near average in most of Arizona and New Mexico through the July–September period (Figures 10a–c). For the summer months, forecasts have been less accurate during the monsoon season. Consequently, the CPC has no basis to favor wetter- or drier-than-average conditions and gives an equal chances outlook for the June–August and July–September periods. In the August–October period, odds are slightly increased for below-average rain, most notably in southeastern Arizona (Figure 10d). Although not reflected in the official forecasts, studies have demonstrated that dry winters with low snowpack similar to this year often are followed by wet summers. Some studies suggest shorter-lived snow cover enables the land to warm sooner in the summer, which, in turn, instigates incursions of moisture from the Gulfs of California and Mexico that spark monsoon storms.

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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