The Climate Prediction Center’s (CPC) three-month outlooks (both September-November and October-December) show a tendency for warmer than usual temperatures and mostly normal or drier than normal—except for the Pacific Northwest, which has a greater chance of being wetter than normal. The closer-term forecast does have greater odds of wet weather in North Dakota and Nebraska (see maps.)
The major factor in the outlook is La Nina, which developed in July and is expected to continue through early 2011. For September, “this leads to a pretty strong tilt toward above normal temperatures in the Mid-Atlantic, Southeast, and the Ohio and Tennessee Valley,” says Mike Halpert of CPC.
Other factors such as long-term trends may suggest forecasts that conflict with those tied to La Nina. Expectations of a very active hurricane season raises the odds of a wetter-than-usual season along the Gulf and South Atlantic coasts, he adds. However, the maps show equal chances (EC) of normal, wetter than normal or drier than normal because the various tools considered are not considered skillful enough to support a different forecast.