Over the past couple of months, sea surface temperatures across the equatorial Pacific Ocean have begun to cool slightly across most areas. This cooling indicates that the El Nino, which has been in place since late summer 2009, is beginning to weaken. The latest climate models suggest that further weakening of the El Nino phase is likely and that a transition from an El Nino phase to more of a neutral El Nino-Southern Oscillation (ENSO) phase will take place this summer.
As a result, the June-July temperature outlook was largely based on this weakening El Nino, as well as a negative North Atlantic Oscillation (NAO), which is forecasted to persist for the rest of May and possibly into part of June. Past climate records and trends indicate that when a weak El Nino to neutral phase is in place, slightly warmer than normal temperatures are likely across a good portion of the western United States during the first half of summer.
Above average temperatures are also expected across the southeastern United States during June and July. In both regions, a surplus of cooling degree-days of between 60 and 120 is expected. Energy costs with respect to cooling might be somewhat higher than normal across these two areas. Meanwhile, slightly cooler than average temperatures are predicted for parts of the upper Midwest and northern plains during the first half of summer. Lower than average cooling degree-days, as well as energy costs with respect to cooling, are expected throughout this portion of the country. Much of the Northeast, Midwest and southern-central plains will see mean temperatures that average closer to normal for the first part of summer.