During the past couple of months, sea surface temperatures across much of the equatorial Pacific Ocean have continued to decrease. Most of the equatorial Pacific Ocean now has sea surface temperatures that are near or slightly cooler than average.
As a result, all indications are that the transition from a more neutral El Nino Southern Oscillation (ENSO) phase to a La Nina ENSO phase will continue. This transition to La Nina might be complete by August. A developing La Nina was strongly figured into the August-September temperature outlook. Looking at past years in which El Nino transitioned all the way to a La Nina during the summer (specifically 1983, 1995 and 1998) reveals that the Desert Southwest and southern Rockies often see slightly warmer than average temperatures during late summer and early fall when this occurs.
Other climate indices such as the North Atlantic Oscillation (NAO) suggest that a good portion of the eastern third of the country will see above average temperatures during August and September. The NAO is finally expected to become positive within the next month or two after an unpredicted long duration of being negative. A positive NAO usually leads to an upper-level weather pattern that supports warmer conditions across the eastern United States.
A surplus of cooling degree-days is forecasted across the Desert Southwest, southern Rockies, Southeast, Ohio Valley, Mid-Atlantic and Northeast during August and September. Portions of the Pacific Northwest and northern Rockies, however, might see slightly cooler than average temperatures and as a result lower than normal amounts of cooling degree-days.