Overall, sea-surface temperatures across the Equatorial Pacific Ocean continue to slowly increase with much of the Pacific Ocean seeing at least slightly warmer than average water temperatures. The latest long-range climate models continue to advertise at least a weak El Nino being in place during the upcoming late fall and winter.
As for the October temperature outlook, slightly above normal temperatures are expected throughout portions of the Rockies, Intermountain West, and north-central Plains where a deficit of early-season heating degree days of between 20 and 60 is predicted in October. Parts of the Desert Southwest may see some extra late-season cooling degree days with a surplus of between 20 and 40 possible.
Farther east, climatological trends from past years with a developing weak El Nino, specifically 2006, 2009, and 2012, suggest that temperatures may average slightly below normal from parts of the Ohio Valley down to parts of the Southeast during October. In addition, the NAO (North Atlantic Oscillation) has been primarily positive since May and there are some indications that this index may finally become negative by late September or October.
If this occurs, it could lead to a pattern that allows for cooler conditions across parts of the eastern United States. Portions of the Southeast, Tennessee Valley, and Ohio Valley may see an early-season surplus of heating degree days as a result.