September 2018 Climate Forecast Discussion

September 2018 Climate Forecast Discussion for Oct-Dec through Jan-Mar

The SST forecast shows weak El Nino conditions developing by the initial Oct-Dec season, most likely remaining at weak strength (>0.5C and <1.0C for the Nino3.4 index) through Jan-Mar, with a smaller chance for moderate strength (>1.0C). Much of the subtropical and extratropical North Atlantic and North Pacific SST, as well as the SST in much of the mid- to high-latitude Southern Hemisphere, are predicted to be above-average during all four forecast seasons. Exceptions are noted in the very high latitudes north of North America and south of South America, where below-average SST is predicted for all four forecast seasons. Slightly below-average SST is also predicted in the Southern Hemisphere tropics in a few locations for the Oct-Dec and Nov-Jan seasons. A positive Indian Ocean Dipole is predicted for all four forecast seasons. Near-average SST is predicted for the north and south tropical Atlantic for all four seasons.

A tilt of the odds toward below-normal precipitation is forecast for northern South America, Indonesia, the Philippines and parts of Australia for all four forecast seasons (Oct-Dec through Jan-Mar). Slightly enhanced probabilities for below-normal precipitation are also predicted for central Chile for Oct-Dec and Nov-Jan, parts of southern Africa for Nov-Jan through Jan-Mar, and some Caribbean islands for Oct-Dec and Dec-Feb. Enhanced chances for above-normal precipitation are forecast for parts of the southern U.S./northern Mexico and part of southeastern South America, and varying parts of Asia, for all four forecast seasons. Above-normal is also favored for eastern equatorial Africa for all four forecast seasons, and for the African Gulf of Guinea and part of northern Africa for Oct-Dec and Nov-Jan.

Enhanced probabilities for above-normal temperature are predicted for nearly all of Greenland, most of Australia, and varying parts of Asia, northeastern and/or eastern Europe, northern or southern Africa, North America and South America from Oct-Dec through Jan-Mar. However, the above-normal signal is mainly absent from south-central Africa and western Europe in all four seasons, from southern Africa in Oct-Dec, and north-central Asia in Jan-Mar. Enhanced probabilities for below-normal temperature are very scant, and appear just weakly in part of north-central or northeastern Asia in Jan-Mar. An enhanced likelihood for near-normal temperature is predicted for various small parts of northern South America and in tropical Africa in all four forecast seasons, and in parts of Indonesia in Oct-Dec.