El Nino Onset by December 1st. Discussion: During the past 6 months a titanic effort on part of the warming equatorial Pacific Ocean subsurface to produce El Nino has been underway. However, El Nino has not developed due to the lack of interaction between the atmosphere and the ocean warming. During the past few weeks the lack of cooperation between the coupling of the atmosphere and ocean warming has changed. The primary indicator of the change is the sudden persistent negative phase of the southern oscillation (Fig. 1). The –SOI indicates the convection patterns in the tropical Pacific are favoring an El Nino climate evolving. Presence of –SOI also indicates trade winds are easing or reversing to the westerly direction. The factors identified that favor El Nino approach could only happen if the subsurface anomalous warmth (Fig. 2) was now appearing at the surface of the eastern equatorial Pacific (Fig. 3). Unique to the evolving warming ocean in the Pacific tropics in 2018 is presence of most of the surface and subsurface anomalous warmth near or just east of the Dateline suggesting El Nino Modoki may be developing (Fig. 4). However, the far eastern equatorial Pacific is now warming and the Modoki risk is easing. The ENSO analog years now identify the evolution of 3 conventional El Nino episodes and 1 El Nino Modoki (which eventually became conventional El Nino). The analog years support recent observational diagnostic trends toward El Nino. El Nino onset is likely in 1-2 months (Fig. 5). Fig. 1: The southern oscillation index has shifted to the negative (El Nino) phase during late summer and early autumn. The persistence strongly suggests the atmosphere is responding to ocean warming and an El Nino climate is developing. Fig. 2: Immense subsurface warming has been present much of 2018 near the Dateline. Recently that warmth has extended eastward to the northwest coast of South America causing warming of the surface raising risk of conventional El Nino with onset by Dec. 1st. Fig. 3: Despite subsurface warming the Nino SSTA regions have been neutral this past summer until the past 1-2 weeks when sudden warming occurred. Fig. 4: The Modoki index wondered into positive phase during August suggesting El Nino Modoki was ahead. However, a more traditional El Nino character has developed the past 30 days or so lowering Modoki risk. Fig. 5: The Climate Impact Company ENSO analog years indicate El Nino is imminent and peaks in December then weakens late winter with uncertainty as to whether El Nino continues or dissipates as mid-2019 arrives. Implications: The analog years are a consensus of 3 conventional (weak-to-moderate) El Nino years and 1 El Nino Modoki year. Other factors such as regional ocean sea surface temperature anomaly (SSTA) patterns, snow cover and intra-seasonal oscillation frequency and intensity (i.e. MJO events) are not included in the ENSO only climate implications for winter. The selected analog years continue to indicate colder differences from a conventional strong El Nino regime which would produce a vast anomalous warm winter. The reason for this colder change remains the expectation that the bulk of anomalous warmth in the tropical Pacific surface and subsurface will remain near and just east of the Dateline. The influence on the atmosphere of this ocean temperature pattern on U.S. winter is a warm upper ridge in the West and cold/stormy trough in the East (Fig. 6-7). The northern Plains have a greater tendency to experience mild Pacific westerlies while the Southeast U.S. storm track drives East/South chill. Note that the Tennessee Valley and West U.S. are dry. The colder pattern likely develops mid-to-late winter due to the early winter warming influence of warm SSTA east of the Northeast U.S. The HDD departures for DEC/JAN/FEB 2018-19 range from -70 from Texas to the Appalachians to +70 over the Northwest to the Upper Midwest (Fig. 8). Fig. 6: CIC El Nino analog years and their consensus of anomalous temperature implications for DEC/JAN/FEB 2018-19. Fig. 7: CIC El Nino analog years and their consensus of anomalous precipitation implications for DEC/JAN/FEB 2018-19. Fig. 8: CIC El Nino analog years and their consensus of anomalous heating degree day implications for DEC/JAN/FEB 2018-19.