Extreme El Niño events are among the most consequential climate phenomena, yet their early prediction remains challenging. Here, by analyzing the 1982/83, 1997/98, and 2015/16 events together with a large ensemble of climate model simulations, we identify a slow, eastward-propagating coupled mode in the western-central equatorial Pacific that plays a critical role in the onset of such events. This mode features warm sea surface temperature anomalies coupled with sustained westerly wind anomalies during boreal winter-spring. These westerly wind anomalies drive the equatorward transport of warm water, rapidly increasing equatorial ocean heat content and priming extreme El Niño growth in the following summer and fall. This mode is also observed in early 2026, preconditioning the development of this on-going El Niño event. Ensemble hindcasts further reveal that East Asian cold surges and the North Pacific Oscillation can stochastically excite this equatorial mode, enabling skillful forecasts of extreme events as early as February and helping to overcome the spring predictability barrier.