El Nino Watch
A few days to weeks of winds blowing from the west over the western Pacific, against the usual trade winds. It can launch a warm Kelvin wave.
El Nino basics
Normally the equatorial winds blow from the east. Sometimes, for a few days to a few weeks, they blow from the west instead over the far western Pacific. That is a westerly wind burst. They are often linked to tropical storm activity or to the Madden-Julian Oscillation (a wave of cloud and rain that travels around the tropics every 30 to 60 days).
A strong burst pushes warm surface water east and triggers a downwelling Oceanic Kelvin wave. Several bursts in a row during the build-up of an El Nino often make it stronger.