El Nino is an atmosphere/ocean phenomenon in which the waters of the eastern equatorial Pacific Ocean are warmer than normal for an extended period of time.
This unusual warmth forces changes in the distribution of deep convective thunderstorms over the tropical ocean which, in turn, affect the position and strength of the jet stream. Since the jet stream acts a conduit of winter storms and can regulate intrusions of cold arctic air, changes in jet stream characteristics can have a profound influence on our winter weather.
Generally, El Nino is associated with warmer than normal winters in the western Great Lakes states so we can reasonably expect that this coming winter will be milder than the last two. Though there is a tendency for a bit more precipitation in El Nino winters here, since it is usually warmer than normal, there is usually less than the normal amount of snow.
Currently, the sea surface temperature data in the equatorial Pacific suggest that this year’s El Nino may be the strongest on record. If it continues to grow in intensity as predicted, our winter could become memorably mild. One of us recalls throwing rocks into Lake Mendota in late February 1998, near the end of the last really strong El Nino winter, and imagining that might never happen again. Perhaps it will.