NASA's Earth Observatory posted these two comparison images of sea surface patterns in May of 1997, before that year's infamously terrible El Niño, and this month. The images are startlingly similar, suggesting we might be on the precipice of another tough El Niño year.
During an El Niño, sea surfaces in the central and East Pacific warm up, which in turn leads to (as was the case in '97) heavy storms, floods, and severe weather patterns all around. Using the height of the sea surface, and its changes, scientists can get a pretty good idea of just how much the water has warmed and get some clues as to when we might expect an El Niño to hit.
Even though the odds are looking better and better that we might be looking at something pretty similar to what we saw 17 years ago, scientists still aren't ready to say for certain whether it will be this year or, if it is, if it will be the match of 97's El Niño. Still, they're saying it's more likely than not (65%, to be precise) that it will arrive this summer.