I am a co-author, with lead author John Allen and second author Mike Tippett on a paper that just came out today in Nature Geoscience. This paper quantifies, better than had been done previously, the relationship between El Niño or La Niña events and severe weather – tornadoes and hail – over the United States. The big conclusion is that knowledge of the state of an El Niño or La Niña during winter can be used, in principle, to forecast changes in the probability of severe weather the following spring, several months ahead. The relationship is such that tornadoes and hail are suppressed in an El Niño year (enhanced in La Niña). This seems to be playing out so far this year, as March has been dead tornado-wise and we are indeed in an El Niño.
This paper is getting some media attention. So for more detail, please look at Climate Central, Bloomberg, Phys.org, The Carbon Brief, Science Magazine, or LiveScience.