As the ocean temperature rises, pressure changes over the water have an influence on upper air patterns over North America. The changes in the Jet Stream brings some good and some bad.
First the good, El Nino can supress the Atlantic Hurricane Season. It can also bring a milder Winter to the Northern states and can bring beneficial rain to the desert Southwest.
Now the bad, El Nino can bring destructive storms to California, increasing flooding and mudslides. It can also bring more Winter severe weather to the southern states including Arkansas.
El Nino usually returns every 2-5 years. This one should last through the Winter of 2009-10.