The eastern and central equatorial Pacific Ocean warm up periodically. The onset of this year's El Nio, the first in almost four years, occurred in June.
Since the end of August, its warm temperature anomalies have pushed over the threshold of a severe El Niño, at least 1.5 degrees Celsius, or 2.7 degrees Fahrenheit, higher than typical.
The weather in the United States and elsewhere may be affected by El Nio and its cooler counterpart, La Nia, even though they are separated by thousands of km.
Due in part to a stronger, more southern jet stream course, the traditional El Nio winter tends to be wetter than usual over most of the southern U.S., from sections of California to the Carolinas.
It snowed more than usual throughout a large swath of the West, from the Northern Plains to the upper Midwest. However, most of the Northeast was notably less snowy than typical.
Particularly intriguing is the possibility of more snowfall in the mid-Atlantic. None of the three major East Coast cities—Philadelphia, Baltimore, or Washington, DC—received even an inch of snowfall all season long last year.
The two "Snowmageddon" snowstorms that hit the mid-Atlantic states in early February 2009 were a result of El Nio, one of the more recent stronger El Nios.