Key Facts

• El Niño is a warm phase of a climate pattern in the Pacific Ocean that affects weather worldwide.
• The National Oceanic and Atmospheric Administration (NOAA) predicts a very strong El Niño this fall and winter, with a high chance it could be stronger than past events since 1950.
• The polar vortex is a large area of cold air around the poles that strengthens in winter and can send cold air south to the U.S.
• A strong polar vortex usually brings warmer conditions in the Northern Hemisphere but can still cause short cold spells.
• The polar vortex forms in late summer and usually breaks down by mid-May.
• The effects of the polar vortex on U.S. winter are less significant than those of El Niño and another Pacific pattern called the Pacific Decadal Oscillation.
• If the polar vortex weakens in late winter, it might cause a short period of colder weather in the northern U.S.
• Overall, the polar vortex does not usually set the long-term weather pattern during a strong El Niño winter.