CEDAR RAPIDS, Iowa — A possible early-season polar vortex disruption has forecasters looking back at past winters for clues. The last strong El Niño winter with a November polar vortex disruption was 2009–2010. That winter turned out colder than expected, with December bringing bitterly cold conditions across much of the country. Based on that history, a quick start to winter looks possible this year.
December 2009 Turned Bitterly Cold
Temperature records from December 2009 show below-normal readings across nearly the entire United States. The coldest departures, about 13 degrees below normal, were centered over Montana and North Dakota. Much of the northern and central Plains ran 7 to 10 degrees colder than average.
That included large parts of South Dakota, Nebraska, Kansas, and Iowa. Texas, Oklahoma, and the Rockies were also several degrees colder than normal. Even the East Coast and Southeast ended up slightly below average.
Only Florida Escaped the Cold
Southern Florida was the only area that stayed close to normal that December. Nearly every other region recorded below-average temperatures. The Pacific Coast and the Great Lakes were only a little colder than normal.
The widespread cold surprised many who expected a mild El Niño winter. Strong El Niño winters often run warm across the northern U.S. In 2009, the polar vortex helped flip that expectation.
Why the Polar Vortex Matters
The polar vortex is a large area of cold air and strong winds circling the North Pole high in the atmosphere. When it weakens or splits, Arctic air can break loose and spill south. That can override the usual warmer pattern of an El Niño winter…