U.S. seems to have largely escaped winter

The temperature in Minneapolis didn’t fall to zero degrees this winter until Jan. 12. On Jan. 5., the daytime high in Rapid City, S.D. (a record-setting 71 degrees), was higher than in balmy Miami (69 degrees). And just a couple of days before New Year’s, visitors to Park City, Utah, skied on man-made snow and dined al fresco — without their parkas.

Throughout the continental United States, it’s been a very warm winter.

“The talk across the whole country has been, ‘Where has winter been?’” said Dale Eck, who runs the global forecast center at the Weather Channel in Atlanta.

The answer: A combination of factors has trapped the winter’s cold air in the northern latitudes over Canada and Alaska.

“If you look at U.S. temperatures, you’d say, ‘Wow, it was a warm winter,’” said Dan Cayan, a climate researcher at the U.S. Geological Service and the Scripps Institution of Oceanography in La Jolla. And you’d be right.

“But,” he added, “in the coastal West, it’s been cool.”

Sunshine and nearly 80-degree temperatures in downtown Los Angeles this week — combined with an early January heat wave and vicious Santa Ana winds in late November and early December — might leave locals with the impression that winter has been similarly balmy in Southern California.

But while the season is shaping up to be exceptionally dry, it has not been unusually warm.

In fact, November’s average high temperature of 69 degrees in downtown Los Angeles was four degrees below normal, and December’s average of 66 was two degrees below normal, said Ryan Kittell, a forecaster at the National Weather Service’s Oxnard office.

Overnight low temperatures were also cooler than average, making this December the seventh-coldest (by that measure) since 1877.

In January, however, there have been an unusual number of days when the temperature downtown exceeded 80 degrees — four, as of Friday. January usually has two such days, on average. Those days have pushed the average temperature for the month so far to 70 degrees, which is 2 degrees above normal.

Scientists said the cyclical cooling in the Pacific Ocean known as La Niña was a likely cause for dry conditions in California and across the nation.

There’s an 82% probability of less-than-normal rainfall in a La Niña year, said Bill Patzert, a climate researcher at the Jet Propulsion Laboratory in La Cañada Flintridge.

Most of California has received less than half of its normal precipitation this winter, Cayan said.

According to the National Weather Service, downtown Los Angeles has had 5.06 inches of rain this water year, which began July 1. The average for that time period is 6.74 inches.

La Niña-related dryness might have helped California stay cool at night, Kittell said, because less rain means less water vapor in the air. Water vapor is a greenhouse gas that traps heat near the ground.

“When it’s very dry, you kind of lose that extra layer and the ground cools like crazy,” he said.

Cayan chalked up the cool temperatures on the West Coast to its position on the eastern edge of a La Niña-related high-pressure center over the Pacific Ocean that has created a dry, cool air flow in the region.

La Niña has also helped keep the jet stream on a west-to-east path over Canada, preventing cold Arctic air from dipping into the Lower 48 states, he said.

A phenomenon known as the Arctic Oscillation has reinforced that effect, Patzert said.

The oscillation is a pattern of pressure that wraps itself around the North Pole. When the pressure is low, as it has been for most of this winter, the oscillation captures the cool air that normally breaks out of the Arctic and moves into Canada.

The Arctic Oscillation shifted in January, leading some meteorologists to predict that cold air would soon dip farther south, allowing the winter to finally begin in earnest.

But since La Niña can persist for years, Cayan said he suspected it was unlikely California would catch up on rain and snowfall this year.

“We’re so far behind right now,” he said.

eryn.brown@latimes.com