EXPLANATION: Jet stream blocked, La Nina causing strange weather

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FILE - Kenneth Warner runs through the water to help rescue the driver of a car that entered the flooded Nooksack River on Tuesday, November 16, 2021, in Ferndale, Washington.  Winter has started bizarrely in the United States.  The weather has been shocking across much of the country, with the Pacific Northwest receiving record precipitation and Denver and the Rockies yet to see their first major snowfall of the season.  (AP Photo / Elaine Thompson, file)

FILE – Kenneth Warner runs through the water to help rescue the driver of a car that entered the flooded Nooksack River on Tuesday, November 16, 2021, in Ferndale, Washington. Winter has started bizarrely in the United States. The weather has been shocking across much of the country, with the Pacific Northwest receiving record precipitation and Denver and the Rockies yet to see their first major snowfall of the season. (AP Photo / Elaine Thompson, file)

PA

America’s winter wonderland begins this season like anything but traditional.

The calendar says December, but for much of the country, temperatures invite sandals. Umbrellas, even arches, are needed in the Pacific Northwest, while in the Rockies snow shovels collect cobwebs.

Meteorologists attribute the latest batch of record-breaking weather extremes to a blocked jet stream and the effects of a weather model of La Nina coming from cooling waters in the equatorial Pacific.

Astronomically it’s still fall, but winter begins on December 1 for meteorologists. This year, no one told the weather forecast.

On Thursday, 65 weather stations across the country set high temperature records for December 2, including Springfield, Missouri, reaching 75 degrees Fahrenheit (24 Celsius) and Roanoke, Virginia 72 degrees Fahrenheit (22 Celsius). Cheyenne, Wyoming, and Billings, MT, broke long-standing heat records of 6 degrees.

Parts of Canada and Montana recorded their highest December records in history. Parts of South Carolina and Georgia hit record highs on Friday.

In Washington state, Seattle, Bellingham and Quillayute all set 90-day fall records for precipitation. Bellingham was washed down by almost two feet of rain. The Olympic and Cascade Mountains have been hit hard, with more than 50 inches (127 centimeters) in three months, according to the National Weather Service. Forks, Washington, received more rain in 90 days than Las Vegas in 13 years.

On top of that, there is a blizzard warning over the peaks of the Big Islands of Hawaii with up to 12 inches (30.5 centimeters) of snow expected and wind gusts of over 100 miles per hour (161 kilometers per hour).

Meanwhile, the snow has disappeared in Colorado. Prior to this year, the last recorded first measurable snowfall in Denver was Nov. 21, 1934. There is a slight possibility of snow Monday night, according to the Weather Service. Still, without snow since April 22, it’s the third longest snow-free stretch in the city.

One important factor: the jet stream – the river of air that moves the weather from west to east on a roller-coaster-like path – has just been blocked. This means that the low pressure over part of the stream brings rain to the Pacific Northwest, while the high pressure hovering over about two-thirds of the country is producing drier and warmer weather, Brian said. Hurley, senior meteorologist at the Weather Prediction Center of the Weather Service. in College Park, Maryland.

If the jet stream moves more or bends differently, rain and other extreme weather conditions won’t be as concentrated, Hurley said.

This is a typical weather pattern with a natural weather oscillation from La Nina, he said. The flip side of El Nino, a La Nina is a cooling of parts of the central Pacific Ocean that changes weather patterns across the world. The Ninas tends to bring more rain to the Pacific Northwest and make the south drier and warmer.

These extreme weather episodes are occurring more frequently as the world warms, said meteorologist Jeff Masters, founder of Weather Underground who now works at Yale Climate Connections. But scientists haven’t done the study required to attribute these events to man-made climate change.

In Boulder, Colorado, meteorologist Bob Henson enjoyed a rare bike ride in December Thursday.

Still, “there is a lot of angst about the lack of snow,” he said. “It puts you in a psychic dilemma. You take advantage of the hot weather while keeping in mind that it is not good for the Earth to warm up.

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Follow Seth Borenstein on Twitter at @borenbears.

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