The calendar is about to flip to December, but the atmosphere feels more like mid-October as a dome of anomalous warmth envelops large parts of the Lower 48. Most of the western and central regions of the United States are set to experience temperatures about 10 to 15 degrees above normal for an extended period.
Some of the mild air may sneak into the eastern United States at times, especially toward the second week of December.
November ordinarily features a parade of storminess across the continental United States, but this year’s conga line of season-shifting storms appears delayed as toasty high pressure expands and languishes. Most of the storminess is just skirting very northern areas, such as northwest Washington state, the Great Lakes and, occasionally, the Northeast.
To the south and southwest, it’s unusually mild, and the pattern shows little sign of changing.
Into the second week of December, the National Oceanic and Atmospheric Administration’s Climate Prediction Center is calling for virtually the entire country, save for Alaska, to remain near or above average with respect to temperatures, a prominent red shading engulfing the national map.
The only area not in line for warmth right away is the Northeast and New England, where persistent northerly flow is keeping the encroaching warmth at bay.
On Monday, temperatures were spiking early across the Intermountain West, Front Range and Great Plains. Boulder, Colo., hit 70 degrees at 10 a.m., with only a 1 percent chance of that occurring so early in the day this late in the year. Denver’s average high for late November is 48 degrees. The Mile High City could climb into the 70s on Monday, Wednesday and Thursday.
Denver has yet to see measurable snow this fall, the latest into the season on record for such a lack.
In Oklahoma City, the temperature stood at 64 degrees at noon Monday, seven degrees above the average high for the date. The Sooner State’s capital should also peak near or above 70 degrees every day through the end of the workweek.
By Wednesday, temperatures in the mid- to upper 60s should make it all the way north to the Canadian border. Billings, Mont., is expected to hit 67 on Wednesday and 66 on Thursday, which could tie or break records both days. The city’s average high for early December is in the upper 30s.
Dallas will approach 80 degrees on Friday, about 19 degrees above typical as December is ushered in.
Instigating the warmth is a chain-reaction atmospheric process that Judah Cohen, director of seasonal forecasting at Atmospheric and Environmental Research, says begins in the North Pacific. That’s where high pressure is likely to build in the coming week south of the Aleutians, shuffling large-scale weather systems to the east.
Cohen explained that the high favors a dip in the jet stream, to its east between Alaska and Hawaii, which will keep much of the United States under a high-pressure zone and mild southwesterly flow. He also noted that persistent downsloping, or air subsiding in the lee of a mountain range (in this case the Rockies), will be a factor. Air that sinks is compressed and warmed by increased atmospheric pressure near sea level, resulting in greater temperatures.
While some outlooks foreshadow toasty times for weeks to come, Cohen does see an out — if the high-pressure ridge over the West breaks down. That, he said, would open the doors for some eventual cooling. The question is whether it will materialize.
“I am surprised at the [simulated] stubbornness of the Western U.S. ridge,” he wrote in a Twitter direct message. “I would think it would get replaced with a trough and, though the models predict such a scenario, it never seems to get any closer than 11-15 days out.”
Cohen is also keeping tabs on the polar vortex, or an eddy of frigid air wrapped up in a whirlpool of strong winds encircling the High Arctic. When the polar vortex is strong, as is the case now, much of the bone-chilling air remains bottled up, at bay to our north and out of our hair.
“Overall the polar vortex is relatively strong, which also favors cold temperatures in the North American Arctic including Alaska, but with milder temperatures in the mid-latitudes across the Northern Hemisphere,” Cohen wrote.
Long-range weather models struggle to capture exact details in the longer-range weather patterns but hint that the blob of warmth over the western and central United States may migrate eastward toward the second or third week in December. That’s when the Climate Prediction Center favors warmer-than-normal conditions finally expanding into the Northeast, although confidence in specifics is low that far into the future.
While brief pulses of chilly air are likely to sweep across the country over the next two weeks, there’s little sign of deep, sustained cold in the Lower 48 states through mid-December.