A blast of frigid Arctic air has descended over the Western United States, toppling records far and wide as readings plummet to some 40 degrees below average for this time of year. Temperatures throughout much of the Rockies dipped below zero to start the week, falling as low as minus-29.2 in Potomac, Mont., early Sunday — the coldest temperature ever observed this early in the season across the Lower 48.

The cold has helped whip up a significant winter storm, which is taking shape now from the Four Corners to the southern Plains. It’s dropping heavy accumulations of snow and ice as the frigid air plunges south in its wake.

Winter storm and ice storm warnings are up for most of Colorado, New Mexico, the Texas Panhandle and Oklahoma, while winter weather advisories stretch northeast into Kansas City toward the Iowa border. Behind the system, wind chills will drop to minus-25 in the Intermountain West on Monday night.

Meteorologists sometimes refer to storms like this one as a kitchen-sink storm, a tempest that combines the worst of all seasons in which different air masses collide.

Central Oklahoma is preparing for its worst ice storm since 2015, with the Oklahoma City metro area under an ice storm warning.

Flanking the shot of cold air and snow lays extreme wildfire danger on the West Coast, the result of cool air pushing across the slopes of the Sierra Nevada Mountains, warming, drying and accelerating downhill at speeds greater than 100 mph in some cases.

The record cold

A sharp dip, or trough, in the jet stream across the West has allowed cold air to spill south into the Western and north-central states, bringing frigid readings bordering on the extreme.

In Potomac, Mont., where the minus-29.2 reading was set, the National Weather Service had predicted that the location would hit minus-30, which would have been the Lower 48′s earliest minus-30 reading on record. That didn’t end up happening as some last-minute cloud cover prevented radiational cooling from having its maximum effect.

“The observer has come in, and it did not get that cold,” Corby Dickerson, a meteorologist at the National Weather Service in Missoula, said in an interview. “Surface pressure didn’t get nearly as high, the high was slightly weaker … [and] I think there was just a bit more [cold air] drainage wind through the canyon last night. It’s the subtleties that make the forecast during this sort of cold so challenging.”

Regardless, the ongoing cold is off the charts, with an air mass more typical of December or January than late October.

“I talked with a climatologist in Alaska,” Dickerson said. “He said that there are 14.5 million observations [across the country] in the database between October 1 and October 25, and Potomac was the coldest. It’s truly remarkable. There’s no other way to describe it.”

Two out of the past three mornings have been among Missoula’s coldest three ever recorded in October, and more records will probably fall later in the week.

“I’ve been describing it as a once-in-a-century event,” Dickerson said.

In addition to Sunday’s reading of nearly 30 below, a slew of other records have come crashing down courtesy of the early-season Arctic outbreak. Missoula set a record for its earliest zero-degree reading observed, hitting minus-7 on Monday morning. Anaconda, a town of 9,000 in western Montana, was forecast to drop to minus-23 degrees Monday morning; its previous record was 5 degrees.

Missoula also logged its eighth-biggest snowstorm on record, with a hefty 13.8 inches falling in just two days.

It’s not just the Northern Tier that’s been dealing with record cold. Boulder, Colo., snagged a record Monday morning when it dipped to 5 degrees; the previous record for the date, set in 1997, was 13 degrees.

The core of the cold — some 40 to 45 degrees below average in spots — should diminish in intensity and shift south and east mid- to late week.

The snowfall

Accompanying the cold has been a batch of heavy snow that over the weekend plastered parts of the High Plains and Rockies. Denver hit 8 degrees early Sunday and picked up just under three inches of snow. Boulder wound up with more than a foot.

Despite the heaviness of the snow, Coloradans are no stranger to wintry weather in October. In fact, the 9.9 inches that fell in Boulder on Sunday doesn’t even claim a top spot for an October snowstorm.

Some of the snow did fall on the Cameron Peak and East Troublesome fires, the state’s largest and second-largest wildfires on record. While that will bring about at least a temporary slowdown of the blazes, it is unlikely to completely extinguish them, especially because milder weather will follow this cold snap.

Farther south, almost the entire state of New Mexico was under a winter storm warning Monday, with heavy snow of half a foot or more likely in the majority of locales. Higher elevations in the mountains could see 12 to 18 inches.

The snow had filtered into Texas after a bone-chilling “blue norther” cold front last week, with temperatures plummeting behind an Arctic cold front.

Thundersnow was reported across the Texas Panhandle on Monday morning, and more is on the way. Four to 10 inches could fall Tuesday into Wednesday. That may break the October record for Amarillo’s snowiest calendar day, which stands at 5.5 inches.

The snow came sweeping down the Plains, too. Wichita had accumulated 1.3 inches by lunchtime Monday, the heaviest snow it has experienced so early in a season. Every county in the state was under a winter weather advisory, a combination of snow, sleet and freezing rain making for slick travel. Wintry weather was tapering down and exiting to the east Monday afternoon.

The ice

In Oklahoma, waves of moisture riding along the stalled frontal boundary were producing a messy slop of precipitation. Much of central Oklahoma was under an ice storm warning, including the Interstate 35 corridor north of Oklahoma City. Some parts of the Sooner State could see a half-inch to an inch of ice accretion by Wednesday.

It’s exceptionally early in the season to see an episode like this; in fact, there is no precedent for the Weather Service offices in either Tulsa or Oklahoma City issuing an ice storm warning in October. The warning issued Monday morning by the Tulsa office beats out the previous earliest issuance by six weeks.

Oklahoma City, Lawton-Fort Sill, Okla., and Wichita Falls, Tex., — all home to roughly 100,000 people or more — could see a half-inch to an inch of ice, making this a high-impact event. For some, the freezing rain could mix with or change to sleet for a time, which would limit ice accumulations on trees and power lines.