Friday night’s vicious tornadoes put a deadly exclamation point on the end of an extraordinary year for extreme weather in the United States.

Earlier in 2021, Texas froze and Seattle roasted. Parts of California flooded, burned, then flooded again. A hurricane that slammed Louisiana was so waterlogged that its remnants inundated New York City. A blizzard hit Hawaii.

The weather was wilder than usual this year, and the reasons vary, climate experts say.

Crazy cold snap? Giant hail? December tornadoes? Those happen now and then on a planet with natural variations in weather patterns.

But evidence increasingly shows that historic heat waves, monster rain events and ultra-intense storms are exacerbated by the warmer air and water of our overheating planet.

“The only two truisms when it comes to extremes in climate change are that almost everywhere: The hot hots are getting hotter and more frequent, and the wet wets are getting wetter and more frequent,” said Daniel Swain, a climate scientist at UCLA who specializes in the relationship between climate change and weather.

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The year began with what Swain might call a “wetter wet” against the backdrop of a yearlong drought, and it just got weirder from there.

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January: California floods amid drought

Swain said climate change has created conditions in which — even if the total amount of rain and snow is fairly stable from year to year — the West’s rainy season is shorter, and the moisture often falls in violent bursts rather than long, drought-squelching soaks. It’s like the difference between sipping a cup of water vs. having it thrown at your mouth.

For five days late in late January, California had water thrown at its mouth.

Much of the West’s water comes from atmospheric rivers, which are like fast-moving, airborne conveyor belts that shuttle moisture from the Pacific to the West Coast about a dozen times a year. They are notoriously unpredictable and are often described as giant fire hoses in the sky.

This particular atmospheric river blasted as much as 16 inches of rain and 100-mph winds through the middle of the state. Mudslides and floodwater covered major highways. The Big Sur River overtopped its banks. Debris flowed unchecked through areas recently scarred by wildfires. Higher in the Sierra Nevadas, roads were buried beneath up to 9 feet of snow.

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February: Deadly cold in Texas

It was strolling-around-the-neighborhood weather in much of Texas for the first week of February. Then the next week, frigid Arctic air stretched drastically far south and obliterated low-temperature records from North Dakota to Mexico.

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At least 210 people died in Texas, where the underprepared power grid gave out and more than 4 million households lost power. Travel was nearly impossible, as many areas were covered in snow and ice. Crops were devastated as newly planted seedlings in the Rio Grande Valley couldn’t withstand the hard freeze.

The Texas cold snap may have been an anomaly. But some scientists believe that the warming of the Arctic has disrupted the winds that encircle the pole, unleashing giant blobs of frigid air into lower latitudes.

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March, April and May: Supersized storms

In spring, violently rotating thunderstorms called supercells are common across the country as cold and warm air masses meet and dance around each other. But the three supercells that struck Alabama and Georgia in late March were notable for their power and endurance. All three spun out tornadoes and lasted several hours. One cell traveled more than 400 miles through four states.

Just two days later, another powerful system swept from East Texas to North Carolina, leaving a trail of hail, flooding and tornado damage. A couple of weeks later, more than a foot of rain in one day dumped onto parts of Louisiana that were already saturated from an unusually rainy April.

And though it was a light year for hail overall, three dramatic storms during two days in April flung ice chunks the size of baseballs and softballs through roofs and windows around Fort Worth, San Antonio, Oklahoma City and Norman, Okla.

May and June are usually prime tornado months, and true to form, May began with a three-day bout of twister-spewing thunderstorms that dropped tornadoes from Colorado to Georgia.

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It’s nearly impossible to tie an individual storm to climate change, but the trends show a greater probability of violent storms in a warmer climate, said Victor Gensini, an atmospheric sciences professor at Northern Illinois University who studies tornadoes and other severe storms.

Gensini compared the uncertainty to dice. If you change one side of a die from a five to a six, you will have a much greater chance of rolling a six. But when it happens, you can’t be sure if that die is showing the six you added or the one that was always there.

Though hurricane season was still a couple of weeks away, Tropical Storm Ana became the first of 21 named Atlantic storms of 2021 and made this the seventh straight year that a named storm formed before the actual hurricane season, a trend scientists have linked to warming ocean waters. Ana didn’t make landfall, as eight others would, but it kicked off the second year in row that the National Hurricane Center would use all the names on its list.

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June: Otherworldly heat

Beginning in mid-June, a blanket of unprecedented hot air spread over the typically mild Pacific Northwest, an event scientists say was “virtually impossible” without climate change.

Locations in seven states set all-time heat records. Seattle hit 108 degrees, Portland 116, and Salem, Ore., recorded 117 on June 28, matching the hottest day ever recorded in Las Vegas. Hundreds of deaths were attributed to the heat.

The culprit was an alarmingly strong heat dome, a sprawling mass of high pressure and hot air that muscles out any cooling systems that come near it. And its sheer power rattled experts who study heat waves all the time.

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“That just blew everyone’s minds,” said Swain. “It was just so far beyond what really anybody thought was physically possible in our present climate … What happens if that happens over Chicago in July or Delhi during the dry pre-monsoon season? What does that look like? … That’s very unsettling.”

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July: Storm clouds of fire

On July 14, the largest, most destructive wildfire of the year began on the West Coast about 100 miles north of Sacramento.

The monstrous Dixie Fire was exactly what fire experts had feared after the historically horrible 2020 wildfire season and the extreme June heat. It would become California’s second-largest fire ever, burning nearly 1 million acres in the Lassen National Forest and destroying the town of Greenville before it was finally doused in late October.

Within days after it started in the Feather River Canyon, it produced pyrocumulonimbus clouds — literal storm clouds of smoke and fire — that rose more than 30,000 feet.

Its smoke wafted all the way to New England.

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August and September: Smoke and a string of hurricanes

As Dixie and other large fires across the West continued to belch smoke into the atmosphere, plenty of precipitation was on its way to the other side of the country.

By mid-August, tropical storms and hurricanes had queued up in the Atlantic.

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Tropical Storm Fred was first in line of the August storms, rolling through the Florida Panhandle and causing deadly flooding in North Carolina.

Then came three days during which Tropical Storm Henri made a rare Rhode Island landfall and drenched New England while unrelated storms caused catastrophic flooding in North Carolina and a freak deluge in Tennessee.

That hyper-localized storm dropped about 21 inches of rain in one day over Waverly, Tenn., about an hour west of Nashville. Quick-rising water killed at least 22 people, including a woman who was livestreaming the flooding in her neighborhood.

At the very end of the month, the year’s biggest and most destructive storm rumbled into Louisiana.

Hurricane Ida hit Port Fourchon on Aug. 29 as a strong Category 4 storm with 150-mph winds so powerful that they reversed the flow of a portion of the Mississippi River for three hours.

Ida knocked out power in New Orleans and Houma, submerged the town of Jean Lafitte and damaged or destroyed every structure in Grand Isle while covering it in 3 feet of sand.

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Ida’s wind diminished during its soggy trek north, but it still spun out tornadoes in New Jersey, Pennsylvania and Maryland.

And it still had plenty of water.

In a few hours beginning the night of Sept. 1, Ida’s remnants dumped up to 11 inches of rain over a swath that included densely populated areas of New York and New Jersey.

More than 3 inches of rain fell in Central Park in just an hour, a record, engulfing subway tunnels. Thousands of homes and businesses were inundated, including some in places that rarely experienced high water.

Fifty-two people drowned in the Northeast, including 11 who couldn’t escape from basement apartments in New York City.

Altogether, Ida killed at least 91 people in nine states, according to the Centers for Disease Control and Prevention.

On Sept. 14, the season’s last hurricane landfall occurred near Sargent Beach, Texas. Nicholas, a Category 1 storm, brought heavy rain but did relatively little damage as it meandered along the Gulf Coast. On Nov. 30, the sixth-straight above-average hurricane season officially ended.

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Hurricanes have always occurred, but models indicate that warmer temperatures combined with unchecked greenhouse emissions may make hurricanes wetter, stronger and more likely to veer toward North America.

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October: Historic rivers in the sky

In late October, three atmospheric rivers showed up on the California coast, and the last one was a whopper.

Sacramento ended a record 212-day rainless streak on Oct. 18 and had its wettest-ever day on Oct. 24, when 5.44 inches of rain fell in 24 hours. San Francisco got 4 inches. By the time the water stopped, both cities had set new rainfall records for an October day.

Along the way came the mudslides, rockslides, flooding and power outages that often accompany a deluge in a bone-dry place. In the Sierra Nevadas, snow again shut down highways.

But there was very good news: All that precipitation doused remaining wildfires, including Dixie, and made a decent dent in the drought. Water in Lake Oroville, a major reservoir in northern California, rose 20 feet after falling so low in August that a large hydroelectric plant had to shut down.

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November and December: Late-breaking tornadoes and a blizzard in Hawaii

Tornadoes occur year-round, whenever hot, moist air close to the ground collides with cooler, drier air above and creates thunderstorms. By late fall, the humid, unstable air needed for tornadoes is usually limited, especially outside the South.

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Not so this year.

On Nov. 14, 11 twisters touched down in New York, Rhode Island and Connecticut; the four that landed in Connecticut were the only November tornadoes ever recorded in the state. The instability that produced them came from a strong cold front meeting Atlantic waters that were as much as six degrees warmer than normal.

A few weeks later on Dec. 11, amid record-setting warmth, an even later, larger and more violent outbreak of tornadoes roared through nine states. The twisters sheared houses off their foundations, collapsed huge commercial buildings and tossed train cars through the air. More than 88 people were killed, making it the country’s deadliest December tornado outbreak ever.

One extraordinary supercell produced a colossal tornado — or series of tornadoes — that chewed a 250-mile path of destruction from eastern Arkansas into Kentucky. It tossed debris more than 30,000 feet in the air; some items from damaged homes were reported found more than 100 miles away.

Tornadoes were the big weather story, but not the only weather story, toward the end of the year.

In mid-November some of the same areas of the Pacific Northwest that sweltered through heat and fires in summer and had been doused by rain in October were walloped by the most intense atmospheric river of the season. Flooding was extensive on both sides of the Canadian border.

And in early December, a blizzard hit Hawaii.

Snow isn’t unusual on the Big Island peaks of Mauna Loa and Mauna Kea, but an actual blizzard is, and that is what arrived in a stronger-than-usual Kona storm over the first weekend of the month. Post-storm images showed the mountain summits capped in snow. The lower elevations got more than a foot of rain. Honolulu had its wettest December day on record.

While scientists say man-made warming isn’t the cause of most extreme weather events, they agree it is making many of them more frequent and intense. Heat waves are hotter, downpours are heavier, fires spread more quickly. Generally, more energy is available to storms, increasing atmospheric chaos and portending a turbulent future.

“Every time we have one of these events, it’s like an at-bat,” Gensini said. “But we need to start looking at the batting average.”