Was Northern California bomb cyclone a ‘100-year storm’? The answer’s a bit complicated

The bomb cyclone and atmospheric river that doused a parched Northern California this weekend brought the most rainfall to Sacramento in a 24-hour period Sunday, breaking a record that’s held since 1880.

Now that the rain is slowing, The Sacramento Bee dug into the technicalities. Was this record-breaking storm a so-called 100- year storm? The Bee called UCLA’s Daniel Swain, a climate scientist who studies how climate change drives extreme weather like droughts, floods, and wildfires, as well as the California Department of Water Resources and the National Weather Service for answers.

First, let’s define what it means to call something a 100-year storm.

What is a 100-year storm?

If you did the math on the record and got excited because 1880 was 141 years ago, hold on for just a second.

A 100-year storm does not necessarily mean a storm that happens every 100 years. Instead, it refers to the probability of how likely such a storm is to occur.

A 100-year storm is a weather event that has a 1% chance of happening in any given year. In theory, another “100-year storm” could happen next week (though National Weather Service meteorologist Emily Heller confirmed there’s nothing of the sort on the horizon).

Declaring something to be a “100-year storm” requires context, though, according to Swain. Even without factoring in climate change, part of defining it is taking a look at impacts. It’s not a fixed metric. Layering in climate change, the term becomes even more complicated — it’s turning into a moving target.

“Our climate today is significantly different than it was 100 years ago and a couple of decades now will be significantly different even from today,” Swain said. “And the likelihood of seeing 100-year precipitation events is increasing pretty fast with climate change even in places that aren’t getting wetter on average like California. “

Was the storm in Sacramento this weekend a ‘100 year storm’?

Yes, according to the California Department of Water Resources.

“Sacramento’s 5.44 inches for a 24-hour total is greater than the 5.35 inch estimate for a 100-year storm (right in the ballpark and definitely within the uncertainty bounds provided,” Michael Anderson, a state climatologist, said via email.

That ballpark is 4.50 to 6.49 inches, according to National Oceanic and Atmospheric Administration data that Anderson shared with The Bee.

Though Swain agreed with that designation, he said it’s more of a yes with caveats. While on that narrow definition — focused on daily precipitation for Sacramento — it’s clear the storm counts. But there’s more to it, Swain said.

“Did it produce 100-year level storm impacts in Northern California?” Swain said. “Absolutely not. There was actually not a great deal of flooding considering how much rain occurred.”

That’s not to say there was no flooding or impacts at all, just that these impacts were largely localized to the Insterstate 80 corridor. That might have been different had the storm not happened in the middle of a drought.

Instead of inundating already soggy ground with little capacity for more water, the storm instead functioned like a last minute deposit before your bank account overdrafts, Swain said.

“It’s not a huge amount of money in the long run, but it’s enough to get you through the next paycheck,” he said. “And there was no guarantee that was going to happen.”

Though the prospect of a drier than normal winter is still likely, Swain said he’s less worried about what will happen next year purely because of this single storm.

An impressive storm

The complications in defining a 100-year storm may point to why it’s a designation that doesn’t matter much.

Regardless of whether it fulfills the rights metrics, it was clearly an “impressive storm,” Heller, the NWS meteorologist, said. Heller added that she finds wrapping her head around the term harder to think about than measuring it in other ways.

On that note, Swain pointed out the storm was notable on a number of counts:

▪ It happened in October, and in recent years California Octobers have been leaning toward the dry side of the climate spectrum.

▪ The wettest day on record happened in the midst one of the driest years and after what was, in most of the state, the hottest summer on record.

▪ And even though storms of this magnitude may become more common because of climate change (though usually, they would happen during peak rainy season), that doesn’t mean the region will see more storms overall. The rain last weekend more so reflects a trend that a warming climate will mean more water vapor is available, but it gets released less frequently.

So, Sacramentans, you can confidently tell your friends that you bore witness to a 100-year storm. Just don’t be too surprised if another one comes up soon.

This story was originally published October 25, 2021 at 2:51 PM with the headline "Was Northern California bomb cyclone a ‘100-year storm’? The answer’s a bit complicated."