How does this winter storm compare to October bomb cyclone that drenched Sacramento?

This week’s winter storm in Sacramento comes just two months after a bomb cyclone-atmospheric river combo broke records and took the title of ”100-year storm.”

And while the October storm dropped significant amounts of water in just a day and a half, this week’s storm — which started over the weekend and is expected to last into Thursday — will spread precipitation over several days.

Here’s how the two Sacramento winter storms compare, including storm characteristics, precipitation totals and safety concerns:

Type of storm

This week’s storm and the October event both include atmospheric rivers.

The bomb cyclone and atmospheric river earlier this year, while separate weather events, co-existed and enhanced one another.

What is an atmospheric river?

Atmospheric rivers form over the Pacific Ocean when high-powered winds drag a fire hose of tropical moisture across the ocean’s surface.

According to the University of California, atmospheric rivers are long, narrow bands of moisture in the atmosphere that extend from the tropics to higher latitudes. These rivers in the sky can transport 15 times the volume of the Mississippi River.

And when that moisture reaches the coast and moves inland, it rises over the mountains, generating rain and snowfall.

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The storm currently pummeling Northern California is listed as a “strong” category “AR 3” system. That’s middle of the road, with an AR 5 being the strongest.

What is a bomb cyclone?

Also known as bombogenesis, according to the National Oceanic and Atmospheric Administration, a bomb cyclone is when a midlatitude cyclone quickly intensifies over 24 hours. Within that time, it drops at least 24 millibars, which is a unit that measures atmospheric pressure.

Precipitation amounts

According to meteorologist Eric Kurth with the National Weather Service Sacramento Office, the bomb cyclone that produced a rapid intensity of rain in about a day in a half, doesn’t compare to the rainstorm forecast this week.

Sacramento recorded 5.44 inches in 24 hours back in October, the most rainfall capital had seen since 1880. This storm is expected to produce about 4 inches of rain over a week, Kurth said.

“We’re going to get quite a bit of precipitation spread out over several days,” Kurth said. “So it’s not as concentrated but over the several days we’re going to get quite a bit of rain down here in Sacramento.”

Safety concerns

Atmospheric rivers can trigger extreme flooding and debris flows, which is what happened after the October storm hit Northern California and left Sacramento officials with damage caused by post-wildfire debris flows across the region.

Flooding and debris flows were a concern with the bomb cyclone, said meteorologist Anna Wanoess with the National Weather Service Sacramento Office. This week, she’s most concerned with ponding water and roadways as rain is expected to be its heaviest as most people commute home from work.

Rush hour, specifically between 3 p.m. and 7 p.m. in Sacramento, will see intensified bands of rain and winds gusting between 40 and 50 mph in the area, according to the National Weather Service.

“We’re way less concerned with that than we were,” Wanoess said. “It doesn’t look like we’re going to be seeing the high rain rates that we would need for debris flows to occur.”

The National Weather Service issued a wind advisory Monday morning that will last through the evening.

Portions of the Sacramento Valley, Carquinez Strait and Delta and the Northern San Joaquin Valley can expect gusty winds traveling at 20 to 30 mph with gusts up to 50 mph that could result in broken tree limbs, power outages and difficult travel.

This story was originally published December 13, 2021 at 11:14 AM.