A series of potent storms is poised to deliver torrents of rain and feet of snow across the West over the next few days, including one on Sunday that’s being called a “bomb cyclone” because of its ferocity.
Supercharged by a classic atmospheric river pattern, the storms could lead to flash floods and dangerous debris flows in a wide swath of the region already devastated by wildfires, CNN said.
Still, despite the danger, the rain and snow will be welcome across drought-plagued areas of the West. As of Thursday’s U.S. Drought Monitor, nearly 92% of the western U.S. is in some level of drought. The rain will help stop smoldering wildfires and could ease water restrictions on farmers.
California has been hit especially hard by the drought. It has had some of the hottest, driest months on record in the past year. Some of California’s most important reservoirs are at or near historic lows heading into the rainy season.
“The pattern unfolding this week to next week may be one of the biggest series of storms for the rainy season for California, but there is still potential for a couple of bigger storms over the winter,” AccuWeather forecaster Paul Pastelok said.
Emily Heller, a meteorologist with the National Weather Service in Sacramento, said, “This is definitely going to be one of those ground-soaking events.”
What is a bomb cyclone?
Meteorologists define a bomb cyclone as a rapidly strengthening storm with central pressure that plummets by 24 millibars or more within 24 hours, AccuWeather said. The process is known as bombogenesis.
A millibar is a measure of pressure. The lower the pressure, the more powerful the storm.
The cyclone will rival, in some aspects, the intensity of strong hurricanes from the Atlantic this season. For example, the bomb cyclone could be at the intensity level of Hurricane Larry, a long-lived and intense cyclone that churned across the Atlantic in early September, according to AccuWeather.
Meteorologist Ryan Maue tweeted that “this monster hurricane-force bomb cyclone this weekend will challenge observed historical low pressure off the U.S. West Coast.”
What is an atmospheric river?
Atmospheric rivers are responsible for up to 65% of the western USA’s extreme rain and snow events, a 2017 study said.
“An atmospheric river marked as a category 4 or a 5 is capable of producing remarkable rainfall totals over three or more days, likely to exceed 10-15% of a typical year’s precipitation in some locations,” Marty Ralph, director of the Center for Western Weather and Water Extremes at UC San Diego, told CNN.
This weekend’s atmospheric river could be a rare Category 5 event, he said.
Made visible by clouds, these ribbons of water vapor extend thousands of miles from the tropics to the western U.S. They provide the fuel for the massive rain and snowstorms and subsequent floods along the West Coast.
Though beneficial for water supplies, these events can wreak havoc on travel, set off deadly mudslides and cause catastrophic damage to life and property, the National Oceanic and Atmospheric Administration said.
One well-known nickname for an atmospheric river is the “Pineapple Express,” which occurs when the source of the moisture is near Hawaii.
A single strong atmospheric river can transport up to 15 times the water vapor compared with the average flow of water at the mouth of the Mississippi River, according to NOAA’s Earth System Research Laboratory.
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