In the winter California often gets hammered by storms that can drop inches of rain at low elevations and multiple feet of snow in the Sierra, like the ones that covered the state in mid-January and again in mid-February.
The storms are what's known as an atmospheric river, sometimes called a Pineapple Express, and are usually a warmer and wetter weather pattern that acts like a hose dumping rain on California.
An atmospheric river can be 250-370 miles wide, according to NOAA, and “transports an amount of water vapor roughly equivalent to 7.5–15 times the average flow of liquid water at the mouth of the Mississippi River.”
On average around 30 to 50 percent of the West Coast’s water supply stored in the snowpack comes from just a few yearly atmospheric river events. The long plumes of water vapor take moisture from the tropics and drop it over the West Coast in storms.
An atmospheric river is bringing heavy rain and mountain snow to #California. Check out the conveyor belt of clouds and moisture heading toward the West Coast in this loop from our newly operational #GOES17 (#GOESWest) satellite. Learn more: https://t.co/DhXRshmtyJ pic.twitter.com/fix8G2myrO— NOAA Satellites (@NOAASatellites) February 13, 2019
These storms “produce debris, which clog drains and may cause localized flooding,” Miller added about local streams and wildfire burn areas.
Atmospheric rivers also cause high winds. In the January storm, Sacramento encountered 50 to 60 mph gusts.
An issue with atmospheric rivers is that storms usually drop warmer rain and that can push snow levels to above 7,000 feet. That can be a problem in California, where a third of the water supply comes from snowpack.
“[Atmospheric rivers] likely add to the snowpack, but typically you want to see a lower snow level system to really make a difference,” Miller said.
Interesting weather events can take place within an atmospheric river. The January storm caused a blizzard in the Tahoe region and around 110 mph winds whipped over ridge tops increasing avalanche danger.
A Blizzard Warning has been issued for the Sierra Wed PM-Thurs AM for elevations >7,000’. Dangerous & potentially life threatening conditions will be possible. Blizzard Warnings are saved for only the most serious situations at our office. Stay home, stay safe! #CAwx #NVwx pic.twitter.com/52nEAhq4LR— NWS Reno (@NWSReno) January 15, 2019
Storms caused by atmospheric rivers are now measured for intensity. A new severity scale was put into place in February. The Center for Western Weather and Water Extremes in La Jolla created the severity levels, with one being weak and five being exceptionally strong.
The first storm ranked occured the week of Feb. 10. In this case the system ranked as a "Category Four" in the north of the state and a "Three" in the south.
Julie Kalansky, the center's operations manager, said a storm's water content and duration are considered, but so are situations on the ground like recent wildfire burn scars.
"If one atmospheric river is coming right after another and the soil is already saturated that's another component,” she said. “So, there's these other situational components that could impact how hazardous, potentially, an atmospheric river is.”
Kalansky said the scale is not widely used yet, but that will change. "It will be an evolution in how the scale is used and communicated as forecasters and the general public become more familiar with it," she said.
Like other major storm events, atmospheric rivers often disrupt travel because of mudslides and can cause damage to both life and property. During the February storm officials advised commuters to plan extra travel time, and avoid standing water on roadways.
Mudslide Blocks State Route 26: A mudslide has temporarily blocked State Route 26 at Woodhouse Mine Road (southwest of West Point) in northern @CalaverasCounty. There is no estimated time of opening. Please avoid this area and drive safely! READ MORE: https://t.co/ggWOWX2upE pic.twitter.com/E4M1RyD2Vf— Caltrans District 10 (@CaltransDist10) February 14, 2019