A parade of storms known as atmospheric rivers has dumped massive amounts of rain and snow on California since late December. The storms have produced deadly flooding, crippling snow, dangerous mudslides, severe thunderstorms and tornadoes.
An atmospheric river event for the ages
Atmospheric rivers funnel extreme amounts of moisture over the oceans into narrow bands of clouds. As these clouds are transported over land, they can produce many hours of intense rain and snow.
Precipitable water is an indicator of how much moisture there is in the atmosphere. Higher values of precipitable water correspond to greater potential for heavy rain or snow. The animation of precipitable water forecast above, which spans Jan. 9 to 23, shows the parade of multiple atmospheric rivers lashing California with repeated rounds of heavy rain and snow.
The darkest shades of red and brown represent precipitable water values 200 to 250 percent of normal.
The awe of a bomb cyclone
As is often the case, the worst weather Mother Nature has to offer can often be beautiful when viewed from high above. In this view captured by the National Oceanic and Atmospheric Administration’s GOES-18 weather satellite, the low-pressure center of last week’s “bomb cyclone” can be seen spinning over the Pacific Ocean as it approaches the West Coast from Jan. 3 to 4.
The bomb cyclone, which is a rapidly intensifying storm whose central pressure drops at least 24 millibars in 24 hours, was one of a series of powerful storms that has repeatedly thrust atmospheric rivers into California in recent weeks. It was only a few weeks ago that a different bomb cyclone developed along the Arctic front, blasting much of the country with extreme cold and some areas with blizzard conditions.
Historic rainfall hammers California
The atmospheric rivers have deluged California with copious amounts of rain. In just the past two days, ending at 4 a.m. Tuesday, areas of higher terrain in Ventura and Santa Barbara counties have seen more than 16 inches of rain. The 12.37 inches of rain in San Francisco between Dec. 26 and Jan. 9 qualifies as the third-wettest 15-day period since 1849, according to Bay Area meteorologist Jan Null.
During this period, Sacramento recorded more than 8 inches of rain, and Los Angeles registered more than 4 inches.
The extreme rainfall, which is expected to continue until the expected end of the pattern around Jan. 20, has caused widespread and severe flooding, road closures and mudslides. While the unrelenting rain is wreaking havoc in the short term, it is combining with snow to help put a significant dent in the drought that has long plagued the region.
Surging snowpack across the Sierra Nevada
Snowpack across much of the central and southern portions of the Sierra Nevada is now 200 to 300 percent of normal for the date, as shown in the map above. In some locations the snowpack has already exceeded the April 1 average. (April 1 is typically around the time the snowpack is at its deepest.)
The growing snowpack is helping to ease California’s years-long drought. The most recent U.S. Drought Monitor, released Jan. 5, upgraded central to southern portions of the state from the most severe level, D4 (exceptional drought), to D3 (extreme drought). Additional rain and snow in the next week or so could further ease drought conditions across California.
Raging rivers rise out of their banks
After multiple years of drought, too much rain falling too fast has pushed multiple rivers beyond flood stage. One of the more extreme forecasts is for the Salinas River. The visual above shows that where the river runs near Spreckels in Monterey County, the water level is expected to reach or exceed 30 feet, which is about seven feet above flood stage.
The effects of heavy rain and flooding have been catastrophic, and at least 17 deaths have been attributed to the storms since late December. Rivers that were recently, currently or soon expected to be above flood stage as of Tuesday afternoon include the Russian River, the Salinas River, the Carmel River, the Santa Ynez River and Bear Creek, according to the California Nevada River Forecast Center.