By mid-May, California is usually leaning pretty hard into the summer dry season. But a bizarre series of storms will have weather maps looking more like winter than spring.
Inches of rain, feet of snow, and gusty winds will buffet Northern California over the next week and a half. Records could be broken and it could even be good news for fire season, at least in the short term.
The first in a series of storms should arrive Wednesday morning. The National Weather Service is forecasting up to 3 inches of rain through Thursday from Big Sur to Marin County north of San Francisco and other parts of Northern California, though areas could see locally higher amounts. Umbrellas will be pretty useless as winds gust to 40 mph.
The cool air accompanying the storm also means mountain snow will fall. Winter storm warnings cover the Sierra Nevada, and the National Weather Service is calling for 24 inches of snow, though some high peaks could pick up 35 inches. All this is just from round one, as the parade of storms is set to continue into next week, repeatedly blitzing the northern half of the state. Later next week, Southern California could get in on the action, too.
That means the state will continue to build on its already prolific snowpack from this winter. California Department of Water Resources data show that all parts of the Sierra Nevada currently have snowpack anywhere from 120-133 percent above normal for this time of year, after a winter that saw some locations receive more than 50 feet of snow. That’s helped the state climb out of drought save a small pocket of what the U.S. Drought Monitor calls “abnormally dry” conditions on the U.S.-Mexico border. The new snow and rain will keep things damp and tamp down early season wildfire risk, though the extra vegetation they help grow could eventually end up as flammable fuel later this year.
The source of the storms isn’t quite an atmospheric river, a meteorological phenomenon where fast-flowing air taps moisture from the tropical Pacific and ushers it to California. These rivers in the air are responsible for the majority of California’s winter precipitation. Rather, it’s a strong jet stream blowing across the Pacific that will help usher rain and snow toward the Golden State. The pattern is really weird for this time of year, when high pressure normally builds over California and keeps storms at bay.
There are a couple reasons for the breakdown of that normal pattern. Daniel Swain, an atmospheric scientist at the University of California, Los Angeles who has extensively researched California, pointed to a shift in the Madden Julien Oscillation (MJO) as one potential cause. The MJO is characterized by a swath of clouds and thunderstorms that moves back and forth around the western Pacific and affects weather downstream. Swain wrote that it’s “been very active for the time of year” and has helped strengthen the jet steam enough to steer storms to California.
But climate change could also be playing a role. Swain pointed to the unusual heat that’s been gripping the Arctic as well as disappearing sea ice—both symptoms of climate change—as another potential driver of the weird jet stream pattern. A growing body of research has linked changes in the Arctic to unusual weather patterns to the south, and this could be no exception. At the same time, Swain’s own research also shows that weather whiplash between wet and dry years will become more common for California.