How Terrain Can Make Flash Flooding Worse

After days of heavy snowfall and much-needed snowpack replenishment in the Sierra Nevada, several days of clearer skies are ahead for California as the snowmaker heads into the Four Corners.

A powerful avalanche led to search and rescue crews looking for backcountry skiers on Tuesday around 11:30 am local time.

This avalanche was caused by very high snowfall rates and gusty winds, which weakened the snowpack layer. Snowfall reports have already exceeded 5 feet at Soda Springs with more to come for the Sierra Nevada this week. An avalanche warning remains in effect.

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Interstate 80 has at times been closed in both directions over Donner Summit, due to whiteout conditions and winds gusting as high as 100 mph.

Why The Change This Week?

Most of the winter has seen the jet stream riding well north into western Canada, then plunging southward into the East. This pattern leads to less rainfall and warmer temperatures out West.

The graphic below shows what's happening now. The jet stream is taking its sharp southward plunge into the West, leading to cooler temperatures and an influx of much-needed rain and snow.

Why Snowpack Matters

The snowpack across the West isn’t just for skiers and snowboarders. It also helps provide a freshwater source to the West in the drier months.

This is why the lack of snowfall has been so concerning. The West is dependent on snow, which they hadn't been seeing.

According to the National Resources Conservation Service, dozens of locations from Colorado and Utah to the Pacific Northwest have snowpack that is lowest at this time of year in at least a decade or more.

Storm Reports

The first round of rain and mountain snow hit the state Sunday into Monday. Parts of the coastal ranges of Northern and Central California as well as the Sierra foothills below snow level picked up 2 to 6 inches of rain, while parts of the L.A. Basin picked up 1 to 3 inches of rain.

That triggered flash flooding of some roads and stretches of freeways in Ventura and L.A. County, and debris flows, as well.

Rockslides and debris flows closed a stretch of the Pacific Coast Highway from south of Big Sur to north of San Simeon. A stretch of state route 27 in Topanga Canyon was also closed due to multiple slides. This included some areas burned by the catastrophic January 2025 wildfires. The city of L.A. issued evacuation warnings ahead of Monday's rain for areas in and near the burn areas.

Around midday Monday, several vehicles were stuck in floodwater at the McClure Tunnel to eastbound Interstate 10 in Santa Monica, and significant debris was reported on the 405 freeway in Long Beach near Cherry Avenue.

Severe thunderstorms Monday produced wind gusts to 81 mph in the Malibu Hills, significant wind damage in Ventura, and damaged the roof of a CalFire station in Tranquility, just west of Fresno.

Snow snarled traffic through the Sierra, including on Interstate 80 and U.S. 50, where traffic was held at times or completely shut down to allow crews to clear snow. Numerous snowfall reports over 3 feet have been reported so far, with many more feet of snowfall expected.

Strong thunderstorms swept through Southern California on Tuesday night, leading to downed trees and power lines stretching from Los Angeles to San Luis Obispo. Trees were reported down in Hollywood, Beverly Hills and even the 710 freeway.

By Wednesday afternoon, many areas across the Sierra were approaching 5-6 feet of snow.

- Soda Springs: 68.7 inches

- Blue Canyon: 62 inches

- Twin Bridges: 61 inches

- Donner Peak: 57 inches

- Bear Valley: 56 inches

The AP contributed to this story.

Rob Shackelford is a meteorologist and climate scientist at weather.com. He received his undergraduate and master’s degrees from the University of Georgia studying meteorology and experimenting with alternative hurricane forecasting tools.