What led to California's drought-busting rain this winter?

This winter has been one of the wettest and snowiest on record for California, but what caused so many storms to slam into the state?

The weather pattern this winter has been very beneficial for the exceptional, multi-year drought that has plagued the state, filling water reservoirs and almost completely eliminating the drought in just a few short months.

“It has been a very interesting winter across most of the United States with very stormy and chilly weather in the West,” AccuWeather Long-Range Forecaster Jack Boston said.

The beneficial moisture came as a surprise to the public due to the development of a weak La Niña, a phenomenon that usually results in a drier-than-normal winter across California.

La Niña occurs when ocean water temperatures are below normal across the central and eastern Pacific Ocean near the equator. This is the counterpart to El Niño, which typically favors wetter-than-normal weather across California.

However, there are many other phenomena that occur that can influence the global weather pattern besides La Niña or El Niño.

“The main reason for the persistent, excessive drought-busting rains in California appears to be unrelated to any affects that La Niña might have caused,” Boston said.

“The factor that has caused all of the storminess in California this season seems to be an unusual sea surface temperature distribution in the Pacific Ocean,” Boston said.

Boston explained that water temperatures across the northern Pacific Ocean were significantly lower than normal while water temperatures just south of Hawaii were well above average throughout much of the winter.

This unusually large, north-to-south contrast in water temperature influenced the jet stream, leading to a very active pattern that sent waves of storm systems directed at California, especially during January and February.

The jet stream is a fast river of air high in the atmosphere that is essentially the highway on which storm systems travel.

The temperature of the water just off the coast of the western United States also plays a role in how systems impact the region.

“When temperatures along the coast are above normal, it can cause storms to weaken and cut to the north sending them into Oregon, Washington or even Canada and Alaska,” AccuWeather Meteorologist Jim Andrews said.

This was the case last year during an exceptionally strong El Niño. While El Niño usually favors stormy weather for California, warm water off the coast sent the systems on a more northerly track, missing out on the drought-stricken state.

Water temperatures in this region fell drastically since last winter, allowing storms to track farther south and drop heavy rain and mountain snow over California.

The heavy rain and mountain snow was a double-edged sword for the state, helping to alleviate the severe drought but also triggering flooding, mudslides and avalanches.

Water reservoirs that were well below their historical average quickly filled to the brim, including Lake Oroville, which for the first time in history overflowed over the emergency spillway. The spillway emergency forced evacuations downstream.

Officials also opened the spillway to the Don Pedro water reservoir for the first time in 20 years to prevent water from flowing over the dam’s uncontrolled spillway.

Snow that fell over the Sierra Nevada during the onslaught of storms quickly piled up, forcing mountain passes, such as Donner Pass, to shut down during the storms. This led to significant traffic backups for those attempting to cross the Sierra.

The difference in snowpack from this winter versus last winter was clearly visible from space.

While the rain led to flooding and frequent disruptions, it has been beneficial for the long-term drought that has plagued the state.

“The rain and snow this winter, especially during January, was a huge help in the short and long-term drought,” AccuWeather Western Weather Expert Ken Clark said.

“There should be more water available to the massive farmlands of the Central Valley,” Clark said.

Ski resorts across the region will also reap the benefits of the record snowfall well into the summer.

Mammoth Mountain, one of the largest ski resorts in the United States, recorded over 500 inches of snow through February. This should allow the resort to remain open well into the spring, potentially remaining open even into the Fourth of July weekend.

The stormy winter was also good for other areas over the southwestern United States with above-normal rainfall and snowfall feeding into the Colorado River Basin, eventually reaching Lake Mead.

“This is important not only to Las Vegas but also to the Imperial Valley of California, a huge farming area, and also San Diego [which] draws from that water source,” Clark said.

April 1 marks the end of the wet season from California and the date when the snowpack in the Sierra is typically at its peak.

The snow depth measured this April could play a major role in the decision whether or not to lift water restrictions that were put in place due to the severity of the drought.

“This is the kind of season that was hoped for last year, but it never arrived,” Clark said.

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