A firehose of moisture and a series of potent storms have sent the western United States see-sawing from drought to flood in the past few weeks, with widespread extreme weather continuing on Wednesday.

Weather warnings and advisories, from blizzard warnings to flood watches and warnings were in effect for nine states across the West as of Wednesday morning.

Heavy snow with accumulations greater than 100 inches at the highest peaks of the the Sierra Nevada Mountains are possible through Thursday, with winds of greater than 65 miles per hour likely. The National Weather Service (NWS) has issued a blizzard warning for the Lake Tahoe area, its first for the region in nine years.

So much rain and snow has fallen on and around Lake Tahoe that the surface of the lake has risen by “nearly 1 foot” since Jan. 1, according to the NWS in Reno. This means that about 33.6 billion gallons of water have fallen there.

The avalanche danger in California as well as other states, including Colorado, remains extremely high.

This crushing dump of snow is coming on top of the several feet that fell last week. From the morning of Jan. 9 through early on Jan. 11, Soda Springs, Calif. picked up 79 inches of snow. Squaw Valley received 54 inches during the same period, the NWS reported.

It’s likely that even higher amounts have fallen in the Sierra Nevada Mountains but have not been officially recorded.

The snow was so heavy, and the wind so strong, that many ski resorts will remain shut down on Wednesday.

Interstate 80 remained closed Wednesday between the Nevada border and Colfax, California, after more than 6 feet of snow fell along the mountain corridor. There was no estimate for when it could reopen.

Across several states, mudslides, rockslides and other hazards have also cut off critical roadways.

Evacuations due to flooding have been ongoing in central California, where many rivers are above flood stage. About 2,000 people in Wilton, a rural community near Sacramento, were asked to leave their homes Tuesday evening, as emergency crews worked to try to bolster a Cosumnes River levee.

In Portland, Oregon, a foot of snow fell downtown overnight, knocking out power and toppling trees, as well as grinding the metro area to a halt by Wednesday morning. Residents of the Portland area are not accustomed to such large snowfalls, since winter storms typically bring ice or rain to the lower elevations where the city is located, and removing the snow from roadways could take days.

Media reports indicated that cars were simply abandoned on roadways on Tuesday night as snowfall rates reached 2 inches per hour and lightning flashed in the skies above.

In Oregon, Crater Lake National Park was forced to close due to heavy snowfall and avalanche dangers.

In lower elevations in California, flooding rains as well as tidal flooding caused problems in the San Francisco Bay Area, leading to hasty evacuations and water rescues. Flooding also affected other parts of California, Nevada and other states as rainfall totals for the past two weeks rose well past a foot in some spots to more than 2 feet in others.

Fortunately, rains are becoming lighter at lower elevations, with showers likely through Thursday.

However, precipitation will linger across parts of the West, including the Rockies, through Friday as Pacific moisture continues flowing across the region.

At that point, the firehose of moisture aimed at the West will finally be closed — at least temporarily.

Unrelenting atmospheric rivers

The ongoing storm systems are part of a weather pattern that has helped put a significant — though not decisive — dent in California’s record drought that is now in its sixth year.

The individual storm systems are tied to a weather phenomena known as atmospheric rivers, which are narrow channels of air moving huge amounts of moisture from the tropics to the midlatitudes.

When these highways of water vapor (the gaseous form of water) are wrung out by storms and hit mountain ranges, which force the air to rise, cool and condense into clouds and precipitation via a process known as orographic lifting, they can bring flooding rains and historic amounts of snow.

Much of California’s annual average precipitation tends to come from just a few large atmospheric river events, which sets the state apart from other parts of the U.S.

While other parts of the country receive precipitation from various types of storms, such as everything from low pressure systems and warm fronts to hurricanes along the East Coast, California is highly dependent on atmospheric rivers for its annual precipitation.

Some research shows that extremely prolific atmospheric rivers could become far more common — perhaps twice as common as they are now — in parts of California by the end of the century, though this is still a matter of scientific debate. If this occurs, water managers will have to adjust to accommodate more precipitation extremes and avoid severe floods.

Even with these blockbuster atmospheric river storms, the drought won’t actually be over for California. The state is still deep in a precipitation hole, with a lot more rainfall needed to make up for losses in groundwater and reservoirs.