Yet another atmospheric river to strike US West Coast today

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This animation from NOAA’s Earth System Research Laboratory shows an atmospheric river event in 2017. Atmospheric rivers are relatively narrow regions in the atmosphere that are responsible for most of the horizontal transport of water vapor outside of the tropics. Image via NOAA.

Atmospheric river to continue to pound U.S. West

An atmospheric river has been pummeling northern and central California since the weekend, with wind, heavy rain and snow at high elevations. The locals call such storms a Pineapple Express, as it’s generated by low pressure in the area of Hawaii. These systems propel moist tropical air from Hawaii across the Pacific Ocean on what The Guardian called “blustery westerly winds.” The storm weakened significantly as it moved down the state, but the Los Angeles Times reported that the storm arrived there early Wednesday:

… after delivering a stronger-than-expected pounding to Northern California, breaking rainfall records, swelling rivers and triggering landslides.

And now another atmospheric river is expected to strike the U.S. West today, bringing heavy rain in addition to snow in the higher, mountain elevations. The National Weather Service (NWS) said in an advisory this morning (2:56 a.m. EST or 7:56 UTC):

The next storm system to enter the West Coast and eventually spread wintry weather elsewhere throughout the Lower 48 is forecast to move onshore later today. Initially, heavy coastal/lower elevation rain is expected across California with heavy snow likely into the Sierra Nevada, northern California ranges and southern Oregon. By Friday, the moderate-to-heavy snowfall located just north of an associated area of low pressure shifts into the northern Great Basin and northern Rockies. Total snowfall amounts over a foot (30 cm) are expected across the higher terrain, with multiple feet of heavy snow possible in the Sierra Nevada.

Forecasters are predicting generally minor impacts, such as increased traffic collisions. Thursday’s rainfall in Los Angeles County is expected to be relatively light, similar to the rain that fell Tuesday night and Wednesday morning.

What is an atmospheric river?

On average, 24 atmospheric rivers hit the U.S. West Coast each winter, from 1979 to 2019. But, in 2024, 51 atmospheric rivers hit the West Coast from October through March, which is more than the previous fall-through-spring period.

An atmospheric river is one of those key words or phrases we hear this time of year, especially related to the U.S. West coastal region. But what is it?

An atmospheric river is a narrow band of water vapor, which when it moves onto land, usually the West Coast of the United States, can bring heavy rain and snow in the high elevations, typically during winter. The National Oceanic and Atmospheric Administration (NOAA) says a typical atmospheric river has enough water vapor to be nearly equal to the typical flow of water at the mouth of the Mississippi River. The strongest atmospheric rivers can move 15 times that amount, and it is these events that cause the worst flooding.

Because atmospheric rivers are narrow “rivers” that can stay over one location for a long period of time, flooding due to heavy rain is a frequent concern, in addition to landslides. Landslides can wash out roads, cutting off normal travel in addition to damaging property and posing a threat to people’s lives.

While the strongest atmospheric rivers can pose a threat to life and property, most do not cause catastrophic damage and are a vital part of the water cycle, bringing much needed rain and snow to the Pacific Northwest as well as Northern California, and into the mountains. This snow can be beneficial to winter recreation in these areas, allowing ski resorts not to have to spend money on making or supplementing snow. The Pineapple Express, mentioned above, is one of the best-known atmospheric rivers. It originates near Hawaii, bringing the tropical moisture from that area to the West Coast of the United States.

A diagram showing the trajectory of an atmospheric river. Atmospheric rivers come from the tropics, and strong winds from the low-level jet stream bring that higher content of moisture and water vapor toward the east toward land. The moisture moves over land up the higher terrain it condenses, and then falls as either rain or snow, as shown in the diagram.
Image via NOAA.

Good and bad impacts

Atmospheric rivers have both positive and negative impacts on the environment. While they can add to a much-needed water supply and snowpack along the West Coast, they can also be destructive. This is why scientists study these phenomena. One way we’ve been able to learn more about them is through the use of hurricane hunter aircraft. Even when it’s not hurricane season, these flying laboratories are collecting data to use in weather models and research, so meteorologists can better understand and forecast atmospheric river events. The smaller, Gulfstream jets (which can fly at higher altitudes) collect these valuable data and send them back for more analysis.

In addition to planes, NOAA also utilizes weather satellites to monitor atmospheric rivers.

A Gulfstream G-IV, in the Hurricane Hunter fleet, which also collects data on atmospheric rivers. Image via NOAA.

Bottom line: An atmospheric river has been pummeling California since the weekend, with wind, heavy rain and snow at high elevations. And another one is expected later today.

Sources:
NOAA: What are atmospheric rivers?
NOAA: What Is an Atmospheric River?
NOAA Hurricane Hunters
Read more: An uncommonly strong atmospheric river in Pacific Northwest

Rachel Duensing

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About the Author:

Rachel Duensing is a Certified Broadcast Meteorologist by the American Meteorological Society and is currently a meteorologist and climate reporter at CBS17 in Raleigh, North Carolina. Rachel has previously worked in Fort Myers, Florida and Carterville, Illinois. In addition to daily weather, she has covered tornado outbreaks, hurricanes, and extreme heat and cold.

Rachel received her Bachelor of Science degree in Meteorology and Digital Media from Valparaiso University in Valparaiso, Indiana. While at Valpo, she was a member of the Valparaiso University Storm Intercept Team, and along with professors and classmates, chased tornadoes and severe weather across the United State’s Great Plains and Midwest.

Rachel grew up outside Chester, Illinois, a small town in the southern end of the state (nowhere near Chicago!). When she was only 10 years old, Rachel learned about weather in science class, and when those lessons were followed up by a tornado outbreak outside her town, she took it as a sign and has been in love with weather ever since!

When not forecasting the weather or reporting on weather and climate, Rachel spends her time volunteering at a local cat shelter and watching hockey with her husband.

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