As the new 2023 calendars were going up, a meteorological term was also going up in usage. If you’ve scrolled social media or watched the news, you may have seen the term “atmospheric river” being used in regard to the situation unfolding in California. The recently drought-stricken state has seen wave after wave of low pressure impacting the coast, delivering beneficial rains but at a rate causing major flooding issues. The term “atmospheric river” has been used to describe this onslaught of moisture. This is what an atmospheric river is and how it has been impacting the Golden State.

What is an atmospheric river?

In simplest terms, an atmospheric river is just a conveyor belt of moisture-rich air coming onshore from the ocean. An example of this you may have heard before is the so-called “pineapple express,” describing an atmospheric river originating around Hawaii. That pattern delivers moisture pulled from the tropics around and south of the island chain toward the U.S. mainland.

The oceans routinely pump moisture into the atmosphere. As storm systems develop and concentrate the winds, corridors of high moisture content develop and can be pulled toward land. These bands of concentrated moisture result in areas of heavy rainfall or heavy snow at higher elevations.

The changes in elevation along the coastline help intensify the rainfall when these atmospheric rivers reach land. As the air over the surface of the ocean reaches the coastline, it is forced to rise in the atmosphere when it encounters the higher elevated land. That lift forces rising air into cooler parts of the atmosphere. Cooling of the moisture-rich airmass increases condensation, leading to more precipitation. This process continues as air reaches the much higher mountain elevations. With colder temperatures at the higher elevations, precipitation falls as snow and that snowpack can provide much needed water when that snowpack melts.

NOAA infographic on atmospheric river impacts on the west coast. Courtesy: National Oceanic and Atmospheric Administration

Why have you been hearing about atmospheric rivers so much?

California has been dealing with unrelenting storms for about two weeks now. One after another bringing the atmospheric river of moisture to the state. While rainfall was very much needed, the pattern has resulted in too much too fast. The rounds of torrential rains have resulted in flooding, mudslides and extreme mountain snow. The storm systems impacting the region have been strong, some of them producing extreme wind gusts to near hurricane force. Some roads have been inundated, others washed away. That pattern is continuing. Areas of low pressure will continue delivering rain to already inundated parts of California. While it is great for the reservoirs, problems with flooding, mudslides and sinkholes are likely to continue.

The following is a look at coverage of some of the damages.

Has this pattern had an impact on California’s ongoing drought?

Yes! If there is any positive to all that has been going on, it is that there have been improvements to the drought conditions statewide. Below is a look at radar estimated rainfall over the last seven days, as of Thursday, Jan. 12. A large part of the state has measured rain in inches and that is only the last seven days.

7-day rainfall totals as of Thursday, Jan. 12.

A model depiction of rainfall into next week shows more significant rainfall is likely to occur as storm systems continue to move onshore, pulling in more moisture-rich air.

GFS model output for rainfall between Thursday, Jan. 12 and Friday, Jan. 20, 2023.

The impacts on the drought have been significant. Seven percent of the state was considered to be experiencing an exceptional drought, the highest category on the drought monitor. As of today, no part of California is considered to be experiencing an exceptional drought. The next category down, an extreme drought, encompassed 35.5 percent of the state just two weeks ago. Today, only 0.32 percent of the state of California is considered to be experiencing an extreme drought. Go down one more category to the severe drought category and that encompassed 80.5 percent of the state two weeks ago. The percentage of the state considered to be experiencing a severe drought has now dropped to just 46 percent. Below is a side-by-side comparison of the changes in the drought monitor from last week to this week. Slide the bar to the right to view the outlook released last Thursday, Jan. 5 and to the left to view the most recent status released on Thursday, Jan. 12.

Side-by-side comparison of the changes in the drought monitor from last week to this week. Slide the bar to the right to view the outlook released last Thursday, Jan. 5 and to the left to view the most recent status released on Thursday, Jan. 12.