IN THIS ARTICLE
The western United States and Canada will likely experience excessive rain and heavy snow from a sequence of back-to-back atmospheric rivers beginning this weekend and continuing into next week.
An atmospheric river is like a powerful fire hose with only one person holding it. There is often a narrow path of the heaviest and heaviest precipitation. It can be difficult to determine where the largest stream of water will fall. It might be strong in the San Francisco Bay Area, and little might fall in Southern California, or vice versa.
This early in the forecast, meteorologists are certain that the weather pattern is set up for a series of atmospheric river events – likely in some places reaching a level of three or four on the five-point atmospheric river scale. developed by the SCRIPPS Institution of Oceanography at UC San Diego – along the west coast. But they are less sure where the heaviest precipitation will fall, especially late in the week. There will be at least three atmospheric rivers over the next week, and one more beyond that.
Here’s how things could play out.
The first two atmospheric rivers, one beginning Friday and the other Sunday, will primarily hit northern California, the Pacific Northwest, and British Columbia.
A third will hit the same region midweek but drift south, further affecting northern and central California and potentially much of the western United States.
A fourth could hit Southern California by the end of next week, but that’s less certain.
Overall, according to forecasters at the Climate Prediction Center, there is a high chance of above-average precipitation across the western United States over the next week, which could lead to dangerous heavy precipitation. Localized flooding and landslides are possible, particularly in areas that have recently received heavy rain.
The third atmospheric river is the kicker, meaning the first storms will put the Northwest’s river systems into some sort of high flow, said Marty Ralph, director of the Center for Western Weather and Water Extremes. Depending on a few factors, he said, there could be significant flooding in that area.
The Pacific Northwest has already had a pretty eventful winter. In early December, another series of atmospheric rivers brought significant river flooding, and more recently the region has been hit by snow and ice. These storms will be warmer and any rain falling on existing snow could cause snow to melt and increase the amount of water added to rivers.
What is an atmospheric river?
Atmospheric rivers form when winds over the Pacific draw a filament of moisture from the band of warm, humid air above the tropics and channel it toward the West Coast. When this ribbon of moisture reaches the Sierra Nevada and other mountains, it is pushed upward, cooling it and turning its water into immense amounts of rain and snow.
The name comes from their long, narrow shape and the prodigious amount of water they carry.
Learn more about these types of storms – and how climate change is shaping them – in this article by New York Times climate reporter Raymond Zhong.
Not all atmospheric rivers are harmful.
Not all atmospheric rivers lead to disastrous situations. The weaker events benefit the water supply in the West: in fact, these atmospheric river events provide half of California’s water supply and almost all heavy precipitation events.
They can become dangerous when too much rain falls at once, as happened this week in San Diego, where the edge of a river with a weaker atmosphere broke through ocean water that had been made warmer than average by the effects of El Niño. This increased the amount of rain that fell in a short period, causing flooding in the city. They are also more likely to be a problem when consecutive atmospheric rivers fall in the same location.
Last winter, California experienced the longest period of continuous atmospheric river conditions in the 70 years that records have been collected of these events. With nine consecutive atmospheric rivers from December to January, farmlands turned into lakes and snow accumulated far above homes in the Sierra.
This season has been less dramatic, with below-average seasonal precipitation. That could change next week as an atmospheric river flows south toward California.
More than a foot of snow, or more, could fall in the Sierra, significantly increasing the snowpack this season.
When will this end?
This question is a little more complicated.
Dr. Ralph said the computer models he uses for guidance disagree on what might happen in three weeks. This means there is no clear indication as to whether the rest of February will resemble the wet start the West Coast will experience to start the month.