Seattle under Flood Watch as rare June back-to-back atmospheric river storms take aim at Pacific Northwest (2024)

FAQs

In what months does the Pacific Northwest experience the most frequent atmospheric rivers? ›

Atmospheric Rivers are common during the Fall and Winter months, even into the Spring but are less common this late into the year. ARs are rated on a scale of 1 to 5 and this week's AR was a level 3. In addition to the rainfall will be gusty winds, over the highest peaks upwards of 70 mph.

The average lifecycle of an atmospheric river is between three to seven days, Cordeira said. What usually stops it is rain. "Things that remove water vapor from the sky will cause an atmospheric river to die," Cordeira said. Once the river hits land, it usually rains itself out in about a day, he added.

The storm was driven largely by an "atmospheric river" over California. ( Although atmospheric rivers come in many shapes and sizes, those that contain the largest amounts of water vapor and the strongest winds can create extreme rainfall and floods, often by stalling over watersheds vulnerable to flooding.

The strongest atmospheric rivers can move anywhere between seven and 25 times as much water as the flow of the Mississippi River, which is the second longest river in North America and has a watershed that reaches 32 states, according to the national park service.

Category 5 (exceptional): A Category 5 atmospheric river is primarily hazardous. The USGS gives the example of an atmospheric river that lasted over 100 hours over the Central California coast during the 1996-97 New Year's holiday period. The heavy rain and runoff caused over $100 billion in damage.

The Great Storm of 1861-1862, often referred to as the Great Flood of 1862, was a series of intense storms that battered California for several weeks.

64 Scientific American, drowned: A 43-day atmospheric-river storm in 1861 turned California's Central Valley region into an inland sea, simulated here on a current-day map. the Pacific Ocean began to pound central California on Christmas Eve in 1861 and continued virtually unabated for 43 days.

California is ideally located for atmospheric rivers, as extratropical cyclones formed over the North Pacific Ocean can tap into warm tropical air and, when positioned right, transport a large amount of water vapor from the tropics to the California coast.

Consecutive atmospheric rivers, known as AR families, can cause significant flooding. The first heavy downpours saturate the ground. As consecutive storms arrive, their precipitation falls on soil that can't absorb more water. That contributes to more runoff.

How rare is an atmospheric river? ›

Atmospheric rivers are fairly common in winter in the Western United States. In fact, just a few atmospheric river events each year cause up to half of the annual precipitation on the West Coast. These dumps of rain and snow from atmospheric rivers are an important source of fresh water for the West Coast.

The deep low pressure system driving the atmospheric river also generated historically large and powerful waves along the west coast from Washington to California.

California may have endured an onslaught of tropically-infused atmospheric river storms that filled the calendar for months at the end of 2022 and the start of 2023 and is staring at another atmospheric river this weekend, but those storms pale in comparison to the historic floods during the winter of 1861-1862.

Each of these aircraft are outfitted to fly into and above intense weather to collect huge amounts of meteorological data. The primary way the aircraft collect these data is by dropping about thirty cylindrical instruments called dropsondes into the atmospheric river in each flight.

Quick Facts: The Amazon River in South America is the largest river by discharge volume of water in the world, and by some definitions it is the longest. The portion of the river's drainage basin in Brazil alone is larger than any other river's basin.

Atmospheric rivers primarily occur during winter of the respective hemisphere (i.e., December, January, and February for the northern hemisphere and June, July, and August for the southern hemisphere) when extratropical cyclones are most prevalent.

Typically, the Oregon coast averages one Cat 4 atmospheric river (AR) each year; Washington state averages one Cat 4 AR every two years; the San Francisco Bay Area averages one Cat 4 AR every three years; and southern California, which typically experiences one Cat 2 or Cat 3 AR each year, averages one Cat 4 AR every ...

Prior to that, the most famous atmospheric river in the northern hemisphere was often referred to as the “Pineapple Express” or “Chinook Winds,” and is still referred to that way. This particular atmospheric river pattern brings water from near Hawaii to the west coast of North America.

As can be seen in Figure 1, about half the annual precipitation total arrives in the three-month period from December through February and 90% of the annual precipitation falls between October 1 and April 30. During this time period, winter storms come off the Pacific Ocean delivering rain and snow to California.