Atmospheric flood blasts West Coast as New England braces for storms

“Atmospheric river” was a weather term coined decades ago to describe a large volume of water traveling through the atmosphere. When the winds are strong enough over the warmer tropical regions of the Pacific, the rate of evaporation increases dramatically, drawing significant amounts of water vapor into the atmosphere. If strong enough, these rivers of moisture in the sky can carry as much as 27 times the amount of water flows through the Mississippi or Amazon rivers.

Combined with a “bomb cyclone”—when a storm system experiences a rapid drop in air pressure, causing it to rapidly intensify—an atmospheric river is a formidable force with devastating impacts.

The most popular atmospheric river is called the “Pineapple Express” – when moist air near Hawaii flows to the mainland in the jet stream, leading to massive amounts of precipitation over the Northwest.

This suspended moisture pushes landward and forces the band to rise, cool, and condense, creating heavy rain and snow—especially over the mountainous Northwest coast. If heavy enough, extreme rainfall can lead to floods and mudslides, causing significant damage to life and property.

Atmospheric river moisture generally decreases over land, leading to several days of significant precipitation with multiple bands of storms. Almost half of the annual precipitation in the western United States originates from atmospheric rivers.

These storm events are most common in the second half of the season and usually last until mid-spring.

New England sees a good amount of storms that originate from atmospheric rivers.

“Atmospheric rivers account for more than 90 percent of extreme precipitation events over the Northeast in winter, spring and fall,” said Ben Moore, a research meteorologist at the National Oceanic and Atmospheric Administration.

As climate change continues to warm the oceans and atmosphere, evidence has shown a significant increase in the frequency of atmospheric rivers during the winter over the eastern part of the country. The equation is simple, but devastating – the warmer the ocean, the more moisture can evaporate into the air. The warmer the atmosphere, the more water the air can hold, which naturally leads to more precipitation.

Much of the evidence points to the Pacific-North American pattern, a recurring weather pattern in which changes in the atmosphere in one place will affect the weather elsewhere. The PNA highlights low and high pressure systems across the continent that swing from negative to positive phases, affecting the position and strength of the jet stream. The example below shows the air pressure in the lower atmosphere during positive and negative phases of PNA.

When the PNA pattern is in a negative phase in late fall and winter, the jet stream is usually pushed northward and may allow more moist air from the Gulf of Mexico or even the western Atlantic to move northward. This could result in significant rain and snow in New England or fuel existing storms crossing the country.

“Atmospheric rivers often accompany mid-latitude (storm systems) that affect the northeastern United States,” Moore said. “They can contribute to potential extreme precipitation when the moist air is forced to rise over atmospheric fronts with an active system or, in some parts of the Northeast, mountainous terrain.”

One of the most impactful atmospheric river-driven storms occurred in December last winter, dumping inundating rainfall. You can see the precipitable water flowing into the Northeast, while a separate atmospheric river hit California.

Since 1980, you can see the correlation of negative PNA phases (blue shading) with atmospheric river events (black dots) in the eastern United States.

It will be important to note that PNA is not an indicator. It is simply a piece of the puzzle. The jet stream usually becomes weaker and wavy when we are in a negative PNA phase. “Large surges in the jet stream tend to increase the frequency of atmospheric river-driven storms over the Northeast,” Moore said.

As we move closer to December, it looks like the PNA index may remain mostly in a negative phase and will be worth watching the deeper we get into the meteorological winter. New England could be in for another messy winter.

Ken Mahan can be reached at [email protected]. Follow him on Instagram @kenmahantheweatherman.