What Is an "Atmospheric River"?

A flooded playground in San Jose, California, on February 22. Blame the high water on an atmospheric river. Image Credit: Noah Berger/AFP/Getty Images

 
Storms make for attention-grabbing headlines, and almost every disaster has a meteorological term that makes a hazardous situation sound 10 times as terrifying. A derecho tore through the Mid-Atlantic back in 2012 and had such a profound psychological impact on the affected residents that you could cause mass hysteria by just mentioning the term. Then came the dreaded polar vortex, an ever-present large-scale wind pattern that encircles the North Pole and occasionally gets wavy and injects bitterly cold air into southern Canada and the United States. It was nothing new—but it sounded scary, so the name took off.

The recent deluges in California highlighted the latest captivating weather-y buzzword: an “atmospheric river.” Like its counterparts, this scary-sounding phenomenon is not as uncommon as it seems. It's responsible for almost all of the rain on the West Coast this winter.

Clouds outline the atmospheric river stretching from Hawaii to California in the storm that affected the West Coast on February 17, 2017. Image Credit: Dennis Mersereau

 
So what is it? An atmospheric river is a long, narrow band of deep tropical moisture that can span thousands of miles in length. They occur from the tropics to the mid-latitudes. Atmospheric rivers aren’t actual rivers, of course, but it’s a pretty good description of a feature that resembles a river on satellite imagery and can bring torrents of water to the unlucky communities caught beneath one as the system comes ashore.

These ribbons of tropical moisture are the result of sprawling low-pressure systems that form just far enough south that their counter-clockwise circulation scoops up water vapor from the tropics and transports it northward. The storms that cause atmospheric rivers to form in the first place are usually able to generate enough upward lift to create precipitation. Mountains can play a role—they're very effective at wringing moisture out of the atmosphere as wind travels up the side of their terrain. Whether it’s rain or snow, any precipitation that forms within that band of elevated moisture levels can be quite heavy, producing steep rainfall totals and many feet of snow in extreme cases.

NASA’s Global Precipitation Measurement (GPM) mission captured three weeks of heavy rainfall slamming into the West Coast between February 1 and February 20, 2017. Watch it happen in the video below.

California has a reputation for calm, sunny weather, but the state never really has it easy when it comes to dealing with nature’s temper tantrums. For the past couple of years, the state has been mired in a devastating drought, a cycle of dryness that was finally broken this winter as one storm after another roared in from the Pacific and drenched the state with unmanageable amounts of heavy rain. The driving force that gave each storm its bulk was an atmospheric river. Without it, there wouldn’t have been much moisture for the storm systems to work with.

An atmospheric river that affects the West Coast—and California in particular—is usually nicknamed the “Pineapple Express” since the source of the tropical moisture is the area around Hawaii. Though they go without a popular nickname, these features are also common over the eastern half of the United States during the warm season. Many of the major flash floods that occur in the eastern U.S. during the summer months are the result of intense thunderstorms tapping into the bountiful moisture present in an atmospheric river flowing over the region.

All weather is the result of nature trying to balance out inequality, and atmospheric rivers, just like every other weather condition, serve this purpose. Wind blows from areas of high air pressure to areas of lower pressure in an attempt to erase the inequality of more air molecules over one spot than another. The jet stream is the direct result of sharp temperature differences between the tropics and the poles. Hurricanes exist to transport heat from the tropics to the poles. Atmospheric rivers exist to take moisture out of the tropics and spread it around the world. Though they can seem difficult to enjoy, we’d be in some pretty big trouble without them. In 1998, a study by MIT scientists reported that 90 percent of all the moisture transfer between the tropics and the rest of the world each year occurs within these narrow bands of evaporated paradise.

What Is a Bomb Cyclone?

Maddie Meyer/Getty Images
Maddie Meyer/Getty Images

The phrase bomb cyclone has re-entered the news this week as parts of the central U.S. face severe weather. Mountain and Midwestern states, including Colorado, Nebraska, Wyoming, and South Dakota, all fall in the path of a winter storm expected to deliver tornadoes, hail, heavy snow, flooding, and hurricane-force winds on Wednesday, March 13 into Thursday. It seems appropriate for a storm that strong to have bomb in its name, but the word actually refers to a meteorological phenomenon and not the cyclone's explosive intensity.

According to The Denver Post, the bomb in bomb cyclone stands for bombogenesis. Bombogenesis occurs when a non-tropical storm experiences at least a 24 millibar (the unit used to measure barometric pressure) drop within 24 hours. Low pressure makes for intense storms, so a bomb cyclone is a system that's built up a significant amount strength in a short length of time.

This type of storm usually depends on the ocean or another large body of water for its power. During the winter, the relatively warm air coming off the ocean and the cold air above land can collide to create a sharp drop in atmospheric pressure. Also known as a winter hurricane, this effect has produced some of the worst snowstorms to ever hit the U.S.

The fact that this latest bomb cyclone has formed nowhere near the coast makes it even more remarkable. Rather, a warm, subtropical air mass and a cold, Arctic air mass crossed paths, creating the perfect conditions for a rare bombogenesis over the Rockies and Great Plains states.

Central U.S. residents in the bomb cyclone's path have taken great precautions ahead of the storm. Over 1000 flights have been canceled for Wednesday and schools throughout Colorado have closed.

[h/t The Denver Post]

Watch a Rare ‘Ice Tsunami’ Slam Lake Erie

Clean Lakes Alliance, Flickr // CC BY 2.0
Clean Lakes Alliance, Flickr // CC BY 2.0

A combination of freezing cold temperatures and high winds is creating an unusual phenomenon along Lake Erie. As KDKA reports, ice tsunamis are toppling onto lake shores, and many locals have been asked to stay inside and even evacuate their homes.

On February 24, 2019, the National Weather Service in Buffalo, New York issued a warning about dangerous wind gusts in the Lake Erie area. The service urged citizens to seek shelter indoors and avoid traveling if possible. Winds peaked at 74 mph earlier this week, the level of a Category 1 hurricane, and tore down trees and power lines throughout the region.

People who got close to Lake Erie during the windstorm witnessed a rare event known as an ice tsunami. When wind pushed ice on the lake's surface toward the retaining wall, the sheet broke apart and dumped massive ice chunks on the shore. The video below captures the phenomenon.

In some areas, the ice piles grew so large that roadways had to be closed. Residents of Hamburg, New York's Hoover Beach area were asked to voluntarily evacuate due to the encroaching ice.

Ice tsunamis, or ice shoves, are rare, but in some cases they can be life-threatening. In 2013, waves of ice shards from a Minnesota lake destroyed people's homes.

[h/t KDKA]

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