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.

Denver's Temperature Dropped a Record 64 Degrees In 24 Hours

Leonid Ikan/iStock via Getty Images
Leonid Ikan/iStock via Getty Images

One sure sign summer is over: On Wednesday, residents of Denver, Colorado were experiencing a comfortable 82-degree day. Just before midnight, the temperature dropped to 29 degrees. Between Wednesday and Thursday afternoon, the Denver airport recorded a differential of 79 degrees down to 24 degrees. At one point on Wednesday, a staggering 45-degree drop was seen in the span of just three hours.

All told, a one-day span saw a 64-degree change in temperature, from a high of 83 to a low of 19, a record for the state in the month of October and just two degrees shy of matching Denver’s all-time record drop of 66 degrees on January 25, 1872. On that date, the temperature plummeted from 46 degrees to -20 degrees.

Back to 2019: Citizens tried their best to cope with the jarring transition in their environment, to mixed success. On Wednesday, the city’s Washington Park was full of joggers and shorts-wearing outdoor enthusiasts. Thursday, only the most devoted runners were out, bundled up against the frigid weather.

The cold snap also brought with it some freezing drizzle which prompted several vehicular accidents, including 200 reported during Thursday's morning commute. It’s expected to warm up some in the coming days, but residents shouldn't get too comfortable: Melting ice could lead to potholes.

[h/t KRDO]

Fall Foliage Is Running Late This Year

Free art director/iStock via Getty Images
Free art director/iStock via Getty Images

The August arrival of the pumpkin spice latte might have you feeling like fall is in full swing already, but plants aren’t quite so impressionable. According to Travel + Leisure, the best fall foliage could be coming a little later than usual this year.

Historically, the vibrant transformation starts to sweep through northern regions of the Rocky Mountains, Minnesota, and New England in mid-September, and reaches its peak by the end of the month. Other areas, including the Appalachians and Midwest states, don’t see the brightest autumn leaves until early or mid-October. The Weather Channel reports that this year, however, the forecast from the National Oceanic and Atmospheric Administration predicts unseasonably warm temperatures for the next two weeks, which could impede the color-changing process.

Warm temperatures aren’t necessarily bad for fall foliage, as long as they occur during the day and are offset by cool nights. Since meteorologists don’t expect the overnight temperatures to drop off yet, plants will likely continue producing enough chlorophyll to keep their leaves green in the coming days.

The good news is that this year’s fall foliage should only be about a week late, and meteorologist David Epstein thinks that when leaves do start to change color, we’re in for an especially beautiful treat. If the current weather forecast holds, he told Boston.com, we'll "see a longer season than last year, we’d see a more vibrant season than last year, and it would come on a little earlier than last year, which was so late.”

Though poor weather conditions like early snow, heavy rain, drought, or strong winds can cause leaves to fall prematurely, most trees right now are in a good position to deliver a brilliant display of color after a healthy, rain-filled summer.

Find out when you’ll experience peak fall foliage in your area with this interactive map.

[h/t Travel + Leisure]

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