What are the ‘atmospheric rivers’ hitting the US West Coast?

Enormous ‘atmospheric rivers’ have resulted in torrential downpours, excessive winds and flood watches.

Channels of moisture known as “atmospheric rivers” are presently stretching alongside the US West Coast to central California from the tropical skies north of Hawaii.

The centre of this moisture plume is about 200 to 300 miles (320-480km) extensive.

The climate system spawned by a low-pressure system off the Pacific Northwest has resulted in persevering with torrential downpours alongside the coast, with many areas positioned beneath a flood watch.

What’s an ‘atmospheric river’?

An “atmospheric river” is a comparatively lengthy, slender area that transports columns of water vapour away from the tropics within the environment.

Based on the Nationwide Oceanic and Atmospheric Administration (NOAA), these columns can carry roughly 7.5 to fifteen occasions the typical move of water on the mouth of the Mississippi River.

When the atmospheric rivers attain land, they typically launch these columns of water vapour within the type of rain or snow.

On common, the West Coast receives between 30 p.c and 50 p.c of its annual precipitation by “atmospheric rivers”, in accordance with the NOAA.

Are ‘atmospheric rivers’ harmful?

“Atmospheric rivers” are a typical phenomenon alongside the US West Coast and are normally innocent.

Weaker storms present helpful quantities of rain for the water provide. In addition they assist improve California’s mountain snowpack, which gives a 3rd of the state’s water provide.

“Atmospheric rivers” are ranked on a scale of 1-5 primarily based on how a lot moisture they transport from the tropics to the mid-latitudes.

Nonetheless, class 4 or 5 “atmospheric rivers” that produce giant portions of rain may end up in extreme flooding, induce mudslides and trigger catastrophic harm to life and property, in accordance with the NOAA.

The Middle for Western Climate and Water Extremes has ranked the persevering with “atmospheric river” occasion as a Stage 4, which corresponds to “excessive” moisture transport.

How have they affected the West Coast this week?

The “atmospheric rivers” have resulted in torrential downpours within the San Francisco Bay Space since Monday night time, with quite a few experiences of roadway flooding and downed bushes and branches.

Some components of central California have already registered greater than 5 inches (12.7cm) of rain.

Pacific Gasoline & Electrical’s web site confirmed quite a few energy outages scattered throughout Northern California.

A second “atmospheric river” is forecast for later this week and weekend and is anticipated to convey additional heavy rainfall and a menace of flooding.

A number of toes of snow are anticipated within the Sierra Nevada mountain vary. Backcountry avalanche warnings have been issued for components of the mountain vary.

The current rain and snowfall are additionally anticipated to supply some reduction to drought-stricken areas and supply some surplus water for doubtlessly drier months.