AUSTIN (KXAN) — The West Coast is bracing for a deluge of flooding rain, snow and mudslides. Many areas could see nearly a foot of rain with several feet of accumulating snow expected in the high elevations of the Sierra Mountains through Friday morning. Emergency officials ordered mandatory evacuations for areas that face a high risk of flooding as California Governor Gavin Newsom declared a State of Emergency in advance of this latest storm.

Watches, Warnings & Advisories in effect

This flooding rain is all thanks to a powerful storm and a southerly dip in the jet stream which is carrying warm, moist and tropical air across the Pacific and into the West Coast. This plume of moisture being pulled from Hawaii all the way to the West Coast is called an atmospheric river.

Flooding rain expected for many

What is an atmospheric river?

An atmospheric river is a narrow corridor of concentrated moisture or water vapor in the atmosphere. They can stretch hundreds to even thousands of miles in length while usually being much narrower in width (less than 500 miles or so). As this water vapor gets pushed up the mountains along the West Coast, this moisture is forced upwards creating cooling, condensation and a continuous torrential train of falling heavy precipitation. The faster the atmospheric river, the more intense these precipitation rates can be.

What is a Pineapple Express?

This specific atmospheric river event that is occurring in the Pacific Northwest down through California is called a ‘Pineapple Express’. The source of tropical moisture must originate or pass near Hawaii to earn the name. It is coined a Pineapple Express due to the storm’s Hawaiian origins and the long symbolic history the state has with pineapples.

The Pineapple Express is always considered an atmospheric river event, but not all atmospheric rivers are a Pineapple Express.

Climate change

With increasing sea surface temperatures and water vapor in the air, a result of climate change, atmospheric rivers are expected to continue to increase in intensity and in frequency resulting in more flooding and economic loss.