Austin (KXAN) — The West Coast is preparing for flooding due to rain, snow and landslides. Nearly a foot of rain could fall in many areas, with several feet of snow expected in the high Sierra Mountains until Friday morning. Emergency officials have ordered mandatory evacuations from areas at high risk of flooding as California Gov. Gavin Newsom declared a state of emergency ahead of this latest storm.
This torrential rain is caused by a strong storm and a southerly descent jet stream that carries warm, humid, and tropical air across the Pacific Ocean to the west coast. This plume of moisture, stretching from Hawaii to the west coast, is called an atmospheric river.
What is an atmospheric river?
An atmospheric river is a narrow corridor of concentrated moisture or water vapor in the atmosphere. They can span hundreds or even thousands of miles in length, but are usually much less wide (less than 500 miles or so). As this water vapor rises up the mountains along the west coast, this moisture is pushed up, causing cooling, condensation and a continuous torrential string of heavy rainfall. The faster the atmospheric river, the more intense this precipitation can be.
What is Pineapple Express?
This particular atmospheric river phenomenon that occurs in the Pacific Northwest through California is called the “Pineapple Express”. A source of tropical moisture must originate or pass near Hawaii to merit the name. It is named the Pineapple Express because of the storm’s Hawaiian origin and the state’s long symbolic history with pineapples.
Pineapple Express is always considered an atmospheric river event, but not all atmospheric rivers are Pineapple Express.
Changing of the climate
With rising sea surface temperatures and rising air water vapor as a result of climate change, atmospheric rivers are expected to continue to increase in intensity and frequency, resulting in increased flooding and economic loss.