It is December in Oregon. Temperatures are rising, the wind is blowing, and lots of rain is falling. Welcome to the Pineapple Express, an atmospheric river bringing moist tropical air from Hawaii to the West Coast of the USA. The amount of water carried in this tropical blast is equivalent to the average flow of water at the mouth of the Mississippi River. When the Pineapple Express hits California and Oregon, it dumps that moisture in the form of rain and snow.
The number is big, and each day this atmospheric river brings 380 billion gallons of water ashore, large portions of which transform into precipitation. In 2010 the Pineapple Express dumped up to 25 inches of rain in some areas of California over 12 days.
What is an atmospheric river?
An atmospheric river (AR) is a narrow, elongated flow of moist air in the lower atmosphere. The flow corridor is greater than 2,000 km in length, typically 400 – 600 km wide, and up to 3 km deep. ARs like the Pineapple Express form along the front edge of slow-moving, low-pressure weather systems related to the polar jet stream. The cyclone nature of these weather systems in the northern hemisphere causes the winds to flow from the SW to the NE. Hence, the warm moist air from the tropics reaches as far north as Washington and Oregon.
Warm air and snow
The winter months between December and February are when the Pineapple Express usually develops. The air coming across the Pacific from Hawaii is not only very moist, but it is also exceptionally warm. Winter snowpacks in the mountains, warm air from the ocean, and lots of rain are the perfect ingredients for epic flooding.
The great flood of 1862 resulted from a prolonged battering of the West Coast by a series of ARs. During December of 1861 and January of 1862, areas from Oregon to San Diego were inundated with rain and snow from the Pineapple Express. Over 43 days, the equivalent of 10 feet of rain fell on California. The precipitation came in the form of both rain and snow.
The culmination of this event was a blast of very warm air that melted the high mountain snowpacks. C
It is nice to get a little warmth during an Oregon December, but not too much.
What are atmospheric rivers? (NOAA) – https://www.noaa.gov/stories/what-are-atmospheric-rivers
Atmospheric Rivers (NASA – Earthdata) – https://ghrc.nsstc.nasa.gov/home/micro-articles/atmospheric-rivers
The Great Flood of 1862 left devastation in its path across the state (Mark Landis – The Sun) – https://www.sbsun.com/2018/11/12/the-great-flood-of-1862-left-devastation-in-its-path-across-the-state/