The Pineapple Express and California’s Rain

Another week and another big storm, so another post.

Welcome back! California has seen some flooding rains the past few days. How much rain? San Francisco received 3.75 inches in one day, about one-third of the total received during the 2013-14 rain year. Hurricane force winds have been reported, major highways were closed, and 4.5 inches of rain fell on Big Sur in only three hours. Unfortunately, soils as dry as they are currently in California do not absorb moisture very well. The majority of the rain is running off and contributing to the flooding. To any of those dealing with the flooding as they read this, please do not take chances with the flood waters. Please Turn Around, Don’t Drown!

With such strong winds and flooding rains, this is clearly no ordinary storm. It is a storm pattern unique in the United States to the Pacific coastline. Like the nor’easter along the Atlantic coast, the patten has a name: The Pineapple Express. It was given this name because the rain comes from the general area of Hawaii. Also like nor’easters, the storm occurs in the wintertime or adjoining months. The Pineapple Express is an example of an atmospheric river: a narrow band of atmosphere transporting a large amount of moisture. Pineapple Express occurrences account for between one-third and one-half of California’s annual precipitation, although the size of this storm and the lack of rain previous will probably push this year’s value over 50%.

To understand how they occur we need to quickly go over the Madden-Julian Oscillation (MJO). The MJO is a climate pattern with a period between 1 and 3 months. We will not delve into the reasons for the MJO here, but its most important feature is the region of enhanced convection (thunderstorm, heavy rainfall activity) that moves west to east along the equator. Although the MJO pattern wraps around the entire planet, its effect is greatest in the Pacific Ocean.

Image created by Baker of NWS Boulder
Image created by Baker of NWS Boulder

It is this region of thunderstorm activity that both triggers the Pineapple Express and is the source of its moisture. Below is the best visual explanation of the process I could find. The image is unfortunately small so I will explain it myself. The entire process takes between ten days and two weeks.

Image again created by Baker of NWS Boulder
Image again created by Baker of NWS Boulder

We start with the MJO thunderstorm region moving east along the equatorial Pacific. Also in place at the time is the Pacific High, a semi-permanent region of high pressure in the northeast Pacific that is most prevalent in the summer. In the wintertime months it is weaker and more easily eroded. That will be important.

As the storms move closer to the high pressure, the westerly winds create a split in the jet stream. The southern half of the jet begins the pull moisture further northeast and weakens the Pacific high. A trough (low) forms off of Canada’s west coast.

Over the next week the trough grows, the high weakens, and the jet stream migrates further south along the west coast. The strengthened jet quickly pulls a large amount of moisture from the MJO moisture plume. The moisture is stretched into a think band known as the atmospheric river, or Pineapple Express. Additionally, the strengthened low is now powerful storm the brings the moisture onshore in the form of heavy rain and high winds.

Atmospheric river moisture band. Image from the great Cliff Mass Weather Blog at
Atmospheric river moisture band. The low pressure center is to it’s northwest. Image from the great Cliff Mass Weather Blog at

I should also mention that an El Nino year makes this occurrence more likely . Low pressure that will tend to sit in place of the Pacific High steers moisture towards the western United States. Currently we are not in an official El Nino, but eastern equatorial Pacific temperatures are above average, putting us in an El Nino Watch.


Since the MJO has a period of 30-90, it is possible to have multiple events in the same winter season. With up to ten inches of rain falling from each instance, you can see how important these events are to the climate of the west coast, especially California. These storms are dangerous ones, but they are beneficial ones. The entire state of California is in drought. This one storm will not remove the drought, but it will bring much needed rain to the state’s aquifers and build a snowpack for the spring.

Let us hope to see less exceptional drought in the next map.
Let us hope to see less exceptional drought in the next map.

Another storm explained, successfully I hope. Comment with any questions and please follow the blog!



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