‘Pineapple Express’ Drenches Pacific Northwest

Written by Gavin White

Last updated 12/7/2023, 6:50:31 PM

If you thought weather in 2023 couldn’t get any weirder, prepare yourself for this unfamiliar weather term. A ‘Pineapple Express,’ as it’s known to local forecasters, has formed in the Pacific Northwest, dumping loads of moisture in the form of rain to the Pacific coasts of Oregon, Washington, and Canada. As much as 4-8 inches of rain has fallen between Monday and Tuesday thus far, with the higher amounts over mountainous regions.

As a result of the heavy rainfall, especially over river origins in the mountains, rivers in Washington have overflowed, flooding towns and neighborhoods. Flood warnings and advisories are in effect throughout the state Tuesday (December 5) and Wednesday (December 6).

Flooding in western Washington on December 5. Photo courtesy of Snohomish County Sheriff’s Office (KOIN 6 News).

Floodwater rescue conducted in Washington on December 5. Photo courtesy of the U.S. Coast Guard (CBS News).

The ‘Pineapple Express’ is a strong type of atmospheric river, with its moisture originating from the Hawaiian Islands (hence the name). The event is triggered when the jet stream over the Pacific Ocean dips southward, which is known as a trough in the jet stream. The trough collects an ever-growing pool of warm, saturated air from the Hawaiian region. The jet stream then carries the extremely moist air across the ocean, dumping it on the doorstep of the West Coast.

The image below depicts the GFS model’s precipitation accumulation prediction from 7:00 PM EST December 4 through 7:00 PM EST December 5. Take note of the appearance of a river-like band of rainfall from the Pacific, leading directly to the Pacific Northwest. That river-like band of intensely colored moisture is the forecast model’s representation of this atmospheric river or ‘Pineapple Express.’ According to this particular model run, most affected areas were predicted to see 2-4 inches of rain over that 24-hour period.

The ’Pineapple Express’ of moisture has brought warmer, tropical-like temperatures to the region, too. Thanks to the atmospheric river’s collection of moisture from the Hawaiian region, the moist air bringing the intense rainfall has led to an immediate temperature difference in the region. The regions in the direct path of the expressway have temperatures 10-15 degrees warmer than the immediate surrounding areas, at the time of writing. These temperature differences are pictured below, overtop of a radar frame. The red polygons indicate active flood warnings at the time of writing.

The good news is that the event should begin to calm down over the next 24-36 hours. While the devastating effects of the flooding will be widespread, intermittent breaks from the rain are in sight, allowing opportunities for recovery and cleanup.