Pineapple Express Continues to Persist Across the West Coast

A Pineapple Express is unleashing heavy rain and snow across a good portion of the western U.S. How much moisture has hit the region so far and how much more is on the horizon? Read on for all of the details.

Atmospheric River Forecast to Linger Through Tuesday in Parts of the West

An atmospheric river coming from near the Hawaiian islands pushed toward the West Coast last week, bringing in mass amounts of moisture in the form of both rain and snow. This deep flow of tropical moisture set up late Thursday and has been targeting the Pacific Northwest and Northern California since that time.

In addition to beneficial moisture, the Pineapple Express is also raising the threat of flash flooding, high winds, and avalanches. While this is typically the rainy season for this corner of the country, it had been a particularly dry January for much of the region.

Forecasters are predicting that Northern California will continue to see the greatest impacts from this atmospheric river through at least Tuesday. Rain will fall across the lower elevations while snow will be the story for the higher terrains.

It has been raining steadily in San Francisco since late last week. The Bay Area could see up to a month's worth of rain by the time this event wraps up. The average rainfall for the entirety of February in San Francisco is 3.96 inches. The metro area is bracing for rainfall amounts of 2 to 4 inches through Tuesday.

A similar situation is unfolding farther inland in places such as Redding, California where heavy rain continues to persist heading into the new work week. The average amount of rain for the month of February in Redding is 5.48 inches. This metro area is expecting about 4 to 8 inches. Torrential rain across the northern and central parts of the Golden State will also raise the threat of landslides in the rugged terrains.

This heavy rain will creep up into the southwestern corner of Oregon, reaching as far as the college town of Eugene. The heaviest rainfall in this region could hit up to 12 inches over the course of the multi-day event. The northern edge of the Sierra Nevada will also see some of this rain thanks to warmer temperatures.

While this amount of rain seems excessively high, it is not out of the norm when a Pineapple Express digs into the deep wells of moisture in the Pacific. This is because the torrent of moisture travels for thousands of miles, picking up loads of precipitation along the way before depositing it over the West Coast.

Colder temperatures in the top terrains of the Sierra will result in snowfall measurements of 3 to 6 feet. Elevations over 7,000 feet will pick up the bulk of the snow, creating dangerous travel conditions.

For instance, the greater Lake Tahoe area is under a winter storm warning from midday Monday through early Wednesday. Snow accumulations are expected to hit between 1 to 3 feet in elevations above 7000 feet while 6 to 12 inches will fall below 7000 feet. Winds could hit speeds as high as 90 mph across the Sierra crest while speeds of 35 to 40 mph are likely in the lower valleys. The Monday evening and Tuesday morning commutes are forecast to be especially dangerous in this area.

High winds will also complicate the travel situation in metro areas such as Reno. Winds are forecast to be whipping around at speeds of 20 to 30 mph from the southwest on Monday with some gusts reaching 40 mph. Temperatures in the mid to upper 50s will keep any moisture falling as rain. This moisture will pick up in the overnight hours and into Tuesday in Reno. You can expect rainfall of about a half of an inch on Tuesday. Colder temperatures overnight will mean that the rain will likely switch over to snow.

Looking to the Middle of the Week

The worst impacts of the atmospheric river will begin to dissipate by late Tuesday. However, another weather maker is waiting in the wings for the West Coast. This stream of moisture is likely to fire up by late Wednesday and into Thursday, bringing a renewed chance of rain and snow.

This midweek storm is expected to track farther to the south, meaning that Southern California may see some of the precipitation. Any rain will be welcome in the areas still at risk of wildfire development, however, too much rain too quickly could trigger dangerous mudslides and debris flows.

Los Angeles has rain in the forecast on both Wednesday and Thursday with the chance of lingering showers into early Friday. Check back later in the week for a more detailed look at how much rain the City of Angels can expect.

Snow for the Lowland Areas of the Pacific Northwest

The weekend weather event also brought rare snow to the lowland areas of the Pacific Northwest, including western Washington, northwestern Oregon, and the southwestern edge of British Columbia, Canada. A surge of Arctic air dipping down into the Northwest paired with the influx of moisture to send the flakes flying over parts of Seattle and Portland. While accumulations were minimal, it was enough to create slippery spots on the roads for a brief period of time.

It has been over a year since accumulating snow fell across Portland. You have to go back to January 13, 2024, to find the last time the Rose City saw measurable snow. Seattle picked up a small bit of accumulation in early March of 2024. Seattle typically sees 6.3 inches of snow during an average winter while Portland records 4.3 inches.

It was a difficult situation in Vancouver, Canada. This city is continuing to see steady snow with up to 4 more inches possible on Monday before the system moves out of the region. Daily highs will temperatures hover around the freezing level over the next few days with lows landing in the 20s.

Vancouver generally sees more snow than its neighbors to the south. However, it has been more than 70 days since this metro area has recorded measurable snow. Vancouver last saw snow on November 19 when 1.6 inches fell.

Understanding the Pineapple Express

How does the Pineapple Express get its name and how does it relate to the typical atmospheric river? It is not unusual for the West Coast to see the impacts of weather systems that originate in the northern part of the Pacific Ocean. This happens because the jet stream pattern typically flows west to east.

However, every now and then, the jet stream will plunge far enough to the south across the central Pacific Ocean to pick up moisture that is circulating closer to Hawaii. When this happens, the flow of warm moisture from the warmer part of the Pacific is brought to the north by the jet stream and dumped across various parts of the western U.S.

The name Pineapple Express comes from the fact that the fruit is well known in Hawaii and due to the speed of the moisture moving up from the islands and to the mainland. As such, the Pineapple Express is simply an atmospheric river that gets its start in the tropical Pacific.

A Pineapple Express event can fire up at any time of the year but they are most common during the fall and winter months as the weather pattern is generally more active at this time. Although residents may experience disruptions when the express ignites, there is no doubt that its associated moisture can be beneficial in the long term.

This is especially true in parts of central and southern California where precipitation is hard to come by during the dry season. The parade of atmospheric rivers during the winter months is what replenishes the water in the state's lakes and reservoirs.

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