Pineapple Express Threatens California and Southwest With Flooding

Heavy rainfall and potential flash flooding threaten Southern California and the Desert Southwest over Halloween weekend as an atmospheric river carrying tropical moisture from near Hawaii moves toward the region. The weather system could affect Arizona, Nevada, New Mexico, Utah and surrounding areas following destructive flooding across the Southwest last week.

According to The Weather Channel meteorologist Jonathan Erdman, the incoming system is a relatively weak version of the phenomenon known as the "pineapple express." Despite its lower intensity, the unusually high moisture content could produce significant rainfall across areas already affected by recent storms.

The National Weather Service forecasts atmospheric moisture levels across parts of Arizona, Colorado, Nevada and Utah reaching two to three times the average for late October. These levels could approach records for Halloween weekend.

The incoming precipitation will arrive in two main rounds. The first is expected through Friday, affecting western Texas and parts of New Mexico and Arizona. The second will develop over the weekend, with Southern California rainfall increasing Saturday and reaching its peak early Sunday.

Across the broader Southwest, rain and some high-elevation snowfall are expected to intensify late Saturday, continue Sunday and persist in certain locations through Monday.

Western Texas communities, including Lubbock and Midland, could experience localized flash flooding during the first round of rainfall. However, the more significant flooding concern involves the Desert Southwest, where repeated precipitation could overwhelm already saturated ground.

Last week's storms delivered between 2 and 5 inches of rain across affected areas, triggering major flash flooding in northern Arizona, New Mexico, southern Colorado and southern Utah. The additional rainfall could increase the danger because some locations will experience both rounds of precipitation within approximately three days.

The areas facing particular concern include the northern and eastern portions of the Phoenix metropolitan area, the Four Corners region, and the foothills and high-elevation areas of San Diego County.

Recent wildfire burn scars also present an additional hazard. Rain falling over these areas can trigger rapidly moving debris flows, even when precipitation amounts are relatively modest.

Slot canyons and other narrow waterways in southern Utah, southern Colorado and Arizona could become especially dangerous. Rapidly rising floodwaters may sweep through these locations with little warning, creating potentially life-threatening conditions for hikers.

Mountain travel could also become more difficult as precipitation falls as snow at higher elevations in the Sierra. According to the forecast, slippery conditions are possible along mountain passes, particularly from Sunday into Monday.

The weather pattern is developing as the jet stream dips southward off the West Coast, directing a concentrated stream of tropical moisture toward the Southwest. This moisture corridor, known as an atmospheric river, originates near Hawaii and is commonly referred to as the "pineapple express."

Although the incoming system is not classified as an especially strong atmospheric river, its moisture supply could be exceptional for this time of year. The National Weather Service indicates that atmospheric moisture over parts of the Southwest could reach two to three times the late-October average, approaching record levels for the Halloween period.