A southward plunge of the jet stream along the West Coast is pulling a ribbon of deep tropical moisture — an atmospheric river nicknamed the 'pineapple express' for its origin near Hawaii — into the Desert Southwest and Southern California over Halloween weekend.

Both The Weather Channel and SSBCrack report that rain ramps up Saturday night into early Sunday, with the heaviest downpours forecast for the greater Phoenix area and pockets of rain reaching as far north as Elko, Nevada. Los Angeles and San Diego could also see some rainfall.

By Sunday, an area of low pressure charged by the atmospheric river moves onshore, spreading heavy rain across the West.

The surprising element is the sheer amount of water in the air.

According to the National Weather Service, total atmospheric moisture over parts of Arizona, Colorado, Nevada and Utah could run 2 to 3 times the average for the end of October — near record amounts for Halloween weekend — even though this is not a particularly strong version of the pattern.

The Weather Channel notes this could be a harbinger, given the tendency for a wetter Southwest during most stronger El Niño winters, tying the event to the Super El Niño of 2026-27 coverage.

The timing is dangerous: the region is still soaked from 2 to 5 inches of rain last week that triggered major flash flooding in northern Arizona, New Mexico, southern Colorado and southern Utah.

Sources flag the greatest concern in southwestern Colorado and the Four Corners below snow level, the Phoenix metro's north and east sides and the Mogollon Rim, and the canyonlands of southern Utah.

Ground scorched by recent wildfires is especially prone to fast-moving debris flows, rockslides and mudslides even in modest rain. The Weather Channel also reports that precipitation may fall as snow over higher Rockies and Sierra peaks, making mountain passes slippery mainly Sunday into Monday.

Both outlets urge anyone planning weekend hikes in the parks of southern Utah, southern Colorado and Arizona to reconsider — fast-moving, life-threatening flash floods can sweep rapidly through slot canyons.

The National Weather Service in Flagstaff, Arizona, warns that rivers such as the Little Colorado and Verde could run high for days after this weekend's soaking.

Flagstaff, Zion National Park and parts of northern New Mexico and southern Colorado could see the greatest rainfall and rainfall rates, while Phoenix remains at a level 2 of 4 flood threat.