An atmospheric river nicknamed the pineapple express is set to bring heavy rain to parts of Southern California and the Desert Southwest over Halloween weekend, with the heaviest rain expected to build from Saturday night into early Sunday. The system is expected to intensify as the weekend goes on and then move onshore Sunday, spreading rain farther across the West.
The main focus is the greater Phoenix area, where the heaviest rain is expected, but pockets of heavy rain could also reach as far north as Elko, Nevada, with Los Angeles and San Diego seeing some rainfall too. Phoenix just misses the heaviest rain region, yet it remains under a level 2 of 4 threat on the map, a reminder that even the edge of the storm could still bring disruptive weather.
Farther west and north, the greatest rainfall and rainfall rates could hit Flagstaff, Zion National Park and pockets of northern New Mexico and southern Colorado. Higher elevations may also see snow over the higher peaks of the Rockies and Sierra, setting up slippery travel mainly Sunday into Monday over mountain passes. In other words, this is not just a rain event for the lowlands; it is a multi-hazard setup stretching from wet highways to snowy mountain roads.
The concern is amplified by what the ground is already carrying. Last week, 2 to 5 inches of rain soaked parts of the area and triggered major flash flooding in northern Arizona, New Mexico, southern Colorado and southern Utah. With the soil still saturated, new rain is more likely to run off quickly instead of soaking in, which raises the chance of flash flooding and keeps rivers elevated after the storm.
According to the National Weather Service in Flagstaff, rivers such as the Little Colorado and Verde could run high for days after this weekend’s latest soaking. That lingering risk matters because the heaviest rain is arriving on top of already wet ground, not on a dry landscape that can absorb it. The system is originating near Hawaii, and meteorologists call this kind of moisture tap the pineapple express, a setup that can deliver a lot of water in a short time.
By Sunday, the area of low pressure charged by the atmospheric river is expected to move onshore and spread heavy rain across the West, making the heaviest impacts more widespread even as the weekend travel window narrows. For residents and travelers across Arizona, Nevada, New Mexico, Utah and Southern California, the question is no longer whether the storm will arrive, but how much runoff, flooding and travel disruption it leaves behind once it does.
