SF weather is turning stormy over Halloween weekend as an atmospheric river known as the pineapple express sends heavy rain toward the Desert Southwest and Southern California. The heaviest rain is expected to focus on the greater Phoenix area, while other pockets spread north into Nevada and west into California.
The system arrives after last week’s 2 to 5 inches of rain soaked the region and triggered major flash flooding in northern Arizona, New Mexico, southern Colorado and southern Utah. Ground conditions are already primed for runoff, and the National Weather Service in Flagstaff says rivers such as the Little Colorado and Verde could run high for days after the weekend rain.
Phoenix and Flagstaff timing
Rainfall will really get going across much of the Southwest from Saturday overnight into early Sunday. Phoenix just misses the heaviest rain region on Sunday, but it remains in a level 2 of 4 threat area, while Flagstaff, Zion National Park, pockets of northern New Mexico and southern Colorado could see the greatest rainfall and rainfall rates.
Los Angeles and San Diego could also see some rainfall, but the core of the event is aimed farther inland. Pockets of heavy rain could reach as far north as Elko, showing how wide the moisture plume stretches from one weekend system.
National Weather Service in Flagstaff
An area of low pressure charged by the atmospheric river will move onshore on Sunday and spread heavy rain across the West. Meteorologists call the pattern the pineapple express because the moisture tap is originating from near Hawaii, and atmospheric rivers often fuel heavy rain events in the West, especially in a stronger El Niño winter.
Over the higher peaks of the Rockies and Sierra, precipitation may fall as snow rather than rain. That raises the chance of slippery travel mainly Sunday into Monday over mountain passes, while fast-moving, life-threatening flash floods can sweep rapidly through slot canyons in the lower elevations.
People in the Desert Southwest should plan around heavy rain, possible flash flooding and slower mountain travel before the weekend ends. The heaviest rain is expected in the areas already hit hardest last week, and the next round could keep rivers elevated for days.
