Wet ground, a second soaking and a Halloween weekend

Just over a week after destructive flash floods in Arizona, Nevada, New Mexico and Utah, the US Southwest faces more rain¹. Forecasters at weather.com expect two rounds: on Thursday and Friday over western Texas, New Mexico and Arizona, then from Saturday into Sunday over mainly Southern California, with rain and some high-mountain snow in the Southwest lingering into Monday in places¹.

The source is a low-end “pineapple express”, an atmospheric river drawing tropical moisture from near Hawaii¹. It is not a strong one. The concern is the ground it lands on. Last week’s 2 to 5 inches of rain left parts of northern Arizona, New Mexico, southern Colorado and southern Utah saturated, and some areas will get both rounds within three days¹. The places of greatest concern, the forecasters say, are the north and east sides of the Phoenix metro, the Four Corners, and the foothills and high country of San Diego County¹. Ground burnt by recent wildfires can produce fast debris flows even in modest rain¹.

The US National Weather Service is cited as saying the total moisture in the air over parts of Arizona, Colorado, Nevada and Utah could be two to three times the late-October average, near record for Halloween weekend¹. The forecasters add that stronger El Niño winters tend to make the Southwest wetter from autumn into spring¹. That is their outlook, not ours, and it is a reason to check, not to panic.

The practical advice is old and good. If you planned to hike the slot canyons of southern Utah, southern Colorado or Arizona this weekend, the forecasters suggest you consider cancelling: flash floods move through them fast¹. Plans can be rebooked. A desert in flood cannot be outrun.