A slow-moving low pressure system that’s been hanging around for an unusually long two weeks is set to bring locally heavy rains and possible flash floods to parts of the Southwest this weekend. Parts of Southern California have already seen flooding and mudslides from the system.

The flood threat comes just a month after the remnants of a tropical system unleashed another bout of flooding in Arizona, Southern California and New Mexico.


The set-up this time around is different: This low pressure system originated over southwestern Canada before it wound its way southward over California, wandered around the Southwest, then plunged south again into Mexico. It was thrown out into the Pacific Ocean before it came back over land again, where it is now dropping rain on drought-stricken California. When heavy rains hit such dry soils, the land can’t absorb the water quickly enough and flooding can result, particularly where steep terrain causes even more runoff.

Loop showing the progress of the low from the morning of Friday, October 2nd, to Thursday morning, October 15th

Loop showing the progress of the low from the morning of Friday, October 2nd, to Thursday morning, October 15th.


It is unusual for a low pressure system like this to have such longevity, as dips (or troughs) in the jet stream usually come along and absorb them. So far the jet stream has remained well away from the low, allowing it to meander around, but a trough is expected to absorb the system later this weekend, according to the National Weather Service. That trough brings further chances for strong storms.

Currently parts of Southern California, southern Nevada, southern Utah and northwestern Arizona are under a flash flood watch.

This region is one that is expected to see wetter-than-normal conditions this winter thanks to a strong El Nino, which tends to alter the position of the jet stream over the continental U.S. The Southern Plains and Southeast, especially Florida, are also expected to see a wet winter.

Heavy downpours are also expected to happen more often as the climate warms and the air has more available moisture in it to fuel rains, even in areas that are expected to see an overall drying trend, like the Southwest.