Southwestern US braced for severe flooding

As Hurricane Odile weakens to a tropical storm, there are growing concerns about the risk of flooding across the desert regions of the southwestern US.

Odile hit Mexico’s Baja California as a category 3 storm on the five point Saffir-Simpson scale – the most powerful cyclone to have hit the peninsula in recorded history.

It has subsequently weakened, but it has left a trail of damage across the region.

Now crossing the Gulf of California, Odile remains a potent threat in terms of rainfall. Official rainfall totals throughout Baha California have been surprisingly modest, generally in the range of 100 to 150mm. This may have been due to the speed of movement of Odile, which exceeded 30kph at times.

It is now moving at about 12kph and it will continue to slow as it heads across northern Mexico and into the US.

The prospect of persistent torrential rain has caused the US National Weather Service to issue flash flood watches for southeastern California, southern Nevada, southern Utah, southwestern California and northern New Mexico and Arizona, including Tuscon.

Rainfall accumulations could reach 75 to 150mm, but locally, totals could exceed 250mm. To put this into context, Tuscon can normally expect just 42mm of rain during the entire month of September.

 The dry, hard-baked desert soils of the region, subject to years of drought, will encourage run-off of surface water and in more mountainous regions, it is thought that there could be rockslides and mudslides.

For some parts of the Southwest, September could turn out to be the wettest on record, as Odile comes just one week after Tropical Storm Norbert brought torrential rain, with 83mm falling in Phoenix, Arizona.