Once-mighty Hurricane Rosa is now just a low-end tropical storm. Nevertheless, it is still poised to pack a punch — in the form of heavy, flooding rains that are targeting the Desert Southwest.
Rosa is set to make landfall along Mexico’s Baja Peninsula Monday evening, likely as a weak tropical storm or depression. But the reduction in winds doesn’t mean the threat is waning — in fact, concern is growing about potentially dangerous flooding.
Flash-flood watches stretch a whopping 750 miles, extending from the Mexican border northward through Utah. ‘‘These rainfall amounts may produce life-threatening flash flooding,’’ wrote the National Hurricane Center on Monday. ‘‘Dangerous debris flows and landslides are also possible in mountainous terrain.’’
Isolated debris flows will be possible as far west as the Mojave Desert as well, ‘‘particularly over fresh burn scars, where heavy rain can cause a cascade of rocks [and mud],’’ the National Weather Service said.
An initial batch of light to moderate rain had already broken out along the Interstate 10 corridor of southern Arizona Monday. This ‘‘appetizer’’ round of storminess will give way to heavy rain bands late Monday night into Tuesday morning. The highest rainfall amounts will fall just north of the center of circulation, which looks to track roughly from Yuma to Flagstaff in Arizona through Tuesday afternoon.
Yuma picked up 0.65 inches of rain Sunday alone as moisture was drawn inland ahead of the storm. Yuma averages a mere 3.78 inches per year, meaning it received 62 days’ worth of rain in just 24 hours.
The National Weather Service in Phoenix is warning that a widespread 1 to 2 inches (with localized 4-inch amounts) could come down before all is said and done. ‘‘Many of the heavier amounts will likely be focused over favorable upslope areas to the north and northeast of Phoenix,’’ it wrote in a discussion Monday morning. That’s where mountains in the Tonto National Forest could tap into moisture aloft and enhance the rainfall even further.
Flagstaff will get its share of the action Tuesday too, with downpours arriving around sunrise and persisting through lunchtime Wednesday. The jackpot totals should remain just to the south, with up to 2 inches still expected in the city.
While the rain dragged in by Rosa will begin to shut off north of Interstate 40 in Arizona, disturbed weather at high altitudes will keep scattered afternoon downpours in the offing Tuesday into Wednesday from Nevada to western New Mexico, especially around the Four Corners region.
This upper-level swirl left behind by Rosa will then sweep into Utah mid- to late-week, bringing cloudy skies and the chance of a few showers. Sunshine will return to waterlogged areas farther south.
Though tropical cyclones rarely affect Arizona, they are not unheard of. Generally speaking, they arrive once every couple of years. Most spin down into tropical rainstorms by the time they emerge over the mountains, but at least eight have met the wind requirements for tropical-storm status since 1965. The most recent of these — Nora — dropped more than a foot of rain on Harquahala Mountain in 1997. Achieving that kind of feat in a desert is rare.
The Eastern Pacific has been a hot spot for hurricane activity this year. Eight storms have already reached major hurricane status, reaching Category 3 or higher on the Saffir-Simpson wind scale. Rosa peaked at Category 4 with 145-mile-per-hour winds before weakening. The average for any given year is just over three. The most active years for Eastern Pacific hurricanes tend to be during El Niño patterns, but this year is an unusual exception.
Meanwhile, recently christened Tropical Storm Sergio is likely to become a major hurricane over the open ocean in the Eastern Pacific into Thursday.