Rosa and non-tropical storm to deliver dangerous flooding, drought-busting rain in southwestern US

By Alex Sosnowski, AccuWeather senior meteorologist
October 03, 2018, 2:25:40 AM EDT


Rosa will spread the risk of life-threatening flash flooding over the interior Southwest, while a non-tropical storm spreads soaking rain into much of California this week.

(AP Photo/Ross D. Franklin)

A pedestrian walks through a flooded street with a hand truck to get sand bags to deliver to local businesses during a flash flood as a result of heavy rains from tropical storm Rosa Tuesday, Oct. 2, 2018, in Phoenix.

(AP Photo/Ross D. Franklin)

Vehicles wait to get towed from a flooded street during a flash flood as a result of heavy rains from tropical storm Rosa Tuesday, Oct. 2, 2018, in Phoenix.

(AP Photo/Ross D. Franklin)

A Jeep drives through a flooded street to get sand bags to deliver to local businesses during a flash flood as a result of heavy rains from tropical storm Rosa Tuesday, Oct. 2, 2018, in Phoenix.

(Image via Instagram/azchumley)

The remnants of Hurricane Rosa dumping rain across Arizona.

(Image via Reed Timmer)

Standing waves in flash flood in Gila Bend, Arizona.

(Image via Reed Timmer)

Flash flooding in Scottsdale, Arizona along Indian Bend Wash after heavy rainfall from Rosa.


Rosa to bring half a year to a year's worth of rain

Even though Rosa was once a Category 4 hurricane, its days of producing high winds are over. However, it will remain a very effective rain machine into the middle and latter part of this week.

Rosa swiftly weakened as it approached shore on Monday night, losing tropical storm status early Tuesday morning. It will remain a tropical rainstorm through midweek as it moves through the southwestern U.S.

Static Rosa Track Final 11 am


In terms of direct rainfall from Rosa, enough rain is forecast to be both problematic and beneficial. The swath of rain will continue lead the center of the storm by hundreds of miles.

A general storm total of 1-3 inches of rain is expected to fall on low-elevation, desert locations in the direct path of the storm. There can be 3-6 inches in a narrow swath, especially in the mountains, with an AccuWeather Local StormMax™ of 8 inches.

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In some desert areas, this is as much rain that typically falls in six months to a year.

The swath of drenching rain is forecast to fall from Arizona to central and western New Mexico, southern Utah and central and western Colorado.

Normally dry stream beds, known locally as arroyos, will fill with rushing water. Some canyons popular among hikers may become raging torrents. Significant rises are possible on some of the rivers in the region.

Expect debris flows, especially along secondary roads in the outlying areas.

Water will wash across roads without notice and some roads may be heavily damaged and/or clogged with rocks and other debris.

Significant problems are expected in urban areas and along major highways as well.

Motorists should anticipate additional urban flooding and travel disruptions in Phoenix, Holbrook, Tucson and Flagstaff, Arizona. Travel can become slow on interstates 8, 10, 17 and 40, and flooding can lead to closures.

As of 11:00 a.m. MST, this October was already the fourth wettest on record in Phoenix with 2.61 inches so far. Tuesday now ranks in the top 10 wettest days ever on record in Phoenix. Records date back to 1895.

It appears that Las Vegas will avoid the heaviest rain from Rosa. However, a couple of showers and a thunderstorm may still develop and bring slick travel into Wednesday.

Download the free AccuWeather app to see when rain is coming to your location and to track the latest advisories for your area.

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Much of Rosa's rainfall will target an area of extreme to exceptional drought in the Four Corners region. The rainfall should be enough to significantly wind back the severity of the drought in much of this area. The rain is likely to help fill area lakes and reservoirs.

Some of Rosa's moisture will be quickly strung out across the Plains and Great Lakes region at midweek. By the time moisture reaches the Midwest, it may be indistinguishable from a non-tropical storm racing across the region.

Because Rosa's winds generated larges swells in prior days, surf and rip currents will likely remain dangerous along the Southern California beaches into Tuesday. Onlookers should avoid standing on jetties and breakwaters as there will be an elevated risk of sneaker waves.

Non-tropical Pacific storm to bring rain to large part of California

A separate storm not associated with Rosa will spin ashore from the Pacific Ocean into Wednesday.

Overall the rain from this feature will be much more gentle than that of Rosa and will help to wet the landscape and lower the risk of wildfire ignition in the short-term.

Static Pacific Non-Tropical Rain California


Enough rain will fall to slow travel from north of San Francisco to south and east of Los Angeles.

"In the swath from San Francisco and Sacramento to Los Angeles this is likely to be the first measurable rain since May," according to AccuWeather Senior Meteorologist Kristina Pydnynowski.

Measurable rain is any rainfall of 0.01 of an inch or greater.

The rain may become intertwined with part of Rosa's moisture that got left behind over the Great Basin on Wednesday and Thursday.

Static SW Wednesday


In addition to the drought and fire benefits, wet roads may create hazards for motorists who are not used to driving in such conditions. Minor flash flooding and debris flows can occur in burn scar areas.

Eastern Pacific to remain active with tropical activity well into October

A relatively new tropical storm by the name of Sergio is forecast to become a power hurricane well off the west coast of Mexico this week.

"Steering winds are forecast to keep the storm at sea this week," Travis said.

"It is possible, if the storm survives, it could be guided toward northwestern Mexico during the middle of October."

In this case, there is the possibility of another dose of heavy rain being drawn into part of the southwestern U.S.

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