Flash Flood Watches Up For The American Southwest As Hurricane Rosa Approaches Mexico

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Infrared satellite loop of Hurricane Rosa on September 29, 2018.NOAA

Tropical storm warnings are in effect for parts of Mexico’s Baja Peninsula ahead of Hurricane Rosa. The storm, which is accelerating toward the northeast, is expected to come ashore on Monday and race inland toward the United States early next week. The storm will bring heavy rain to the southwestern states, potentially leading to flash flooding in desert and mountains areas.

Hurricane Rosa was once a powerful storm, one of many such storms in the eastern Pacific this year that exceeded expectations and took advantage of a favorable environment to reach its full potential. Rosa exploded into a picture-perfect category four hurricane earlier this week, briefly achieving maximum sustained winds of 145 MPH before it began to weaken.

The storm’s power was ultimately short-lived. Rosa made a hard-right turn on Friday and began accelerating into a less-favorable environment. Despite its increasing raggedness, Rosa still has a formidable appearance on satellite imagery this weekend. Forecasters expect Rosa to weaken to tropical storm status as it makes landfall on the Baja Peninsula on Monday, and it should be a depression by the time it reaches mainland Mexico later the same day.

Forecast precipitation between September 29 and October 6, 2018.Dennis Mersereau

While Rosa will lose its tropical characteristics as it moves inland, its moisture will remain in place. Heavy showers and thunderstorms over the desert southwest early this week will bring the potential for flash flooding across much of Arizona and neighboring states. Gusty winds in the southern part of Arizona could also knock down some trees and lead to some scattered power outages, but wind shouldn’t be as big of a hazard as heavy rain.

The Weather Prediction Center’s forecast on Saturday evening showed the potential for a widespread area of the southwest and Intermountain West to see more than an inch of rain. Some higher elevations in central Arizona could see several inches of rain from the storm. Not everyone will see rain early next week, but the ingredients will be there for storms to form and tap into a relatively deep reserve of moisture. The heaviest rain will fall in areas that see thunderstorm training, or storms that repeatedly move over the same areas.

That might not seem like much rain in the grand scheme of things, but the soil that makes up the desert ground in the southwest causes most rain to simply run off rather than soak into the earth. This leads to a situation where heavy rain can easily lead to flash flooding. Flash flood watches are in effect for early next week from northern Utah to the U.S./Mexico border in California and Arizona. This flash flood watch includes the cities of Phoenix, Tucson, Flagstaff, Las Vegas, and Salt Lake City.

Despite its hot and arid climate, flash floods are common enough in the desert during monsoon season that the region knows how to prepare for and deal with them. Arizona even has a law on the books—nicknamed the “Stupid Motorist Law”—as a way to deter people from putting themselves in hazardous situations. The law gives authorities in the state the option to pursue charges that allow agencies to recoup the cost of the motorist’s own rescue if they drive across a flooded roadway and get stuck.

The easiest way to prevent this—and something that should go without saying—is not to drive into a flood in the first place. It’s terrifyingly easy to misjudge the depth and speed of water flowing across a road. More than half of all flooding deaths in the United States occur in vehicles, and it can take less than a foot of swiftly-moving water to sweep a car downstream. It’s not worth your life, and it’s not fair to unnecessarily risk the lives of the rescuers who have to retrieve people swept away by rising waters.

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