Flash Flood Threat in the Southwest; Phoenix, Las Vegas at Risk

By By Alex Sosnowski, expert senior meteorologist.
August 27, 2013, 5:45:56 AM EDT

The combination of the ongoing monsoon and moisture from Tropical Storm Ivo over the eastern Pacific will bring heavy rainfall to portions of the Southwest Sunday and Monday.

The weather pattern has the potential to bring drought-busting rain to some locations but also packs the risk of urban flooding and a flash flooding disaster.

Tropical Depression Nine formed on cue Thursday morning and is projected to drift northward near the coast of Baja California, Mexico, this weekend. From that position, the system would be able to pump a great deal of moisture northward. As forecast, T.D. Nine became Tropical Storm Ivo Friday morning.


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Initially, northwestern Mexico and part of Southern California to southern Nevada, Arizona and New Mexico will be targeted with the downpours.


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However, the rainfall is likely to spread northward into part of the Sierra Nevada, northern Nevada, much of Utah and the Colorado Rockies, even though the center of circulation from the storm may dissolve along the Baja California coast.

A couple of inches of rain could fall over a few hours, which is more than enough to cause dry stream beds to turn into raging rivers and overwhelm storm drains in towns and cities.

Motorists should be prepared for not only rapidly changing weather conditions but also hazards on the roads. Downpours miles away can lead to rapid flooding and mudslides.

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How much, if any rain reaches areas from San Diego to Los Angeles and Sacramento is questionable. However, areas from near Palm Springs, Calif., to Las Vegas, Phoenix, Flagstaff, Ariz., and Salt Lake City are more likely be hit with torrential downpours.


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The rain and higher humidity will lower the risk of wildfires for a time in the Southwest.

However, the bulk of the drenching rain is forecast to stop short of or diminish over the area where massive wildfires are burning in portions of Idaho, Oregon and northern California.

The rain is also forecast to hold up west of much of Texas and the southern High Plains.

There is a chance of a second tropical system coming northward, possibly spreading more rain over part of the Southwest U.S. and Northwest Mexico during the Labor Day Weekend.

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