Tuesday, July 13, 2004 | 9:03 a.m.
As the Southwest's monsoonal moisture moves into Southern Nevada this week, a flood control official urged people caught in an afternoon thunderstorm to stay put during the downpour.
Floodwaters can rise at a rate of one foot per minute, Regional Flood Control District spokeswoman Betty Hollister said. In the time it takes a traffic light to change on a Las Vegas street, floodwaters can rise as much as three feet during a summer storm.
And in the heat of a summer's afternoon at rush hour, traffic can tie up quickly, a problem intensified by floods, Hollister said.
The Regional Flood Control District is asking motorists to sit out the storm in a safe, comfortable place, rather than contribute to traffic problems.
"Read the newspaper and drink a latte while you wait out the storm," Hollister said. "Why endanger your car's safety, and most important, your life?"
The National Weather Service forecasts the heat and the humidity to combine in the right mix by Wednesday afternoon to give Southern Nevada a chance of thunderstorms.
The heat is definitely in place. Monday's high temperature of 109 degrees recorded at McCarran International Airport is the hottest so far this year, National Weather Service meteorologist Larry Jensen said.
The second-hottest day this year was July 6 at 108 degrees.
There's no way of knowing how much rain the expected monsoon might bring, or if it will rain in the urban areas once the monsoon arrives here, Jensen said.
"We don't know if there's going to be a thunderstorm in the valley," Jensen said on Monday.
Last year on Aug. 19 the northwest valley was pounded with a storm that damaged homes and triggered dozens of rescues as motorists were trapped in rushing floodwaters.
The surge of moisture from Mexico is already pushing into Arizona, weather service forecaster Andrew Gorelow said.
"This is the early part of it," Gorelow said. Southwestern monsoons generally run from mid July until mid September.
Computer models indicate that the best chance for thunderstorms in Southern Nevada will be on Wednesday and Thursday, with a chance of isolated storms through Saturday.
In the Southwest, the winds are subtle and hard to detect when they change, so forecasters watch the humidity levels, Gorelow said.
"I'd take 110 degrees with low relative humidity, rather and 100 degrees and high humidity," Gorelow said.
In Tucson, when the dew point averages 54 degrees for three consecutive days, the monsoon has started. This year Tucson entered the monsoon season on July 8. The dew point is a number that measures the absolute amount of water vapor in the air.
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