Forecasters warn of possible flash flood: Similar conditions led to 1999 deluge

Friday, July 6, 2001 | 11:06 a.m.

A flash flood watch was in effect for Southern Nevada today until 9 p.m., National Weather Service meteorologists said, because of two streams of air ladened with moisture merging over the Southwest.

Light showers fell in the deserts and mountains south and west of the Las Vegas Valley this morning, but forecasters said unusual conditions brought the typical monsoon moisture from Mexico together with a wet wave from the Pacific Ocean.

Weather forecasters extended the flood watch to northwestern Arizona and Southern California this morning, predicting up to 3 inches of rainfall from afternoon and evening thunderstorms.

Today's weather is a reminder of a 100-year flash flood from an intense summer storm on July 8, 1999, resulting in roughly $20.5 million in public property damage and two deaths.

The average rainfall in Las Vegas, measured by the weather service, is about 4 inches a year. So far this year the rainfall has totaled 3.30 inches. In 1999 the total measured at McCarran International Airport -- the official weather station -- was 3.73 inches.

So why did it flood in 1999? The Clark County Regional Flood Control District's General Manager Gale Fraser noted that 1.25 inches of rain fell in a two-hour period that day. That's nearly one third of the average annual rainfall, he said.

A 100-year flood can happen in any year. There's a 1 percent chance at any given time, flood control district spokeswoman Betty Hollister said.

The weather service issued its flash flood watch for today to alert drivers, pilots and those who might enjoy outdoor activities with temperatures reaching the high 90s today to be aware of a sudden change once the sun heats the clouds. That's when thunderstorms can create downpours that could flood surrounding mountains and urban streets.

"It's better to err on the side of caution," Hollister said.

Although the flood control district has built 57 detention basins and 270 miles of channels and underground storm drains since 1989 -- nine of them in the past two years -- the valley can still flood depending on how intense the storm becomes and where it falls, she said.

With a $1.1 million bridge completed in June at Duck Creek and Robindale Road, 29 acres of homes are protected from a flood similar to the 1999 flood.

Another critical project on Flamingo Wash is a bridge that protects Boulder Highway where the wash crosses. The entire project will cost $7.7 million and be completed in June 2002 and will protect the Miracle Mile Mobile Home Park, whose residents watched as their homes floated away in 1999.

Another project at Spencer Street and the Flamingo Wash is a bridge and upstream channel that will begin construction in January 2002.

The flood control district with the city of Las Vegas also will capture water in five miles of underground storm drains under construction between Sahara Avenue and Alta Drive to keep floodwaters from ever entering the Charleston Underpass, notorious for catching motorists in its rising waters.

That $36 million project will not only protect Charleston Boulevard, but also businesses in downtown Las Vegas.

The Charleston Underpass project has about a year to go before it is completed, so motorists in the meantime need to be cautious this year, because the system is not completed and the underpass can still flood, Hollister said.