Monsoon season spawns deluges

Sunday, June 18, 2000 | 10:16 a.m.

The season officially opens in mid-July but you'll know it has really started when you see towering cumulonimbus clouds with dark, flat bases boiling up over the Las Vegas Valley.

What you'll be seeing is a clear and sometimes ominous sign of the worst weather Mother Nature annually flings at Southern Nevada -- the Southwest monsoon season.

July 8, 1999, was a perfect example.

One minute it was broiling hot, the next it was a boiling flood that killed one man and caused $25 million in property damages. And that's not even the worst monsoon flood on record, just the most recent.

Blame it on a unique convergence of moisture-ridden streams of air flowing inland from the Gulf of California and the Gulf of Mexico running head-on into a wall of hot, dry desert air.

The results of that climatic collision can be spectacular and devastating.

Southwest monsoonal storms are usually accompanied by intense lightning. During last July's monsoonal storm, the National Weather Service's sophisticated equipment that monitors lightning strikes couldn't keep up with the electrical barrage.

But it's the gully-washing rainfalls that do the damage. Last July's storm dumped 1.29 inches on the valley in less than two hours.

And that was hardly a record.

On June 13, 1955, a mammoth "supercell" storm system covering 500 square miles of the Southwest pounded Las Vegas with 3 inches of rain in about an hour, causing the city's worst flooding since official record-keeping started in 1937.

Recent major floods also occurred in the Las Vegas Valley in 1975 when 300 cars were washed away from Caesars Palace and in 1984 when streets ran with floodwaters for weeks.

But even those deluges pale in comparison to other monsoonal storms that have doused the desert around Las Vegas.

National Weather Service forecaster Ron McQueen says two back-to-back storm systems in August 1981 unleashed between 6 and 7 inches of rain in less than three hours on Moapa Valley, about 45 miles northeast of Las Vegas. No one was killed in the ensuing flood but 300 cows drowned.

The summer monsoons can have deadly consequences for people caught in the wrong place. A severe thunderstorm on Aug. 12, 1997, dumped heavy rain near Page, Ariz., and sent a 50-foot-high wall of floodwater thundering down Antelope Canyon, killing 11 hikers who were touring the narrow canyon that drains into the Colorado River.

The word monsoon derives from the Arabic word, "mausim" meaning "season" or "wind shift." The term first applied to winds off the Arabian Sea that blow for six months from the northeast and for six months from the southwest.

The Southwest monsoon season normally lasts from July through September, McQueen said. The weather system typically affects Nevada, Arizona, Utah and New Mexico, but the thunderheads can travel as far north as Idaho and Wyoming, he said.

Weather patterns

Weather experts predict the monsoon season is going to be even more severe in coming decades.

The American Southwest and the Las Vegas Valley could become much wetter, if climate patterns change as rapidly as predicted by the National Assessment of Climate Study released last week.

The study, ordered by Congress in 1990, predicts major changes in weather patterns in the next 50 to 100 years, including more rain and flooding in the Southwest in winter months as well as the summer months from July through September.

For the Clark County Regional Flood Control District, keeping up with the growing population has become more than a full-time job even without a climate change to contend with.

While it cost more than $20 million to repair public projects damaged last summer, the control system put in place over the past 10 years did save an estimated $100 million in damages, officials say.

The Regional Flood Control District Board has approved a 10-year construction program this month for about $455 million to update the plan paid for by a quarter-cent sales tax collected since May 1987.

The district has spent $507 million since 1987 to build basins and channels. It needs another $1.1 billion to control floods throughout the county, Director Gale Fraser said.

And another $100.5 million has been spent since last summer on eight major projects in the valley, such as the Gowan South Detention Basin at the southwest corner of Cheyenne Avenue and Tenaya Way, he said.

That basin has been expanded to prepare for this summer's storms.

Flooding is still fresh in the minds of some Las Vegas Valley residents who got first-hand looks at the awesome power of a flashflood during last year's storm.

Mountain View Hospital President and CEO Mark Howard got a bird's eye view of the waters rising in the basin from the unfinished roof of the hospital's north tower.

"I remember walking up on top of the hospital and saying, 'Wow! That's full,' " Howard said of the basin.

Then it started to overflow.

"We had to move some mothers and babies into ICU (intensive care unit), but it was only for a day," Howard said.

He hates to think what would have happened if the first half of the Gowan basin had not been in place.

The speed in which that basin filled is a testament to how much floodwater a monsoon storm can generate. Regional Control District spokeswoman Betty Hollister said the Gowan North Detention Basin filled with 12 feet of water in a mere 15 minutes.

Flood powers

The storm had the same effect on streets and roadways in the valley. In less than 12 hours that day, Metro Police received 150 emergency calls for help from stranded motorists and people who tried to cross flooded streets and drainages.

Metro Search and Rescue Supervisor Bill Cassell said people underestimate the power of a flood.

Two feet of water moving down a local street creates 42 pounds of pressure against each leg, he said.

"When you stop and do the math and physics, you are not going to win. People do not understand the awesome power of water, the hazard they put their families in as well as the contamination in the water," he said.

Flood basins are also doubly dangerous, Cassell said, because not only do they fill rapidly, but a drain at the downstream end sucks the water out and across the valley into the Las Vegas Wash.

Those runoff waters contain bacteria, oil, brake fluids, fertilizers, pesticides and worse. "Everything from particulates to Pontiacs go into those basins," Cassell said. "It's a public health issue."

In the meantime, Cassell promises that when the next monsoon storm rolls over Las Vegas, the helicopters will be warmed up and ready to go when the inevitable calls come for rescuers to pluck people out of harm's way.