Las Vegas Sun

March 29, 2024

Monsoon season over before it starts

This year's monsoon season produced an average rainfall pattern that left National Weather Service meteorologists in Las Vegas high and dry by mid September.

"It wasn't an active season," Weather Service meteorologist Andy Gorelow said.

Compared with 2003, this year's monsoon season -- from July to mid-September -- was tame.

July 2003 was the wettest July on record, since the Weather Service started keeping records in Las Vegas in 1934, with a total of 1.08 inches of rain falling during the month. The total rainfall for August 2003 was 0.83 inches, almost double the normal for the month.

And on Aug. 19, 2003, thunderstorms developed over the northwest part of the city, causing rainfall of more than 2 1/2 inches in only 25 minutes and flooding homes and businesses there. The rest of the valley escaped unscathed.

This year, 0.05 of an inch of rain was recorded in July, well below the average for the month of 0.44 of an inch.

There was no rainfall recorded in August this year at McCarran International Airport, the official weather station where records are kept.

"The airport is probably one of the driest spots in the valley," Gorelow said.

The heaviest monsoon rainfall since the start of the record-keeping occurred in August 1984, when 2.48 inches flooded most of the valley.

"It's hard to flood the whole valley," Gorelow said.

When another powerful storm occurred on July 8, 1999, unofficial rainfall totaled some three inches falling in some parts of Las Vegas in less than an hour when two storm systems merged over the valley. McCarran's gauge recorded 1.05 inches of rain for the same time period.

On Aug. 10, 1997, rains created a flash flood that drenched Henderson and Boulder City, southeast of Las Vegas, causing $8.5 million worth of property damage.

A Henderson man, 58-year-old Harold Dittmer, drowned during that flood, which brought 2.28 inches down on Henderson in an eight-hour period, the National Weather Service said. Boulder City received the most rain in the same eight hours with 3.03 inches.

Each year around the second week in July moisture-ladened winds pushing from the west or southwest across the Southwestern states, including Nevada, shift and come from a southerly direction, signaling the start of the monsoon season.

"It depends on upper wind patterns that bring moisture in from Mexico," weather service meteorologist Charlie Schlott said.

The monsoon season is sparked by a flow of warm, moist air from Mexico, which then collides with a high-pressure ridge over the Rocky Mountains and a low-pressure system over Southern Nevada.

Westerly winds this year pushed most of the monsoon east of the western states.

While the summer's afternoon thunderstorms may be gone for this year, climate watchers recording air and sea temperatures see a warming trend that may portend more intense, flood-producing storms in the Las Vegas Valley in years to come.

The National Center for Atmospheric Research kept a scientific eye on moisture-laden skies from July into September as part of the largest study of the North American monsoon ever attempted.

Data from rain gauges and radar are being combined to help the scientists gather better read the monsoonal flow.

The research may change the way the National Weather Service forecasts the onset of the monsoonal moisture in the southwest.

"A long-term goal of the project is to produce forecasts of the monsoon's onset with perhaps more than a week of lead time," said NCAR's David Gochis, a lead investigator for the North American Monsoon Experiment, or NAME.

The experiment began in July and will take eight years to complete.

David Mitchell of the Desert Research Institute's Division of Atmospheric Sciences in Reno is taking the temperature of the sea's surface in the Gulf of California.

The temperatures in the upper gulf waters can signal a drenching rain may be on the way to Las Vegas. The higher the sea's temperature goes, the more rainfall generally drops in the Southwest.

When sea surface temperatures jump to 90 degrees Fahrenheit or higher in the upper gulf, the Las Vegas Valley is likely to experience monsoonal surges, Mitchell said.

Most of the year, the moisture is bottled up over Mexico, but as the sea's surface heats and the winds shift, nature peels the lid off the monsoonal flow and it moves north, he said.

This year the central Gulf of California's surface temperature had warmed to 90 degrees Fahrenheit on July 11; four days later, the drenching rains began to fall in Utah, New Mexico and Colorado. The rain clouds of the first storm rode the mountain ridges along eastern Nevada, Mitchell said.

Mitchell has been studying sea surface temperatures in the Gulf of California for years.

On Aug. 16, 2003, the sea's surface temperature reached 90.1 degrees Fahrenheit, the highest temperature recorded in the upper gulf since August 1996 when it was a tenth of a degree higher.

"That's like super-hot," Mitchell said.

Three days later, on Aug. 19, the northwest Las Vegas Valley experienced a 100-year storm that flooded homes and businesses.

From information dating back to 1980, it appears that the gulf's waters are warming about 1 1/2 degrees every 20 years.

"That's not very fast to us, but in geological time, that is really fast," Mitchell said.

Scientists are paying attention to sea temperatures in attempts to hone weather forecasting and to look for evidence of global warming.

Unfortunately, Mitchell said, no one can predict precisely where the rains and consequent flooding will occur in any specific location. Shifting winds and changes in temperature determine where storms will go.

"We think we can predict a trend," however, Mitchell said of the research.

The trend from a seasonal event like the southwestern monsoon could help scientists forecast hurricanes and other severe weather in both the Atlantic and Pacific oceans.

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