Las Vegas Sun

September 30, 2014

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Sure, it’s monsoon season, but what does that mean?

Image

Steve Marcus

An abandoned car sits in the westbound lanes of Craig Road after a powerful thunderstorm dumped three inches of rain in about 90 minutes in the northwest section of the city Wednesday, August 19, 2003. The torrential downpour caused flash flooding that swamped neighborhoods and trapped people in their cars.

Flash Floods

Pedestrians wade across Craig Road near U.S. 95  after a powerful thunderstorm dumped three inches of rain in about 90 minutes in the northwest section of the city Wednesday, August 19, 2003. The torrential downpour caused flash flooding that swamped neighborhoods and trapped people in their cars. Launch slideshow »

Lighting Storms

Lightning strikes behind the Luxor, left, and Excalibur hotel-casinos as a thunderstorm passes through Las Vegas early in the morning of September 11, 2011. Launch slideshow »

Las Vegas Monsoon Season Storms

Storms were moving from the south into the Las Vegas Valley. This view looks toward the south-southeast from the 16th floor of the Clark County Regional Justice Center in downtown Las Vegas. Launch slideshow »

With more thunderstorm activity expected to move into our region today, this is an appropriate time to reprise an explanation of the monsoon season that delivers high humidity — at least by our standards — to Las Vegas every summer.

Let’s get it right. A monsoon is not a storm. It’s a season.

This is the plea that Michael Staudenmaier has made to Las Vegas’ television weathermen. Staudenmaier is the meteorologist in charge of the local National Weather Service office, a concrete-block bunker, the interior of which is a cross between an insurance office and NORAD missile control. The office, like its counterparts across the country, provides most of the serious weather data and forecasting for the valley — measurements, analysis, 24-hour monitoring. When your local weatherman speaks in front of a mind-bogglingly lush and expensive display of computer graphics, chances are that he’s hewing to the weather service’s line.

And during a friendly little meeting complete with home-baked cookies, Staudenmaier asks the weathermen to please not addle the public mind by issuing “monsoon alerts” for every summer storm. Because that’s not what a monsoon is. It’s a seasonal wind shift.

Here’s how it works.

Starting in late May or early June, the sun warms up the deserts in northwestern Mexico and the southwestern United States. The air gets so hot and excited that it can’t wait to go share its energy with the cold upper atmosphere. This rising air lowers the air pressure above the desert, creating a bit of a vacuum, especially compared with the colder air over the ocean (the ocean does not get nearly so excited about the summer).

Because nature always balances its forces, the cold high-pressure air from the ocean wants to blow into the desert. When it comes, it brings water with it.

But there’s a problem: At the same time all of this is going on, a ridge of high-pressure air sets up shop in Mexico. In June, this ridge mostly blocks moisture from flowing north from the Gulf of California and Gulf of Mexico.

By mid-July, however, this ridge migrates north into the United States, where it bums around New Mexico and the four corners area. The farther north it gets, the more it allows moist air out of the gulfs into Southern Nevada — hence the humidity that sets your air conditioner intermittently wheezing through the end of September.

Now, when that air of the gulfs is especially moist, it’ll bump up against our mountains and condense into thunderheads as improbably high and menacing as the mold in a bachelor’s jar of olives.

In the late afternoon or early evening, the now-saturated clouds burst: lightning, sudden downpours, howling winds and sometimes hail. Roads flood, cars crash, and every now and then, a person is swept down a wash and drowns.

On the upside, it cools Las Vegas to slightly below the surface temperature of Mercury.

­— Brendan Buhler / June 5, 2009

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