Las Vegas Weather: What is the Monsoon?

Introduction

At the most basic level, the monsoon is a wind pattern change. Wait…wind and not rain? Yes and no. More moisture is a direct result of the wind pattern change. For Las Vegas, this can mean sporadic bouts of thunderstorms in the summer months, usually July through September. In this post I will detail how the wind pattern changes, what happens in Las Vegas when the monsoon arrives, and why the Las Vegas monsoon is more sporadic than other parts of the world or even the rest of the Southwest.

Changing Winds

The word monsoon is derived from the Arabic word mausim, which means season. In this case we are talking about a season of rain. During the summer months, the land gets very hot which creates low pressure. To fill the low pressure, wind flows toward it. If the wind comes from a direction where there is water, it will increase the moisture in the area causing thunderstorms and rain. Once thunderstorms produce rain it adds more moisture to the air which can continue the cycle. In the fall, the land begins to cool - breaking the cycle. During the winter months wind will flow toward the water coming from the dry land.

The images above created by the National Weather Service in Tucson, Arizona is an excellent example of the fundamentals of the monsoon. In this case over India. You can see wind flowing south across India from the the dry interior parts of Asia. On the right, hot temperatures in the summer create low pressure over India and moist wind from the Indian Ocean flows toward the low. This is exactly the same pattern in the Southwest US except there is not as much water due to the Mexico landmass. Moisture does make it from the Gulf of California, the Gulf of Mexico, and some from the Pacific Ocean. There are also a lot of mountains that can block some of the moisture. The best way for Las Vegas to see monsoon rain is a direct southerly wind which brings moisture north from the Gulf of California.

The Las Vegas Monsoon

Las Vegas is quite a distance from the moisture that sits just south of Arizona in the summer. For Arizona, it doesn’t take very strong south winds to bring the moisture north due to its close proximity. Thunderstorms are quite common in southern Arizona in the late summer and this makes up the majority of their annual rainfall. It takes a much stronger southerly wind to pull that moisture north all the way to Las Vegas. Eventually it does happen but it’s usually later and weaker than Arizona. The monsoon does not make up the majority of annual rainfall contrary to popular belief. Las Vegas is far enough north that winter storms coming in from the Pacific Ocean tend to drop more rainfall. The likely reason people mistake the monsoon for the majority of annual rainfall has to due with the intense but short thunderstorms. It can seem like a crazy amount of rain because of the intensity and often flooding, but monsoon storms do not occur every day. If they occurred more regularly, Las Vegas would be more like Arizona. The other reason is the word ‘monsoon’ makes it sound worse than it is. People tend to equate monsoon with lots and lots of rain.

Las Vegas weather July 2023. Monsoon conditions started around July 17th.

In the chart above you can see it was very hot throughout July last year peaking at 112F/44C for three days in a row. Then on the 17th, suddenly there was rain nearly every day. This is an excellent example of the monsoon kicking in. And what it notable is the amount of rain that fell. The most was 3/4 of inch on the last day of the month but most days had pretty light precipitation. This is typical of the Las Vegas monsoon where it rains hard briefly but we’re not talking about inches of rain at a time typically. The air can still be pretty dry and much of the rain evaporates before hitting the ground. Occasionally, a thunderstorm can produces inches of rain if they are extra wet or slow moving. Of the top ten wettest days (below) 8 out of 10 were due to a monsoon thunderstorm.

Monsoon Safety

Visitors and residents of Las Vegas seem to forget how dangerous it is when there is heavy rain. Due to the urban concrete and the poor desert soil, whatever water falls out of the sky tends to just run off…FAST. Just 6 inches of fast flowing water is enough to knock over and adult and just 12 inches can carry a car away. People often see water flowing over the road and think it will be no problem to just drive through. They don’t realize under the water it may be deep because the road washed away or it is designed to be deeper (a wash). 26 people have died in the Las Vegas Valley due to flash flooding since 1960 according to the Regional Flood Control District.

Here are some safety tips when flash flooding is possible

Stay Informed: Pay attention to weather forecasts, especially during the monsoon season. Stay updated on any flash flood watches or warnings issued by the National Weather Service.
Know Your Risk: Understand if you live in a flood-prone area or if you're traveling through one. Be aware of low-lying areas, canyons, washes, and other regions susceptible to flash flooding.
Have an Emergency Plan: Create and discuss a family emergency plan that includes what to do in case of a flash flood. Establish meeting points and evacuation routes.
Avoid Flooded Areas: Never walk, swim, or drive through flooded areas, especially if you can't see the bottom.
Turn Around, Don't Drown: If you encounter a flooded road while driving, turn around and find an alternate route. It's not worth risking your life or your vehicle's safety.
Monitor Water Levels: Keep an eye on water levels in nearby streams, rivers, and drainage channels. Be prepared to evacuate if water levels rise rapidly.
Be Cautious at Campsites: If camping near waterways, be aware of the potential for flash flooding, especially during heavy rain. Move to higher ground if flooding occurs.
Heed Warnings: Take flash flood warnings seriously. If authorities advise evacuation, follow their instructions promptly.
Stay Away from Dry Washes: Dry washes can quickly become raging torrents during flash floods. Avoid camping or parking in these areas during the monsoon season.
Prepare an Emergency Kit: Have an emergency kit ready with essential supplies, including water, food, first aid supplies, flashlights, batteries, and a portable weather radio.

This flash flood northeast of Las Vegas on Interstate 15 killed two people. This happened right before I moved to Las Vegas so I was very interested to understand what exactly happened. Not only was it a tremendous amount of rain in a short period of time, but there was only a small pipe under the freeway which quickly became overwhelmed. This lead to the water flowing over the freeway sweeping cars off. People continued to try and drive through it at their peril.

Conclusion

In the summer when Vegas heats up, it invites moist air from the south into the region causing the monsoon. The Las Vegas monsoon is weaker and more sporadic than even Arizona just to the south due to its distance from this moist air. Once it does happen, localized flash flooding is often a result. While it may seem like this must be the majority of the rain every year, there is actually more in the winter. When it happens, avoid flooded roads at all costs or you are betting against the house - mother nature.