Flooding In Las Vegas Casinos?

A high school classmate sent me a video of clouds in her backyard. Because I am a meteorologist, she knew that I would be fascinated by them. It was a manifestation of the North American Monsoon. An active and wet monsoon season was expected, and those forecasts have been accurate. In the past few days, a casino flooded in Las Vegas, and there have been flood-related fatalities in the region too. Many are calling it one of the worst monsoons in a decade. What is the North American Monsoon anyway?

Before I dig into that question, let’s review the term “monsoon” because it is one of the most misused weather terms by the public. I often hear people say things like, “It is a monsoon out there” when describing very rainy conditions. Technically, the term monsoon is more about wind than rain. The American Meteorological Society (AMS) Glossary defines the term as, “A name for seasonal winds.” It is derived from the Arabic word “mausim.” Monsoons are generally caused by temperature and atmospheric pressure variability associated with how landmasses and large bodies of water warm (or cool) through the seasons. The Indian or Asian Monsoon is very well-known, but we also have one that affects the U.S. too.

The National Oceanic and Atmospheric Administration (NOAA) climate website describes the North American Monsoon this way – “As the summer heat builds over North America, a region of high pressure forms over the U.S. Southwest, and the wind becomes more southerly, bringing moisture from the Pacific Ocean and the Gulf of California. This meteorological pattern is conducive to thunderstorms and rainfall in is responsible for most of the yearly rainfall total (50 to 75%) in places like Arizona, New Mexico, and northwestern Mexico. The NASA satellite-derived precipitation (see below) reveals some of the North American Monsoon rainfall activity from the past week. By the way, this data comes from an algorithm associated with the Global Precipitation Measurement (GPM) mission, which I was Deputy Project Scientist for during my tenure at NASA Goddard Space Flight Center.

The North American Monsoon typically starts around June to July and ends around September as winds shift to a more westerly pattern. The monsoon season is a vital source of rainfall in the region and studies suggest that they are critical in extinguishing wildfires, which ironically may be caused by lighting in “dry” monsoon thunderstorms. This year the monsoon has provided some relief from historic levels of drought in parts of the American West.

We have been in the midst of a “triple-dip” La Niña pattern. La Niña features cooler than normal temperatures in the eastern Pacific that can affect weather patterns around the world. El Niño is the warm temperature “brother.” Both 2020 and 2021 were La Niña years and 2022 is exhibiting La Niña characteristics as well. Such a “triple-dip” La Niña has been quite rare in the past 75 years. I bring this up because some scientific studies argue La Nina may be associated with “wetter” North American Monsoon seasons whereas other studies are not as conclusive. It is worth noting that the 2021 North American Monsoon ranked among the top 10 wettest according to NOAA.

While rains associated with the monsoon can be beneficial, there are often fatalities, mosquito outbreaks, and other hazards associated with the North American Monsoon. For more information on it, check out this video produced by the National Weather Service.