Monsoon Moisture Returns to the Southwest, Increasing Storm and Flash Flood Potential

After weeks of hot and mostly dry weather, the Southwest is beginning its transition into the annual North American monsoon season. A shift in the overall weather pattern is allowing deeper moisture to move north from the Pacific and Gulf, setting the stage for a daily chance of showers and thunderstorms across parts of Arizona, New Mexico, southern Utah, southern Colorado, and Nevada.

The North American monsoon is a seasonal wind shift that occurs each summer, transporting moisture from the eastern Pacific Ocean and Gulf of California into the Desert Southwest. As the moist air combines with intense daytime heating and mountainous terrain, thunderstorms become much more common during the afternoon and evening hours.

The first surge of monsoonal moisture is expected to increase storm coverage through the coming days. While many of these storms will produce beneficial rainfall for areas experiencing drought, they will also be capable of dropping heavy rain over a short period of time. Because the Southwest’s dry soils do not absorb water efficiently, runoff can develop quickly, creating a heightened risk of flash flooding in washes, slot canyons and other low-lying locations.

Recently burned areas will be particularly susceptible to flooding and debris flows. Vegetation lost to wildfires leaves hillsides unable to absorb rainfall, allowing water, mud and rocks to move rapidly downhill during intense thunderstorms.

In addition to heavy rainfall, some storms may generate strong, gusty outflow winds that kick up blowing dust and sharply reduce visibility along area highways. Frequent lightning will also accompany many thunderstorms, posing a danger to hikers and anyone spending time outdoors. Lightning can also spark new wildfires in areas that receive little rainfall from passing storms.

The arrival of the monsoon may provide some relief from the persistent heat that has gripped the region in recent weeks. Increased cloud cover and scattered thunderstorms can help lower daytime temperatures, although conditions will remain warm and humid compared to the typically dry desert climate.

As the monsoon continues to strengthen through July, residents and visitors across the Southwest should be prepared for rapidly changing weather conditions each afternoon. Even storms that last only a short time can produce dangerous flash flooding, making it important to avoid flooded roadways and never enter fast-moving water.