Monsoonal Rain Spread Flooding Risks Across the West

The North American monsoon will be in full swing this week, drenching the Mountain West with plenty of rain and bringing concerns for flooding.

In the late summer months, a strong ridge of high pressure sets up over the Four Corner states. Winds within a high pressure system spin clockwise, which sets the stage for a large-scale, long duration transportation of moisture from the Pacific Ocean and Gulf. The two surges of moisture merge over the Southwest, leading to heavy rain, flooding and dangerous thunderstorms. This is known as the North American, or Southwest, monsoon.

This monsoon pattern has finally set up and has already been transferring a significant amount of moisture into the Mountain West. It was also responsible for the gusty thunderstorms that produced a potent haboob, or dust storm, that moved through the Phoenix metro area on Monday.

Monsoonal rain and thunderstorms will remain widespread throughout the Southwest and Mountain West during the second half of the work week. While possible throughout the day, the best chance for rain and thunderstorms will occur in the afternoon and evening. The heaviest rainfall amounts will occur in northern Utah, eastern Idaho, southwestern Montana, most of Wyoming, and eastern Colorado. Here, rainfall amounts of 1 to 3 inches will be common, with locally higher amounts up to 4 inches are possible.

Excessive rainfall may result in flash flooding, especially on recent burn scars. Minor flooding of streams, creeks, urban areas and low-lying areas. Flood and Flash Flood Watches are scattered throughout the Great Basin and Rockies.

As if the flooding rains wasn’t enough, gusty winds will be likely within thunderstorms. Areas that pick up less rainfall will be at risk for “dry thunderstorms.” Dry thunderstorms are usually defined as thunderstorms that produce little to no rainfall. A common risk with gusty “dry thunderstorms” is the risk for wildfires to begin and spread rapidly. If you are outdoors, make sure to take precautions to prevent the development and spread of wildfires.

The rain and thunderstorms will slowly shift east this weekend, with the heaviest rainfall occurring along the Front Range and central/southern High Plains. Meanwhile, drier air filters into the Great Basin and the Mountain West.

Backcountry hikers and those headed through rugged terrain should pay close attention to the weather. Runoff from heavy thunderstorm rain could result in debris flows, mudslides, and rapid flooding of canyons, normally dry washes, slickrock areas and burn scars.

If you are out and about, be sure to keep an eye to the sky and look out for rapidly changing cloud coverage. If you encounter flooded roadways, remember to “Turn Around, Don’t Drown!” In some cases, the thunderstorm that leads to flooding at your location could be many miles away.