Technical Update vol. 25 no. 27 - Monsoon Season

Did you know that Colorado has a monsoon season? The frequent mid-to-late afternoon thunderstorms Colorado experiences during July, August, and September are part of the North American Monsoon (or NAM) weather system. This shift in wind patterns allows continuous moisture from the Gulf of California to blow across the normally arid southwest. The monsoons occur in Arizona, New Mexico, and the four corners region, which includes Colorado. Depending on the strength of the winds, the monsoon can stretch across a large portion of our state, bringing lightning, flash flooding, and hail.

Lightning

Colorado ranked number four in the country for lightning fatalities from 2008 to 2018, even though it is number 18 in the country for overall lightning. Most of these deaths occur in July during the monsoon season. Sudden summer storms can catch people working or engaging in outdoor activities by surprise. Lightning can strike up to 60 miles away from the nearest rainfall. If you can hear thunder, you are within striking distance of lightning and should seek shelter.

The National Park Service recommends that people do the following if they are caught outside during a storm:

• Seek shelter in a building with walls, a roof, and a floor. A hard-top vehicle with all doors closed and windows completely closed can also be an appropriate shelter. Do not go back outside for at least 30 minutes after hearing the last thunderclap.
• If in a group and no shelter is available, spread out at least 50 feet apart to minimize the chance of everyone being struck.
•  If caught outside, adopt the lightning position – crouch down on the balls of your feet, keeping them as close together as possible. Don’t allow other body parts to touch the ground and cover your ears.

Flash Flooding

A flash flood is a dangerous, sudden rise of water along a river or creek, in a canyon, or over normally dry land. The climate and terrain in Colorado make the state susceptible to flash flooding. Wildfires and droughts can harden soil, making it difficult for rain to soak into the ground. At the same time, rugged terrain and changes in elevation can cause storms to stall over certain areas, with disastrous results such as the Big Thompson Flash Flood of 1976, which killed 143 people.

The majority of flash flooding fatalities occur in vehicles. It only takes a foot or two of swiftly moving water to carry most vehicles away. Flash flooding can also wash away roads and bridges, which is why you should never drive through floodwaters.

What This Means for Counties

 County employees working outside during monsoon season should pay attention to the weather forecast and be prepared to take shelter or move to higher ground should they be caught in a sudden summer storm. For more information about severe weather preparedness, contact CTSI at 303 861 0507.

A PDF of this Technical Update is available here.