Summer Prep: Monsoon Season
The North American monsoon, also known as the Southwest monsoon, Mexican monsoon, New Mexican monsoon, or Arizona monsoon, is a weather pattern characterized by a significant increase in thunderstorms and rainfall across large areas of the southwestern United States and northwestern Mexico. Typically occurring between June and mid-September, this seasonal change in atmospheric circulation is driven by the intense summer heat over the continental land mass.
Throughout most of the year, the prevailing wind in northwestern Mexico, Arizona, and New Mexico blows from the west, bringing dry conditions. However, as the summer sun heats up North America, a high-pressure system forms over the US Southwest, causing the wind to shift and bring moisture from the Pacific Ocean and the Gulf of California. This shift in circulation leads to the development of thunderstorms and increased rainfall in the monsoon region, with northwestern Mexico receiving over 75% of its average annual precipitation from the monsoon, and Arizona and New Mexico receiving more than 50% during the period from July to September.
Contrary to the popular notion of monsoons as prolonged periods of heavy rain, the North American monsoon displays unique characteristics. Rainfall follows a distinct diurnal cycle, with predominantly dry mornings, followed by the development of storms throughout the day, and most rainfall occurring in the afternoon and evening. Some of these thunderstorms can be intense, delivering heavy rain and frequent lightning. The monsoon rainfall tends to occur in bursts, with rainy days interspersed with drier periods rather than continuous rainfall. Additionally, the influence of occasional tropical storms originating in the eastern Pacific can further enhance monsoon moisture and rainfall.
The impact of the monsoon extends beyond precipitation levels, as there is a significant relationship between rainfall and temperature. Typically, greater rainfall results in cooler conditions, while lower rainfall leads to hotter temperatures. This season also brings about a range of destructive phenomena, such as flash floods, dust storms, strong winds, lightning, and fires. Research conducted by the University of Arizona indicates that the North American monsoon has historically influenced the duration of wildfire seasons in the US-Mexico border region. In the Southwest, lightning strikes have been responsible for igniting over 2,300 fires annually since 2001, resulting in the burning of approximately 277,000 acres each year.
Preparation for severe weather is key to staying safe and minimizing impacts. Check out these tips from the National Weather Service:
- Be Weather-Ready: Check the forecast regularly to see if you’re at risk for severe weather. Listen to local news or a NOAA Weather Radio to stay informed about severe thunderstorm watches and warnings. Check the Weather-Ready Nation for tips.
- Sign Up for Notifications: Know how your community sends warning. Some communities have outdoor sirens. Others depend on media and smart phones to alert residents to severe storms.
- Create a Communications Plan: Have a family plan that includes an emergency meeting place and related information. Pick a safe room in your home or business, such as a basement, storm cellar, or an interior room on the lowest floor with no windows. Get more ideas for a plan here.
- Practice Your Plan: Conduct a severe thunderstorm drill regularly so everyone knows what to do if a damaging wind or large hail is approaching. Make sure all members of your family or staff know to go there when severe thunderstorm warnings are issued. Don’t forget pets if time allows.
- Prepare Your Home and Business: Keep trees and branches trimmed near your house or business. If you have time before severe weather hits, secure loose objects, close windows and doors, and move any valuable objects inside or under a sturdy structure.
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