OPINION: A primer on New Mexico monsoon season

Why is the North American Monsoon (NAM) important to New Mexico?

First: Roughly 30-40% (more in the south and less in the northwest) of the annual average precipitation across New Mexico falls during the heart of monsoon season — July through August, or 17% of the year; the National Oceanic and Atmospheric Administration has defined the NAM season from mid-June through mid-September, or about quarter of a year.

Second: Water is a precious resource across New Mexico and a drier monsoon season puts strains on water resources across the state and facilitates/worsens droughts.

Third: Monsoonal moisture fluxes also increase cloud cover and help keep temperatures from getting well above normal — years with below average monsoon moisture directly correlates to significant heat waves that can lead to hefty societal impacts. Heat-related death and hospitalizations are increasing with an increasingly warmer climate and the absent/dry monsoon can exacerbate these impacts.

Fourth: The outcome of the NAM season also significantly impacts the agricultural community and the availability of water to the general public.

Fifth: Fire danger and risk for large destructive wildfires are typically mitigated by the NAM. Inversely, monsoon seasons with well above average precipitation lead to increase instances of flash flooding, which can threaten human life and infrastructure.

What is the NAM?

Simply put, a monsoon is a seasonal change in wind patterns across a region, typically bringing an increase in atmospheric moisture — in the specific case of the NAM an increase in moisture to the Desert Southwest.

Each summer, a strong ridge of high pressure develops in the general Four Corners/Desert Southwest region typically shutting off stronger/drier springtime westerly flow aloft. This ridge of high pressure typically begins developing in northwest Mexico in May and begins to expand and shift north during the summer months; typically centering near/over New Mexico in July and August. As this strong ridge of high pressure builds over New Mexico and the general region it can open the door to atmospheric moisture transport from two separate sources regions:

• The eastern tropical Pacific and the Baja of California
• The Gulf of Mexico.

The first source region provides the dominate moisture source for Arizona and far western New Mexico — roughly west of the continental divide. The second source region is the remainder of New Mexico and parts of southern Colorado — east of the continental divide.

This seasonal increase in moisture, strong daytime heating, and occasionally embedded disturbances/shortwaves combine together to create a sufficiently unstable atmosphere and the result is rain showers and thunderstorms. Mountain ranges across the region/state typically help initiate the first updrafts that develop rain showers and thunderstorms (provided sufficient atmospheric moisture is present) during the late morning and early afternoon hours, then showers and thunderstorms can potentially drift or initiate off the mountains into surrounding valley locations during the mid-afternoon and evening hours. This allows for a higher frequency in rain showers and thunderstorms and more rainfall over the mountains, with showers/thunderstorms and rainfall typically decreasing the further you move away from the mountains. Fairly light winds aloft under the ridge of high pressure allow for slow moving and pulsating thunderstorms that can lead to flash flooding and localized heavy rainfall. Additionally, the presence of embedded disturbances/shortwaves will lead to greater coverage of rain shower/thunderstorm activity and can lead to an increased chance for nocturnal rain showers/thunderstorms.

Specific thunderstorms on a given day are not technically a monsoon, but are a potential outcome from the increase in atmospheric moisture directly related to monsoonal moisture influxes.