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Monsoon Season Officially Kicks Off Next Week in the Southwest, but It's Typically Slow to Start

The Weather Channel logoThe Weather Channel 6/10/2021 Chris Dolce and Linda Lam

The Southwest monsoon season officially kicks off next week, but it's typically slow to start for this region that's in dire need of rainfall in the upcoming summer.

Monsoon season starts June 15 in the southwestern U.S. and lasts through Sept. 30, according to NOAA. However, in most of the region, it doesn't really kick in until late June or early July.

The monsoon is a seasonal change in winds, which helps to draw moisture into the Southwest. This can fuel the development of showers and thunderstorms in the region, with the coverage of those storms varying day-to-day or week-to-week.

Most of northwestern Mexico and the southwestern U.S. receive over half of their annual precipitation from the monsoon.

But not every year is equal, and the monsoon can produce different results each summer.

Last year's monsoon was the driest on record for parts of the Southwest, sending the region into a serious drought that has persisted heading into this summer. The 2019 monsoon did not fare much better and ranked ninth-driest on record.

This year's forecast calls for equal chances of the monsoon producing near, above or below average rainfall, according to the National Weather Service. That's because there is no clear signal for how the monsoon pattern might set up this summer.

How the Monsoon Develops

Hot and dry conditions, with low humidity, prevail in June before the monsoon really gets going.

Fire danger is high due to these conditions and heat can be oppressive, particularly in the deserts of Arizona, southeastern California and southern Nevada.

The seasonal wind shift happens gradually through summer when a thermal low develops due to intense heating of the land, but large bodies of water nearby do not warm as quickly. In addition, a ridge of high pressure builds over the Rockies or Plains.

Eventually, pressure differences between the warm land and cooler water cause more humid air from the gulfs of California and Mexico to be drawn toward the Southwest. This flow from moist ocean waters is a change from the usual flow from land areas to the ocean waters.

The result is thunderstorm development.

Rain from these storms causes humidity to increase, which triggers more storms. This cycle continues until early fall when the land finally cools and water temperatures reach peak warmth, which reduces the pressure difference.

Beneficial, but Also Hazardous

Rainfall from thunderstorms reduces the risk of wildfires and brings much-needed rain to the region for reservoir replenishment and vegetation.

The monsoon pattern also brings concerns, including flash flooding, lightning, downburst winds and dust storms.

Flash flooding is the leading thunderstorm-related killer and most flash flood deaths occur in vehicles. Never drive through flooded roadways. Lightning also causes injuries and fatalities every year in the United States.

Downbursts are another concern – they can result in an outward burst of damaging winds at the ground. Outflow boundaries created by strong thunderstorm downburst winds can cause blowing dust and potentially dust storms, which can significantly and quickly limit visibility.

The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.


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