The Southwest monsoon began on June 15 and lasts through Sept. 30, according to NOAA, but the rain really ramps up now that we're in July.

The science behind monsoons

The monsoon is a seasonal change in winds. This 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 don't warm as quickly.

Pressure differences between the hotter land and cooler water draw more humid air from the Gulf of California and Eastern Pacific Ocean. This flow from these water bodies is a reversal of the usual flow from land areas to the ocean. 

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Then, a ridge of high pressure builds over the Rockies or Plains that taps more moisture in the atmosphere from the Gulf of Mexico or Southern Plains, as you can see in the graphic above.

More frequent thunderstorms result. This fuels the development of showers and thunderstorms in the region, with the coverage of those storms varying day-to-day or week-to-week.

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. Once this happens, the pressure difference is reduced.

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

Rain from thunderstorms reduces the risk of wildfires and brings much-needed drought relief to the region for reservoir replenishment and vegetation.

However, these thunderstorms can also generate huge dust storms known as haboobs, flooding rain and prolific lightning, as we detailed in this previous article.

What to expect this summer

The monsoon outlook is trending above normal. The seasonal forecast is leaning toward above-average overall precipitation this summer for the southwest U.S.

This year's forecast calls for much more precipitation than the average skew in much of the Southwest, according to AG2 and The Weather Company. However, the signal, as is often the case in summer weather, isn't all that clear.

Faiza Mohammed is a meteorologist intern at weather.com. She studied geography and worked with the Weather Dawgs team at the University of Georgia. She also received her master's in Earth and Atmospheric Science from the Georgia Institute of Technology.