Southwest Monsoon Season Set To Begin, But 2024 Could Trend Dry

M​id-June marks the official start of the Southwest U.S. monsoon, which provides an increase in moisture that fuels thunderstorm activity across the region through summer. Indications are that the 2024 version might trend drier than usual, according to an updated outlook.

The monsoon is a seasonal wind direction change, not an individual thunderstorm. You might have heard of someone saying they got "caught in a monsoon" when referring to a downpour they encountered, but that's not what it means. Monsoon is a term used in multiple parts of the world that refers to a seasonal wind shift that has wet and dry phases.

I​n the Southwest U.S., winds change from a more westerly direction during the dry phase to a southerly direction during the summertime wet phase, which pumps moisture from the Eastern Pacific and Gulf of Mexico into the region.

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More frequent thunderstorms result from the monsoon-fed moisture. June 15 is when the monsoon officially begins, but in many areas, it takes until late June or even July for the coverage of storms to grow. The thunderstorm activity then varies in intensity from day to day or week to week through August and September.

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 precipitation to the region for reservoir replenishment and vegetation.

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

R​ainfall is forecast to be below average across parts of Arizona, New Mexico, Utah and Colorado for this year's monsoon as a whole, according to the outlook below for July through September from The Weather Company and Atmospheric G2.

L​ast year's monsoon had drier-than-average conditions in a large part of the Southwest. However, portions of California, Nevada and western Arizona saw above-average rainfall, largely because of former Hurricane Hilary's moisture in August and heavy rain events in September.

Other recent years show how monsoon rainfall can vary greatly.

2020 was the driest monsoon on record for much of the Southwest. That was followed by one of the wettest monsoons in 2021 and another wetter-than-usual monsoon in 2022.

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The wild card is the influence of the Eastern Pacific hurricane season. Occasionally, remnant spin and moisture from former Eastern Pacific tropical storms or hurricanes can be pulled into the Desert Southwest, enhancing heavy rainfall.

We saw this last year with the previously mentioned former Hurricane Hilary, and in 2022, when former Hurricane Kay fueled flooding rainfall in parts of Southern California and Arizona.

I​f any Eastern Pacific storms or their remnants can send moisture into the Southwest this year, it might help boost rainfall for some time across the region. On the flip side, NOAA is forecasting the Eastern Pacific hurricane season to have below-average activity this year, which might decrease the odds of that scenario playing out in 2024.

Chris Dolce has been a senior meteorologist with weather.com for over 10 years after beginning his career with The Weather Channel in the early 2000s.