Why is it extra dry in May and June in Arizona?

PHOENIX — You may have heard our 12News meteorologists mention that May and June are historically the two driest months of the year in Arizona. There is a reason for that.

At this time of year, Arizona is geographically isolated from tropical moisture. The main storm track (jet stream) has migrated north for the summer.  Prevailing winds along the west coast originate from the subtropical Pacific high-pressure zone, where the air is already naturally sinking, compressed, and stripped of deep moisture before it ever hits the coast.

Meanwhile, the Gulf of Mexico is too far east, and the tropical easterlies that power the summer monsoon have not yet shifted far enough north.

The result? Relative humidity across the state routinely drops into the single digits, and the landscape dries out ahead of the mid-summer rainy season. This is why Arizona typically reaches a peak in fire danger, when conditions are at their hottest and driest of the year.

In contrast: A stormy Central Plains

While Arizona is locked down under a dome of dry, sinking air, the Central Plains are experiencing the exact opposite: An incredibly active atmosphere primed for heavy rainfall and severe weather.

The single most important player in the Plains’ wet season is the Gulf of Mexico. As the weather pattern shifts, it pumps a continuous stream of warm, humid, tropical air northward into the center of the U.S. This moisture acts as fuel for daily thunderstorm development.

This atmospheric setup doesn’t last forever. By late June and early July, the intense heat building over the Arizona desert creates a localized thermal low. At the same time, the “four corners high” shifts just far enough east to change the steering winds.

A moist southeast wind flow then develops, helping to break the dry spell by kicking off the Southwest Monsoon.  That shifting atmospheric pipeline finally brings much-needed relief to the desert.  It’s not a continual surge of showers, but at times we’ll see active rounds of rain with lulls in between through September.

>> Download the 12News app for the latest local breaking news straight to your phone.

You can now watch 12News content anytime, anywhere thanks to the 12+ app! 

The free 12+ app from 12News lets users stream live events — including daily newscasts like “Today in AZ” and “12 News” and our daily lifestyle program, “Arizona Midday”—on Roku, Apple TV and Amazon Fire TV. 

12+  showcases live video throughout the day for breaking news, local news, weather and even an occasional moment of Zen showcasing breathtaking sights from across Arizona. 

Users can also watch on-demand videos of top stories, local politics, I-Team investigations, Arizona-specific features and vintage videos from the 12News archives. 

Roku: Add the channel from the Roku store or by searching for “12 News KPNX.” 

Amazon Fire TV: Search for “12 News KPNX” to find the free 12+ app to add to your account, or have the 12+ app delivered directly to your Amazon Fire TV through Amazon.com or the Amazon app.