Fox News Weather Center

Storms this week signal start to monsoon season in southwestern US

Topic driven playlist

The uptick in spotty, drenching thunderstorms, associated with the southwestern United States monsoon, will continue in the coming days and weeks.

Storms this week are an early look of what the monsoon season will bring.

Tucson, Arizona, was hit with flash flooding that required swift water rescues on Sunday evening. Phoenix hasn't recorded any measurable rain since May 8, but storms this week could deliver similar downpours.

Storms will ramp up in intensity as the height of the season approaches. Not all of the storms produced in the pattern will be created equal, however.

Some of the storms will bring torrential downpours. The downpours can cause temperatures to drop up to 25 degrees Fahrenheit rapidly, but they will also deliver rainfall capable of producing flash flooding.

Other storms will bring little or no rainfall and numerous lightning strikes. The lightning can not only be a source of ignition for wildfires, but it can catch unwary outdoor enthusiasts off guard.

Wind gusts produced by the storms with little rainfall can also kick up a sudden haboob.

For these reasons, people spending time outdoors and those driving through the region should stay alert for rapidly changing weather conditions.

Those in the Southwest will notice not only a growing number of afternoon and evening thunderstorms each day but also an increase in humidity levels well into the summer.

Each year a buildup of heat over the Western states helps to draw moisture northward from Mexico.

"The monsoon begins during late June to early July and reaches a peak in August," AccuWeather Meteorologist Jim Andrews said.

"The storms produced by the higher humidity and heat generally form over the mountains during the afternoon and tend to drift toward the lower desert areas, such as Phoenix and even Las Vegas," Andrews said.

"In some cases, the storms can hold off until late in the day and at night," he said.

Fluctuations in the stream of moisture from Mexico and intrusions of dry air from Canada can cause the area of thunderstorms to expand or shrink, even during the middle of the season.

Occasionally, storms can develop as far to the western parts of California, including the Sierra Nevada and San Joaquin Valley, as well as the interior northwestern U.S.

As heat begins to diminish over the interior West late in the summer, the flow of moisture from Mexico subsides and the amount of storms taper off.