Determining when to issue an Excessive Heat Warning depends on the science

Forecasting extreme heat is one thing, but knowing when to issue an Excessive Heat Warning requires a deeper look into the science behind these weather patterns.

A dangerous heat wave has hit the Valley once again this summer and meteorologists with the National Weather Service in Phoenix say it's staying put for now.

"We have this really strong ridge of high pressure, which is essentially compressing the atmosphere," Matt Salerno, meteorologist at the National Weather Service said.

"The high pressure is gonna stick around, mainly over the Great Basin area for the next several days and it's not gonna move much at all."

Excessive Heat Watch issued for Fourth of July

The latest summer scorcher prompted an Excessive Heat Watch from July 4 through July 9. 

It's determined by a heat risk system that weighs the risk on a scale from minor to extreme.

Signs posted at Piestewa Peak warn that hiking is not allowed with dogs on the peak trail.

"We use heat risk to determine the implication of how hot we are in comparison to our normals. So our daily normal high and low temperatures each day," Salerno said.

The temperatures earlier this week were hot enough to trigger an Excessive Heat Watch, but not enough for an Excessive Heat Warning.

"Now in July, our average high temperature in Phoenix is 107 degrees, so we already have a pretty high baseline to start. So to reach that major heat risk category, it's a little bit of a higher threshold," he added.

Salerno say not to wait for a Watch or Warning to play it safe in the heat.

"Pretty much every single day during the summertime, there's gonna be some level of heat risk across the desert Southwest," Salerno said.

What happens during Excessive Heat Warnings?

There are protections in place for animals in the Valley. 

Phoenix hiking trails are closed to all dogs when temperatures reach 100 degrees or above but there are no restrictions for kids.

The Phoenix Fire Department called for trail closures to be put into effect during Excessive Heat Warnings last year after a record number of rescues. 

However, Camelback Mountain and Piestewa Peak are the only two trails closed when the National Weather Service issues those warnings.