A long-duration heat wave is taking shape over the western half of the U.S. and is forecast to stick around in the days ahead. 

A high-pressure system in place over the West is keeping precipitation out, which is also allowing for early-season heat across a large region. Triple-digit temperatures are baking the Desert Southwest, and cities even closer to the Pacific Ocean are also seeing unseasonably high temperatures.  

More than 60 record-high temperatures were hit Tuesday, including 103 degrees in Palm Springs, California. 

Phoenix, Arizona, had its first 100-degree high of the year on Wednesday, the earliest that has ever happened. The last such record was set on March 26, 1988. Record keeping in the Valley of the Sun has been reported since 1896. 

The above-normal temperatures are expected through the weekend with more record-breaking daily highs and overnight lows, according to the National Weather Service. The region could see prolonged heat into early next week, forecasters said.

What is a heat wave, technically?

A heat wave is defined as a period of unusually hot weather that typically lasts two or more days. The temperatures also must be outside the historical average for a given area. For instance, a 100-degree temperature is pretty typical in Phoenix during the summer, but before the official start of spring would be classified as a heat wave.

Extreme heat warnings and heat advisories are in effect through at least Sunday across the Southwest and could possibly be extended. These alerts are in effect for major cities such as San Francisco, Los Angeles, San Diego, Palm Springs, Las Vegas, Phoenix and Tucson. 

Heat is the No. 1 weather-related killer compared to all of other weather phenomena combined (tornadoes, flooding, lightning, etc). If you have to be outside for extended periods of time, drink plenty of water, take breaks in shade or inside an air-conditioned building, and limit any strenuous activities.

Hot car deaths of young children also rise during heat waves, and this could be a cause for concern well before the summer months when it is more common.