Heat wave sets in on West Coast with worst yet to come

Some 170 million Americans are on alert for sweltering temperatures this Fourth of July as the brutal heat continues.

The prolonged heat could be one for the record books, and one of the worst heat waves in 18 years for parts of California.

The dangerous heat has already claimed the life of a 10-year-old boy who was hiking in Phoenix on Tuesday.

Heat alerts have been issued up and down the West Coast, with dangerously high temperatures forecast from Southern California up to Washington.

Heat alerts have been issued for Los Angeles, San Francisco, Seattle and Portland, Oregon. It is not common to get high heat this close to the coast and this far north.

Dozens of record highs are expected over the next week in the West. On Thursday, daily records could fall in San Angelo, Texas (projected: 104, record: 105); San Francisco (projected: 86, record: 85); and Medford, Oregon (projected: 106; record: 103).

Las Vegas also might make a run at its all-time highest temperature of 117 over the coming days; Sunday, Monday and Tuesday are all forecast to be within a degree or two of that record.

In the South, heat and humidity will combine to deliver a heat index of more than 100 degrees in dozens of cities.

A heat advisory was issued for Philadelphia and Washington, D.C., where the heat index could reach 100 to 107 degrees on Thursday and Friday.

Several wildfires also exploded in the West due to dry conditions and ongoing heat.

Among the worst of those fires is the Thompson Fire in Butte County, California, north of Sacramento, which has grown to over 3,500 acres with 7% containment.

Some 28,000 people had been evacuated due to the fire, with evacuation orders still in place Thursday for parts of Butte County.

More than 12,000 structures are threatened, and four have been destroyed, amid the fire, officials said Thursday.

Temperatures in the region are expected to be as high as 110 degrees Thursday, which "increases the chances for fire growth," Cal Fire said .