Fire alert in the southern and central US 0:45
(CNN) --
The official start of summer is 10 days away, but more than 60 million Americans are under heat alerts this Saturday from California to Louisiana, with triple-digit temperatures expected to exceed daily records throughout the Southwest.
Temperatures in California's Central Valley and the Southwest will be dangerously high as they climb into triple digits on Saturday afternoon.
"Extreme heat will significantly increase the potential for heat-related illness, particularly for those who work or participate in outdoor activities," warned the National Weather Service (NWS) office in Hanford, California.
"Overnight lows may provide little relief."
A "dangerous and deadly heat wave" is approaching the southwestern US, warns the National Weather Service
The record-setting series will stretch from Sacramento to Houston.
In fact, more than 140 US cities could break daily records in the coming days, and even overnight low temperatures are expected to reach dangerous levels.
A sample of the record temperatures arrived on Friday
High temperatures in the central and southern plains this Saturday will also rise to 32 degrees Celsius and above.
But heat index values -- "feel temperatures" -- could approach 110 degrees Fahrenheit, especially in South Texas.
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Several dozen cities could see record temperatures between Saturday June 11 and Wednesday June 15.
A taste of the dangerous and life-threatening heat came on Friday as several major cities set new records.
What is the difference between heat exhaustion and heat stroke?
Yes, the desert is a hot place, but Death Valley in eastern California hit a new daily record high of 50 degrees Celsius, surpassing the previous high, in 1994.
Phoenix reached 45°C, eclipsing the previous July 10 record of 43°C set in 1978.
Austin reached 39°Cs, higher than the previous record set in 2008.
At least 140 cities could set new daily records
More than 220 million people, about 70% of the lower 48, will see temperatures of 32°C or higher over the next week.
More than 45 million of those people, or about 15% of the population of the lower 48, will experience temperatures in the triple digits [in degrees Fahrenheit] over the next week, primarily in California's Central Valley, desert Southwest and the southern and central plains.
That means 140 cities could set new daily records from Saturday to Wednesday.
Relief from uncomfortably hot and humid weather will begin in Northern and Central California on Sunday and Monday with the arrival of a pair of cold fronts.
But the heat dome will simply move into the central and eastern states, bringing scorching temperatures to the Plains and Midwest through early next week.
On Monday, most of the record temperatures will stretch between Denver and Raleigh, North Carolina, but could extend as far north as Wisconsin and Michigan by midweek.
Chicago could top 90 degrees on Tuesday and Wednesday.
And overnight lows, which typically allow the body to cool down adequately, can be just as unforgiving as daytime highs.
"Your body needs to cool down at night and, in fact, expects it while you sleep," says Jenn Varian, a meteorologist at the National Weather Service's Las Vegas office.
"When we have very hot night temperatures, your body just can't cool down properly, which in itself can cause complications, but it will also make you less prepared for the heat of the day."
CNN's Ray Sanchez contributed to this story.