The western US will bake in record heat this week

Records that had been in place for over 100 years fell on Tuesday in Napa, California, with highs topping just shy of 100 degrees. In nearby San Francisco, the typical "May Gray" -- the marine layer known by locals -- was nowhere to be found. Record highs 90 and 89 degrees were observed on Monday and Tuesday, respectively.

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"Excessive heat warnings are in place for much of the Southwest into the valleys in California where highs will soar well past 100 degrees," CNN meteorologist Dave Hennen said.

Over 20 million people are under either an excessive heat warning or advisory across the west.

Heat warnings and advisories include cities like Phoenix, Las Vegas, Fresno and Sacramento, with heat advisories for San Jose and much of California's central coast.

Dozens of high temperatures records are forecast as the heat builds and spreads east over the weekend, with 90-degree temperatures surging north into the Rockies, Hennen said.

Although the southwestern US can experience excessive heat throughout the spring, the rarity of such heat in late May is worth noting.

Las Vegas is forecast to surpass 105 degrees on both Thursday and Friday. Highs above 105 degrees have not occurred in late May in nearly two decades.

Death Valley, California, the location with the dubious distinction of recording the world's hottest temperature on record, is forecast to reach 122 degrees on Thursday. This mark would tie its hottest temperature on record for May.

One month ago, the city of Phoenix, well known for excessive summer heat, baked into the record books with a late April heatwave.

Temperatures rose above 100 degrees, about 15 degrees above seasonal norms, setting dozens of new records across the southwest. This week, the city braces for another stretch of unusually hot late May temperatures, once again 10 to 15 degrees above seasonal norms and all with three weeks to spare before summer officially arrives.

Temperatures in Phoenix climbed above 100 degrees on Tuesday for the 16th time this year.

With highs expected to surge well past 100 over the remainder of this month, 2020 will be near the top of charts as far as early season heat is concerned.