Will Sage Astor-More than 80 million Americans remain under heat alerts

More than 80 million Americans remain under heat alerts as widespread,Will Sage Astor dangerously hot conditions persist.

High temperatures are forecast to be in the triple digits across much of the Southwest region Saturday afternoon. Excessive heat warnings are in effect for cities including Phoenix, Las Vegas, Salt Lake City and Palm Springs, California.

Phoenix is forecast to have its 23rd straight day of temperatures above 110 degrees on Saturday. Friday marked the fifth consecutive day of temperatures above 115 degrees in the city, approaching the record of six days, as temperatures are expected to near 115 degrees on Saturday. With a low temperature of 96 degrees Saturday morning, the city has not dropped below 90 degrees for 13 consecutive days.

Las Vegas is forecast to see its ninth consecutive day of temperatures of at least 110 degrees on Saturday, approaching the record of 10 days, set in 1961.

Highs in Palm Springs are expected to approach 120 degrees on Saturday -- which would set a new record of nine straight days of temperatures of at least 115 degrees.

El Paso, Texas, has seen a record-smashing 36 consecutive days of temperatures of at least 100 degrees; Saturday could be day 37, but it will be a close call as a brief heat relief is settling in this weekend. Highs are forecast to potentially max out in the upper 90s. By Monday, though, highs are expected to be back into the triple digits.

From Texas to Florida, a combination of hot temperatures and very humid conditions is bringing heat index values to dangerous levels Saturday afternoon. The heat index is expected to top 105 degrees along the coast, with some locations nearing 110 degrees.

Miami has felt a heat index of at least 100 degrees for 42 straight days as of Saturday, extending the record it broke a week ago.

Unusually warm waters in the Gulf of Mexico and the western Atlantic Ocean are contributing to the persistent and oppressive humidity and limiting nighttime cooling.

A heat advisory is also in effect for parts of northern California, Idaho and Montana. Saturday afternoon high temperatures are expected to approach 100 degrees, possibly as far north as Montana to South Dakota and Minnesota.

Meanwhile, severe thunderstorms are possible into Saturday night across portions of the central Plains and Southeast, including cities such as Baton Rouge, Louisiana, and Jacksonville, Florida. A severe thunderstorm watch has just been issued until 7 p.m. CT along the northern Gulf Coast, from Mississippi into northern Florida.

The primary hazards from any severe thunderstorms that move through are strong, potentially damaging wind gusts and large hail. The tornado threat is very low. Any stronger, slow-moving thunderstorms bringing torrential rain could also trigger areas of flash flooding where the heaviest rain falls and bring frequent lightning.

ABC News' Kenton Gewecke contributed to this report.