SAVANNAH, Ga. (AP) — The unofficial start of summer feels more like its blistering peak in parts of the Deep South, where temperatures at or near 100 degrees are setting heat records during the Memorial Day weekend.

Heat and humidity were forecast to combine for a dangerous heat index of 105 degrees Fahrenheit (40.56 Celsius) on Sunday across roughly 20 counties in southeastern Georgia and South Carolina, said Emily McGraw, a National Weather Service meteorologist in Charleston, South Carolina.

The weather service issued a similar heat advisory for a portion of the Florida Panhandle. McGraw said the forecast showed little relief in sight, and there was potential for some cities to see their hottest days ever in the month of May.

Savannah reached 100 degrees (37.78 Celsius) Saturday, tying a record and marking the coastal Georgia city's earliest triple-degree day since 1953. Columbia, South Carolina, on Saturday also hit 100, just one degree shy of a record from 2000.

"Unfortunately, it's going to be fairly hot for the next couple of days, staying in the 90s through the end of the week," McGraw said.

Even Southern cities that escaped triple-digit heat over the weekend set records that were 10 degrees or more above normal temperatures.

Atlanta hit 94 degrees (34.44 Celsius) Saturday, breaking a record set in 1960. That same day Charleston, South Carolina, and Tuscaloosa, Alabama, both reached a record 97 degrees (36.11 Celsius), while Pensacola, Florida, set a new record with a high temperature of 95 (35 Celsius). Nashville, Tennessee, tied a 1926 record of 93 degrees (33.89 Celsius).

McGraw blamed the intense heat across the region on a wave of high pressure that forces air from the upper atmosphere to sink lower to the ground and become hotter. She said portions of South Carolina and Georgia have about a 20% chance of some cooling rain later in the week.

Meanwhile, she urged people to take precautions if they're outside through the Memorial Day holiday Monday.

"Certainly if you're outdoors stay hydrated, keep putting on sunscreen, and take breaks from the heat if you can," McGraw said.