It’s the hottest weekend this summer, so far. When do we get relief?

Chris Ware, Lexington Herald Leader/MCT
Chris Ware, Lexington Herald Leader/MCT MCT

The hottest weekend so of the summer is here, but forecasters are promising relief soon.

The worst of the heat wave covering the eastern two-thirds of the country is expected in the Great Lakes and Northeast, but National Weather Service meteorologists say there will be plenty of hot temperatures and high humidity.

The heat index, a measurement of the combined impact of temperature and humidity, could reach 105 degrees this weekend, according to forecasters.

“Heat index values will easily surpass 100, nearing the advisory threshold of 105 in the hottest spots -- main in the southern Lakelands and south of Charlotte,” National Weather Service meteorologist Jake Wimberley says.

“With folks heading out for weekend activities, extra caution is certainly advisable.”

Highs have been in the 90s all but three days July, with average highs and lows of 92 and 72 in Rock Hill. Afternoon highs have hit 95 degrees the past few days in Rock Hill, Chester and Lancaster.

Afternoon highs of 95-98 degrees are forecast through Sunday, and overnight hours will provide little relief, with lows only expected in the mid 70s.

Conditions are even worse to the north.

Heat indices are forecast to reach 113 degrees this weekend in Philadelphia, with readings of 110 predicted for New York City, Washington, Cleveland and Chicago. In Chicago, the morning low Friday was 81 degrees.

But relief is coming.

Forecasters say a trough, or dip in the jet stream, will develop across the eastern United States by late in the weekend. The effects won’t be obvious in the Rock Hill area until Tuesday, when highs will be dialed back to the low and mid 80s. The cooler conditions are expected to last through next Thursday or Friday before gradual warming returns.

As is typical in summer, a few thunderstorms are expected to develop each day this weekend, although computer data indicates storm activity Saturday could be minimal.

Chances of showers and thunderstorms will increase Monday and Tuesday, as the cold front approaches the region.

Steve Lyttle on Twitter: @slyttle