The heat wave baking the D.C. metro region is part of a sprawling dome of high pressure sizzling a swath of the nation from Texas to Boston.

The large expanse of sinking, sultry air is not forecast to slink away from the mid-Atlantic region until early next week.

On Thursday, about 148 million people in 32 states and the District were covered by heat warnings or advisories.

Temperatures near or over 100 are likely again Friday and Saturday along the East Coast, and the heat index — how hot the air feels because of the combined effects of temperature and humidity — could reach or surpass 115 to 120 in some places.

An excessive heat warning issued by the National Weather Service and in effect for the Washington area through Saturday warned that “the combination of hot temperatures and high humidity will create a dangerous situation in which heat illnesses are likely.”

In addition, a Code Red air-quality alert issued by the Metropolitan Washington Council of Governments warned of “air pollution concentrations . . . unhealthful for the general population.”

The heat wave, which began early in the week across the central United States and upper Midwest, has been notable for its scorching temperatures, as well as unusually high humidity surging north to the Great Lakes.

Chicago hit 100 degrees Wednesday and 101 Thursday, its first back-to-back 100-degree days since the deadly 1995 heat wave. On Tuesday, Minneapolis tied its all-time heat-index record at 119.

Other notable heat-index values include 123 in Council Bluffs, Iowa; 121 in Atlantic, Iowa; 121 in Newton, Iowa; and 120 at Grissom Air Reserve Base, Indiana, on Tuesday — and a stifling 131 in Knoxville, Iowa, on Monday.

The high humidity has slowed nighttime cooling, and 992 record-high minimum temperatures were tied or set across the country between Monday and Wednesday. During that span, 255 record maximum temperatures were tied or set.

In Washington, yesterday’s high temperature of 99 at Reagan National Airport, the city’s official observing station, fell short of the record for the day (104 in 1926). But an afternoon heat index of at least 112 was Washington’s highest in nine years and higher than any heat index recorded last summer, the city’s hottest on record, with 67 days at or above 90 degrees.

While heat index records for Washington are incomplete, according to Steve Zubrick, science operations officer at the ­Baltimore/Washington National Weather Service Forecast Office, the heat index reached at least 122 on July 16, 1980.

Before Thursday, the last time Washington’s heat index reached 110 was Aug. 3, 2002. The last time it reached 115 was July 15, 1995.