A searing heat wave was expected to expand and intensify across much of the United States through the weekend, as a heat dome traps hot air close to the ground.

Many records are projected to fall. Here are the ones that forecasters are watching closely.

How many days at 100 or above?

Many places will reach 100 degrees for two, three or even four days straight. On some of those days, a few places may even hit 105 or higher, with a heat index up to 115.

Washington, D.C., was predicted to hit 100 or above on Thursday, Friday and Saturday. The city could also reach 100 as early as Wednesday. The city has recorded four days in a row of 100 degrees or higher only two other times.

Philadelphia’s longest run of consecutive days with temperatures of 100 or more is three, which happened in July 1966 and July 1993. This week, the city was projected to tie with the record, and there is a chance to break it.

New York City could hit 100 for first time in 14 years.

The thermometer at Belvedere Castle in Central Park, the city’s official weather station, hasn’t recorded 100 since July 18, 2012. This week, the city is poised to hit 101 on both Thursday and Friday.

Hundreds of daily highs will be broken.

A daily high record is the highest temperature ever officially recorded on a certain date at a specific weather station. These records are often the first to fall, because there are 365 opportunities each year, versus only 12 chances with a monthly one.

“It’s really just about simple matters of probability,” said Michael Mann, presidential distinguished professor in the department of earth & environmental science at the University of Pennsylvania.

More than 100 new records are expected through Saturday, at locations spread from Tennessee and Ohio, east to Washington and all the way north into Maine.

Nighttime records will also fall.

Sometimes, the temperature at a weather station stays hotter at night than ever before on that date. Meteorologists call that a “daily record high minimum.”

This week, meteorologists at the National Weather Service said that more than 200 such records could be broken. They’ll likely fall over a broader geographic area than the daytime records — from Texas to North Carolina and up to Maine.

These records get broken when a hot air mass over an area prevents temperatures from cooling down as they usually do overnight, which is what’s expected to happen this week.

These conditions are especially dangerous because people’s bodies typically get a chance to recover from the heat of the day when the sun goes down and the air cools down. This week, overnight lows are expected to drop only into the 70s to near 80s, providing little relief. People without air-conditioners will be at particular risk for heat illness.

There could be a few new all-time records.

All-time record high temperatures mark the highest temperature since record keeping began at a designated location. They are broken only rarely, especially at places where the records go back more than 100 years — as they do in many of the big cities along the East Coast.

A new one is a sign of an extreme heat event.

This week, all eyes are on Washington, where a new all-time high is not impossible. Ronald Reagan National Airport, the city’s official weather station since World War II, has a record high of 105, set in July 2012. The projected high in Washington this week was a moving target on Tuesday, but the latest official forecast from the Weather Service predicted the temperature would top out at 103 on Friday.

The all-time high in Concord, N.H., is 102, set on July 4, 1911, and again on July 3, 1966. That record may have a better shot at falling, with a high of 102 expected on Thursday.

It’s a longer shot in New York City, where the highest temperature ever recorded at Central Park is 106 and this week’s high is most likely to top out at 101 on Thursday. It’s a similar story in Boston, where the all-time high is 104, and the week’s high is likely to be 99 degrees, also on Thursday.