Since the past weekend, the first intense heat wave has kept a tight, sweaty grip on the eastern United States. As of Tuesday morning, about 80 million Americans were under some type of heat advisory. Forty-four of the 50 states expected to reach at least 90 degrees Tuesday afternoon. Expect both of those statistics to grow through the end of this week, as the brutal heat wave intensifies and expands to include the western part of the country.

On Monday, Washington, Philadelphia and New York all reported heat indexes — or “feels like” temperatures — in excess of 100 degrees. Record-warm overnight low temperatures were set in Albany and Burlington, Vt.

Tuesday could end up being even warmer — or at least it will feel warmer — since humidity was higher across the Mid-Atlantic and Northeast.

The Midwest has been roasting since the end of last week. Heat indexes this past weekend in Chicago were regularly in the 105-to-115-degree range. The excessive heat caused at least one road to buckle in Wisconsin, according to a contributor on Reddit’s r/weather forum. And a bridge was deformed because of the heat in Chicago, according to WREX.

In the West, the main concern has been wildfires. A fire that ignited Saturday in California has burned 109 square miles of land in Yolo and Napa counties, about 75 miles north of San Francisco, according to Cal Fire. Meanwhile, fires continue to burn in parts of Colorado and Utah, as the region suffers through an extended drought. With the combination of gusty winds and low humidity, forecasters posted red flag warnings in both states. Conditions are favorable for more fire development over the next several days, the Weather Service said.

The culprit for the extended stretch of heat and humidity over the Midwest and East Coast is a large area of high pressure over the eastern United States. Areas of high and low pressure can form both at the surface and in the upper levels of the atmosphere. The area of high pressure responsible for the heat exists in the upper atmosphere.

Broadly speaking, when a center of high pressure, or ridge, forms in the upper atmosphere (about 3 1/2 miles above the ground) large scale flow causes huge amounts of air to sink toward the surface. As air sinks, it compresses because of the increased pressure  and heats up. These types of heat waves are sometimes called “heat domes” because of the way the high pressure traps and compresses the air.

Perhaps the most important thing to understand about strong high pressure is that it’s very stubborn. Once it develops in the upper atmosphere, it’s very difficult for other weather systems to chip away at it. High pressure domes like this tend to dominate the atmospheric landscape, shunting the jet stream far to the north and keeping everything under them locked in a pattern of hot and humid weather.

Not only is the current upper-level high going to stick around, but also it’s going to become stronger and larger over the next several days. Over the weekend, the same ridge of high pressure that has been dominating the weather over the East will expand west. Forecast models expect the dome of high pressure will eventually encompass almost the entire Lower 48.

The extent of this heat wave over the weekend not only will be uncommon, but also the high pressure is going to be intense. So intense, in fact, it may lack historical precedent.

Long-term weather records don’t include the kind of upper-level measurements needed to track records, but in at least one sense, it’s fair to say this weekend’s ridge will be unprecedented. One forecast model the National Weather Service uses is predicting a ridge that will be record-intense for this time of year. This particular model is calculating records based on data back to only 1979, but, nonetheless, the forecast is significant. Peak intensity in the contiguous United States will be Friday, the model suggests.