Heat wave and high humidity will blast much of the eastern US this week, meteorologists say

A long and dangerous heat wave will blast a large swath of the United States this week, the National Weather Service said, with temperatures rising ahead of the July Fourth holiday and feeling even hotter because of the high humidity that's arriving with it.

Several days of high temperatures will settle in across the lower Great Lakes, the mid-Atlantic and the Mississippi and Ohio River valleys, the weather service said.

A number of big cities could see their highest temperatures of the year so far as they host World Cup matches and celebrations for America’s 250th anniversary.

Feeling the heat will be the East Coast cities of New York, Philadelphia, Washington and Baltimore, and Midwestern and Great Lakes cities including Chicago, Indianapolis, St. Louis and Detroit. Southern cities including Dallas, Little Rock, Arkansas, and Memphis, Tennessee, will also see high temperatures.

High heat will last into next weekend across the Great Plains, the Southeast and the mid-Atlantic, the weather service said.

Temperatures will reach well into the 90s and low 100 degrees Fahrenheit (mid to high 30s Celsius), the weather service said. High humidity will lead to heat indexes of 100 to 110 degrees F (40 C to 43 C), and as high as 115 F (46 C).

The heat index, which factors in humidity and is included on many weather forecasts, provides a sense of how hot it really feels — and what’s dangerous for prolonged exposure or strenuous activity.

AccuWeather meteorologist Tyler Roys said temperatures will be significantly above normal. For instance, in Washington, highs around July Fourth average around 89 degrees F (32 C), while Indianapolis averages around 85 degrees F (29 C). But this week, both cities will be hotter by 10 or 11 degrees F, Roys said.

The high pressure system — an area of dry, sinking air — creating the heat will act like a “rock” that will force storms to go around it and limit rainfall across the East, Roys said. That's sometimes called a “heat dome.”