Heat wave and high humidity will blast much of the US this week, weather service says

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.

Feeling the heat will be the East Coast cities of New York, Philadelphia, Washington and Baltimore and Midwestern and Great Lakes cities including Chicago, 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, southeast and 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 indices of 100 to 110 degrees F (40 C to 43 C), and as high as 115 F (46 C). Nightly lows in the 70s F (21 to 26 C) won't provide much relief, the weather service said.

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.

Parts of the U.S., especially Phoenix, Las Vegas, central Texas, and much of the southwest, were already experiencing temperatures around 100 F.

During extreme temperatures, limit outdoor activity, stay hydrated and ensure access to air-conditioning and other cooling areas, the weather service said.