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.

It's the first widespread heat wave of the summer, impacting more than half of the country, forecasters said. The heat wave is expanding across the central and eastern U.S., while the Four Corners remain under dangerous fire weather conditions. At the same time, severe thunderstorms continue across the northern Plains and Upper Midwest, with flooding concerns stretching from the Rockies into the Mid-Atlantic. 

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. The Centers for Disease Control and Prevention says heat can impact anyone. Signs of overheating include muscle cramping, dizziness, unusually heavy sweating, shortness of breath, headaches, weakness and nausea.

While large portions of the U.S. confront high temperatures, much of the West remains unusually cool, with high temperatures running 20-35 degrees below average across portions of the Northwest and northern Rockies, according to weather reports.

Europe has been sizzling for days, with the temperature hitting a record 106 degrees Fahrenheit in Germany on Saturday, the AFP reported. Switzerland, the Czech Republic, Denmark and France have also been facing high temperatures. In France, dozens of drowning deaths were recorded this month as swimmers turned to the water for relief from the heat.