Nearly 200 million Americans are under heat advisories or excessive heat warnings as dual "heat domes" affect the Pacific Northwest, Central states and East Coast.

Why it matters: Extreme heat can kill, and it can also greatly aggravate wildfire conditions, making it even harder for thousands of firefighters to contain California's Dixie Fire, the state's second-largest on record.

• Although it is summer, it's unusual to see so much of the Lower 48 states experiencing extreme heat simultaneously.

The big picture: An area of upper level high pressure, also known as a heat dome, is parked over the Pacific Northwest, just off the coast of Washington State. The air circulation around this high is bringing winds off land areas land areas in British Columbia, rather than the typical cooling ocean breezes that this region is more known for.

• The Northwest is a region that has already seen a record-shattering heat wave that set all-time temperature milestones in late June into early July.
• High temperatures in Portland, Ore., are forecast to reach 98°F Wednesday, and 100°F on Thursday and Friday before cooling down for the weekend. The typical high temperature in Portland at this time of year is 83°F.
• Red flag warnings are up for wildfire zones in northern California and parts of Oregon, and excessive heat warnings stretch from extreme northern California into Washington State. A state of emergency due to the heat wave is in effect in Oregon.
• The heat is also worsening fire conditions in British Columbia, where blazes started during the June heat wave.
• Heat advisories also extend from Michigan to Texas, with high humidity making for especially dangerous conditions near the urban heat islands of Kansas City and St. Louis.

Threat level: Heat advisories also stretch from North Carolina to Maine, which are under the influence of a "Bermuda High," so named for its tendency to be located near Bermuda or between Bermuda and the East Coast at this time of year.

• Currently, the high pressure area is located over the Southeastern U.S. and the southwesterly flow of air up the East Coast is bringing the heat and humidity.
• Excessive heat warnings, which are a more severe type of alert, are in effect for New York City and Philadelphia, where heat indices will reach or even exceed 105°F on Wednesday and Thursday, with the hottest conditions expected Thursday.
• "Extreme heat and humidity will significantly increase the potential for heat related illnesses, particularly for those working or participating in outdoor activities," the National Weather Service said.
• Washington, D.C. could hit 100°F on Thursday, with a heat index higher than that.

Context: Climate change caused by the burning of fossil fuels for energy is causing a significant rise in the intensity and probability of extreme heat events, a landmark U.N. sponsored scientific panel in a report released Monday concluded.

• It warned of even more "unprecedented" heat events, like the one in the Pacific Northwest in June, to come as global warming continues.

What's next: The heat in the Pacific Northwest should abate during the next several days, while conditions gradually moderate in the East as well. However, an overall pattern of above average temperatures in the West, in particular, is likely going to continue, in large part due to the severe drought in place in the region.