Temperatures could spike beyond 100F in parts of the Pacific Northwest, the mid-Atlantic and the Northeast.

New York, July 7 (RHC)-- More than 130 million people in the United States are under threat from a long-running heatwave that has already broken records with dangerously high temperatures, forecasters said, adding that scorching heat will be felt from the East Coast to the West Coast.

The heat and humidity could team up to spike temperatures above 100 degrees Fahrenheit (about 38 degrees Celsius) in parts of the Pacific Northwest, the Mid-Atlantic and the Northeast, said Jacob Asherman, a meteorologist with the National Weather Service (NWS).

In the Pacific Northwest region, records could be broken in Oregon state in cities including Eugene, Portland and Salem, Asherman said. Dozens of other records throughout the US could fall, he said, causing millions to seek relief from the blanket of heat in cooling centres from Bullhead City, Arizona, to Norfolk, Virginia.

The NWS said on Saturday it was extending the excessive heat warning for much of the southwest into Friday.  Three-digit temperatures are likely, higher by 15F to 30F (8-16C) than average in the west of the US into next week, the NWS said.

The eastern U.S. could also be bracing for more hot temperatures. Baltimore and other parts of Maryland are under an excessive heat warning as heat index values could climb to 110F (43C), forecasters said.

“Drink plenty of fluids, stay in an air-conditioned room, stay out of the sun, and check up on relatives and neighbors,” read a National Weather Service advisory for the Baltimore area. “Young children and pets should never be left unattended in vehicles under any circumstances.”

Last week, hot, dry, and windy conditions in the west of the US prompted fire officials and forecasters to issue warnings about the heightened risk of wildfires.

What is being called the Thompson Fire in Butte County, California, 105km (about 65 miles) north of Sacramento, has scorched almost 1,554 hectares (3,840 acres) of scrub and bush since it started on Tuesday.

Some 362 km (225 miles) south, firefighters battled the French Fire that threatened Mariposa, a gateway to Yosemite National Park. The fire spread over 323 hectares (800 acres) was 5 percent contained as winds calmed on Friday, helping firefighters make some progress.

Southern Texas faces a different threat early next week when remnants of Hurricane Beryl are expected to dump heavy rains on the region.

The U.S. Midwest has been facing extreme heat since June, with temperatures climbing beyond 100F (37.8C).  Experts attribute this heatwave to high levels of humidity and the presence of heat domes in the atmosphere.