Heat wave builds from coast to coast, worsening wildfires

• It will also cause an uptick in fire risks in the West, where large, fast-moving and deadly wildfires are already burning.

Zoom in: As a strong area of high pressure aloft, or heat dome, intensifies and stretches its influence across the Lower 48 states, cities from Washington to Sacramento could near or surpass 100°F as early as Friday.

• The heat is forecast to persist the longest in the West, where the National Weather Service shows at least some risk of extreme heat through Aug. 10.
• The heat index is likely to max out in the humid Mississippi River Valley during this event at around 115°F, according to the NWS.
• "The combination of hot temperatures/high heat indices, as well as very warm morning lows only dropping into the mid- to upper 70s, will be dangerous to anyone without access to adequate air conditioning," the NWS warned in a Friday forecast discussion.
• The West will also face hazardous air quality due to ongoing fires, the NWS warned.

Stunning stat: Last month will go down in history as not just the hottest July on record for many cities in the West — particularly in California — but the hottest month overall since records began.

• The NWS' Sacramento office said in a post to X Thursday that preliminary data showed "all our official climate sites recorded their hottest July on record (according to average temperatures for the month)!"
• The NWS' Las Vegas office noted on X Thursday that the monthly average temperature was 99.9°F, "setting a new highest monthly average temperature record for July."
• It added: "The month also saw the hottest high temperature in Las Vegas's climate record of 120 degrees on 7/7."

Threat level: The increase in temperatures will yield conditions that are conducive to further growth of the historic Park Fire in northern California, as well as the significant Borel Fire in southern California.

• The Park Fire was at 394,953 acres and 20% contained as of Thursday morning. That is the state's fifth-largest blaze on record, but only a few thousand acres short of its fourth-largest wildfire.
• Firefighters from the U.S. and abroad are battling blazes across the West, and there is concern among forecasters that a surge of moisture from the Gulf of California this weekend could yield an outbreak of thunderstorms.

• The heat and dry vegetation combined with lightning strikes may start new wildfires, a development seen in past fire seasons.

The big picture: Other states are also seeing large wildfires, with Oregon fighting the highest number of large fires, with 35, followed by California, Idaho and Washington, per the National Interagency Fire Center.

• Heat waves and related wildfires are not only a threat to human lives and property; they are an economic peril as well.
• A recent report from the California insurance commissioner on seven recent heat waves found that each caused between $7.7 million and $210 million in labor productivity losses.

Context: Human-caused climate change is leading to more frequent, severe and longer-lasting heat waves, and is also driving an uptick in large wildfires in the West, among other trends.

• According to Climate Central's Climate Shift Index, 148 million Americans are likely to see temperatures on Friday that were made at least three times more likely due to climate change.
• The index uses peer-reviewed methods to measure the influence of human-caused climate change on daily temperatures.

Go deeper:

• Dozens of wildfires scorch U.S. West as another heat wave builds
• Extreme wildfires doubled in frequency, magnitude since 2003
• The seas are not alright: "Unusual warmth" hits nearly all of world's oceans

Editor's note: This a breaking news story. Please check back for updates.