Heat Wave Expected to Bake California Through July 4th Weekend

A significant heat wave will begin in California on Tuesday, with searing temperatures that could last well through the long holiday weekend, forecasters said.

The heat wave is expected to be long even by “California standards,” said Alex Lamers, the chief of forecast operations for the National Weather Service’s Weather Prediction Center.

The state’s deserts and valleys will experience the hottest temperatures, with highs over 100, and even coastal areas may not escape the abnormal heat.

Here are some key things to know about this heat wave.

• The heat will ramp up starting Tuesday and continue to worsen through the 4th of July weekend.

• It isn’t just California. By July 4, other Western states including Washington, Oregon and Nevada will feel temperatures 10 to 20 degrees above average.

• The heat wave in the West will be particularly dangerous because of warm nighttime temperatures. Overnight cooling is not expected to provide the usual level of relief typical of early July.

• A separate heat wave is continuing across the southeastern United States and will last into the holiday weekend. Combined with humidity, sweltering temperatures along the Gulf Coast will feel even hotter than their measurement.

“Given that there is little overnight relief expected throughout this event, this is where it really becomes a marathon,” forecasters in the San Francisco Bay Area said on Monday. “This is not just for individuals sensitive to heat, but rather, the entire population,” they warned.

On Monday, the California Insurance Commissioner, Ricardo Lara, released a report that concluded that seven recent extreme heat events had cost the state $7.7 billion and were responsible for about 460 deaths.

The effect of heat on the body is cumulative. As each day goes by, the risk of heat-related illness grows. And when temperatures don’t cool adequately at night, the body can’t recover. In Las Vegas, where high temperatures of around 115 degrees are forecast this weekend, lows may not fall far below 90 degrees at night.

The length of the heat wave could break records for consecutive days when scorching temperatures are reached. In an interview, Mr. Lamers named just a few of what could be many locations in California that tie or break such records. He said that San Jose was expected to tie its longest run of triple-digit temperatures, five consecutive days, and that Redding may tie or break its longest stretch of 110-degree heat of six days.

The abnormal heat will most likely continue beyond what local weather offices currently forecast. The Climate Prediction Center, where meteorologists are already looking toward next week, noted on Monday that the excessive heat could continue through July 11, if not further, with a slight chance of the withering temperatures lasting until July 15.

Alexandra E. Petri contributed reporting.