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Cal Fire is canceling firefighters’ days off, local parks are closing, Bay Area health officials are warning of potential heat deaths, and residents of the region are making plans to head for the beaches as the Bay Area and much of California suffer under a “heat dome” that is only going to get hotter.

On Friday, the meteorological phenomenon dubbed the dome resulted in highs of 98 degrees in Livermore, 96 in Concord, 82 in San Jose, 79 in Redwood City, 75 in Oakland and 68 in San Francisco, according to the National Weather Service. Temperatures were expected to tick up a few degrees Saturday before climbing to a peak Monday when San Jose is forecast to hit 96 and Concord an eye-popping 110.

Temperatures this high are not unheard of in the Bay Area, but three to four days of heat that severe comes only about once a year, said San Jose State University meteorologist Jan Null.

But what’s a heat dome?

“It all has to do with high pressure,” Null said — a heat dome is a colloquial term for a strong, persistent region of high pressure. Under such a dome, air circulates less and becomes stagnant; as the atmospheric pressure increases, the air column is compressed, and the downward pressure from the compressed air creates heat in the lower regions of the column that gets trapped by the pressure.

The pressure also squashes the “marine layer” of cooler, moister air from the Pacific Ocean, keeping it too low to get past coastal hills and deliver cooler temperatures beyond San Francisco and Oakland to inland areas such as Concord and Livermore, he said.

Climate change — which raises temperatures world-wide and creates heat waves more often — is not known to create heat domes, Null said, but some scientists believe it has intensified the domes’ impact.

This particular dome started over the Four Corners region east of Las Vegas and spread “because there’s nothing really getting in its way,” said National Weather Service meteorologist Sarah McCorkle.

Heat domes form regularly in the U.S. during summer when the sun’s rays are more intense, she said. “What’s making this system different is the fact that it’s just a stronger high pressure,” McCorkle said.

The heat dome also has dramatically boosted the risk of wildfire, and Cal Fire is ramping up to address the danger, said battalion chief Jon Heggie. “We are bringing people back from off duty. We are cancelling days off for firefighters. We are doing everything we can to ensure that we are extremely well prepared for any and all incidents,” Heggie said. “No one is safe when it comes to high temperatures: Fire can happen in urban areas as much as rural.”

Heggie urged residents to avoid mowing yards during the heat wave because mowers hitting rocks can create sparks. People planning to drive RVs should make sure no chains are dragging, he said. Fires should be made only in designated and permitted areas, and residents should ensure that their homes are fire-safe and that they have a “go bag” and an escape plan in case of fire, Heggie said.

Temperatures over 100 with humidity below 20% create a high risk for wildfire, according to Cal Fire. Areas and cities on the east side of the East Bay hills and parts of southern Santa Clara County including Hollister, Gilroy and Morgan Hill will likely exceed those thresholds Monday, Null said.

East Bay parks officials on Thursday announced that many parks and park areas, including Mission Peak and Tilden, will be closed Sunday and Monday because of fire risk.

While stagnant air under the heat dome cuts the risk of wind helping fire spread, it also means air pollution is expected to linger over the Bay Area. The Bay Area Air Quality Management District has issued a “Spare the Air” alert for Saturday and is asking residents to drive less.

Meanwhile, Bay Area health officials warned that the heat wave could bring tragedy. “This one is being taken particularly seriously because of how hot and how long it’s going to last,” Contra Costa County Health Officer Dr. Ori Tzvieli said. “Heat is a very threatening event. Very many people can die, and they can die anonymously. A lot of people, especially elders, may not have air conditioning and can be particularly vulnerable.”

State workplace regulator Cal/OSHA is planning special worker-safety inspections through Wednesday to ensure that workers in construction, agriculture, tree-trimming, landscaping, car washing and warehousing are safe from heat, the agency said Friday.

Because the atmospheric pressure is higher to the south in California, residents there will swelter more, including in Paso Robles, expected to hit 113 on Monday, Null said.

The Bay Area is expected to start cooling Tuesday, beginning with coastal areas and locations around the San Francisco Bay, with many inland areas less affected by the marine influence lagging behind, Null said. Still, highs will remain in the 80s and 90s in much of the Bay Area on Wednesday and Thursday, according to the National Weather Service.

Staff writers Rick Hurd and Jason Green contributed to this report.

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