Extreme heat is about to make a return to the Bay Area in time to bid May adieu, but it won’t be hanging out for very long, according to the National Weather Service.

“This one is coming with some complexity,” NWS meteorologist Nicole Sarment said early Wednesday of an approaching heat wave.

First, the simple part: Temperatures will begin to rise starting today and will peak Friday as they top 100 degrees in far eastern Contra Costa County. It could reach triple digits in Concord, Walnut Creek, Livermore, Pleasanton and Morgan Hill.

In San Jose, the thermometer will reach 90 degrees; along the Peninsula, some spots may see the 80s; in Oakland and Richmond, it will reach the high 70s.

Yet just as quickly as it rises, temperatures will drop by Saturday by 6-8 degrees in some areas. On Sunday, the hottest spots in the region won’t get past 85 degrees, according to the weather service. By the middle of next week, the hottest spots are forecast to be in the 70s.

Not exactly a typical heat wave — but then again, few things about the recent weather pattern have been normal.

“What we’ve had for a while now is a series of short-wave (high-pressure) ridges with short upper-wave (low-pressure) troughs, and that’s created a temperature roller coaster,” Sarment said. “Now the long-range pattern is starting to evolve into what we call a ‘dirty ridge,’ because there’s a low-pressure trough off the California-Mexico border near Baja that’s affecting how it evolves.”

Sarment also said that the so-called dirty ridge will be oriented over 75% of the state. That could mean record-breaking temperatures across the far northern part of the state and temperatures of 105 degrees in the Central Valley, according to the weather service. The Central Coast also is expected to bake, Sarment said.

Fire conditions in the Bay Area are expected to increase beginning today and through Saturday. Winds are not expected to be a problem, though Sarment said that areas of the region, particularly the Central Coast, “will have a wind period.”

“It going to get extremely dry,” she said.

It will be over by Sunday, and the forecast does not indicate that any other heat waves are arriving soon.

“By the time we get to Sunday, it will definitely be a welcome relief,” Sarment said. “It should last a while.”