The heat is about to return to the Bay Area again, just in time for the end of the work week. Come the weekend, however, the National Weather Service said another blast of coolness will move in, quickly smothering the high temperatures.

That pattern has repeated itself often enough over the past few weeks that it may seem like déjà vu all over again. This time, though, there is a subtle difference.

“Essentially, we’ve been cooler than normal (since Memorial Day), and the reason being is that there’s been a low-pressure trough up in the atmosphere and over the ocean,” NWS meteorologist Dylan Flynn said. “That trough is finally moving.”

As a result, Flynn said the overall weather pattern may become more conducive to a longer heat wave as the calendar pages turn. For now, the heat blast, like others before it, is expected to last only a couple of days.

“The center of the high pressure is offshore,” Flynn said. “When it’s like that and it slides toward (the desert southwest) or toward the Rocky Mountains, the high pressure moves right over us, and that’s when we start to see it get — and stay — really warm. We’re not going to be that warm, because we’re still getting cool air from over the ocean, and that air is cooler than normal.”

So it is that the warmup, though significant, is not expected to last very long.

As usual, the hottest spots will be in the far inland parts of the East Bay. Cities in eastern Contra Costa County, including Antioch and Pittsburg, are expected to break the 100-degree barrier today and then perhaps again Friday, and Concord is expected to peak at 99 on Friday. In Livermore, generally the hottest spot in Alameda County, the temperature is expected to reach 96 on both days. Pleasanton is likely to reach 93.

In the South Bay, Morgan Hill is forecast to reach 95 today and 91 on Friday. In San Jose, the thermometer is expected to max at 87 today and 88 Friday. Near the Bay, San Mateo is expected to reach 76 today and 74 on Friday, and Oakland is likely to hit 74 today and 72 on Friday.

All of those temperatures are normal for this time of year, Flynn said.

“Normal is still warmer than it’s been,” Flynn said. “We’ve been below normal now for weeks. So even though this is just a return to normal, it’s going to feel a lot warmer. It takes the body time to acclimate. Last year, when we had the long extended heat waves, this would not have been a big deal. This year, it is a big deal.”

Flynn said light winds from 5 to 10 mph will accompany the heat and that because they are blowing in from the ocean, “They’ll carry a bit more humidity.” Thus the fire danger is expected to remain as low as it can remain during a hot summer day.

“We still have a lot of vegetation drying above the marine level,” Flynn said. “It’s dryer than normal in the higher elevations and further inland.

“So we’re getting primed at some point for a big fire-weather week. We’re just not there yet.”