The onset of what weather forecasters have said will be the longest, most intense heat wave of the summer arrived a bit warmer than the National Weather Service anticipated, and they said temperatures may rise above expected temperatures again on Thursday.

“If the trend of Wednesday continues, it’s going to be hotter than advertised,” NWS meteorologist Dylan Flynn said.

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Bay Area temperatures are about about to rise a little higher

As advertised, the temperatures still are going to be hotter for a longer stretch than they’ve been at any point since the summer solstice. They are expected to rise to 101 in Antioch, 99 in Livermore and 98 in Concord, generally the three hottest places in the interior East Bay. In the South Bay, the thermometer is expected to reach 97 in Morgan Hill and 91 in San Jose.

The temperatures closer to the water won’t run quite as hot, but they won’t be as cool as normal, either. Richmond is expected reach 84, San Mateo 80, Oakland 79 and San Francisco 71.

Those figures led the NWS to issue the region a heat advisory for the first time this summer.

“By this time last year, we’d already had 23 summer days under a heat advisory, and most of those turned into a warning,” Flynn said. “So this year, 100 degrees is going to feel a lot different. It’s going to feel a lot hotter. People need to be aware of that and be careful.”

The agency extended the advisory early Thursday. It now ends at 9 p.m. on Saturday in the East Bay, South Bay and North Bay Mountains, as well as in the interior of Monterey and San Benito counties. The same heat advisory ends at 10 p.m. Thursday for the North Bay valleys, the shoreline of the San Francisco Bay and the Santa Cruz Mountains.

As hot as it will be, the temperatures still will fall short of what long-term models showed last weekend, when forecasters first thought the hottest temperatures might rise above 105.

This heat may also come with some humidity, Flynn said. High clouds may cover the region at times Thursday and Friday, an indication that monsoonal moisture from the southwestern desert is moving north. Flynn said dew points — the temperature to which air needs to cool in order to produce droplets — may rise into the low 60s around the region. Generally, dew points this time of year are in the low 50s, he said.

The monsoon influence will team with some instability in the upper atmosphere that could lead to dry lightning strikes, Flynn said, though he added that the possibility is probably “less than 5%.”

“It needs a trigger, and right now we’re missing the trigger,” he said. “So if we see any at all, it’s probably going to be confined to the higher terrain in the Sierra.”

The temperatures will start to fall on Friday and continue to do so on Saturday and into Sunday. But Flynn said the cooldown likely will be a more gradual one than initially forecast.

“The cooler air is going to come from west to east, but it won’t get to places like Morgan Hill or far into the East Bay until Sunday,” he said. “It won’t be a sharp drop off.”

The hottest places in the region Sunday are expected to reach 86 degrees.