Mercury Climbs as Heat Is On : Weather: County records a high of 107 in Van Nuys. And, thanks to a misplaced high-pressure system, it’s not cooling off this weekend.

How hot was it? That line used to guarantee laughs when Ed McMahon delivered it to Johnny Carson in Burbank, but they’re off the air now and nobody in the San Fernando, Santa Clarita or Antelope valleys found much mirth in the weather Thursday.

By 1 p.m. the temperature was already 107 degrees in Van Nuys and it stayed right there for almost three hours, according to Marty McKewon, a senior meteorologist at WeatherData Inc., which supplies weather information to The Times. That 107 was the official high for Los Angeles County on Thursday, but well short of the 121 degrees that made Death Valley the hottest spot in the country.

Every official reporting station in the three local valleys recorded three-figure temperatures. In descending order: Woodland Hills hit 106, Lancaster and Newhall reached to 103, Palmdale hit 102 and Burbank got to an even 100.

It’s not going to get much cooler anytime soon. “I’d say we can expect temperatures to be lower by about three to five degrees in the San Fernando Valley on Friday and through the weekend,” said McKewon, checking his computer. “About the same, then, for Monday and then it looks like we’ll start getting back to seasonal temperatures around Tuesday.”

In other words, if you don’t have air conditioning, plan to spend a lot of this weekend at your local mall or supermarket, perhaps near the frozen goods section.

You can blame these temperatures on a high-pressure system that is supposed to be over the Texas Panhandle this time of year but instead has migrated, at least temporarily, westward.

“What we are seeing is that the ridge of high pressure began to drift on Wednesday,” McKewon said, “settling over the Four Corners area where Utah, New Mexico, Arizona and Colorado meet.”

Because the system suppresses the downward flow of air, it inhibits the formation of clouds, “allowing full solar heating over the Southern California area,” he said.

To make matters worse, the offshore airflow that normally moderates temperatures is almost nonexistent.

“It’s common in the summer to have a low-pressure system over the deserts and this causes air to blow in from the ocean toward the lower pressure,” McKewon said. “This cools off the coast and to a certain extent, the valleys.

“But the air mass is basically stagnant right now. You’re just getting intense heating.”

If the temperatures do slightly dip as McKewon predicted, it will be thanks to an increase in that air flow.

“For Friday, Burbank will have a high of about 97 and then on Saturday and Sunday it will be about 95,” McKewon said. “In Lancaster and Palmdale the high will be about 97 for the weekend days with possible isolated thunderstorms.”

And what about sweltering Van Nuys?

“For Friday, we’re looking at a high of 103 and then it will get to about 100 on Saturday,” he said.