Armistead Maupin's book, The Days of Anna Madrigal, is deep in his series about connected characters in San Francisco. That means that many readers will come to this first sentence already trusting the author, already connected to the characters, already eager to read.
Did that allow the author to open the door to his story with less urgency? This I cannot know. However, I will note that the first sentence of the first book of the series, Tales of the City, contains a person and their circumstance in more classic first sentence form: "Mary Ann Singleton was twenty-five years old when she saw San Francisco for the first time."
So Maupin knew the form and chose to do something else here.
Let's see what we have here. He starts with the weather! He makes the bold choice to start with the most widely shared topic of conversation. Weather is a polite topic. Everyone experiences it, everyone has some interest, and yet no one becomes exercised over it. So we can kindly exchange a few words about it, extending a social interaction, at very little risk of acrimony.
Weather, in other words, unless extreme, is the opposite of dramatic.
The words up to the comma are common. Summer temperatures do vary, and, as averages have been trending hotter, there's nothing unusual about a summer that's warmer than usual. This is an ordinary observation, that any acquaintance might express to another.
Do we feel bored by that? Or do we feel at ease with it?
After the comma, the language becomes stronger. "Heat" is more than "warmth" and "throbbed" is more intense than either. It's a distinctive word, seldom applied to weather. I'm quite a bit less likely to tell my neighbor "Throbbing day today, isn't it?" Throb is a word associated with pain or passion – my neighbor might find that question odd or laden with innuendo.
Then there's the poetic phrase "coaxing pale fingers of fog." That's four uncommon words in a set of five. "Coaxing" is normally something people do, as "fingers" are something people have – this makes it as if both the East Bay and the fog are human. At this point, we've left the direct, commonplace language of the first clause entirely.
Any self-assured city might be "the city" to those who live in its orbit. Adding summer fog and "the East Bay" outlines San Francisco.
Altogether, this sentence gives us a location, starts conversational, and moves into poetry. It's promising the pleasures of company and language. Instead of grabbing, it seduces.