Weather forecasters seem to agree that by Wednesday of next week, it should be feeling very warm throughout California. But a heat dome taking shape over the western half of the US will either bring us a major heatwave, or just a mild warming trend in the Bay Area.

It's all about position. Where the heat dome — a strong ridge of high pressure — ultimately takes shape and comes to rest for a couple of days will determine how much of California gets bathed in extreme heat. As the Chronicle's meteorology team explains, multiple forecast models are in agreement about the heat dome, which will form by Wednesday and stick around through Friday, but it's too soon to say where it will be positioned.

That will mean the difference between hot city days and nights and guaranteed warmth at San Francisco beaches, and just your average, warm-ish, sunny days at the beach with strong onshore breezes and temps in the 70s.

The National Weather Service's Bay Area bureau doesn't sound too confident in the strength of the heatwave as of now. Their forecast for the coming days, in a tweet, goes like this: "Thu-Fri: Nice and warm. Sat-Mon: Nice and maybe still a little warm. Wed: ...a little hot? Unless you live along the coast, of course."

They're predicting temps in the 90s in places like Santa Rosa, San Jose, and Cloverdale by Wednesday. But they're not going beyond that just yet. And their forecast still has SF with a high of 73 on Wednesday.

Apple's Weather app also currently shows a high of just 74 on Wednesday, with highs in the 60s next Thursday and Friday.

But if the heat dome really settles in over California, and not more to the east, that could mean 100- to 108-degree temperatures in the Central Valley, and perhaps SF's highs will creep into the 80s. (The European forecast model currently favors this, but the American model does not.)

Meanwhile, another weather service meteorologist, Brayden Murdock, tells Bay Area News Group that this will be "the biggest heat event of the year, so far." And people in inland areas, especially, should brace themselves for "sneaky" heat.

The Chronicle also notes that there's an "unusual component" to this possible early season heatwave, which is the possibility that subtropical moisture well get dragged in behind the heat dome, presenting the possibility of thunderstorms in California late next week. But it's too early to say where or if these will actually occur.

In any event, it should be a lovely weekend in San Francisco — not too hot for those who don't like the hot, and those of us without air conditioning. Today (Thursday) is the warmest day for now, with a forecast high of 68 in the city, and a cooling trend coming ahead of the heat dome, if it arrives.

Photo: Jeremy Ricketts