I use windy.com. Select the Air Quality layer, then PM2.5. You can play a 5-day forecast. A clean air window is opening for a few days starting tomorrow.
They're just guessing. The current condition presents a completely new set of inputs that the existing models cannot handle. A good example is heavy smoke keeping temps in the central valley lower than normal, thus suppressing the onshore winds that normally keep the bay clear this time of year.