Heat from the Central Valley is critical to generating the winds in the San Francisco Bay Area. The heat in the valley creates low pressure, which creates a pressure gradient that draws cool ocean breezes through gaps in the Coast Mountains and our launch site.
But too much heat could cause a Central Valley low to expand over the coast, producing weak winds. This image and prediction show how this happens:
With the exception of brief outboard peaks at Stick and Coyote and 3rd, the winds weakened significantly at all locations, especially offshore. This sudden wind attenuation occurs when:
1. The upper ridge at about 18,000 feet (500 MB) moves toward the west coast.
2. This high pressure dumps hot air over California, even creating heat warning conditions near the coast and shattering marine stratus clouds.
3. This heating causes the Central Valley low to expand over the coast, weakening the pressure gradient.
4. Low pressure causes high-altitude north-northeast winds to blow northwest sea breezes away from our coast and bring hot air into the Bay Area.
5. Later that day, the low pressure retreated inland near the East Bay coast.
6. The resulting weak pressure gradient brings winds in the mid-to-teens to the middle of the bay, but hot bubbles expanding from the hot land keep most of the water away from the coast.
