The long awaited heat dome is hanging right over California today. But you wouldn’t know it if you woke up this morning in San Francisco, which has stayed cool today. The “fog” season is over, so what’s causing this mild weather? It is largely due to the shifting winds. A heat dome—a powerful high-pressure system that sweeps away any cold air in its wake—creates an interesting phenomenon known as marine cloud compression. The sea breeze blows cool, Pacific hazy clouds and fog into the communities along San Francisco Bay for most of the summer. This changes by September, when the winds in Northern California begin to blow from the north and northeast. The warm, dry air from the Central Valley acts like a hair dryer on neighboring Bay Area communities. Cities like Oakland, Napa and Sonoma are suddenly losing the patchy fog that usually blows into parts of the North Bay and East Bay from the sea breeze. Marine clouds are compressed (or compressed) back to the coast by these drier winds. And this holiday weekend’s heat dome is only accelerating that process. San Francisco is the final destination for these clouds before they are released again. With nowhere to go, the cool air and its clouds that would normally spread to all corners of the Bay are concentrated in the only part of the Bay Area where it can still stick: San Francisco. Because the city is surrounded on three sides by water, it takes longer for the heat dome to compress the marine layer further. That’s not to say San Francisco can’t experience triple-digit heat like its neighbors to the east. It saw plenty of that during the historic heat wave of 2017. But that means the city’s geography acts as the last stop for the sea layer whenever warm, dry air blows in from the rest of California. The good news for San Francisco is that this heat dome is nothing like the one we saw in 2017. But over the holiday weekend, this cool marine layer will be increasingly crushed by the heat dome. So expect fog to become more limited on the west side and temperatures to rise into the late 70s and maybe even 80s for some spots east of Sutro Tower over the next few days. Gerry Díaz is the meteorologist for The San Francisco Chronicle. Email: [email protected] Twitter: @geravitywave
