A nagging upper high pressure has kept the San Francisco Bay Area hot for days. But far to the south it has been clear for several days that a Marine Surge was gathering steam for a lunge northward to the Golden Gate.
(Here is a time-lapse video of a Marine Surge as seen from the Berkeley hills.)
It is always very hard to forecast a Marine Surge and even harder to get the timing as to when it will hit the Golden Gate and shoot a flood of strong cool wind into the Bay ending a heat wave.
Typically I watch the satellite imagery to watch the surges progression from near Southern California. The problem with that approach is that only deeper fog shows in the satellite imagery so the leading edge of the Marine Surge can be 50 miles in advance of the imagery.
So I also watch the cams along the coast to get a feeling of how the fog is moving. But today, for only the second time in 25 years in forecasting, a Marine Surge arrived with no visible fog except near Monterey way to the south. But crazy strong winds and temperatures dropping fast made it clear a surge had arrived.
Since I knew there was a chance of such an event I added a note to many sites forecast saying “Much stronger if Marine Surge arrives.