"The coldest winter I ever saw was the summer I spent in San Francisco"
- Attributed to Mark Twain, but source unverified
This year's U.S. Open is being held this weekend at The Olympic Club in San Francisco. The Olympic Club sits just inland from the Pacific Coast and south of Lake Merced Park, as illustrated by the two Google Maps images below.
During the summer, high pressure and low clouds are a semi-permanent feature off the California Coast, whereas low pressure dominates over the Central Valley and Great Basin. Indeed, this is the case today.
This leads to large contrasts in surface weather from the Pacific Coast, which is inundated by cool marine air, and the Central Valley, which has a more continental summertime climate. For example, maximum temperatures yesterday increased from the 50s along much of the Pacific Coast to as high as 101ºF at one MesoWest site in Sacramento.
The Point Reyes Lighthouse on the top of Point Reyes National Seashore (left side of the image above) is figuratively one of the coldest places I've ever been. It experiences strong northwesterly flow for most of the summer with temperatures in the 50s. It is a major shock to the system in July! San Francisco often features large (~10ºF) variations in temperature depending on location.
The only MesoWest site near the Olympic Club is a citizen weather observer station (K16BSW) about 2 miles to the north. Over the past five days, temperatures even in the afternoon have been in the 60s, whereas morning temperatures have been near 50. Winds have been predominantly southerly, although there was a period of stronger northwesterly flow late yesterday with gusts over 20 miles per hour.
The weather during the US Open will be strongly affected by the battle between marine and continental airmasses. When the marine air dominates, cold, cloudy, windy conditions prevail. When it doesn't, it's still cool, but cloud cover is less prevalent and temperatures can be a few degrees warmer.
For the first round today, the marine airmass will continue to dominate. It will be a cool, cloudy start this morning. However the forecast models call for low pressure to develop over southern California on Friday.
Over northern California, this should yield large-scale offshore flow and, for the Olympic Club, improving weather during the day on Friday that should persist through at least Saturday. The players should be quite happy with this shift as it will probably be less cloudy and warmer than one would expect based on climatology.