Temperature records in San Francisco began in 1874 and this August will tie as only the 4th time that there was not a daily maximum temperature all month that did not exceed 70 degrees. In 1942 the highest reading for the month was only 67 degrees, while in 1917 and 1882 the highest was 69. On August 8, 2016 the maximum was just 70 degrees, tying it with 1881.
So far in August 2016 the average maximum temperature has been 64.6, and given forecast maxima of around 64 degrees for the remainder of the month this will change little. The normal August high temperature in San Francisco is 68.1. But even with prolonged coolness, the average for the month will only rank as about the 35th coolest on record.
The major player in the mild weather has been a persistent weak trough of low pressure over the West Coast for most of the month and a likewise persistent marine layer. It should be noted that the sea surface temperatures along the coast west of San Francisco remained above normal for the entire month.
The story is different farther inland and away from the cool coastal air with August monthly maximums generally above normal.
Finally I would be remiss if I did not point out that Mark Twain NEVER SAID "the coldest winter I ever spent was summer in San Francicsco". (per http://www.marktwainproject.org/). But if he had, it would likely have been a year like 2016.
Jan Null, CCM
Golden Gate Weather Services