September is typically the warmest month on the central California coast, but be aware that Asilomar is on the open ocean, subject to marine air flows. The average September high is 70 F (21 C) and the average low 53 F (12 C). If the fog rolls in while you are strolling the beach, you will regret having failed to pack a heavy sweater and windbreaker. Recall the misattribution to Mark Twain “The coldest winter I ever spent was a summer in San Francisco.” Rain, however, is very rare in September – an average of 0.6 cm.
Asilomar’s daily weather web site is http://www.visitasilomar.com/monterey-conditions.aspx