That's quite a bit different from Saturday, when the wayward whale wasn't officially observed until late afternoon.
The USCG official, who asked not to be identified, said the most recent sighting was around 1:15 p.m. near the San Diego-Coronado Bay Bridge.
Boaters were urged to give wide berth to the roughly 30-foot-long whale, which was first discovered in the bay on Tuesday.
Officials hope it will find its way back into the ocean to join its fellow cetaceans on their annual migration from the lagoons of Baja California, where they calf and mate in the winters, to Alaskan waters, where they spend summers.
The wayward whale was spotted near the North Island Naval Air Station about 5:30 p.m. Saturday. It made only two appearances all day, disappointing hundreds of would-be whale watchers.
A whale expert from the U.S. National Marine Fisheries Service has been observing the whale, and the U.S. Coast Guard has been warning boaters to stay at least 100 yards away.
Joe Cordero of the Marine Fisheries Service said the whale, believed to be 1 or 2 years old, may be migrating by itself for the first time. And, like a juvenile human, the whale is probably just curious. It could have chased some food into the bay and become disoriented.
It is uncommon for whales to stray from migration routes, but it happens. In 1992, a roughly 35-foot gray spent about two weeks in San Diego Bay, until it was found dead with a gash to its head. The whale apparently had been struck by a boat.
Gray whales, which can reach more than 50 feet in maturity, travel some 10,000 miles annually. Around the end of February, southbound stragglers mix with whales already heading north; so it's hard to say which way the whale in San Diego Bay was headed when he left the open ocean, Cordero said.