• Lewis Samuels, who missed the alert about a possible tsunami and the later cancellation, heads out to surf at Ocean Beach in San Francisco Tuesday. The alert didn’t cause too much of a stir in San Francisco. Photo: Paul Kuroda, Special To The Chronicle

When a 7.9-magnitude earthquake shook the Gulf of Alaska on Tuesday, tsunami experts knew almost immediately that a turbulent surge of water had been set in motion and just how fast it was traveling. The wave was on pace to strike California in four hours, and several cities, including San Francisco, alerted residents that danger could be headed their way.

It was a false alarm — no blast of seawater arrived. The problem, experts say, is that scientific models that are good at projecting the trajectory of tsunamis are not so good at forecasting their strength.

“In a sense, it’s like what happens with sound in a Gothic cathedral,” said Lori Dengler, a professor of geology at Humboldt State University. “We have a good picture of what it looks like, but the sounds echo in very different ways. … How many wave surges are there going to be? When is the largest one going to happen? These sorts of details are hard to assess.”

Tuesday’s tsunami was unleashed by a 1:32 a.m. quake off the southern coast of Alaska, near Kodiak Island. No major damage was reported, but within minutes, the National Tsunami Warning Center sent out a warning — the highest level of notification — to residents on coasts of Alaska and British Columbia. It told residents to head to higher ground immediately, prompting people to jam roads leading away from the coast and flock to evacuation centers.

For California, Oregon and Washington state, the warning center issued a tsunami watch, the lowest of its three levels of alerts. The notification, which means the threat is still being evaluated, is based only on the size of the earthquake and the location. Scientists at the warning…