While warnings for travel to the United States aren't new, they have picked up significantly in the last few weeks, following these events, said Ed Daly, who oversees the content for the Global Intelligence Division of iJET International, a travel intelligence firm based in Annapolis, Md. "After a lull, there was a rash of incidents which happened one after the other and led to these advisories," he said.

On Oct. 16, for example, the Canadian government updated its travel advisory for the United States. In the safety and security section, the advisory says that travelers should be aware that, "The possession of firearms and the frequency of violent crime are generally more prevalent in the U.S. than in Canada." In addition, it advised against nonessential travel to Sonoma, Napa and Lake Mendocino in the wake of recent wildfires in California and Nevada.