Homeowners insurance does not cover flood, and storm surge is flood. A flood policy bought when a storm is in the Gulf usually will not help with that storm.
FEMA’s own wording: "Typically, there’s a 30-day waiting period from date of purchase before your policy goes into effect." FEMA links to a page of exceptions, and your agent can tell you whether one applies, but the rule of thumb is simple. If you want flood coverage for September, buy it by August at the latest, and June is better.
Surge can reach farther than expected
Around Mobile Bay, the Weather Service has recorded surge of 12 to 14 ft at Bayou La Batre in Katrina and 7 to 9 ft in the back bays of southeast Baldwin County in Sally. Our guide to flood zones and flood insurance shows how to look up your address and what the zones mean.