The traditional pattern is for tropical depressions to form off the coast of Africa and slowly move up the Atlantic. This gives American households enough time to prepare.

But a new pattern is emerging. Instead of forming up off the African coast, some storms now start off the U.S. coast, in the Gulf of Mexico or the Atlantic Ocean — like last week’s storm.

This means there is less time to prepare.

Storms that develop just offshore could easily hit the coastline within two or three days. And these pop-up storms could be on the rise, according to some forecasters.

The most well-known pop-up storm is Hurricane Humberto which hit Texas in 2007. It developed and intensified into a category 1 hurricane within one day. Damage caused: $50 million.

The presence of warm water in the Gulf and the Atlantic Ocean — where waters heat up faster than waters off Africa — could be a key factor in the formation of these pop-up storms. Once conditions are favorable, the pop-up storms can quickly reach category 4 or 5 status.

There are other forecasters who do not believe pop-up storms are increasing in frequency. But even these forecasters think pop-up storms should still be a cause for concern.

They are likely to contribute to excessive rain and flash floods. Judging from storm behavior in recent times, most deaths from tropical storms have been caused by storm-related floods, according to the National Hurricane Center based in Miami.

And because there is less time to warn people about them, more people might be at risk.

Storm surge specialists advise people not to wait until a storm forms off the coastline. There may no longer be enough time to think about what to do. The best thing is to prepare emergency plans beforehand.

Safety Tips:

• Prepare a Hurricane Kit. Stock it with: several changes of clothes, extra cash (some prepare $400-$500), flash lights, batteries, bottled water, personal hygiene items, non-perishable foods (such as cereal, canned goods) good for several days, medicines used regularly, candles and matches, baby supplies (if applicable), board games, crayons and other supplies to amuse children.

• Pack important documents and special photos in secure plastic. Personal identification and important papers cannot be replaced easily so make sure to secure them: birth certificates, social security cards, deeds to property, diplomas and GED certificates, etc.

• Make sure to know the vulnerabilities of your home to storm surge, wind, and flooding. Identify safe areas in the home or the community for each possible hazard your family will face.