It's hard to forget last year's hurricane season and the lessons we all should have learned from it.

Here are two facts I bet you didn't know: Floods occur in all 50 states. And most homeowners insurance does not cover flood damage.

Here's another fact: The Atlantic hurricane season begins June 1. It's important to remember that date because flood insurance requires a 30-day waiting period to take effect. This means that if you have damage from a flood, you'd better hope and pray that your policy was in place 30 days before the damage, or you are not covered.

Only about half of homeowners living in some of the most flood-prone areas of the United States buy federal flood insurance, leaving millions of families at risk for severe financial losses when floods strike, according to a study released this year by the Rand Corporation, a nonprofit research organization.

''Forecasters say this hurricane season could be as destructive as 2005, the worst on record," said Jesse Muñoz, acting director of the transitional recovery office for Mississippi's Hurricane Katrina efforts.

For most homes, apartments, and businesses, the only insurance protection against damage from rising water is flood insurance underwritten by the National Flood Insurance Program (NFIP). This program is greatly underused, even though flooding is a major source of loss to throughout the United States, according to the Rand study.

Most homeowners who buy flood insurance do so because they are required by their lenders. That's because they live in areas considered most vulnerable to flooding, the study found. Just 20 percent of homeowners living in the most flood-prone areas buy federal flood insurance when they are not required to, the study found.

One reason many people don't buy flood insurance is simple: They don't think they need it, said Lloyd Dixon, senior economist at Rand and lead author of the study.

But roughly 25 percent of all claims paid by the NFIP are for policies in low- to moderate-risk communities, according to the Federal Emergency Management Agency.

If you don't live in an area with a high flood risk, your premium could be as low as $112 a year, which would cover about $20,000 in building damage and $8,000 in contents. You have the option of selecting coverage in a range of yearly premium amounts. For example, for $180 a year, you can get $50,000 in building coverage and $20,000 in content coverage.

If you want more protection, and, again, if you aren't in a high-risk area, you can expect to pay as much as $317 a year for $250,000 worth of building coverage and $100,000 in content coverage. That's about $26 a month. It would cost two people more than that to go to a movie with popcorn and sodas.

Here's something else you need to know and probably don't: Flood insurance does not cover improvements to a basement, such as finished walls, floors, carpeting, and personal belongings.

It does cover your home's foundation. Functional items in your basement are covered under your policy's building coverage, such as plumbing, furnace, water heater, and circuit breakers.

Other limited items, such as a washer and dryer or a freezer, are covered under your contents coverage, which must be purchased in addition to building coverage.

In fact, Dixon said that was another reason some people gave for not buying flood insurance. This was particularly true for people who live inland.

But even with its limitations, flood insurance is worth getting.

If your community participates in the NFIP, you can purchase flood insurance from a licensed private insurance company or through an independent property and casualty insurance agent in your state.

For information on flood insurance, go to www.floodsmart.gov.

Last year, as I read reports of Katrina families and their hardships, I made a mental note to purchase flood insurance. But I didn't do it. This year, I will. I don't want to learn my lesson the hard way.

Michelle Singletary is a columnist for The Washington Post.