Motorists drive along a snow covered Interstate-94 in Detroit, Jan. 2, 2014. (Joshua Lott/Reuters)

As a massive winter storm bears down on the Northeast United States, shoppers are heading to stores from New Jersey to Maine, buying everything from batteries to cans of Campbell's soup before the snow piles up.

What do you really need when a storm hits? A disaster preparedness expert breaks it down for ABC News.

1. Food and water: People in the Northeast should get ready to stay in their homes for several days if necessary, which means they should stock up on enough food and water to last at least three days, according to Irwin Redlener, the director of the National Center for Disaster Preparedness at Columbia University.

"Think about things that are non-perishable, in case your refrigerator's not working," Redlener told ABC News. This storm will be relatively short, though, which means fruit and vegetables should be OK, he said.

2. Lights out: In case the power goes out, people should have flashlights and battery-operated lanterns ready to switch on, said Redlener, who recommended checking to make sure you have enough batteries to power them. "Make sure your cell phones are charged," he said. "Make sure your gas tank is filled up."

Stay away from burning candles, though, he added - they could become a fire hazard.

3. Medications: If you take medications, now would be a good time to check and make sure you have enough stocked up to last you the next week, Redlener said.

4. Diapers, diapers, diapers: For those taking care of babies, it's important to purchase enough diapers, formula and baby food to last through the storm, according to Redlener. Same goes for the elderly and others with special requirements - make sure their special needs are covered, Redlener advised.

And don't forget those cats and dogs. A quick check to ensure you have enough pet food for the next several days would be a good idea, according to Redlener.

5. Stay informed: If the electricity does cut off, a battery-powered radio or crank radio can keep you informed, Redlener said.