What should you do during a flash flood? Never walk or drive through floodwaters

• Flash flooding swept across the Northeast US this week, causing one death and millions in damage.
• Flash floods are often caused by slow-moving thunderstorms or tropical storms and can happen in an instant.

Heavy rainfall inundated the Northeast this week, causing deadly flash flooding, submerging buildings, and turning streets into murky waterways.

On Monday, more than 9 million people stretching from Alaska to Maine were under flood alerts, according to CNN.

A 35-year-old woman died in Orange County, New York, after floodwaters swept her away, and a county executive in her town said he expects the damage to be tens of millions of dollars, CNN reported.

So what can you do to prepare for a flash flood ahead of time and protect yourself during one?

Here's everything you need to know about the types of flood alerts, what causes flash floods, and how to stay safe:

What are flash floods?

Flash floods are different from regular floods because of how quickly they develop, as the name suggests. They can form in a few hours or even minutes, according to the National Weather Service, and are the deadliest weather-related event in the US.

Flash floods happen after heavy rainfall hits a region — like during a slow-moving thunderstorm or a tropical storm — and can worsen depending on how long the rainfall lasts, according to the NWS.

The topography of an area and the condition of the soil can also affect the formation of flash floods, and even lead to mudslides, the NWS says.

What is a flash flood warning?

There are a few different levels of flood warning that the National Weather Service uses to alert Americans of risk factors — flash flood warnings and flood warnings are at the top of that list.

The service issues a Flash Flood Warning or a Flood Warning when flooding is "imminent or already happening," according to the NWS website. The agency says people living in flood-prone areas should immediately "take action" and move to higher ground.

A Flood Advisory is one level below a warning when the flooding will likely only be a "nuisance," according to the NWS. Under this type of warning, the service says people need to "be aware" and exercise caution.

And below that is a Flood Watch, when the weather conditions might result in a flood. The service says under this warning, people should "be prepared" for potential flooding.

What to do before a flash flood

There are also a number of ways you can prepare for a potential flash flood before it happens.

First, you should make a plan with your roommates or family members for an emergency situation, including how to escape if necessary. You can find detailed planning resources on the American Red Cross's website.

It's also important to have an emergency go-bag filled with essentials ready in the event of a crisis, like a flash flood. The New York Times suggests filling your bag with items like bottled water, a portable phone charger and cord, nonperishable food, a first-aid kit, and even cash.

And above all, make sure you stay up to date on the latest information about where and when a flood is advancing, and how to get away from a dangerous area.

What to do in a flash flood to stay safe

One of the most important things to remember during a flash flood is that no matter how shallow or slow-moving the flooding may appear to be, you don't always know how deep or fast it actually is. The National Weather Service says you should never walk, drive, or swim through swift or flowing water.

"If you come upon flood waters, STOP! TURN AROUND AND GO ANOTHER WAY," the NWS's website says.

The service also advises you to get to higher ground if you are able to before flooding reaches your area.

And if your car stalls while it's in floodwater, the service says you should "abandon it immediately and seek higher ground," according to its website.

For more information on flood safety and preparedness, review the American Red Cross's guidelines.