Why Storm Surges Are Worse Than Normal Flooding

A hurricane’s category only tells you about its wind speed, but much of its devastation comes from flooding. With Hurricane Hilary approaching the West Coast, some areas could see as many as 10 inches of rain plus a storm surge that, the National Hurricane Center says, could come with “large and destructive waves.”

What is a storm surge?

The winds of a hurricane push water around, even while the storm is still in the ocean. Water that is pushed onto land becomes the storm surge.

Think of how the tide can roll in from the sea and cover up land that was previously bare. It’s like that, except you have tides in addition to the storm surge, raising water levels even higher. (The surge plus the tide are called a storm tide.)

What is the difference between a storm surge and a flood?

Technically, a storm surge is a flood, but it’s a specific kind. In a storm surge, sea levels rise far above where they normally would from an ordinary high tide.

Storm surges can flood large areas along the coast, the National Severe Storms Laboratory explains. Surges are, they say, “often the greatest threat to life and property from a hurricane.”

Where does the storm surge happen?

Surges are expected to be particularly bad in a stretch of the Baja peninsula of Mexico, from Punta Abreojos north to Ensenada. Coastal areas of Mexico and Southern California could see surf as high as 5 to 9 feet.

But flooding from a surge is not just an issue at the coast. Storm surges can cause flooding in upstream rivers, or in any low-lying areas the surge water reaches. Arroyos are expected to flood, and the Hurricane Center says that urban flooding and flash floods are likely, due to the surge and to rainfall. They describe the potential flooding as “rare and dangerous.”

How much flooding should we expect?

Total flooding depends on the storm surge, the tides, and rainfall. It varies depending on the exact contours of the land. And your safety in the storm depends on other factors as well: Perhaps your house will be dry, but if all the roads are flooded, that doesn’t help you much. Bottom line, if you or a loved one are in the area, the most important map is an evacuation map.

But if you’re curious about the predicted flooding, here are the Key Messages for Hurricane Hilary from the National Hurricane Center. This is the first time a Tropical Storm Watch has been issued for southern California, so the available maps and predictions don’t seem to be as detailed as they sometimes are for Atlantic hurricanes. Keep an eye on public advisories from weather.gov, listen to local warnings and evacuation orders, and read our guide for the safest places to be during a hurricane.

This post was originally published in 2018 and was updated on Aug. 18, 2023 in reference to the approach of Hurricane Hilary.