Winter Storm Jonas, Storm Surge, and the Science of Coastal Flooding

Winter Storm Jonas, poised to pound the mid-Atlantic this weekend, could bring record-breaking conditions. Some of the greatest danger will exist along our coasts, as high winds and high tides contribute to storm surge and potentially massive waves.

With every major weather event these days, people understandably want to know if there’s a climate change connection. So here goes.

To get at the coastal flooding connection, let’s get the questions of snow and storms out of the way. On snow, please see my UCS colleagues’ excellent summary of the relationship between global warming and snow. In a nutshell: we should expect snow and snowstorms in a warming world, and sometimes in big ways because of increased extremes. In terms of extremes, please see the most recent National Climate Assessment for a discussion of the effect of global warming on the frequency and intensity of extreme weather, including major storms.

This leaves us with one direct connection between anticipated flooding and climate change, which we’ll get to below.

Here’s what to expect from Jonas and why:

Expect serious flooding

Across the mid-Atlantic and into southern New England, Jonas could cause minor, moderate, and even major flooding. The most severe flooding is currently expected in both coastal and back bay areas of Delaware and New Jersey. As the National Weather Service warned residents across large areas on Friday, “THIS COASTAL FLOOD WARNING MEANS THAT MODERATE OR MAJOR TIDAL FLOODING IS IMMINENT. BE PREPARED FOR RISING WATER LEVELS AND TAKE APPROPRIATE ACTION TO PROTECT LIFE AND PROPERTY.”

Blame storm surge

Jonas will create storm surge, which in turn drives water ashore, causing flooding. There are several key things that drive storm surge—the shape of the coast, the direction from which the storm approaches, the ocean bottom, and others—but the primary driver is wind. As winds push surface water toward the coast over a long expanse of ocean, that water builds up and you have a “surge.” Stronger winds cause bigger surge. Larger storms push water over a larger expanse of ocean—a distance known as “fetch.” The larger the storm, the longer the fetch; the longer the fetch, the bigger the surge.

• Jonas is currently expected to cause storm surge of 4 to 5 feet in Delaware and 3 to 4 feet along the Jersey Shore.
• As the National Weather Service warned earlier in the week “High potential exists for widespread moderate to major coastal flooding. We only need a 1.5 foot storm surge near the time of Saturday morning high tide along the NJ and DE Atlantic Coasts.”
• So we’re starting with some forecasts of storm surge. Let’s add on….

Beware the waves

It is one thing to get wet; it is another to be pounded by the awesome power of waves. And amidst high winds and storm surge, very large and dangerous waves can result.

If we dust off the cobwebs, we recall that ocean waves are not the movement of water, but the movement of energy; this movement happens in an orbital motion across long distances of ocean. But in storms, we’re talking about a LOT of energy being transferred from the atmosphere to the ocean’s surface waters. And as this orbiting wave energy nears shore and encounters the ocean bottom, it is pushed higher, creating the bulging and cresting waves we see from the beach.

During a storm, the water can be pushed ashore with great velocity and to alarming heights. At approximately 1,700 pounds per cubic yard, it can do tremendous damage to what lies in its path. When these waves ride atop storm surge they can not only give the shore a greater pounding, but can reach farther inland on the back of the surge. (For more on the destructive force of waves, please see this 2013 blog by my colleague, climate scientist Brenda Ekwurzel.)

Jonas is currently predicted to generate wave heights of 15 and 20 feet along the Jersey and DelMarVa (Delaware, Maryland and Virginia) coasts. Now we have storm surge + waves. See this helpful (but scary) animation that shows how they combine.

But there’s more.

“Storm tide”—timing is everything

The extent to which sea water actually floods our coasts this weekend will depend on storm surge, waves, and tides. (If this storm were bringing rain, instead of snow, flooding would also be determined by that immediate freshwater input. Let’s hear it for snow?)

The impact of storm surge and the waves that ride atop it is far greater if the storm strikes at high tide. If a Category 3 hurricane arrives at low tide in a region with a 10-foot tidal range, far less water will reach onto land than if it had arrived at high tide. This is one reason that New York City was hit so hard by Hurricane Sandy and Boston was largely spared. Sandy made landfall in New York at high tide, and in Boston, close to low tide.

Jonas is expected to linger over parts of the East Coast through three high-tide cycles. So, high tide + storm surge + waves.

Full-moon storm tide—timing is everything, and this timing is bad

Twice a month, during the new and full moons, the earth, sun, and moon align and the combined gravitational force they exert on the earth’s oceans cause high tides to rise somewhat higher and low tides to drop somewhat lower.

Jonas is expected to hit during the full-moon tide, meaning that the high tide in places like Cape May, NJ, will be running roughly a foot higher than it was last week. So Jonas will have an additional foot of water, yet another, to work with along the mid-Atlantic thanks to the extra-high tide. So now we have full-moon high tide + storm surge + waves.

The unfortunate sea level rise boost

Global warming, by melting land-based ice and increasing water temperatures and causing thermal expansion, has caused an average sea level rise of about 7 inches around the world, since 1880. But that amount varies widely, based on local and global factors. The East Coast has seen some of the fastest increases, and the mid-Atlantic some of the fastest of all. During World War II, when my grandfather was sailing destroyers out of Norfolk Naval Base, a typical high tide there would have been roughly a foot lower than it is today.

• In Atlantic City, where the flood risks are high this weekend, sea level is 9.5 inches higher today than it was just 50 years ago, giving Jonas that much more water to work with.
• In Washington, DC, and New York City it is 7 inches higher.
• The rate of increase is accelerating, with many East Coast locations projected to see another 12 inches in just the next 30 years.
• Thus, today, we have sea level rise + full moon high tide + storm surge + waves

So we have a dangerous weekend ahead, and we have a long, concerted push ahead to make our coastal communities safer and more resilient to inevitable storms and flooding. Stay safe everyone.

Featured image: Michael Dwyer/Associated Press