The wind is the part of a hurricane you can hear coming. The water is the part that does most of the killing. Storm surge is the sea driven up onto the coast by the storm, a rise of ocean that climbs above the normal tide and comes inland over streets, dunes and first floors. It is the single deadliest hazard a hurricane carries, and it is the one the wind category on the news does not measure at all.
What storm surge is
Storm surge is the abnormal rise of seawater during a storm, measured as the height of the water above the level the tide would be at that moment with no storm present. It is not the tide and it is not the waves riding on top. It is the whole body of the sea lifted and shoved ashore. When forecasters add the surge to the ordinary tide they call the total the storm tide, and that combined number is what decides how deep the water gets and how far inland it reaches.
What drives it
The main engine is wind. The storm's winds blow across the ocean for hours and push the surface water ahead of them, piling it up against the coast where it has nowhere left to go but up and inland. The low pressure at the storm's center lifts the sea a little as well, but that effect is small. It is the wind, pushing on the water for a long time over a wide area, that builds the surge.
What makes one surge bigger than another
Several things set the height. A stronger storm pushes harder. A physically larger storm drives a wider, deeper mound of water than a small intense one. A storm that crawls ashore slowly keeps piling water onto the coast longer than one that races in. The angle of approach matters, and so does the shape of the shoreline and the slope of the sea floor just offshore. This is why two storms of the same category can leave very different water lines, and why the surge forecast is made for each stretch of coast rather than for the storm as a whole.
Why it matters
Most people who die in hurricanes drown. The surge comes fast, it comes with force, and a depth of moving water far shallower than people expect will carry a car or knock an adult off their feet. This is the reason coastal evacuations are built around surge zones and not wind. You can often ride out the wind where you are. You cannot ride out the sea arriving in your living room. When the order to leave a surge zone comes, the water is the reason, and the water does not negotiate.
Know when the water is coming for your coast.
The surge forecast is made for each stretch of shore, and the window to act on it is short. Magna Earth watches the storm against your exact coordinates and tells you when the surge threat reaches your zone, so you move on time instead of waiting for the water to decide for you.
Watch the water over your coast — from $14/mo