Storm surge and storm tide: when storms raise the sea

What storm surge is

A tide chart tells you when high and low tide will come and how high the water will get because of the Moon and the Sun. That’s the astronomical tide, and it can be calculated years ahead. But the weather also moves the sea, and during a storm it can move it a lot. The National Hurricane Center (NHC) defines storm surge as an abnormal rise of water generated by a storm, over and above the predicted astronomical tide.

A few things to know about it:

• It can reach far inland. The NHC notes that surge can even travel up rivers and canals, well inland from the coastline.
• It can also be negative. When strong winds blow offshore, the water drops below the predicted tide, which can leave boats aground in shallow harbors.
• It’s the deadliest part of a hurricane. The NHC calls storm surge the leading cause of hurricane deaths.

On an ordinary day, the weather only nudges the tide. According to NOAA’s comparisons of predicted and observed water levels, the typical difference at Charleston or the Battery in New York is around 6 to 7 inches (15 to 18 cm). In a storm, the surge can run to several feet.

What drives it: wind, pressure and waves

Wind does most of the work

The UK Met Office puts it simply: the main cause of storm surge is strong wind pushing seawater toward the coast, where it piles up. When that wind blows for hours over a wide stretch of ocean, the water has nowhere to go.

How high it gets depends on the storm and the coast. The NHC lists these factors:

• Intensity: stronger winds produce a higher surge.
• Size: a larger storm produces a higher surge.
• Forward speed: a faster storm pushes a higher surge at the immediate coast, while a slower one floods farther inland.
• Angle of approach: a storm moving straight onshore tends to produce more surge than one running along the coast.
• The continental shelf: a wide, gently sloping shelf allows a higher surge. That’s why the coasts of Louisiana and Mississippi are so exposed, and Florida’s southeast coast, with deep water close to shore, is less so.
• Coastline shape: surge runs higher where a storm hits a concave coastline, and features like barrier islands, inlets, sounds, bays and rivers change how the water flows.

Low pressure lifts the sea

Air has weight. When pressure drops, the sea rises a little, an effect called the inverse barometer effect. The rule of thumb is about 1 centimeter of rise for every millibar (hectopascal) below average pressure, roughly 0.4 inch per millibar.

Pressure	Approximate change in sea level
1035 mb (strong high)	about 9 in (22 cm) lower
1013 mb (average)	no change
1000 mb	about 5 in (13 cm) higher
980 mb (strong storm)	about 13 in (33 cm) higher
960 mb	about 21 in (53 cm) higher
940 mb (intense hurricane)	about 29 in (73 cm) higher

This is an approximation, since the sea takes time to adjust. But it shows why pressure alone doesn’t explain a big surge: an intense hurricane of 940 mb lifts the sea only about 2½ feet this way. The rest comes from the wind.

Waves on top

Storm waves aren’t part of the surge itself, but they ride on top of it. They break against seawalls and dunes that would normally be out of reach, and breaking waves push extra water toward the shore, called wave setup. The NHC counts the tide, wave setup and freshwater flow from rivers as parts of the total water level.

Storm surge vs. storm tide

These two terms sound alike but mean different things:

• Storm surge is the rise caused only by the storm.
• Storm tide is the total water level during the storm: the astronomical tide plus the surge.

Storm tide = predicted tide + storm surge, and the waves come on top of that.

Hurricane Sandy shows the difference. At the Battery, on the southern tip of Manhattan, the surge was 9.40 feet (2.87 m). Because it came near high tide, the storm tide reached 14.06 feet (4.29 m) above mean lower low water, the zero of US tide charts. That was 4.36 feet higher than the previous record at a gauge with records going back to 1920. If you’re not sure what that zero means, our guide to chart datum and MLLW explains it.

Now picture the same surge arriving at low tide. In New York, the difference between low and high tide is around 4 to 5 feet, so the storm tide would have been several feet lower.

Why timing and spring tides matter

On coasts with a big tide, when the storm peaks matters as much as how strong it is. The Met Office notes that in places with large tides, a couple of hours’ difference can decide whether an area floods or stays dry.

The worst case is a storm that peaks at high tide during spring tides, around the new and full moon, when high tides are highest. NOAA points out that the highest storm tides often occur when a storm coincides with one of those periods. Sandy struck during the fall full moon period, which the NHC says enhanced its effects, and the great North Sea surge of 1953 arrived together with a high spring tide.

So when a storm is coming, check two things on the tide chart: the time of the next high tides and whether it’s a spring tide. Our guide to spring and neap tides explains when they happen.

Storms that show how it works

Hurricane Katrina, 2005

According to the NHC, Katrina’s surge reached about 24 to 28 feet (7.3 to 8.5 m) along a 20-mile stretch of the Mississippi coast, with a peak high-water mark of 27.8 feet at Pass Christian. Katrina had weakened to Category 3 by landfall, yet its surge was enormous. The NHC explains why: it was a huge storm, with hurricane-force winds reaching at least 75 nautical miles east of its center, and big waves had already built up the water level along the Gulf coast before landfall. Hurricane Camille in 1969 was stronger at landfall but much more compact, and its extreme surge covered a much narrower stretch.

Hurricane Sandy, 2012

Sandy’s surge reached 12.65 feet at Kings Point, on Long Island Sound, and 9.40 feet at the Battery. Storm tides set records across the New York area. Of the 72 direct deaths in the US, 41 were caused by the storm surge, according to the NHC.

The North Sea flood, 1953

On the night of January 31, 1953, a storm surge in the North Sea coincided with a high spring tide. More than 2,500 people died around the North Sea, including 307 in England and 19 in Scotland, according to the Met Office. Afterward, the UK invested heavily in sea defenses such as the Thames Barrier and in warning systems.

Big surges don’t need a hurricane. Winter storms such as nor’easters, unusually high tides and long spells of strong onshore wind also flood the coast, which is why the National Weather Service issues coastal flood warnings for them.

How storm surge is forecast and warned

In the US

• Storm surge watches and warnings are issued by the NHC with tropical, subtropical and post-tropical storms on the Atlantic and Gulf coasts. A watch means life-threatening inundation from rising water moving inland from the shoreline is possible, generally within 48 hours. A warning means that danger exists, generally within 36 hours.
• Coastal flood advisories, watches and warnings come from local National Weather Service offices for all other coastal flooding: winter storms, unusually high tides or long spells of strong onshore wind.

The forecasts come from computer models of the ocean driven by the storm’s forecast track, size and winds. The NHC’s main one is called SLOSH (Sea, Lake and Overland Surges from Hurricanes). Before Katrina, NHC advisories called for a surge of 18 to 22 feet, locally as high as 28 feet, near where the center made landfall, and the high-water marks found afterward showed those forecasts were quite accurate. The weak point is the storm itself: if its track or strength changes, so does the surge.

In the UK and Canada

• In the UK, the Met Office forecasts how the weather will raise the sea, and the Environment Agency uses those forecasts to issue coastal flood warnings in England. Wales and Scotland have their own agencies, Natural Resources Wales and SEPA.
• In Canada, Environment and Climate Change Canada issues storm surge warnings.

What TidePlanet shows

The times and heights on our tide pages are the astronomical tide, like any tide chart, and we say so next to each day’s tide curve: wind and air pressure can raise or lower the water, more so in a storm.

When a storm is coming, the Tides and Beach pages also show a notice: “The sea may rise higher than the tide forecast.” It tells you how much higher the sea may rise above the predicted tide and when, such as “about 14 in tomorrow morning,” looking 48 hours ahead.

How it works:

• Where it comes from: the sea level forecast of Météo-France’s ocean model for Copernicus Marine, via Open-Meteo. We take out the tide and keep what the storm adds.
• When it shows up: only when the forecast rise reaches at least 10 inches (25 cm) and stands out clearly from the model’s margin of error at that spot. On a normal day you won’t see it.
• Where it shows up: at places whose tide comes from an official or nearby station (precision levels A and B), such as New York, Galveston or Charleston. Also at places with no significant tide in the Mediterranean, the Sea of Marmara and the Baltic, such as Marseille or Helsinki, and at Mediterranean places with no station nearby, such as Piraeus: there’s no tide to take out there, so the notice says how far the sea may rise above normal. Not where the tide comes from the global model, nor in the Black Sea or the Danish straits, where the model falls short or we couldn’t check it.
• Drops: the Tides page also shows “The sea may fall lower than the tide forecast” when offshore winds and high pressure are set to leave the sea 10 inches (25 cm) or more below the predicted tide. Low tides will be lower than the chart shows, which matters for anchoring, beaching a boat or entering harbor.
• In strong storms: from about 20 inches (50 cm), the notice adds that the sea can rise (or fall) even further and that you should follow official warnings, because models tend to fall short in the strongest storms.

It isn’t an official warning. On US place pages, we also show the National Weather Service alerts in effect for the area, such as coastal flood and hurricane alerts, each linked to the full official text.

What to do when a surge is forecast

• Check the time of high tide and whether it’s a spring tide. The highest water usually comes around high tide.
• Follow official warnings from the NHC and your local National Weather Service office.
• Evacuate if told to do so. Under a storm surge warning, check for evacuation orders from your local officials, and keep following their instructions.
• Don’t go to the shore to watch the waves, and don’t drive into flooded roads: turn around, don’t drown.
• On the flats or in tide pools, remember that onshore wind and low pressure can bring the tide in higher and sometimes earlier than the chart says. Our tide safety guide covers the basics.

And if you’ve ever wondered why the water rarely matches the chart exactly even without a storm, see why the tide differs from predictions.