Storm surge is the rise in sea level that a storm pushes onto the coast, over and above the normal tide. The US National Hurricane Center (NHC) calls storm surge the leading cause of deaths from hurricanes, and the water can travel up rivers and canals well inland. In 2005, Hurricane Katrina’s surge reached 27.8 feet (8.5 m) above the normal tide at Pass Christian, Mississippi.
The same storms often bring heavy rain. When the sea is high, rain on low coastal ground cannot drain away by gravity, so a street can flood from the sea and from the rain in the same storm. Scientists call that compound flooding.
- Storm surge is sea water a storm pushes ashore above the normal tide. Storm tide is the total water level: the tide plus the surge.
- Onshore wind drives most of a storm surge. A shallow, gently sloping sea floor, the shape of the coastline and the tide when the storm arrives decide how high it gets.
- Heavy rain makes it worse. A high sea can cover storm drain outfalls, so rain collects in low streets. Scientists call this compound flooding.
- Sea level rise raises the starting point for every storm surge.
- To check a coastal site, read the ground height, a coastal flood map and where rain collects, each with its limits. The worked example is southwest Hoboken, New Jersey.
Storm surge meaning: sea water pushed ashore above the normal tide
The NHC defines storm surge as “an abnormal water level rise generated by a storm over and above the predicted astronomical tide.” The Intergovernmental Panel on Climate Change (IPCC) gives the same idea in its glossary. Storm surge is a temporary rise in the height of the sea at a place, caused by low air pressure, strong winds or both. It is measured above the level the tide alone would bring.
Storm surges come from tropical cyclones, which are called hurricanes or typhoons in different oceans, and from strong winter storms. On 5 December 2013, a large storm surge flooded parts of the east coast of the UK, caused by a mix of low pressure, high winds and a high tide, according to the Met Office.
Storm surge vs storm tide
Storm tide is the total water level during a storm: the normal tide plus the storm surge. A surge that arrives at high tide produces a higher storm tide than the same surge at low tide, and the NHC notes that storm tides can reach 20 feet or more in some cases. Neither figure counts the waves on top, which can add more feet.
What causes storm surge: wind, pressure and the shape of the coast
Strong onshore winds cause most of a storm surge. Over deep water, the storm’s winds set up a circulation in the ocean with no sign of a surge. Near the coast, the shallow sea floor blocks that circulation, so the water moves up and inland, the NHC explains.
Wind and low air pressure
Wind does most of the work. Low pressure at the centre of a storm adds a little more, about 1 cm of sea level for every 1 millibar fall in air pressure, according to the Met Office. The Met Office calls this the inverse barometer effect.
The shape of the sea floor and the coastline
A wide, gently sloping continental shelf lets a higher surge build. The NHC names the coasts of Louisiana and Mississippi as especially exposed because the sea floor deepens slowly offshore, while Florida’s southeast coast drops off quickly and is less exposed. A coastline that curves inward, such as Apalachee Bay in Florida, sees a higher surge than one that curves outward, such as North Carolina’s Outer Banks.
Storm strength, size, speed and angle
Stronger winds produce a higher storm surge, and a larger storm pushes on a larger area of ocean for longer. A fast storm raises a higher surge right at the coast, and a slow storm pushes the surge farther inland. A storm that moves straight onto the coast tends to produce a higher surge than one that runs along it at an angle.
The tide when the storm arrives
The tide sets the starting level. In places with large tides, the Met Office notes that “just a couple of hours’ difference” in timing can decide whether an area floods. The 1953 North Sea flood struck at the same time as a high spring tide.
Check How High a Coastal Site Sits
Storm surge reaches low ground first. Look up the ground height at any address, see the terrain around it in 3D, and draw a cross-section toward the water.
Storm surge vs tsunami, tides and other coastal flooding
Three other kinds of high water are often confused with storm surge:
- Tides rise and fall every day and are predictable. Storm surge is the extra water a storm adds on top of them.
- A tsunami is “a series of waves caused by earthquakes or undersea volcanic eruptions,” in NOAA’s words. Storm surge comes from a storm’s wind and low pressure.
- Coastal flooding is the wider term for sea water flooding coastal land. Storm surge is one cause of it. High tides, waves and rivers swollen by rain are others.
How high storm surge gets: notable storm surges on record
| Event | What happened | Source |
|---|---|---|
| North Sea flood, 1953 | A storm surge arrived with a high spring tide. More than 2,500 people died around the North Sea coastline, including 307 in England and 19 in Scotland. | Met Office |
| Hurricane Katrina, 2005 | The surge reached 27.8 ft (8.5 m) above the normal tide at Pass Christian, Mississippi. | NHC Tropical Cyclone Report |
| Hurricane Sandy, 2012 | The surge at The Battery in New York was 9.40 ft (2.87 m) above the normal tide, and about half of the city of Hoboken, New Jersey, was reported flooded. | NHC Tropical Cyclone Report |
| Hurricane Ian, 2022 | The NHC attributes 41 deaths to the storm surge. | NHC |
The 1953 flood led the UK to invest in sea defences such as the Thames Barrier and in coastal flood warnings. The NHC’s storm surge risk maps show the risk “extending many miles inland from the immediate coastline in some areas.”
Storm surge and heavy rain: how compound flooding makes it worse
The IPCC describes compound flooding as “a storm surge in combination with extreme rainfall and/or river flow.” A large surge floods the coast directly, and the rain adds to it. A smaller surge can cause flooding too. Wahl and colleagues (2015) describe a surge that floods nothing by itself but blocks the drains. In their words, it is “high enough to fully block or slow down gravity-fed storm water drainage, such that precipitation is more likely to cause flooding.”
Their paper starts from the point that when surge and heavy rain coincide, “the potential for flooding in low-lying coastal areas is often much greater than from either in isolation.” Their study of US coasts found the risk of compound flooding higher on the Atlantic and Gulf coasts than on the Pacific coast. Their New York example is a day with 120 mm of rain and a 115 cm surge. The expected gap between such days shortened from about 105 years in the 1940s to about 42 years in recent decades. The authors link that change to a shift toward storm weather that brings both surge and heavy rain.
Rivers add a third driver. Where a river meets the sea, flood maps that ignore how river flow and the sea interact “can underestimate flood extent and depth,” Ward and colleagues (2018) note. The risk depends on the place, though. The IPCC notes estuaries where the river peak and the surge peak did not arrive together.
Sea level rise raises the starting point for every storm surge
Storm surge rides on top of the sea, so a higher sea lets the same storm reach higher ground. The IPCC expects that in coastal cities “the combination of more frequent extreme sea level events (due to sea level rise and storm surge) and extreme rainfall/riverflow events will make flooding more probable” (high confidence). For the projections themselves, see our guide to sea level rise.
How to check a coastal site’s storm surge and flood exposure
Three readings, taken together, describe a coastal site’s exposure. Each answers a different question, and none of them is a forecast.
Ground height and distance to the sea
Start with how high the ground sits and how close the site is to the sea, a tidal river or a canal. Surge reaches low ground first, and it can travel up rivers and canals. You can check the ground height at any address with our free elevation tool.
Coastal flood maps by return period
Global coastal flood maps, such as WRI’s Aqueduct Floods, show where the sea could reach in a storm of a given rarity, such as a 1-in-100-year event. They leave out sea walls and other defences. They work in cells about 1 km across, so they screen a neighbourhood and can miss narrow low ground. For US coasts, the NHC’s storm surge risk maps are built from tens of thousands of simulated hurricanes run through its SLOSH surge model.
The rain side: where water collects when it cannot drain
A rainfall simulation on the site’s terrain shows where storm rain would pond if nothing drained away. A simulation of this kind cannot add a high sea or a storm surge, and it cannot block drains at high tide. Read it as the rain side only, next to the coastal map. The water’s behaviour on the ground is covered in our guides to pluvial flooding and flash floods.
Worked example: storm surge and rain in southwest Hoboken, New Jersey
Hoboken shows both halves of the problem on the same ground. The city’s lowest area, in the southwest, sits less than three feet above sea level. Sandy’s surge flooded about half the city in 2012, and Sandy brought less than an inch of rain. Ordinary heavy rain floods the same streets. Several of the city’s storm water outfalls sit below the high tide level, so when heavy rain coincides with a high tide in the Hudson River, the water cannot drain out. The sewer authority reported flooding in southwest Hoboken on 7 August 2026, when rain arrived during the high tide.
We ran one-hour storms on the terrain of southwest Hoboken, using the rainfall depths NOAA publishes for that point. They range from 27 mm (about once a year) through 48 mm (1-in-10-year), 66 mm (1-in-50) and 73 mm (1-in-100) to 94 mm (1-in-500). The model has no drains, no pumps, no soakage and no high sea, so the depths are an upper bound for rain falling on the modelled area.
- At eight of the lowest points in the modelled area (ground 1.1 to 1.6 m), the once-a-year storm collects 0.4 to 1.2 m of water.
- The 1-in-100-year storm collects 0.6 to 1.3 m at the same points, and the fastest of them passes 0.3 m about 6 minutes after the rain starts.
- Almost a quarter (23%) of the ground lying below 3 m in the modelled 2 km area passes 0.3 m in the 1-in-100-year storm.
- The water gathers in hollows with no surface way out, so at six of the eight points the depth is still at its highest when the run ends, an hour after the rain stops.
The sewer authority’s pumps exist to push that water into the Hudson against the high tide, and a floodwall against storm surge is under construction, expected to run through summer 2027. Neither is in the simulation, and building outlines cover only part of the area, so the result follows the terrain. The direction matches the city’s own account. Low-lying areas of western Hoboken can flood when more than 0.8 inches of rain falls in an hour, less than the 1.08 inches that falls in one hour about once a year.
Continuuiti’s Flood Risk API returns the published coastal flood map at four return periods and runs the rainfall simulation on site terrain, as two separate readings for each location.
Storm surge exposure for coastal sites has two halves: the sea and the rain
Storm surge is the extra sea water a storm pushes ashore above the tide, driven mostly by onshore wind and shaped by the sea floor and the coastline. In some areas it reaches many miles inland, and sea level rise raises its starting point. Heavy rain makes it worse where a high sea stops storm water draining, so the exposure of a low coastal site has two halves. Check the ground height, the coastal flood map and where rain collects, and read each with its limits in mind.
Sources
- US National Hurricane Center (NHC): Storm Surge Overview; National Storm Surge Risk Maps.
- NHC Tropical Cyclone Reports: Hurricane Katrina (Knabb, Rhome and Brown; updated 2023), p. 9; Hurricane Sandy (Blake et al., 2013).
- NOAA Ocean Service: What is storm surge?; What is a tsunami?
- Met Office: Storm surge.
- IPCC (2021). Climate Change 2021: The Physical Science Basis: Summary for Policymakers (C.2.6), Chapter 9 (section 9.6.4.2) and Annex VII Glossary.
- Wahl, T., Jain, S., Bender, J., Meyers, S.D. and Luther, M.E. (2015). Increasing risk of compound flooding from storm surge and rainfall for major US cities. Nature Climate Change 5, 1093-1097.
- Ward, P.J. et al. (2018). Dependence between high sea-level and high river discharge increases flood hazard in global deltas and estuaries. Environmental Research Letters 13, 084012.
- Ward, P.J. et al. (2020). Aqueduct Floods Methodology. World Resources Institute technical note.
- Hoboken: City of Hoboken, Flooding information (archived 2019) and City of Hoboken opens New Jersey’s largest resiliency park (12 June 2023); North Hudson Sewerage Authority, The Facts About Hoboken and Wet Weather (2019) and Storm report, 7 August 2026 rain event; New Jersey DEP, Rebuild by Design Hudson River Final EIS, chapter 2 (2017) and Rebuild by Design Hudson River FAQs (2025).
- Rainfall depths: NOAA Atlas 14, Volume 10, 60-minute partial-duration estimates at the Hoboken site (NOAA Precipitation Frequency Data Server).
- Hoboken model: Continuuiti Pluvial Flood Solver on GEDTM30 terrain (OpenGeoHub, CC BY 4.0) with Microsoft GlobalMLBuildingFootprints (CDLA Permissive 2.0). Method and limits in our flood risk methodology.
Frequently asked questions
Are storm surge and flooding the same thing?
No. Storm surge is one cause of coastal flooding: sea water a storm pushes onto land above the normal tide. Coastal land can also flood from very high tides, waves and rivers swollen by rain, and heavy rain can flood low coastal streets when a high sea stops the drains emptying.
How far inland can storm surge go?
In some areas, many miles. The NHC’s storm surge risk maps show the risk extending many miles inland from the immediate coastline in some areas, and surge can travel up rivers and canals. A slow storm pushes the surge farther inland than a fast one.
Can heavy rain make storm surge flooding worse?
Yes. A high sea can block or slow the drains that carry storm water out to sea, so rain that would normally drain away collects in low streets. Researchers studying US coasts note that the potential for flooding in low-lying coastal areas is often much greater when surge and heavy rain occur together than from either one alone.
Does sea level rise make storm surge worse?
It raises the level a storm surge starts from, so the same storm can push water higher up the shore. The IPCC expects more frequent extreme sea level events, from sea level rise and storm surge, together with extreme rainfall and river flow, to make flooding in coastal cities more probable.
What is the difference between storm surge and a tsunami?
A tsunami is a series of waves caused by earthquakes or undersea volcanic eruptions, according to NOAA. Storm surge is a rise in sea level driven by a storm’s wind and low air pressure, on top of the normal tide.
