Simple Tides / Guides / Storm Surge vs. Tide vs. Sea Level Rise

Storm Surge vs. Tide vs. Sea Level Rise: What's the Difference

News coverage during a hurricane or coastal storm often uses these three terms almost interchangeably, but they describe three genuinely different things — and the worst flooding happens when they stack on top of each other.

Astronomical tide

This is the predictable, twice-(or once-)daily rise and fall caused by the gravitational pull of the moon and sun, shown on any standard tide chart. It's known well in advance and doesn't depend on weather at all — NOAA can predict it years out.

Storm surge

Storm surge is a temporary rise in water level caused by a storm itself: strong onshore wind physically pushes water toward the coast, and the storm's low atmospheric pressure allows the sea surface to bulge upward slightly. Surge is added on top of whatever the astronomical tide happens to be doing at the time — it isn't a separate water level, it's extra height stacked onto the tide.

Why "surge at high tide" is the worst-case phrase

A storm surge that arrives during low tide may cause much less damage than the same storm arriving a few hours later at high tide, because the surge is added to a much higher starting point. This is why forecasters pay close attention to a storm's exact landfall timing relative to the tide, and why a king tide combined with a storm is treated as a serious flood risk.

Sea level rise

This is a slower, longer-term story: the average sea level itself has been gradually rising over decades, mainly from the thermal expansion of warming ocean water and melting land ice. Sea level rise doesn't cause a flood on its own, but it raises the baseline that both the daily tide and any future storm surge build on top of, which is why the same storm can cause more flooding today than an identical storm decades ago.

How this shows up in real data

Real-time water level gauges at NOAA stations — see our station guide — are what actually capture all three effects combined during a storm, which is why live gauge readings can run well above the predicted tide when a storm is offshore.