How can a storm push the sea onto land?

Strong winds blowing toward a coast transfer momentum to the sea surface. Water can pile up against the shore rather than moving away freely. The storm-related rise above the water level expected from the tide is called storm surge.

Low air pressure in the storm also contributes: less downward atmospheric pressure permits a higher water surface. Near many coasts, wind-driven piling is the larger contribution. The shape and depth of the seafloor can make a large difference to the resulting rise.

Storm surge is not just one tall wave. It raises the water level over which waves arrive. Astronomical tide adds another variation, so the same surge at high tide can produce a different total water level than at low tide.

Once the elevated sea reaches low land, it can flow inland through streets, channels and openings. Rainfall and river flooding may add still more water. Explaining coastal flooding therefore requires several linked processes rather than treating every rise as a tsunami or an unusually large breaking wave.