What is storm surge? Understanding the difference between surge and inundation 

Hurricane Milton is placing the entire state of Florida at risk; however, impacts from the storm can be attributed to several factors.

Storm surge is an abnormal rise in seawater level caused by a storm, primarily hurricanes, and Inundation, on the other hand, refers to the flooding of land areas that are normally dry.

Storm surge is defined as the abnormal rise of water generated by a storm over and
above the normal astronomical tide and is expressed in terms of height above
predicted or expected tide levels.

Since storm surge represents the deviation from normal tide levels, it is not referenced to a vertical or tidal datum. By contrast, storm tide is defined as the water level due to the combination of storm surge and the astronomical tide and is expressed in terms of height above a vertical or tidal datum.

A vertical datum is simply a base elevation used as a reference from which to measure heights (or depths). Similarly, a tidal datum is a base elevation defined by a certain tide phase. The definitions of various tidal datums can be found here.

Inundation is the total water level that occurs on normally dry ground as a result of the
storm tide, and is expressed in terms of height of water, in feet, above ground level.

Inundation provides the most clearly and commonly understood method for
communicating storm surge-driven coastal flooding. As such, the National Hurricane
Center’s (NHC) official forecasts provide storm surge-induced flooding information regarding inundation or feet of water above ground level.

However, many customers and partners have requested the relationship between inundation and various tidal or vertical datums. This is particularly important in understanding the relationship between the water level observations at a tide station and the official NHC forecast.

The tidal datum Mean Higher High Water (MHHW) is the best possible approximation of the threshold at which inundation can begin.

At the coast, areas higher than MHHW are typically dry most of the time.

Storm surges and inundation are two important but distinct effects of severe weather. Recognizing the differences between these two can help communities better prepare for and respond to the challenges posed by storms and flooding.

As Weather Authority Meteorologist Zach Maloch put it: “There is no rationalizing this. You must evacuate if you live below 10ft elevation or are in an Evacuation Zone.”