Storm Surge Forecast Interpretation

Effects and Forecast Types

Forecasts about storm surge differ from other hurricane forecasts in that they are not predictions about the storm itself, but about its effects on the ocean's interaction with land.

Storm surge is seawater or lake water pushed ashore by hurricane winds, resulting in the inundation of a coastal area. The height of the surge at any point on land depends not only on the meteorological attributes of the hurricane but also on the configuration of the coastline with which it is interacting. Gradually descending coastlines (small slopes underwater close to shore) have the tendency to pile up more water onshore, leading to higher storm surges. Coastlines shaped like funnels have the ability to concentrate the water on one location, increasing the destructive potential of the surge, provided the storm comes in at the correct angle.

Astonomical high tides that may coincide with a surge event are not part of the storm surge per se, but are added to it to get the total height of the water. Storm surge also doesn't include the effects of accumulating rainfall from the hurricane or the height of the wind-driven waves breaking atop the water dome.

Storm surge forecasts of the categorical type are issued within the public advisory, together with the hurricane watches and warnings. This storm surge forecast was issued with Hurricane Katrina Advisory #19 on the night of August 27, 2005 - with Katrina as a low Category-3 storm but expected to intensify - about a day and a half before its landfall along the Gulf Coast:

COASTAL STORM SURGE FLOODING OF 15 TO 20 FEET ABOVE NORMAL TIDE LEVELS...LOCALLY AS HIGH AS 25 FEET ALONG WITH LARGE AND DANGEROUS BATTERING WAVES...CAN BE EXPECTED NEAR AND TO THE EAST OF WHERE THE CENTER MAKES LANDFALL.

Like other forecasts, storm surge forecasts are made by human forecasters using the output from numerical models as guidance to their efforts. The SLOSH model - "Sea, Lake, and Overland Surge from Hurricanes" - is a numerical model that predicts storm surge heights resulting from hypothetical hurricane impacts in highly localized coastal areas. Its projections have been found to be precise to within about 20 percent.

Meteorological factors capable of influencing storm surge heights include the hurricane's intensity, both as central pressure and maximum sustained winds, its direction of approach, its radius of maximum winds, and its forward speed. In the example above, the forecast for a maximum 15-20 ft storm surge would have been based in part on Katrina's projected intensity, direction of approach, and forward speed at landfall, as well as the land and near-shore characteristics of the coastline.

The forecast above singles out the area "near and to the east of where the center makes landfall" for the highest surges because these tend to occur ahead of the right side of the storm, relative to its direction of motion, where the highest winds are found (northern hemisphere). In the case of a storm moving in an approximately northerly direction, like Katrina, the storm's right side is east of center. For a westbound storm, the storm's right side is north of center. The right side of a hurricane in the northern hemisphere is sometimes referred to in common parlance as the "dirty side" of the storm. Although the winds and therefore the storm surges may be generally less on the left side of the storm, that doesn't in any way diminish their ability to cause massive destruction of the kind witnessed in New Orleans, which had taken the "clean side" of Hurricane Katrina.

In addition to the categorical surge forecast, there is also a separate probabalistic surge forecast, as represented by this graphic (below). It describes, using colors for visual ease, the probability (in tens of percentage points) that a specific tropical cyclone will produce surge heights of five feet or greater above normal tide at a range of locations during a period just exceeding three days.

Example of a probabalistic storm surge forecast

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