Surge Heights

Because of track forecast uncertainties, the SLOSH storm surge model is run for a variety of possible storm tracks. These many possibilities are condensed down into families of storm tracks, each representing one of the generalized directions of approach (e.g., west, north-by northwest, north, northeast, etc.) that would be logical for a hurricane to follow in that area.

A set of scenarios for a Category-3 hurricane moving toward the northeast at 15 mph might consist of a composite of model runs using a series of parallel storm tracks - one track per model run - with all the tracks pointing in a northeasterly direction, for instance, but impacting the coastline at a different points within the basin. In this way, the likely maximum storm surges may be predicted for a variety of hurricane tracks with the same general direction of approach.

SLOSH model composite data (Maximum Envelope of Water) for the Tallahassee, Florida surge basin

In one such set of scenarios at left, run for the Tallahassee, FL surge basin, the diagonal black lines with arrows indicating the direction of storm motion describe these parallel tracks. A different set of scenarios might be compiled for a family of Category-4 storms with easterly tracks approaching land at a speed of 10 mph; this combination would produce an entirely different distribution of maximum storm surge heights.

These composites help forecasters to analyze a wide variety of hurricane impact scenarios, in which intensity, direction of approach, and forward speed may be varied to produce different distributions of maximum surge values. Maximum surge heights for each point on the basin grid, regardless of the specific track to landfall that produced it, show up in the composite as the maximum possible at that location for a storm of a particular intensity, direction of approach, and forward speed.

Composites are then made of the composites, to enable the calculation of the maximum possible surges for any location within a basin from any Category-4 storm, for instance, regardless of its precise track through the basin, direction of approach, or forward speed. During the approach of an actual storm, those composites deemed most likely relevant to the current situation can be revisited and the maximum surge heights represented in those composites may be taken into account in the warning and evacuation processes.

Real-time individual SLOSH model runs are also performed using track forecast data; this is done just hours before an expected landfall, when track forecast uncertainties are at their lowest. However, the output from the real-time runs may still vary substantially from the actual surge heights, depending on exactly where the center makes landfall. Even at 12h, there is still uncertainty, however seemingly minimal, in the track forecast, and storm surge effects are highly sensitive to the exact point of impact. Hence the composite maximums, which were designed precisely to take these track forecast uncertainties into account in making predictions about surge heights and locations for a variety of landfall scenarios.

SLOSH model composite data (Maximum of Maximums) for the New Orleans surge basin

The second-level composites, the Maximum of Maximums, like this one for New Orleans at left, look very much like the first-level composites as in the earlier image, except that they represent combinations of the data represented in many such first-level composites taken together. The Maximums of Maximums indicate the likely maximum surge heights across a surge basin, but have already integrated the maximums for every contributing factor to storm surge except the hurricane's intensity, or category. That is why the diagonal lines that are present in the earlier first-level composite image are gone in this one. It is these second-level composites that are used to construct the surge maps for a coastal area.

Since the maximum surge heights in the Maximum of Maximums are constructed to vary solely by storm category, one is compiled for each category of storm, for a total of five for any given surge basin. All five will, in a general sense, show the greatest maximum surge heights for land areas closest to the sea, bay, or lake that would be the water source for the surge, as those are, generally speaking, the most exposed places. However, these likely maximum surge heights are not, at a finer level, solely determined by distance from the coast, because topographical factors also play a role in degree of exposure at any given place.

The Maximum of Maximums composites are then used by the Army Corps of Engineers to make surge maps for a large, usually multicounty, area as part of their hazards analysis for that area. Together with the analysis of traffic and population data, theose surge maps are used to make the evacuation zone maps for emergency management agencies to use in the planning of future hurricane evacuations for their areas of responsibility.

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