Forecasters and emergency preparedness officials have struggled for years with a potentially life-threatening issue when it comes to warning people about the dangers of an approaching hurricane.
The problem is that the simplest and most commonly used way of describing a hurricane, the well-known category system based on wind speed, gives little indication of how badly its storm surge will flood coastal areas.
That issue was demonstrated dramatically in 2012, when the relatively minor Hurricane Isaac caused massive and widespread flooding.
This year, the National Weather Service is trying to tackle the problem by redirecting the focus from wind speed to storm surge.
Storms still will be referred to by the Saffir-Simpson Hurricane Wind Scale, which ranges from low-intensity Category 1 to devastating Category 5. But forecasters now also will release detailed maps showing how severe the flooding could be when the storm comes ashore — at least outside the levee system that surrounds much of Orleans, Jefferson and St. Bernard parishes.
Inside that protective system, residents will be warned about the possibility of flooding but without the level of detail available in outlying areas.
The new modeling, combined with earlier and broader analyses that will be shared with local officials as they decide whether to call for evacuations, will be rolled out nationwide this storm season.
It is aimed at driving home the potential dangers of a storm and providing residents with a more accurate sense of the threat, said Kenneth Graham, meteorologist-in-charge of the National Weather Service’s office in Slidell.
The Weather Service will begin issuing maps of projected storm surge 48 hours before a hurricane is due to make landfall. They’ll be released about 80 minutes after the official wind speed and direction forecasts, which are issued every six hours at that point.
Because multiple variables can affect the severity of a storm’s surge, the system runs thousands of potential models for each storm, tweaking its exact track, intensity, forward speed and other factors.
“If we knew exactly how big, how fast and the actual track of the storm ahead of time, we could give a real exact portrayal of how much surge we’d have. But we don’t know that yet,” Graham said.
The most severe projected impacts from those models then will be stitched together to form what Graham described as a “reasonable worst-case scenario,” one depicting flooding levels that forecasters are 90 percent sure will not be exceeded. Those maps will show what areas are likely to flood and — outside of federal levee systems — how deep that flooding probably will be.
As long as forecasters have been issuing warnings about tropical storms, they have focused on a storm’s predicted track and the speed of its winds. That has led to situations where many residents did not take warnings about Category 1 or Category 2 hurricanes seriously enough, not realizing that even relatively weak storms can lead to massive flooding.
That danger was brought to the forefront during Hurricane Isaac in 2012.
“Isaac was barely a hurricane, and look at the impact from it,” said Graham, who recalled conducting interviews at the time in which he repeatedly warned people to ignore the storm’s category and evacuate in the face of potential flooding.
Many people, however, still looked at the storm’s Category 1 rating and figured they could ride it out.
Isaac’s impact would prove to be far greater than the force of its winds had led many to anticipate. LaPlace in St. John the Baptist Parish flooded, more than a hundred residents were stranded in homes in Braithwaite in Plaquemines Parish, and five people died in Louisiana and Mississippi.
Storm surge maps already were being talked about then, but Isaac increased the urgency of the issue.
In addition to the maps showing anticipated flooding levels, the National Weather Service also will release separate maps showing storm surge watches and warnings and the risk of individual threats from the storm such as winds, tornadoes and rain.
Overall, the aim is to give a more complete picture of a storm’s likely effects, creating a better sense among the public of its dangers and potentially bolstering the effect of evacuation calls by local officials.
For the New Orleans region and other areas surrounded by major levee systems, forecasters will be modeling likely storm surge and issuing warnings, but they will not predict how flooding will actually play out at a detailed level.
As long as the levees surrounding the region hold and the pumps keep operating, the primary flooding risk comes from the system being overtopped by a storm surge that is higher than the protective features. That creates a challenge for trying to pinpoint where flooding will occur.
It’s difficult to predict the rate at which overtopping will occur, said Heath Jones, chief of emergency management for the U.S. Army Corps of Engineers’ New Orleans office and one of the officials involved in designing the new maps.
And even if officials know how much water will make it over the levees, determining where it will go once inside them is a complicated and time-consuming task.
Models of the city that can be run quickly enough to be useful in the event of a hurricane are relatively basic and assume water will simply flow to the lowest points and fill up those areas, Jones said. That’s not necessarily a safe assumption, however, particularly if there’s a relatively minor amount of overtopping.
The area’s flat topography means seemingly minor features — a raised railroad line, a slight ridge — may be high enough to redirect the flow of floodwaters, pushing them to unexpected areas, Jones said.
Forecasters are working on that issue.
“The state of the science may be there one day, but we’re not there yet,” Graham said.
While the new models will show far more detail than in the past about the likely effects of a storm, Graham warned there are a number of things they won’t account for. Those include the amount of rainfall and the flow of water after the storm has passed, both issues that could lead to flooding.
Graham also warned that despite the level of detail on the storm surge maps, residents shouldn’t try to figure out if their individual properties will flood.
“If local officials say to go, go,” he said.
Understanding the limitations of the maps is key, Graham said. If an area sees less flooding in a hurricane than had been shown on the maps beforehand, it may be a sign the hurricane followed a slightly different track or had other characteristics that reduced its impact. That doesn’t mean, however, that residents shouldn’t take future forecasts seriously, Graham said.
“They were in danger. They just didn’t get hit,” he said.
Follow Jeff Adelson on Twitter, @jadelson.