Before he took over as director in June, the center was already experimenting with a new storm surge warning and maps intended to show potential inundation levels above ground level. Isaac, Knabb said, has given forecasters motivation to expand the testing.
Surge forecasts pose significant complexities, with the size, speed and approach angle of the storm factoring in with an array of local conditions —the timing of tides, the depth of coastal waters, the shape of the coast line, even the contour of the sea bottom. The main challenge now, said Knabb, is to quickly produce analyses and easy-to-understand maps.
While Knabb said the center welcomes feedback to improve future warnings, he also strongly defended its work on Isaac. Those complicated official advisories, for instance, are written to serve a broad array of users, from shipping interests to the military and local TV weathercasters, and are just one way information is delivered to the public.
In the days before Isaac approached South Florida and the Gulf Coast, Knabb was leading daily briefings with federal, state and local emergency managers, and he and other forecasters were regular faces on national and local news broadcasts. The NHC also was pumping out regular social media updates in more digestible nuggets on Facebook and Twitter. In all of it, the repeated message to the Gulf Coast was to worry more about water than wind.
Knabb acknowledges Isaac was a challenging storm for forecasters — a sprawling but poorly organized system that danced at the threshold of hurricane strength for much of its long journey across the Caribbean and into the Gulf of Mexico. Because it never developed a tight center core, computer models also spit out widely varying tracks for a storm with eerie timing, approaching South Florida just after the 20th anniversary of Andrew and Louisiana just before the seventh anniversary of Hurricane Katrina.
In the end, Knabb said, the center’s team of forecasters generally nailed Isaac’s track. Forecasters admittedly struggle with intensity predictions and Isaac was no exception, remaining on the low side of wind speed estimates. The center’s hurricane watches and warnings reflected that, with only the Keys falling under a hurricane warning as the storm’s core passed just south of Key West — but there was also a prediction of heavy rains, with up to a foot in some spots.
Still, there was grousing from the Keys that the storm had been overhyped, quickly followed by shock at what a brute Isaac became in Palm Beach County and Louisiana.
Knabb, who spent eight years as a senior hurricane specialist and science and operations officer at the National Hurricane Center and two years as deputy director of the Pacific Hurricane Center in Honolulu, calls that typical.
“After every storm, there are folks who express that what happens isn’t what they expected,” he said. “They either didn’t get it as bad as they expected or they got it worse.”
It seems especially pronounced for so-called minimal hurricanes below Category 3 “major” storm with winds of 111 mph or more.
Hurricane Ike in 2008, a huge Category 2 hurricane, surprised Texas when it flooded much of the coast and inundated Galveston. The storm wound up killing 112 people in the U.S. and doing nearly $30 billion in damage, making it the second-costliest hurricane in U.S. history. Last year, Category 1 Hurricane Irene caused some $7 billion in damage, most of it in New England states unprepared for torrential rains and flooding.