The 2017 hurricane season will be remembered for its brutality and endurance, but it also goes down in history as having the most accurate storm track forecasts since modern mapping of tropical cyclones began nearly 50 years ago.
While official verification of forecast accuracy won’t be released until early next year by the National Hurricane Center, University of Miami storm researcher Brian McNoldy tallied preliminary results to find the smallest track error rates on record.
For every measurement, which includes days 1 through 5 before estimated landfall, the 2017 forecasts topped the hurricane center’s running 5-year average by as much as 52 miles at the 120-hour mark.
On this last day of the hurricane season, storm experts said the improvements likely saved lives.
“It’s certainly noteworthy that the track forecasts beat the previous record at every lead time,” McNoldy said. “It wasn’t just that the longer-range forecasts were good, they were all good.”
One of the most precise forecasts in terms of path was for the aberrant Hurricane Irma, which raged as a 185-mph Goliath for a record-shattering 37 hours.
The hurricane center’s average forecast track error five days out is a 225-mile spread. This season, the average during the same time period was 173.6 miles.
But for Hurricane Irma, the 5-day track error was just 155 miles.
The 48-hour error rate as Irma closed in on Cudjoe Key was 58 miles, about 23 miles less than the overall average for that time period.
“Irma’s track forecast was very good. It was just a very well-behaved storm,” McNoldy said.
An estimated 6.5 million Floridians evacuated for Hurricane Irma, a storm that tormented the Caribbean for a breathless 11 days before the Sept. 10 landfall in the Keys as a dangerous Category 4 cyclone. For those who felt chased – first to the west coast, then back to the east coast, or out of the state entirely – the track forecast may have seemed flawed.
But James Franklin, the National Hurricane Center’s former chief of the hurricane specialist unit, said the direction of Irma’s approach and Florida’s narrowness meant even the most minor track errors exacerbated evacuation confusion.
“Maybe expectations are a little unrealistic to think we can distinguish two days in advance between Palm Beach and Tampa,” Franklin said. “They’re not that far apart.”
At its widest, Florida’s peninsula is about 160 miles coast-to-coast.
After Irma’s second Florida landfall on Marco Island at 3:35 p.m. Sept. 10, hurricane-force winds extended 80 miles from its center with tropical-storm force winds extending 415 miles.
“People can argue about uncertainty in the track, and whether one model did better than another, but it misses the point that Irma was just such a physically large storm,” said Chris Davis, an associate director at the National Center for Atmospheric Research.
Davis reiterated Franklin’s concerns about expectations for pinpoint forecast accuracy being too high, but also that people still focus too much on the center of the forecast cone to make decisions.
“The center of Irma went over Naples, Jacksonville flooded,” Davis said. “The cone covered a huge part of the state and there were hurricane-force gusts on each coast.”
The reasons for the increased accuracy in forecasting the tracks of 2017’s 17-named storms are multiple.
Pieces of code that go into computer models representing processes in the atmosphere, such as radiation and clouds, have gotten better. Computers are running at higher resolutions. More sophisticated satellites circle the Earth beaming down images so clear forecasters see towering cloud tops and fields of Saharan dust like never before.
And, simply put, 2017’s storm paths were easier to read.
“Deep tropical long-track storms tend to be a little easier to forecast track for because they move, for the most part, west-northwest at a fairly constant speed,” McNoldy said. “The trick then is when and how quickly they will recurve.”
Storm intensity can also play a role, with stronger storms forging clearer paths, Franklin said.
This season had its share of strong storms with six major hurricanes of Category 3 or higher. That’s triple the average season. The major storms included hurricanes Harvey, Irma, Jose, Lee, Maria and Ophelia. Harvey, Irma and Maria made landfall in Texas, Florida and Puerto Rico, respectively.
“The atmosphere surrounding strong storms is simpler in structure whereas complicated environments result in weak storms,” Franklin said.
Still a challenge this hurricane season was forecasting rapid intensification of storms. Of an estimated 40 incidents of rapid intensification, just six were accurately forecast, according to preliminary results from the NHC. Rapid intensification is defined as an increase in wind speeds of 34 mph or more over a 24-hour period.
Franklin said intensity forecasts were improving in the early years of the Hurricane Forecast Improvement Project between about 2010 and 2013. But the project, which was initiated after the devastating hurricane years of 2004 and 2005, has since suffered budget cuts.
Weather Underground co-founder Jeff Masters said maybe the active 2017 will spur interest in rebuilding hurricane research budgets.
“Wake up America, we have a hurricane vulnerability problem,” Masters said. “We should expect to see more years where we get pounded like this.”
2017 tropical cyclones and maximum sustained winds:
Tropical Storm Arlene, 50 mph
Tropical Storm Bret, 45 mph
Tropical Storm Cindy, 60 mph
Tropical Storm Don, 50 mph
Tropical Storm Emily, 45 mph
Hurricane Franklin, 85 mph
Hurricane Gert, 105 mph
Major Hurricane Harvey, 130 mph
Major Hurricane Irma, 185 mph
Major Hurricane Jose, 155 mph
Hurricane Katia, 105 mph
Major Hurricane Lee, 115 mph
Major Hurricane Maria, 175 mph
Hurricane Nate, 90
Major Hurricane Ophelia, 115 mph
Tropical Storm Philippe, 60 mph
* Major hurricane is a Category 3 or higher
(Source: National Hurricane Center)