Thanks to cooler waters in parts of the Atlantic and a weaker-than-expected “El Nino” equatorial wind pattern, it’s been a quiet Atlantic hurricane season. Each year, it’s like a turn of the roulette wheel – only when the Cape’s number comes up, it won’t be cause for celebration.
While the National Oceanic and Atmospheric Administration issues a seasonal outlook each May for the upcoming hurricane season, its predictions are limited by climate science. Forecasters are able to predict, with some success, the number of named tropical storms, hurricanes and major hurricanes that will form in the entire Atlantic basin between June and November.
“But that’s where the reliable long-range science stops,” according to a NOAA blog post on Aug. 20. “The ability to forecast the location and strength of a landfalling hurricane is based on a variety of factors, details that present themselves days, not months, ahead of the storm.”
Every mile of the Atlantic and Gulf Coast – from Brownsville, Texas to Eastport, Maine – has some hurricane vulnerability, but some areas have higher odds than others. National Hurricane Center scientists use computer models that quantify those chances into a “hurricane return period,” or the frequency at which a hurricane can be expected to pass within 50 nautical miles of a given locale.
The highest return periods, representing the areas of lowest risk in a given year, are in Maine; the area of Portland has a return period of 50 years. By comparison, return periods in southern Florida and the Outer Banks of North Carolina are between five and seven years.
According to the models, Cape Cod has a return period of 16 years, and Nantucket has a period of 13 years. That puts the region on par with the South Texas gulf coast or the Delmarva Peninsula when it comes to the frequency of hurricanes. That prediction is based on the likelihood of a hurricane of any strength passing within 50 nautical miles.
It’s worth noting that hurricanes can have significant effects even if the center passes more than 50 nautical miles away, as Hurricane Bob did in 1991.
The predicted frequency of major hurricanes is understandably lower. According to the models, a hurricane with winds of 111 miles per hour or higher – a Category 3 storm on the Saffir-Simpson scale – passing near to the Cape is 58 years. The only places with higher return periods are the coast of Maine, with Portland not likely to see a major hurricane more often than once every 290 years.
“Regardless of the odds, everyone living or vacationing in a hurricane-prone location has to be prepared,” National Hurricane Center Director Ken Graham said.