September 10 marks the statistical peak of the Atlantic hurricane season. Based on data gathered since 1851, it’s been more likely for a hurricane [or tropical storm] to be active on this date than on any other in the entire season — from June 1 to November 30. Climate scientists, meanwhile, take a longer view: how will hurricane numbers and
MoreSpring is in full bloom in the U.S., which means summer isn’t far behind. And with this march of the seasons, the risk for hurricanes returns. Inevitably, there is a desire to know how active the season is going to be as far in advance as possible.
This season looks to be just slightly less active than average, which translates to a slightly reduced risk of a hit on the U.S. coastline. But it doesn’t mean that such a storm can’t, or won’t, do so.
The Colorado State University Tropical Meteorology Project has been developing hurricane season outlooks since 1982. The outlooks were pioneered by the late Dr. William Gray, and they are now overseen by Dr. Philip Klotzbach, a former graduate student of Dr. Gray.
Given the high variability from year to year in the Tropical Atlantic and the difficulty in making an accurate projection, the team is often asked why they take on such a task in the first place.
“Everyone should realize that it is impossible to precisely predict this season’s hurricane activity in early April. We issue these forecasts to satisfy the curiosity of the general public and to bring attention to the hurricane problem,” the team said in a statement. “There is a general interest in knowing what the odds are for an active or inactive season.”
The outlook is not intended to single out a specific area of the United States as a target of a hurricane. But some groups, like the insurance industry, can benefit from an accurate outlook.
This year, the group expects the season to be less active than normal, but not by much. They are projecting 11 named storms and four hurricanes, compared to the 1981-2010 median for those two values of 12 and 6.5, respectively. (Storms are named when they reach tropical storm status.)
It follows that they expect a less than average chance of a strike on the U.S. coastline, estimating it at 42 percent. The average odds of a U.S. hit over the last century are 52 percent.
While this may seem comforting to coastal residents, the team always sends this reminder, which is echoed by the entire meteorological community: “Coastal residents are reminded that it only takes one hurricane making landfall to make it an active season for them, and they need to prepare the same for every season, regardless of how much activity is predicted.”
The rationale for the outlook is based on current water temperatures in the Atlantic Ocean and the potential for a new El Niño to develop this summer into the fall. Hurricanes only form over warm ocean water, so water that is a cooler than normal is slightly less supportive of hurricane development.
El Niño is largely a Pacific Ocean phenomenon, but the wind patterns associated with it reach beyond the Pacific. These wind patterns introduce shear in the tropics, which means winds are changing direction and speed from the lower atmosphere to the upper atmosphere. Thriving hurricanes need to be away from wind shear, so that their energy can be concentrated within the storm. Too much wind shear effectively tears the storm apart.
Sea surface temperature anomalies. Blue areas indicate water colder than normal.
The Colorado State team’s outlooks have had mixed results historically, but in 28 of the 35 years they’ve been doing them, their methods have correctly indicated whether the season was above average or below average. They had tremendous success in projecting the active 2005 season, but badly over-forecast the numbers for the 2013 season.
Longer term, the ways in which climate change may impact hurricanes is uncertain and potentially mixed. Hurricanes (one name of a class of storms called tropical cyclones) form over other oceans of the world as well. Globally averaged, there may not be a big increase in the number of tropical cyclones by the end of the century, but there may be an increase in the number of the strongest storms.
And in any event, as the sea level continues to rise, the same strength of storm can do more damage in the future from storm surge, which is that onshore push of water as the eye of the storm nears the coast.
The CSU team will update its outlook on June 1, the official beginning of the Atlantic hurricane season. The National Oceanic and Atmospheric Administration typically releases its outlook in late May.
