CSU slightly under-predicts relatively active 2016 Atlantic hurricane season

hurricane matthew

This image of Hurricane Matthew along the southeastern U.S. coast was taken by NOAA’s GOES-East satellite on Oct. 8, 2016. 

The 2016 hurricane season had somewhat above-average activity – a bit more than predicted by the CSU Tropical Meteorology Project forecast team.

“The 2016 hurricane season had activity at levels slightly more than predicted. Overall, our predicted numbers for storm formations were quite close to what was observed, but our forecast for Accumulated Cyclone Energy was a bit too low,” said Phil Klotzbach, lead author of the forecast. Seasonal Accumulated Cyclone Energy (ACE) was approximately 140 percent of the 1981-2010 median, due in part to long-lived, intense Hurricane Matthew in late September through early October.

The report summarizes all tropical cyclone activity in the Atlantic basin during the 2016 hurricane season and compares the team’s seasonal and two-week forecasts to what actually occurred.

Several factors likely combined to make the season have the levels of activity that were observed. The primary reason why tropical cyclone activity was somewhat limited, especially during September, was due to persistent mid-level dryness that pervaded both the tropical Atlantic and the Caribbean. However, unlike 2014 and 2015, vertical wind shear levels (the change in wind direction with height) were somewhat below average, due in large part to the absence of El Nino.

Borderline weak La Nina conditions were present during August-October. La Nina is characterized by cooler than normal water in the central and eastern tropical Pacific. The tropical Atlantic was slightly warmer than normal throughout most of this year’s hurricane season, providing an additional slight enhancing factor.

The Colorado State team, founded by the late William Gray and led by Klotzbach, made its long-range seasonal forecasts, which called for a near-average hurricane season, on April 14 and June 1. Two additional updates during the hurricane season were issued on  July 1 and Aug. 4. Each of these updates continued the call for a near-normal season.

In the first forecast issued on April 14, the team called for 13 named storms, six hurricanes and two major hurricanes. The June 1 forecast called for 14 named storms, six hurricanes and two major hurricanes. The July 1 and Aug. 4 updates called for 15 named storms, six hurricanes and two major hurricanes. Observed activity was 15 named storms, seven hurricanes and three major hurricanes. Near-average ACE was predicted by all of CSU’s seasonal forecasts; however, the observed level (135 ACE) was higher than forecast (93 predicted in April, 94 predicted in June, 95 predicted in July and 100 predicted in August). For comparison, 1981-2010 median ACE is 92.

The team bases its annual forecasts on 60 years of historical data and includes factors such as Atlantic sea surface temperatures and sea level pressures, levels of vertical wind shear (the change in wind direction and speed with height), El Nino (an anomalous warming of waters in the central and eastern tropical Pacific) and other factors. While these forecast factors generally work well and explain approximately 50-60 percent of the year to year hurricane variability in these 60 years of historical data, there still remains 40-50 percent of this variability which is not explained.

Hurricane statistics for 2016 contained in the report include the following:

  • Three major hurricanes formed in 2016. This is the first year with at least three major hurricanes since 2011.
  • Hurricane Hermine made landfall in the Big Bend of Florida on Sept. 2 – ending Florida’s record-long hurricane drought at 3,966 days.
  • Hurricane Matthew became the first Category 5 hurricane in the Atlantic basin since Felix (2007).
  • No major hurricanes made United States landfall in 2016, although Hurricane Matthew came within about 50 miles of breaking this streak. The last major hurricane to make U.S. landfall was Wilma (2005), so the U.S. has now gone 11 years without a major hurricane landfall. The U.S. has never had another 11-year period without a major hurricane landfall since records began in 1851.

Although the Atlantic has seen a large increase in major hurricanes during the recent period of 1995-2016 (average 3.4 per year) in comparison with the prior 25-year period of 1970-1994 (average 1.5 per year), the U.S. has been fortunate that few major hurricanes have made U.S. landfall (except for the two very damaging years of 2004-2005). The Atlantic basin has had 30 major hurricanes since Wilma, with no major hurricane landfalls.  The 20th century average is that approximately 30 percent of all major hurricanes forming in the Atlantic make U.S. landfall.

Klotzbach and Gray have attributed the upturn in major hurricane activity since 1995 as well as the earlier increase in major hurricane activity from the late 1940s through the mid-1960s to natural multi-decadal variability in the strength of the Atlantic Multidecadal Oscillation (AMO) or Atlantic Thermohaline Circulation (THC). A concomitant increase in a number of favorable hurricane-enhancing parameters occur in the tropical Atlantic during the positive phase of this oscillation – while these same parameters tend to suppress hurricanes during the negative phase of this oscillation.

There is the potential that the active Atlantic hurricane era is coming to an end given three relatively quiet seasons of 2013-2015, although the fairly active season we have just witnessed puts the current status in doubt.

The Tropical Meteorology Project has been issuing forecasts for the past 33 years.  A brief qualitative outlook for the 2016 hurricane season will be issued on Wednesday, Dec.  14, with a first full forecast issued in early April 2017.