Hurricanes Harvey and Irma: What's going on with this Atlantic Hurricane Season?

Hurricane Harvey and Hurricane Irma have definitely made an impact in Texas and Florida causing massive damage with flooding, storm surge and high winds. This season is the first season on record that the U.S. has had two, Category 4 (maximum sustained winds >=130 mph) hurricanes make landfall. So, what’s brewing in the Atlantic this year to make for such an active season?

The stats for the Atlantic hurricane season (so far) are as follows:

  • 11 named storms
  • 6 hurricanes
  • 3 major hurricanes (with maximum sustained winds >= 111 mph)

We’re currently near the climatological peak of the Atlantic hurricane season, which is typically regarded to be September 10 (Figure 1). The reason for a more active season boils down to a combination of the following factors:

  • Warmer ocean temperatures provide more fuel for hurricanes (see Figure 2)
  • Low levels of vertical wind shear (the change in wind direction with height in the atmosphere) help the hurricane maintain a well-organized structure and sustain its deep thunderstorm activity
  • High levels of mid-level moisture promote deep thunderstorm activity near the center of the hurricane circulation.

Following the very active 2004-2005 hurricane seasons where the U.S. was hit by 7 of the 13 major hurricanes that formed in the Atlantic basin, the 2006-2016 timeframe had 30 major hurricanes form in the Atlantic with 0 U.S. landfalling major hurricanes. On average, about 25% of all Atlantic major hurricanes make U.S. landfall. From 2006-2016 however, the U.S. tended to have large-scale atmospheric patterns that steered storms away from the U.S. mainland. This year however, the subtropical high over the North Atlantic Ocean has been stronger, causing storms like Irma to track further westward and impact the U.S. mainland.

Potential impacts from Jose – the 3rd major hurricane of the 2017 season (although currently a Category 2) are too early to determine. Some forecast models have Jose going out to sea without posing a threat to the U.S., while others show a significant threat. Jose is projected to do a loop in the Atlantic over the next few days, and storms doing loops have historically low predictability. At this point, the best approach to take is to follow the latest forecasts from the National Hurricane Center to see if any U.S. impacts should be expected.

 
Figure 1: Number of hurricanes and tropical storms forming by calendar date from May 1 to December 31. Figure courtesy of NOAA


Figure 2: Current ocean water temperature anomalies across the Western Hemisphere. Orange and yellow colors indicate warmer-than-normal ocean temperatures. Figure courtesy of NOAA

About the Author:
Dr. Phil Klotzbach has received national and international attention for his work in researching weather patterns and forecasting hurricanes. For the past 15 years, Dr. Klotzbach has co-authored Colorado State University's Atlantic basin hurricane forecasts with his esteemed colleague the late Dr. William Gray.