The Atlantic hurricane season officially started on June 1, and now two months in, we've already seen four named storms. The season got off to a record pace with two storms being named in May, an occurrence not seen in 107 years. Two more were named in June, with the latest being Tropical Storm Debby on June 24. Debby has been the most notable thus far in the season, having crossed Florida, providing drought busting rains. Chris, one of the four named storms, briefly became the season's first hurricane well out to sea.
After the rapid early season pace, the Atlantic went quiet, with no named storms in July. This is actually not at all unusual. Below you will see the tracks of the four storms so far this season, followed by a graph depicting the frequency of hurricanes through the entire six month season. This clearly shows things really don't begin ramping up until we get into the middle of August.
So the question now is what does the remainder of the season hold? Here in the United States, we've been extremely fortunate in recent years. After being pummeled by several major storms in 2004, 2005 and 2008, the U.S. Atlantic and Gulf of Mexico coasts have only seen one landfalling hurricane since. Last year, hurricane Irene made landfall in North Carolina as a category 1 storm, which then took a track into New York and New England as a weakening tropical storm, causing major flood damage.
The last two seasons totaled 38 named storms, an incredible number to only have one hurricane impact the states. How long can our luck hold out?
So as we head into the heart of the hurricane season, which typically is mid-August through September, we should expect to see an uptick in the number of storms in the next 8 weeks, if this turns out to be a typical year. There are MANY factors that need to be considered when forecasting hurricane activity. One of the most common is the cycle of the ENSO climate pattern, or what is more commonly referred to as "El Nino" or "La Nina". Earlier this year, a long lasting La Nina episode finally came to an end. Over the last several months, water temperatures in the equatorial Pacific have been gradually warming, leading to strong speculation that the ENSO cycle could already be making a transition to an El Nino. The image below shows the current sea surface temperatures across the equatorial Pacific and also how much they deviate from average.
Scientific consensus is that El Nino is in the developing stages which is expected to grow in the coming months. Computer guidance is in line with a weak to perhaps border-line moderate episode by later this fall. If this occurs, the peak of the hurricane season and beyond could very well be negatively affected, which would mean fewer storms. It is well documented that the El Nino cycle tends to inhibit tropical cyclone formation and intensification in the Atlantic Ocean. This is due to an increase in wind shear. The strength of the episode correlates well to the adverse impact. So far though, its development remains slow.
Another factor is Atlantic sea surface temperature. In general, hurricanes need warm water of 80 degrees or higher to develop and sustain themselves. Since 1995, the Atlantic has been in a positive phase of the AMO, or Atlantic Multidecadal Oscillation. This is a mode of variability in sea surface temperature that runs cooler or warmer than average for decades at a time. Water temperatures this year remain above long term averages across a good part of the Atlantic hurricane breeding grounds, but have cooled some compared to the last few years. In general, if other conditions are favorable, larger areas of abnormally warm water would favor more, and potentially stronger storms.
There are other factors, including dry, dusty air that comes off the African continent and sweeps across the Atlantic, which has been observed at times already this season. As I think you can see, forecasting the hurricane season can prove quite challenging. NOAA's early season forecast called for an average season, which is 11 named storms, 6 hurricanes, 3 of which become major hurricanes.. category 3 or higher on the Saffir/Simpson hurricane intensity scale. Their forecast is a broader range, indicating 9-15 storms are expected for the season. An update to their seasonal forecast is expected in the next week as we head into the heart of the season. As of this writing, Tropical Storm Ernesto has been named in the Central Atlantic.. and is expected to move into the Caribbean in the next few days..