Atlantic & Pacific Hurricane Season

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.

This year, not so much. 2014 has been one of the least active seasons on record, with only four named storms so far. Compare that with 2005, when there were a whopping 28. That’s one reason we’ve been hearing a lot more about eastern Pacific tropical systems lately, including a hit and a near-miss on Hawaii in August by Iselle and Julio, rough seas pounding southern California by Marie and record heavy rain flooding the Southwest from Norbert. The eastern Pacific hurricane season ends on November 30 as well, but begins a bit earlier - on May 15. There’s also a much less pronounced peak in the eastern Pacific, but its storms tend to be likeliest (by a small margin) on or about August 29.

We don’t talk as much about eastern Pacific storms in the continental U.S. because they usually don’t do much damage here. Atlantic hurricanes travel west and slam into the Americas; Pacific hurricanes form off Mexico and head northwest into wide open ocean. They sometimes double back to hit the mainland, but Pacific Ocean temperatures are much cooler than the warm Gulf Stream waters that help drive Atlantic storms, sapping the storms of their strength as they travel. One notable exception is the hurricane that hit San Diego in 1858 - but that just shows how rare they are.

If an eastern Pacific hurricane should manage to travel far enough west that it crosses the International Dateline before breaking up, it’s rechristened as a typhoon, which are what hurricanes are called in Asia (the name might come originally from a Persian word, or it might be Japanese in origin). It doesn’t happen often, but it happened in early August of this year, when Hurricane Genevieve morphed into Typhoon Genevieve without missing a beat.

One more thing to keep in mind: while hurricane season ends on November 30, hurricanes can’t read calendars and they occasionally form after that date. In 1954, the latest-forming hurricane on record, Alice, took shape on December 30, and persisted until January 6. It was thus both the last hurricane of 1954 and the first of 1955. It’s the only hurricane known to have had a foot in two different calendar years, although Tropical Storm Zeta repeated the feat by leaking from 2005 into 2006.

For information on how climate change is playing a role in tropical activity, take a look at the Climate Matters: Hurricanes that we released at the start of the Atlantic season.

The short term shows that we are in the middle of a quiet stretch of tropical Atlantic activity, but the long term trend shows a different picture. Although there are still questions about how global warming will ultimately influence tropical systems, the recently released National Climate Assessment states: Over the past three decades, “the intensity, frequency, and duration of North Atlantic hurricanes, as well as the frequency of the strongest hurricanes, have all increased.”